- Email: [email protected]
Urban water interfaces
Accepted Manuscript Urban Water Interfaces M.O. Gessner, R. Hinkelmann, G. Nützmann, M. Jekel, G. Singer, J. Lewandowski, T. Nehls, M. Barjenbruch PII: DOI: Reference:
S0022-1694(14)00295-9 http://dx.doi.org/10.1016/j.jhydrol.2014.04.021 HYDROL 19552
To appear in:
Journal of Hydrology
Received Date: Revised Date: Accepted Date:
4 October 2013 14 March 2014 8 April 2014
Please cite this article as: Gessner, M.O., Hinkelmann, R., Nützmann, G., Jekel, M., Singer, G., Lewandowski, J., Nehls, T., Barjenbruch, M., Urban Water Interfaces, Journal of Hydrology (2014), doi: http://dx.doi.org/10.1016/ j.jhydrol.2014.04.021
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
Urban Water Interfaces Gessner MO1,2, Hinkelmann R2, Nützmann G1,3,§, Jekel M2, Singer G1, Lewandowski J1, Nehls T2, Barjenbruch M2 1
Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany Technische Universität Berlin (TU Berlin), Berlin, Germany 3 Humboldt University Berlin, Berlin, Germany 2
Abstract Urban water systems comprise large-scale technical systems and natural and man-made water bodies. The technical systems constitute essential components of urban infrastructure for water collection, treatment, storage and distribution, as well as wastewater and runoff collection and subsequent treatment. Urban aquatic ecosystems are typically subject to strong human influences, and the quality and function of both surface and ground waters tend to be strongly modified, often with far-reaching impacts on downstream aquatic ecosystems and water users. The various surface and subsurface water bodies in urban environments can be viewed as interconnected compartments that are also extensively intertwined with a range of technical compartments of the urban water system. As a result, urban water systems are characterized by fluxes of water, solutes, gases and energy between contrasting compartments of a technical, natural or hybrid nature. Referred to as urban water interfaces, boundaries between and within these compartments are often specific to urban water systems. Urban water interfaces are generally characterized by steep physical and biogeochemical gradients, which promote high reaction rates. We hypothesize that they act as key sites of processes and fluxes with notable effects on overall system behaviour. By their very nature, urban water interfaces are heterogeneous and dynamic. Therefore, they increase spatial heterogeneity in urban areas and are also expected to contribute notably to the temporal dynamics of urban water systems, which often involve non-linear interactions and feedback mechanisms. Processes at and fluxes across urban water interfaces are complex and less well understood than within well-defined, homogeneous compartments, requiring both empirical investigations and new modelling approaches at both the process and system level. We advocate an integrative conceptual framework of the urban water system that considers interfaces as a key component to improve our fundamental understanding of aquatic interface processes in urban environments, advance understanding of current and future system behaviour, and promote an integrated urban water management.
Key words: urban water system, aquatic ecosystems, interface processes, water systems modeling, urban water management
§
Corresponding author 1
52
1. Introduction
53
Extensive landscape fragmentation through human activities has resulted in strong
54
proliferation of interfaces between distinct land-cover types (Cadenasso et al., 2007; Haase,
55
2009). In addition, humans have created numerous intersections between technical and non-
56
technical systems, including natural, though often strongly modified, ecosystems. Nowhere is
57
this perhaps more evident than in the tightly interconnected water systems of urban areas. The
58
technical structures designed for water treatment, storage and distribution, as well as for
59
wastewater and runoff collection, treatment and controlled discharge to receiving water
60
bodies, are vital components of urban infrastructure. They are often well managed, at least in
61
highly industrialized regions and countries, although increasingly also in major cities of the
62
developing world. Man-made channels and other artificial water bodies complement the
63
technical water infrastructure and natural aquatic ecosystems such as streams and rivers, lakes
64
and ground water in urban areas. Urban ‘natural’ aquatic ecosystems are typically subject to
65
strong physical, chemical and biological modifications, often with far-reaching negative
66
consequences for downstream aquatic environments and water users (Roy and Bickerton,
67
2012; Walsh et al., 2005; Meyer et al., 2005). For example, until tertiary wastewater treatment
68
was introduced to reduce the ammonium load generated by the metropolitan area of Paris,
69
France, a huge ammonium plume severely depleted oxygen concentrations in the estuary of
70
the River Seine 100 km downstream of the effluent (Brion et al., 2001).
71
Water use and management in cities is driven by a multitude of goals that may be
72
partially conflicting. They include water supply for domestic and industrial consumption,
73
adequate sanitation, and protection of humans and infrastructure from natural disasters.
74
Ongoing efforts to control pollutant loads to urban surface waters are starting to bear fruit.
75
However, rarely taken into account are impacts such as the disruption of linkages of urban
76
water bodies with their riparian areas and floodplains, disconnection of surface waters from
77
hyporheic zones and aquifers, or outright disappearance of natural surface waters in 2
78
underground pipes. Thus, by managing water resources and land to meet goals dictated by
79
immediate public, industrial or personal demands, humans have profoundly modified urban
80
water bodies (Grimm et al., 2008).
81
The term “urban stream syndrome” has been coined to describe the strong ecological
82
degradation of running waters consistently observed in urban environments (Meyer et al.,
83
2005; Walsh et al., 2005). Classic symptoms of the syndrome include flashy hydrographs that
84
promote physical disturbance in urban running waters; high loads of nutrients, suspended
85
solids and various pollutants ranging from heavy metals to personal care products and drugs;
86
and a simplified and homogenized channel morphology providing limited in-stream and
87
riparian habitat and habitat heterogeneity (Laub et al., 2012). Together, these and other
88
pressures severely compromise water quality and quantity in natural and man-made urban
89
water bodies. Consequences include impoverished biodiversity compared to natural
90
freshwater bodies and increased costs to ensure human water security in terms of water
91
resource supply, wastewater removal, and protection from excessive surface runoff and floods
92
(Grimm et al., 2008).
93
In the future, urban water management will face additional challenges as demographic
94
trends and continued migration of the world population to metropolitan areas leave their
95
imprint on urban water quality and quantity (Endlicher et al., 2011). Global climate change is
96
likely to exacerbate these tendencies in many cities of the world by increasing the likelihood
97
of extreme meteorological events, shifting precipitation regimes, changing surface and
98
groundwater conditions, temperature increases, and other factors (Langeveld et al., 2013). The
99
widespread need to accelerate investments for the maintenance of water distribution and
100
sewer systems adds to this challenge. At the same time, however, growing environmental
101
awareness has prompted measures to improve the quality of urban waters, including heavily
102
modified water bodies, not only in terms of chemical water quality but also in attempts to
103
restore essential structures and functions of aquatic ecosystems as a whole. In Europe, these 3
104
efforts have been fostered particularly by the requirements of the Water Framework Directive
105
(WFD), which stipulates that a good ecological status, or a good ecological potential, is
106
achieved by 2015. The modifications of urban water systems resulting from this stipulation
107
are currently underway or will soon be initiated. Both natural and technical water flows will
108
be affected, with likely consequences on urban water quality and quantity (Braud et al., 2013).
109
Finally, changing behaviours of water users pose their own challenge. Widespread water-
110
saving initiatives are a point in case. They are laudable from a broad-scale sustainability
111
perspective, but run counter at the local scale to the fact that many sewer systems have been
112
designed for larger water volumes than are currently generated with new water-saving
113
technologies.
114
The hydrology of urban areas has been extensively studied in the past (Harremoës,
115
2002; Fletcher et al., 2013). However, as highlighted by Paola et al. (2006), prominent
116
knowledge gaps in the urban water cycle remain, especially with regard to the interaction of
117
water with sediments, solutes and biological communities over a range of spatial and temporal
118
scales (Schulz et al., 2006; Palmer and Bernhardt, 2006; Potter, 2006). This is the domain of
119
ecohydrology, which integrates hydrological and ecological concepts and information
120
(Rodriguez-Iturbe and Porporato, 2004). Debates about sustainable development of urban
121
areas, including urban water systems, have also been informed by ecological concepts. This
122
has led to the notion of urban metabolism (Broto et al., 2012), which refers to the processes
123
by which cities transform raw materials, energy, and water into the built environment, human
124
biomass, and waste. The ecosystem view embodied in the urban metabolism concept has
125
inspired new ways of thinking about how cities can be made more sustainable (Decker et al.,
126
2000). Considering integrative approaches across traditional disciplines in urban design and
127
planning holds potential to improve current management strategies of urban water systems,
128
enhancing understanding of urban ecosystem processes, biodiversity conservation, and partial
129
ecosystem restoration (Alberti, 2005; Pickett and Cadenasso, 2008). 4
130
Here we advocate an integrated water management approach in urban areas that
131
explicitly recognizes the multiple interactions between and among technical water
132
infrastructure and natural and man-made surface waters and aquifers. Given the strong focus
133
on technical water systems in the past when considering urban water issues, the key objective
134
of the present paper is to strengthen the conceptual basis for integrated urban water
135
management strategies that simultaneously consider the technical and natural component of
136
urban water systems and how they interact. A central tenet is that an improved understanding
137
of processes and fluxes at interfaces between system components benefits this integrative
138
approach. Thus, we propose that diverse urban water interfaces within and between natural
139
and technical system components play key roles in the transformation and transport of water,
140
matter, and energy in urban areas. In addition, we present typical urban water interfaces and
141
their roles in the urban water cycle and identify selected areas of urban water research that
142
require particular attention.
143 144
2. Recognizing interfaces as a key feature of urban water systems
145
A prominent feature of urban water systems is that their natural and technical components are
146
tightly meshed, creating multiple interfaces in the urban and peri-urban water cycle (Fig. 1).
147
The transport and transformation of water, matter and energy takes place at and across these
148
interfaces, or boundary zones, between adjacent system compartments, resulting in the
149
exchange of mass, momentum and heat over various spatial and temporal scales (Gualtieri
150
and Mihailovic 2013). Water flow within and across the system components can be
151
adequately described based on the principles of fluid mechanics. Bed load transport can also
152
be modeled based on fundamental physical laws. In contrast, the movement of solutes and
153
suspended solids along the flow paths is superimposed by a variety of biogeochemical
154
processes (e.g. geochemical reactions, biological uptake, enzymatic transformations) and
155
physico-chemical processes (e.g. adsorption, desorption, aggregation) that need to be 5
156
considered for an appropriate system description. Although some of these processes also
157
occur in the bulk phase of water, rates are often highest at interfaces (McClain et al. 2003).
158
We refer to urban water interfaces as the boundary zones between components,
159
subsystems or compartments of the urban water system as a whole. The interfaces may be
160
natural, technical or hybrid depending on the adjacent system compartments. In a strict sense,
161
water interfaces are two-dimensional surfaces of potentially complex and irregular shape.
162
However, three-dimensional structures which are thin relative to the extent of the adjacent
163
system compartments are also included. It is implicit in this definition that interfaces can exist
164
at multiple spatial scales, ranging from tens of micrometres in the biofilm of a sewer or a
165
natural stream, to metres in a drinking water well, or even to entire technical structures such
166
as a wastewater treatment plant that acts as check point between the sewer network and the
167
receiving stream. Often, urban water interfaces are delineated by a gas-liquid or solid-liquid
168
phase transition, linking atmosphere, hydrosphere and pedosphere. However, liquid-liquid
169
interfaces are also common.
170
In general, urban water interfaces are characterized by steep physical gradients, steep
171
chemical gradients, non-linear interactions, and feedback mechanisms, resulting in distinctive
172
biological communities and high turnover rates of organic matter and individual organic and
173
inorganic chemical species, including anthropogenic contaminants. As a result, water quality
174
and quantity are typically altered when water flows across these interfaces (Table 1).
175
Anthropogenic modification of water quality during treatment for human use or release into
176
the environment after human use is substantial. The implication is that differences in water
177
quality are many orders of magnitude greater where different water masses meet at urban
178
water interfaces than is normally encountered in natural environments. Moreover, drinking
179
water supply, wastewater treatment, and storm water management in urban areas all tend to be
180
centralized. This results in large water diversions and, thus, creation of new interfaces and
181
shifts in the location and importance of existing ones. The concomitant massive alterations of 6
182
water flow and quality strongly modify the natural aquatic ecosystems in urban areas (Alley et
183
al., 2002).
184 185 186
3. Examples of urban water interfaces
187
Extensive areas of the urban land surface are totally or partially sealed by buildings,
188
paved streets, sidewalks, esplanades or traffic infrastructure. This results in highly impervious
189
surfaces, reduced ground water recharge, and increased run-off volumes and peak flows
190
(Shuster et al., 2005). Pavements are typically composed of (i) pavers, which can be partially
191
pervious or impervious; (ii) seams between the pavers filled with pervious or impervious
192
materials (Nehls et al., 2008); and (iii) a layer of usually coarse porous material on top of the
193
autochthonous soil whose purpose is to ensure quick drainage and thus prevent frost damage
194
of the pavement. Some of the rainwater falling on paved surfaces is collected and directed
195
into the sewer, especially if the building materials are impervious. To calculate sewer
196
dimensions, storm water run-off in particular has been well described and is frequently used
197
in urban water models (e.g. Mitchell et al., 2001). However, some of the rain water can be
198
stored at the surface (Nehls et al., 2011) before it evaporates or infiltrates through seams,
199
pavers or pavement cracks into the subsoil (Mansell and Rollett, 2009). The portion seeping
200
into the subsoil eventually becomes isolated from the surface because of the pore
201
discontinuity, and infiltrates to the aquifer. Quantifying the flow paths of precipitation on
202
paved surfaces is critically important to understand the urban water cycle.
203
Interfaces between surface and ground water are important even where surface
204
sealing in urban areas is extensive. Though not specific, water interfaces in urban areas show
205
features that often markedly deviate from surface water-ground water interactions in forested
206
or agricultural catchments, partly because of the connection between natural and technical
207
water systems. For example, subsurface water abstraction by bank filtration leads to water 7
208
infiltration from lakes and rivers into the aquifer while suppressing the exfiltration from
209
surface waters into the aquifer. An important variable to consider when managing bank
210
filtration systems is the infiltration capacity. It depends on the well-operation mode and
211
hydraulic resistance of the bed sediments. The latter varies over time. Specifically,
212
fluctuations in temperature and lake stage lead to transient changes in the leakage coefficient
213
(Doppler et al. 2007). Numerical analyses by Wiese and Nützmann (2009) revealed that the
214
water exchange between a lake and the surrounding aquifer follows non-linear behaviour.
215
Although the relationships between surface and ground water are reasonably well understood,
216
the ecological impacts of reversing the flow path as a result of bank filtration are not well
217
known (Nützmann et al., 2011). They could include negative effects on submerged
218
macrophytes and their role in stabilizing clear-water conditions in shallow urban water bodies
219
(Hilt et al., 2011).
220
Ground water in urban aquifers is interconnected with technical water infrastructure in
221
various ways other than water extraction wells for bank filtration (Fig. 1). Leaky sewer
222
systems and water supply pipes are particularly noteworthy, in addition to infiltration ponds
223
for treated sewage and rainwater infiltration systems. Importantly, pipes not only can lose
224
significant volumes of water to the aquifer, especially pressurized drinking water pipes. The
225
hydrostatic pressure in the aquifer also leads to unknown amounts of water infiltrating into the
226
sewer network (Wittenberg and Aksoy, 2010), whereas losses are normally small because
227
sewers are not under pressure and a self-sealing effect of wastewater constituents reduces
228
seepage. Thus, large quantities of ground water can be drained by sewer pipes, especially if
229
wastewater treatment plants are located near rivers that create a positive hydrostatic pressure.
230
Unwanted consequences are wastewater dilution and increased water volumes discharged into
231
wastewater treatment plants.
232
Sewer networks themselves have several technical interfaces. An important one where
233
gas transfer occurs is the interface between wastewater and the sewer atmosphere. If volatile 8
234
hydrogen sulfide (H2S) produced by anaerobic sulfate-reducers escapes the sewer system, it
235
causes odour problems in the urban surroundings. Alternatively, H2S is oxidized within the
236
sewer system, and the product of this oxidation, SO3, which is the anhydride of sulphuric acid,
237
can thus subsequently corrode the sewer pipe construction material. Increased centralization
238
of wastewater disposal has favoured the extension of sewer networks and prolonged the
239
residence time of wastewater in the pipes. At the same time, the specific volume of
240
wastewater per capita has declined, because water saving measures in households has
241
increasingly concentrated the wastewater. This promotes unpleasant odour emission and pipe
242
corrosion by sulphuric acid. Various quantitative relationships have been established to
243
predict H2S production in sewers (e.g. Pomeroy 1970; Nielsen et al., 1998; Urban and
244
Heilmann, 2011). However, application of these empirical relationships is limited to specific
245
conditions, making them ill-suited as a general basis for taking countermeasures to prevent
246
undesirable odour and corrosion (Saračević, 2009). On the other hand, many countermeasures
247
have been developed and applied to sewer networks based on local operational experience.
248
Three main alternatives exist: (i) wastewater treatment to prevent H2S formation, (ii) H2S
249
removal from the water phase or prevention of emissions to the atmosphere, and (iii)
250
treatment of the gas and use of corrosion-resistant material (Barjenbruch et al., 2008). For
251
example, H2S can be chemically bound by iron-based agents, or its formation can be reduced
252
by dosing sewage with nitrate to favour denitrification at the expense of sulfate reduction.
253
Alternatively, moulded trap systems, biofilters and amorphous trap systems are used to treat
254
gas and limit gas emissions (Barjenbruch et al., 2008). Comparisons of the efficiency of these
255
options (e.g. Barjenbruch, 2003; Saračević, 2009; Frey, 2008) have not yielded clear
256
recommendations of which approaches to adopt as effective control measures.
257
Water capture, treatment and supply systems: Protected groundwater is a preferred
258
source of water for human consumption. However, since underground resources are
259
frequently insufficient or polluted in urban agglomerations, water demand usually exceeds the 9
260
locally renewable resources. Therefore, water must be either withdrawn from nearby rivers or
261
brought in from distant sources by man-made water conducts. Local recycling can be a
262
complementary strategy to mitigate water shortage. Two key techniques used to ensure the
263
supply of high-quality water in urban areas are bank filtration and groundwater recharge.
264
Under favourable hydrogeological conditions, these approaches have been widely adopted,
265
both in Germany and elsewhere in the world. For example, in the city of Berlin, Germany, up
266
to 70 % of the water supply occurs via bank filtration and artificial recharge (Grünheid et al.,
267
2005). The contact zone between surface and ground water is an essential interface to ensure
268
safe and sustainable water supply. The capacity of this interface to clean up polluted surface
269
waters is remarkable, offering prospects of long-term reliability and stability of water capture
270
systems based on bank filtration and artificial groundwater recharge (Wiese et al., 2011).
271
Notwithstanding, a number of critical questions remain to ensure high water security
272
for urban populations. These include heterogeneity of redox conditions, which can
273
compromise water quality (Ernst et al., 2012), and the influence of natural organic matter
274
availability on the biodegradation of contaminants (Baumgarten et al., 2011). Furthermore, to
275
enhance water quality when persistent residues are present, additional technical solutions
276
involving interface processes are usually needed after water has been naturally treated at the
277
surface water–groundwater interface. Passage of contaminants over activated carbon has
278
proved effective, taking advantage of adsorption and desorption processes at the interface
279
between the water phase and an artificial solid, although irreversibility of the processes and
280
the desorption kinetics remain incompletely known (Worch, 2012).
281
Sediment-water interfaces of urban surface waters: A key feature of urban water
282
bodies is that they often receive high loads of organic carbon and nutrients, not only from
283
their natural catchments, but also from technical systems such as wastewater treatment plants,
284
as direct run-off from roads and other paved surfaces, and from dry and wet deposition (Nehls
285
and Shaw, 2010). As a result of this abundant resource supply, urban water bodies are often 10
286
highly productive (Yu et al., 2012), unless discharge of toxic waste products limits biological
287
activity (Meyer et al., 2005), and tend to show symptoms of eutrophication exerting negative
288
impacts on water quality, sometimes in extreme ways (Kira, 1993; Yu et al., 2012). Moreover,
289
abundant resource supply fosters heterotrophic activity and oxygen depletion in the unmixed
290
deep water layer above rich organic sediments. Potent greenhouse gases such as methane
291
(CH4) and nitrous oxide (N2O) form in these conditions as products of anaerobic carbon
292
transformations (e.g. denitrification, sulfate reduction, methanogenesis). As a result,
293
significant quantities of gas can accumulate in sediment pore water and lead to strong
294
supersaturation (Casper et al., 2000). Subsequently, the gases either diffuse into the water
295
column or rise as bubbles, a process known as ebullition. It is unknown, however, to what
296
extent rates of such anaerobic processes in urban water bodies and the corresponding gas
297
fluxes across the sediment-water interface differ from those in natural aquatic ecosystems
298
(McKaina et al., 2012).
299
Finally, water surfaces of lakes and running waters also acts as exchange zones for
300
water, energy, particles, solutes and gases (Gualtieri and PulciDoria, 2013). In the case of
301
methane, one of the most important greenhouse gases, only a fraction of the gas released from
302
the sediment is emitted to the atmosphere. This is because methane is efficiently oxidized
303
when reaching the interface between anoxic deep and oxic surface water layers in stratified
304
water bodies. This so-called oxycline, defined by a steep vertical gradient in oxygen
305
concentration, thus functions as a biological methane trap (Duc et al., 2010; Casper et al.,
306
2009). In rising gas bubbles, methane oxidation at the oxycline and in the overlying
307
oxygenated surface water may be effectively circumvented, especially in shallow water
308
bodies. However, whether these fluxes from urban water surfaces are significant at local,
309
regional and possibly global scales, and how they compare with fluxes from technical
310
compartments of urban water systems, is unknown. The temperature rise projected by climate
311
change scenarios will affect such fluxes across water interfaces in the future, influencing not 11
312
only greenhouse gas dynamics but also urban micro-climate and surface water dynamics, not
313
least through shifts in the timing and duration of ice cover on urban lakes.
314 315 316
4. Selected research challenges
317
Understanding the dominant controls of processes is a prerequisite to understand,
318
predict and influence the overall performance of any system (Schulz et al., 2006; Cadenasso
319
et al., 2007). In urban water systems, hydrological and biogeochemical processes have been
320
found to be strongly influenced by physical and chemical characteristics of the subsurface
321
(Paola et al., 2006) as well as by small-scale land-use patterns (Haase, 2009), which generate
322
a multitude of interfaces. It appears, therefore, that understanding interface processes could be
323
particularly important to manage urban water resources efficiently (Aronica and Lanza, 2005).
324
This suggests that concepts, models and data derived from studies in either natural or rural
325
catchments, or from technical systems alone, cannot be readily applied to urban settings,
326
calling for investigations into patterns and processes specifically at interfaces of the urban
327
water system. This includes, but is not limited to, processes at technical-natural interfaces. An
328
emerging research priority to improve urban water resource management is, therefore, to
329
describe and quantify the fluxes and transformations of matter and energy as water flows
330
across interfaces characteristic for urban areas.
331
Meeting this objective not only requires improved analytical techniques to characterize
332
steep physical and biogeochemical gradients, but also advanced computational methods,
333
including numerical modeling. Numerous biogeochemical transformations alter the
334
composition of ground water moving through the aquifer, especially where ground water
335
reaches the sediment-water interface of surface water bodies (Eiswirth et al., 2004).
336
Temperature, pressure, and chemical concentrations of dissolved substances can change
337
substantially as water flows across such interfaces. A difficulty for accurate quantitative 12
338
descriptions of the dynamics beyond the local scale is that locations of high activity are
339
patchily distributed. They also often occur as pulsed events, driven by hydrological conditions
340
that are dictated by local precipitation and run off patterns and, at larger scale, by river flow
341
regimes including severe floods and droughts. Drivers of surface-subsurface water exchange
342
are not restricted to hydraulic gradients, however; temperature, oxygen concentration and
343
biological activity at the sediment-water interfaces can also affect rates of biogeochemical
344
transformations and need to be considered (Greskowiak et al., 2005).
345
Patchiness of the processes and high dynamics imply that measurements at high
346
temporal and spatial resolution are required to identify where and when the processes occur.
347
A range of new and increasingly affordable technologies, both ground-based sensors and air-
348
borne instruments (Lewandowski et al., 2013), are now available to record ecohydrological
349
data in real-time and thus monitor rapid flow alterations or changes in physico-chemical water
350
quality (e.g. during pulse events) as well as detect early indications when and critical
351
thresholds where urban water systems might switch between states (Battin et al., 2003;
352
Argerich et al., 2011). This potential of the new technologies has been insufficiently exploited
353
to date.
354
Even basic hydrological processes in urban areas are not well understood. For
355
example, rainwater partitioning after light rain events is not well investigated. Process-based
356
models embracing the whole range of rain intensities are lacking to date, and important
357
features such as non-linear dynamic filling of surface stores in porous media or the extent of
358
relief storage (puddles, lane grooves) are not currently embodied in such models. Importantly,
359
both run off and evaporation must be represented in the models at sufficiently fine resolution,
360
both in terms of volumes and time, to address questions such as the cooling power of paved
361
surfaces, daily rainwater availability (for toilet flushing, urban greening, and urban gardening)
362
or first-flush toxicity assessments. Thus, an important research need is to investigate the
363
fundamental dynamics of rainwater partitioning, including evaporation from overheated 13
364
pavers into saturated atmospheres, and factors influencing such processes (e.g. hydrophobicity
365
caused by dust accumulation). Coupling of process-oriented rainwater partitioning equations
366
into urban water simulation models could help to get an integrated view of the water
367
dynamics on paved urban surfaces (Rim, 2011).
368
While process-oriented models are important, an additional challenge is to develop a
369
synoptic understanding of the essential processes that characterize urban water systems, with
370
special emphasis on interfaces. One way forward to meet this goal is to engage in building
371
complex models, either comprehensive single models or a suite of coupled models designed
372
for urban water management (Blöschl, 2006). In coping with the task of representing small-
373
scale spatial structures and dynamic processes in these numerical models, it is vital to capture
374
complex information in a small number of meaningful quantities (Schulz et al., 2006). This
375
requires advanced strategies for integrating flow and reactive transport linking two different
376
models on either side of the interface (Dogan, 2011). This problem is similar to modelling
377
concepts developed for porous media and free flow (Khalili et al., 1999). In addition, the new
378
models require laboratory studies to estimate model parameters before tests are run under
379
controlled conditions for initial validation (Hinkelmann, 2005). Next, the models need be
380
subjected to tests in real-world field settings in urban areas to explore the application range
381
for predictions, and to identify significant limitations.
382
Much research effort has been directed towards urban water systems over the past few
383
decades, especially with a view to improve water treatment technology and quantitative urban
384
water management (Aronica and Lanza, 2005). At present, urban water management is
385
generally centralized, based on a small number of drinking and wastewater treatment plants.
386
By and large this approach has proved effective to assure human water security in cities.
387
However, it also suffers from notable weaknesses. In particular, the fact that centralized water
388
supply and wastewater treatment result in extensive water diversions from the original flow
389
systems, contributing to the degradation of urban aquatic ecosystems which often serve as 14
390
signature landscape features of urban areas (Kareiva et al., 2007). Extensive pipe networks
391
also involve important operational costs, particularly for energy consumption of drinking
392
water supply systems, as well as high long-term maintenance costs. Therefore, a shift from a
393
few large treatment plants to a system of small, spatially distributed facilities has been
394
advocated to manage water resources in urban areas (Langeveld et al., 2013). Elements of
395
such a decentralized approach include small-scale engineered systems, but also constructed
396
wetlands for treated wastewater management designed to mimic natural ecosystems, reduce
397
environmental impact (Potter, 2006), and even deliver direct environmental benefits (e.g.
398
habitat for wetland species). Concepts to implement these principles have been proposed
399
(Rodriguez-Iturbe and Porporato, 2004) but they need refinement with built-in flexibility to
400
accommodate a wide variety of site-specific natural, technical and socio-economic variation.
401 402 403
5. Conclusions
404
To conclude, natural, technical and hybrid interfaces play a key role in the urban water
405
cycle. Our central tenet is that analyzing, understanding and modelling processes and fluxes at
406
urban water interface at various spatial scales is key to improve sustainable water resource
407
management in urban areas. Recognizing that natural and technical system components of the
408
urban water cycle need to be viewed in an integrative fashion is an important prerequisite to
409
achieve this goal. Its implementation requires close collaboration of engineers and scientists
410
grounded on a common conceptual basis. Graduate education can serve to develop this basis
411
and catalyse the transfer of the approach from research to urban water management and
412
engineering practice.
413
Beyond the integration of water engineering and science, it is important for success to
414
involve the multitude of stakeholders with vested interests in urban water flows and water
415
bodies, ranging from the water industry and public authorities to taxpayers and a wide range 15
416
of scientists, including social scientists. For example, in catchment management studies, role
417
plays are becoming an important instrument to investigate the behaviour of stakeholders and
418
the consequences thereof (Endlicher et al., 2011). Similarly, integrated system models might
419
be developed not necessarily to provide the best possible representation of a system’s
420
behaviour, but to serve as a communication tool that facilitates understanding complex facts,
421
evaluating their repercussions, and thus reaching consensus among stakeholders as a basis for
422
evidence-based and widely accepted management decisions (Blöschl, 2006).
423 424 425
Acknowledgements
426
We would like to thank our colleagues Peter Casper, Sabine Hilt, Michael Hupfer,
427
Birgit Kleinschmit, Anke Putschew, Ulrich Szewzyk and Gerd Wessolek for their helpful and
428
constructive input during the preparation of this manuscript as a part of a graduate PhD school
429
proposal.
430 431 432 433
References
434
Alberti M. 2005. The effects of urban patterns on ecosystem function. International Regional
435 436 437
Science Review 28: 168-192. Alley WM, Healy RW, LaBaugh JW, Reilly TE. 2002. Flow and storage in groundwater systems. Science 296: 1985-1990.
438
Argerich A, Marti E, Sabater F, Ribot M. 2011. Temporal variation of hydrological exchange
439
and hyporheic biogeochemistry in a headwater stream during autumn. Journal of the North
440
American Benthological Society 30: 635–652.
16
441 442
Aronica G, Lanza L. 2005. Hydrology in the urban environment. Hydrological Processes 44: 1005-1006.
443
Barjenbruch M, Hinkelmann R, Hüttl R, Huhnt W, Krämer T, Nehrig M, Rühmland S, Röben
444
R. 2008. An Online-Monitoring and Operation System to Prevent Odour and Corrosion in
445
Sewer Networks - Feasibilty Study. Report, Kompetenzzentrum Wasserg GmbH.
446 447
Barjenbruch M. 2003. Prevention of odour emergence in sewage networks. Water Science & Technology 47: 357-363.
448
Battin TJ, Kaplan LA, Newbold JD, Hendricks SP. 2003. A mixing model analysis of stream
449
solute dynamics and the contribution of a hyporheic zone to ecosystem function. Freshwater
450
Biology 48: 995–1014.
451
Baumgarten B, Jährig J, Reemtsma T, Jekel M. 2011. Long term laboratory column
452
experiments to simulate bank filtration: Factors controlling removal of sulfamethoxazole.
453
Water Research 45: p. 211-220.
454 455 456 457
Blöschl G. 2006: Hydrologic synthesis: Across processes, places, and scales. Water Resources Research 42: W03S02, doi:10.1029/2005WR004319. Braud I, Fletcher TD, Andrieu H. 2013. Hydrology of peri-urban catchments: processes and modelling. Journal of Hydrology 485: 1-4.
458
Brion GM, Neelakantan TR, Lingireddy S. 2001. Using neuronal networks to predict peak
459
Cryptosporidium concentrations. Journal of the American Water Works Association 93: 99-
460
105.
461 462 463
Broto VC, Allan A, Rapoport, E. 2012. Interdisciplinary perspectives on urban metabolism. Journal of Industrial Ecology 16: 851-861. Cadenasso ML, Pickett STA, Schwarz K. 2007. Spatial heterogeneity in urban ecosystems.
464
Reconceptualizing land cover and a framework for classification. Frontiers in Ecology and
465
the Environment 5: 80-88.
17
466 467
Casper P, Maberly SC, Hall GH, Finlay BJ. 2000. Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere. Biogeochemistry 49, 1-19.
468
Casper P, Albino MF and Adams DD. 2009. Diffusive fluxes of CH4 and CO2 across the
469
water-air interface in the eutrophic lake Dagow, NE Germany. Verhandlungen der
470
Internationalen Vereinigung für Limnologie 30: 874-877.
471
Decker EH, Elliott S, Smith FA, Blake DR, Rowland FS. 2000. Energy and material flow
472
through the urban ecosystem. Annual Review of Energy and the Environment 25: 685-740.
473
Dogan MO. 2011.Coupling of porous media flow with pipe flow. Dissertation, Mitteilungen
474 475
Heft 199, Institut für Wasserbau, Universität Stuttgart Doppler T, Franssen HJH, Kaiser HP, Stauffer F. 2007. Field-evidence of a dynamic
476
leakage coefficient for modeling river-aquifer interactions. Journal of Hydrology 347: 177-
477
187.
478 479 480 481 482
Duc N, Crill P, Bastviken D. 2010. Implications of temperature and sediment characteristics on methane formation and oxidation in lake sediments. Biogeochemistry 100, 185-196. Eiswirth M, Wolf L, Hötzl H. 2004. Balancing the contaminant input into urban water resources. Environmental Geology 46: 246-256. Endlicher W, Hostert P, Kowarik I, Kulke E, Lossau J, Marzluff J, van der Meer E, Mieg H,
483
Nützmann G, Schulz M, Wessolek G (Eds.). 2011. Perspectives in Urban Ecology: Studies
484
of Ecosystems and Interactions between Humans and Nature in the Metropolis of Berlin,
485
Germany. Springer, Heidelberg, pp. 351.
486
Ernst M, Hein A, Asmin J, Krauss M, Fink G, Hollender J, Ternes T, Jorgensen C, Jekel M,
487
McArdell CS. 2012. Water quality analysis: Detection, fate, and behaviour, of selected trace
488
organic pollutants at managed aquifer recharge sites, Chapter 12. IWA Publishing - Water
489
Reclamation Technologies
490
9781843393443): p. 197-224.
491
for
Safe
Managed
Aquifer
Recharge,
2012
(ISBN
Fletcher TD, Andrieu H, Hamel P. 2013. Understanding, management and modeling of urban 18
492
hydrology and its consequences for receiving waters: a state of the art. Advances in Water
493
Resources 51: 261-279.
494
Frey M. 2008. Untersuchungen zur Sulfidbildung und zur Effizienz der Geruchsminimierung
495
durch Zugabe von Additiven in Abwasserkanalisationen. Kassel University Press, ISBN
496
978-3-89958-453-0.
497
Greskowiak J, Prommer H, Massmann G, Johnston CD, Nützmann G, Pekdeger A. 2005.The
498
impact of variably saturated conditions on hydrogeochemical changes during artificial
499
recharge of groundwater. Applied Geochemistry 20:1409–1426.
500 501 502
Grimm NB, Faeth SH, Golubiewski NE, Redman CL, Wu J, Bai X, Briggs JM. 2008. Global change and the ecology of cities. Science 319: 756-760. Grünheid S, Amy G and Jekel M. 2005.Removal of bulk dissolved organic carbon (DOC) and
503
trace organic compounds by bank filtration and artificial recharge. Water Research 39:
504
3219-3228.
505 506
Gualtieri C, Mihailovic DT (Eds.). 2013. Fluid Mechanics of Environmental Interfaces, Second Edition. CRC Press/Balkema, Leiden, The Netherlands, 480 pp.
507
Gualtieri C, PulciDoria G. 2013. Gas-transfer at unsheared free surfaces. In: Gualtieri C and
508
Mihailovic DT (Eds.). 2013. Fluid Mechanics of Environmental Interfaces, Second Edition.
509
CRC Press/Balkema, Leiden, The Netherlands, 145-180.
510 511
Haase D. 2009. Effects of urbanization on the water balance – A long-term trajectory. Environmental Impact Assessment Review 29: 211-219.
512
Harremoës P. 2002. Integrated urban drainage: status and perspectives. Water Sciences
513
&Technology 45: 1-10.
514
Hilt S, Köhler J, Kozerski HP, Scheffer M, Van Nes E. 2011. Abrupt regime shifts in space
515
and time along rivers and connected lake systems. Oikos 120: 766-775.
19
516
Hinkelmann R. 2005. Efficient Numerical Methods and Information-Processing Techniques
517
for Modeling Hydro- and Environmental Systems. Lecture Notes in Applied and
518
Computational Mechanics, Vol. 21, Springer, Berlin, Heidelberg
519 520 521 522 523 524 525 526 527
Kareiva P, Watts S, McDonald R, Boucher T. 2007. Domesticated nature: shaping landscapes and ecosystems for human welfare. Science 316: 1866-1869. Khalili A, Basu AJ, Pietrzyk U, Jörgensen BB. 1999. Advective transport through permeable sediments: a numerical and experimental approach. Acta Mechanica 32: 221-227. Kira T. 1993. Major environmental problems in world lakes. Memorie dell'Istituto Italiano di Idrobiologia 52: 1-7. Langeveld JG, Schilperoort RPS, Weijers SR. 2013. Climate change and urban wastewater infrastructure: There is more to explore. Journal of Hydrology 476: 112-119. Laub BG, Baker DW, Bledsoe BP, Palmer MA. 2012. Range of variability of channel
528
complexity in urban, restored and forested reference streams. Freshwater Biology 57: 1076–
529
1095.
530
Lewandowski J, Meinikmann K, Ruhtz T, Pösche, F, Kirillin, G. 2013. Localization of
531
lacrustine groundwater discharge (LGD) by airborne measurement of thermal infrared
532
radiation. Remote Sensing of Environment 138: 119-125.
533 534
Mansell M, Rollet F. 2009. The effect of surface texture on evaporation, infiltration and storage properties of paved surfaces. Water Science and Technology 60: 71-76.
535
McClain, ME, EW Boyer, CL Dent, SE Gergel, NB Grimm, PM Groffman, SC Hart, JW
536
Harvey, CA Johnston, E Mayorga, WH McDowell, G Pinay. 2003. Biogeochemical hot
537
spots and hot moments at the interface of terrestrial and aquatic ecosystems. Ecosystems 6:
538
301-312.
539
McKaina K, Wofsy SC, Nehrkorn T, Eluszkiewicz J, Ehleringer JR, Stephens BB. 2012.
540
Assessment of ground-based atmospheric observations for verification of greenhouse gas
20
541
emissions from an urban region. Proceedings of the National Academy of Science of the
542
USA 109: 8423-8428.
543 544 545 546 547 548 549 550 551
Meyer JL, Paul MJ, Taulbee WK. 2005. Stream ecosystem function in urbanizing landscapes. Journal of the North American Benthological Society 24: 602-612. Mitchell VG, Mein RG, McMahon TA. 2001. Modelling the urban water cycle. Environmental Modelling & Software 16: 615-629. Nehls T, Jozefaciuk G, Sokolowska Z, Hajnos M, Wessolek G. 2008. Filter properties of seam material of paved urban soils. Hydrology and Earth System Sciences 12: 691-702. Nehls T, Shaw R. 2010. Black carbon in soils - relevance, analysis, distribution. Soil Science Society of America. Soil Survey Horizons 51: 79-84 Nehls T, Rim YN, Wessolek G 2011. Technical note on measuring run-off dynamics from
552
pavements using a new device: the weighable tipping bucket. Hydrology and Earth System
553
Sciences 15: 1379-1386.
554 555
Nielsen PH, Raunkjaer K, Hvitved-Jacobsen T. 1998. Sulfide Production and Wastewater Quality in Pressure Mains. Water Science & Technology 37: 97-104.
556
Nützmann G, Wiegand C, Contardo-Jara C, Hamann E, Burmester V, Gerstenberg K. 2011.
557
Contamination of Urban Surface and Ground Water Resources and Impact on Aquatic
558
Species. In: Endlicher et al. (Eds.) Perspectives in Urban Ecology, Springer, Heidelberg, 43-
559
88.
560 561 562
Palmer MA, Bernhardt E. 2006. Hydroecology and river restoration: Rip for research and synthesis. Water Resources Research 42: W03S07, doi:10.1029/2005WR004354. Paola C, Foufoula-Georgiou E, Dietrich WE, Hondzo M, Mohrig D, Parker G, Power ME,
563
Rodriguez-Iturbe I, Voller V, Wilcock P. 2006. Toward a unified science of the Earth’s
564
surface: Opportunities for synthesis among hydrology, geomorphology, geochemistry, and
565
ecology, Water Resources Research 42: W03S10, doi:10.1029/2005WR004336.
566
Pickett STA, Cadenasso ML. 2008. Linking ecology and built components of urban mosaics: 21
567 568 569
an open cycle of ecological design. Journal of Ecology 96: 8-12. PomeroyRD.1970. Sanitary sewer design for hydrogen sulphide control. Public Works 101 (10): 93-96.
570
Potter KW. 2006. Small-scale, spatially distributed water management practices: Implications
571
for research in the hydrologic sciences. Water Resources Research 42: W03S08,
572
doi:10.1029/2005WR004295.
573 574 575 576 577 578 579 580 581
Rim Y-N. 2011. Analyzing Runoff Dynamics of Paved Soil Surface Using Weighable Lysimeters.PhD Thesis, Technische Universitaet Berlin, urn:nbn:de:kobv:83-opus-30471 Rodriguez-Iturbe I, Porporato A. 2004. Ecohydrology of water-controlled ecosystems: soil moisture and plant dynamics. 1st ed. Cambridge University Press. Roy JW, Bickerton G. 2012. Toxic Groundwater Contaminants: An Overlooked Contributor to Urban Stream Syndrome? Environmental Science & Technology 46, 729-736. Saračević E. 2009. Zur Kenntnis der Schwefelwasserstoffbildung und -vermeidung in Abwasserdruckleitungen. Wiener Mitteilungen 211, ISBN 978-3-85234-103-3. Schulz K, Seppelt R, Zehe E, Vogel HJ, Attinger S. 2006. Importance of spatial structures in
582
advancing
583
doi:10.1029/2005WR004301.
584 585
hydrological
sciences.
Water
Resources
Research
42:
W03S08,
Shuster WD, Bonta J, Thurston H, Warnemuende E, Smith DR. 2005. Impacts of impervious surface on watershed hydrology: A review. Urban Water Journal 2: 263-275.
586
Urban U, Heilmann A. 2011. Möglichkeiten der Belüftung von Druckleitungen zur
587
Minderung von Geruch und Korrosion. 4. OWL Abwassertag, Section of Environmental
588
Engineering Department of Life Sciences, Aalborg University; http://www.sewer.dk/.
589
Walsh CJ, Roy AH, Feminella JW, Cottingham PD, Groffman PM, Morgan RP. 2005. The
590
urban stream syndrome: current knowledge and the search for a cure. Journal of the North
591
American Benthological Society 24: 706-723.
22
592 593 594
Wiese B, Nützmann G. 2009. Transient leakage and infiltration characteristics during lake bank filtration. Ground Water 47: 57-68. Wiese B, Massmann G, Jekel M, Heberer T, Dünnbier U, Orlikowski D, Grützmacher G.
595
2011. Removal kinetics of organic compounds and sum parameters under field conditions
596
for managed aquifer recharge. Water Research 45: 4939-4950.
597 598 599 600 601
Wittenberg H, Aksoy H. 2010. Groundwater intrusion into leaky sewer systems. Water Science & Technology 62: 92-98. Worch E. 2012. Adsorption Technology in Water Treatment: Fundamentals, Processes, and Modeling, De Gruyter. Yu S, Yu GB, Liu Y, Li GL, Feng S, Wu SC, Wong MH. 2012. Urbanization impairs surface
602
water quality: Eutrophication and metal stress in the Grand Canal of China. River Research
603
and Applications 28: 1135-1148.
604
23
605 606 607 608 609 610 611
FIGURE CAPTIONS
Figure 1:
Scheme of an urban water cycle
Table 1:
Examples of urban water interfaces, major processes and key questions
612 613 614
24
615 616 617
Table 1: Examples of urban water interfaces, major processes and key questions
618 Interfaces
Processes
Key questions
Surface water - atmosphere
Release of greenhouse gases
Gas production processes and flux rates are poorly known and differ from those in natural systems
Soil surface - atmosphere
Vapor and heat transport across the interface
Rainwater partitioning, especially from small rain events, storage and evaporation dynamics of paved urban soils
Surface water - sediment groundwater
Bank filtration / exchange between aquifers and urban streams and rivers
Fate of nutrients and pollutants at the interface between surface water – aquifer; Reactivity of this interface
Sewer pipe: wastewater gas space
Transport of sulfurous compounds in sewers
Transport and kinetics of sulfuric acids in sewers are poorly known, especially under variable temperatures
619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647
Surface water - treated wastewater
Interaction of trace substances originating from treated wastewater and freshwater biota
To which extent is the treated wastewater diluted in the surface water and how does it affect the water quality of the receiving water?
648 649 650 651 652 653 654 655 656 657 658 659 25
Figure
660
Highlights
661 662
We identify boundaries involving compartments of the urban water system as urban water interfaces.
663
These urban water interfaces are characterized by steep physical and biogeochemical gradients.
664
We regard these interfaces as hotspots of environmentally relevant fluxes.
665
We discuss a framework to improve our fundamental understanding of aquatic interface processes.
666 667 668
26
S0022-1694(14)00295-9 http://dx.doi.org/10.1016/j.jhydrol.2014.04.021 HYDROL 19552
To appear in:
Journal of Hydrology
Received Date: Revised Date: Accepted Date:
4 October 2013 14 March 2014 8 April 2014
Please cite this article as: Gessner, M.O., Hinkelmann, R., Nützmann, G., Jekel, M., Singer, G., Lewandowski, J., Nehls, T., Barjenbruch, M., Urban Water Interfaces, Journal of Hydrology (2014), doi: http://dx.doi.org/10.1016/ j.jhydrol.2014.04.021
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
Urban Water Interfaces Gessner MO1,2, Hinkelmann R2, Nützmann G1,3,§, Jekel M2, Singer G1, Lewandowski J1, Nehls T2, Barjenbruch M2 1
Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany Technische Universität Berlin (TU Berlin), Berlin, Germany 3 Humboldt University Berlin, Berlin, Germany 2
Abstract Urban water systems comprise large-scale technical systems and natural and man-made water bodies. The technical systems constitute essential components of urban infrastructure for water collection, treatment, storage and distribution, as well as wastewater and runoff collection and subsequent treatment. Urban aquatic ecosystems are typically subject to strong human influences, and the quality and function of both surface and ground waters tend to be strongly modified, often with far-reaching impacts on downstream aquatic ecosystems and water users. The various surface and subsurface water bodies in urban environments can be viewed as interconnected compartments that are also extensively intertwined with a range of technical compartments of the urban water system. As a result, urban water systems are characterized by fluxes of water, solutes, gases and energy between contrasting compartments of a technical, natural or hybrid nature. Referred to as urban water interfaces, boundaries between and within these compartments are often specific to urban water systems. Urban water interfaces are generally characterized by steep physical and biogeochemical gradients, which promote high reaction rates. We hypothesize that they act as key sites of processes and fluxes with notable effects on overall system behaviour. By their very nature, urban water interfaces are heterogeneous and dynamic. Therefore, they increase spatial heterogeneity in urban areas and are also expected to contribute notably to the temporal dynamics of urban water systems, which often involve non-linear interactions and feedback mechanisms. Processes at and fluxes across urban water interfaces are complex and less well understood than within well-defined, homogeneous compartments, requiring both empirical investigations and new modelling approaches at both the process and system level. We advocate an integrative conceptual framework of the urban water system that considers interfaces as a key component to improve our fundamental understanding of aquatic interface processes in urban environments, advance understanding of current and future system behaviour, and promote an integrated urban water management.
Key words: urban water system, aquatic ecosystems, interface processes, water systems modeling, urban water management
§
Corresponding author 1
52
1. Introduction
53
Extensive landscape fragmentation through human activities has resulted in strong
54
proliferation of interfaces between distinct land-cover types (Cadenasso et al., 2007; Haase,
55
2009). In addition, humans have created numerous intersections between technical and non-
56
technical systems, including natural, though often strongly modified, ecosystems. Nowhere is
57
this perhaps more evident than in the tightly interconnected water systems of urban areas. The
58
technical structures designed for water treatment, storage and distribution, as well as for
59
wastewater and runoff collection, treatment and controlled discharge to receiving water
60
bodies, are vital components of urban infrastructure. They are often well managed, at least in
61
highly industrialized regions and countries, although increasingly also in major cities of the
62
developing world. Man-made channels and other artificial water bodies complement the
63
technical water infrastructure and natural aquatic ecosystems such as streams and rivers, lakes
64
and ground water in urban areas. Urban ‘natural’ aquatic ecosystems are typically subject to
65
strong physical, chemical and biological modifications, often with far-reaching negative
66
consequences for downstream aquatic environments and water users (Roy and Bickerton,
67
2012; Walsh et al., 2005; Meyer et al., 2005). For example, until tertiary wastewater treatment
68
was introduced to reduce the ammonium load generated by the metropolitan area of Paris,
69
France, a huge ammonium plume severely depleted oxygen concentrations in the estuary of
70
the River Seine 100 km downstream of the effluent (Brion et al., 2001).
71
Water use and management in cities is driven by a multitude of goals that may be
72
partially conflicting. They include water supply for domestic and industrial consumption,
73
adequate sanitation, and protection of humans and infrastructure from natural disasters.
74
Ongoing efforts to control pollutant loads to urban surface waters are starting to bear fruit.
75
However, rarely taken into account are impacts such as the disruption of linkages of urban
76
water bodies with their riparian areas and floodplains, disconnection of surface waters from
77
hyporheic zones and aquifers, or outright disappearance of natural surface waters in 2
78
underground pipes. Thus, by managing water resources and land to meet goals dictated by
79
immediate public, industrial or personal demands, humans have profoundly modified urban
80
water bodies (Grimm et al., 2008).
81
The term “urban stream syndrome” has been coined to describe the strong ecological
82
degradation of running waters consistently observed in urban environments (Meyer et al.,
83
2005; Walsh et al., 2005). Classic symptoms of the syndrome include flashy hydrographs that
84
promote physical disturbance in urban running waters; high loads of nutrients, suspended
85
solids and various pollutants ranging from heavy metals to personal care products and drugs;
86
and a simplified and homogenized channel morphology providing limited in-stream and
87
riparian habitat and habitat heterogeneity (Laub et al., 2012). Together, these and other
88
pressures severely compromise water quality and quantity in natural and man-made urban
89
water bodies. Consequences include impoverished biodiversity compared to natural
90
freshwater bodies and increased costs to ensure human water security in terms of water
91
resource supply, wastewater removal, and protection from excessive surface runoff and floods
92
(Grimm et al., 2008).
93
In the future, urban water management will face additional challenges as demographic
94
trends and continued migration of the world population to metropolitan areas leave their
95
imprint on urban water quality and quantity (Endlicher et al., 2011). Global climate change is
96
likely to exacerbate these tendencies in many cities of the world by increasing the likelihood
97
of extreme meteorological events, shifting precipitation regimes, changing surface and
98
groundwater conditions, temperature increases, and other factors (Langeveld et al., 2013). The
99
widespread need to accelerate investments for the maintenance of water distribution and
100
sewer systems adds to this challenge. At the same time, however, growing environmental
101
awareness has prompted measures to improve the quality of urban waters, including heavily
102
modified water bodies, not only in terms of chemical water quality but also in attempts to
103
restore essential structures and functions of aquatic ecosystems as a whole. In Europe, these 3
104
efforts have been fostered particularly by the requirements of the Water Framework Directive
105
(WFD), which stipulates that a good ecological status, or a good ecological potential, is
106
achieved by 2015. The modifications of urban water systems resulting from this stipulation
107
are currently underway or will soon be initiated. Both natural and technical water flows will
108
be affected, with likely consequences on urban water quality and quantity (Braud et al., 2013).
109
Finally, changing behaviours of water users pose their own challenge. Widespread water-
110
saving initiatives are a point in case. They are laudable from a broad-scale sustainability
111
perspective, but run counter at the local scale to the fact that many sewer systems have been
112
designed for larger water volumes than are currently generated with new water-saving
113
technologies.
114
The hydrology of urban areas has been extensively studied in the past (Harremoës,
115
2002; Fletcher et al., 2013). However, as highlighted by Paola et al. (2006), prominent
116
knowledge gaps in the urban water cycle remain, especially with regard to the interaction of
117
water with sediments, solutes and biological communities over a range of spatial and temporal
118
scales (Schulz et al., 2006; Palmer and Bernhardt, 2006; Potter, 2006). This is the domain of
119
ecohydrology, which integrates hydrological and ecological concepts and information
120
(Rodriguez-Iturbe and Porporato, 2004). Debates about sustainable development of urban
121
areas, including urban water systems, have also been informed by ecological concepts. This
122
has led to the notion of urban metabolism (Broto et al., 2012), which refers to the processes
123
by which cities transform raw materials, energy, and water into the built environment, human
124
biomass, and waste. The ecosystem view embodied in the urban metabolism concept has
125
inspired new ways of thinking about how cities can be made more sustainable (Decker et al.,
126
2000). Considering integrative approaches across traditional disciplines in urban design and
127
planning holds potential to improve current management strategies of urban water systems,
128
enhancing understanding of urban ecosystem processes, biodiversity conservation, and partial
129
ecosystem restoration (Alberti, 2005; Pickett and Cadenasso, 2008). 4
130
Here we advocate an integrated water management approach in urban areas that
131
explicitly recognizes the multiple interactions between and among technical water
132
infrastructure and natural and man-made surface waters and aquifers. Given the strong focus
133
on technical water systems in the past when considering urban water issues, the key objective
134
of the present paper is to strengthen the conceptual basis for integrated urban water
135
management strategies that simultaneously consider the technical and natural component of
136
urban water systems and how they interact. A central tenet is that an improved understanding
137
of processes and fluxes at interfaces between system components benefits this integrative
138
approach. Thus, we propose that diverse urban water interfaces within and between natural
139
and technical system components play key roles in the transformation and transport of water,
140
matter, and energy in urban areas. In addition, we present typical urban water interfaces and
141
their roles in the urban water cycle and identify selected areas of urban water research that
142
require particular attention.
143 144
2. Recognizing interfaces as a key feature of urban water systems
145
A prominent feature of urban water systems is that their natural and technical components are
146
tightly meshed, creating multiple interfaces in the urban and peri-urban water cycle (Fig. 1).
147
The transport and transformation of water, matter and energy takes place at and across these
148
interfaces, or boundary zones, between adjacent system compartments, resulting in the
149
exchange of mass, momentum and heat over various spatial and temporal scales (Gualtieri
150
and Mihailovic 2013). Water flow within and across the system components can be
151
adequately described based on the principles of fluid mechanics. Bed load transport can also
152
be modeled based on fundamental physical laws. In contrast, the movement of solutes and
153
suspended solids along the flow paths is superimposed by a variety of biogeochemical
154
processes (e.g. geochemical reactions, biological uptake, enzymatic transformations) and
155
physico-chemical processes (e.g. adsorption, desorption, aggregation) that need to be 5
156
considered for an appropriate system description. Although some of these processes also
157
occur in the bulk phase of water, rates are often highest at interfaces (McClain et al. 2003).
158
We refer to urban water interfaces as the boundary zones between components,
159
subsystems or compartments of the urban water system as a whole. The interfaces may be
160
natural, technical or hybrid depending on the adjacent system compartments. In a strict sense,
161
water interfaces are two-dimensional surfaces of potentially complex and irregular shape.
162
However, three-dimensional structures which are thin relative to the extent of the adjacent
163
system compartments are also included. It is implicit in this definition that interfaces can exist
164
at multiple spatial scales, ranging from tens of micrometres in the biofilm of a sewer or a
165
natural stream, to metres in a drinking water well, or even to entire technical structures such
166
as a wastewater treatment plant that acts as check point between the sewer network and the
167
receiving stream. Often, urban water interfaces are delineated by a gas-liquid or solid-liquid
168
phase transition, linking atmosphere, hydrosphere and pedosphere. However, liquid-liquid
169
interfaces are also common.
170
In general, urban water interfaces are characterized by steep physical gradients, steep
171
chemical gradients, non-linear interactions, and feedback mechanisms, resulting in distinctive
172
biological communities and high turnover rates of organic matter and individual organic and
173
inorganic chemical species, including anthropogenic contaminants. As a result, water quality
174
and quantity are typically altered when water flows across these interfaces (Table 1).
175
Anthropogenic modification of water quality during treatment for human use or release into
176
the environment after human use is substantial. The implication is that differences in water
177
quality are many orders of magnitude greater where different water masses meet at urban
178
water interfaces than is normally encountered in natural environments. Moreover, drinking
179
water supply, wastewater treatment, and storm water management in urban areas all tend to be
180
centralized. This results in large water diversions and, thus, creation of new interfaces and
181
shifts in the location and importance of existing ones. The concomitant massive alterations of 6
182
water flow and quality strongly modify the natural aquatic ecosystems in urban areas (Alley et
183
al., 2002).
184 185 186
3. Examples of urban water interfaces
187
Extensive areas of the urban land surface are totally or partially sealed by buildings,
188
paved streets, sidewalks, esplanades or traffic infrastructure. This results in highly impervious
189
surfaces, reduced ground water recharge, and increased run-off volumes and peak flows
190
(Shuster et al., 2005). Pavements are typically composed of (i) pavers, which can be partially
191
pervious or impervious; (ii) seams between the pavers filled with pervious or impervious
192
materials (Nehls et al., 2008); and (iii) a layer of usually coarse porous material on top of the
193
autochthonous soil whose purpose is to ensure quick drainage and thus prevent frost damage
194
of the pavement. Some of the rainwater falling on paved surfaces is collected and directed
195
into the sewer, especially if the building materials are impervious. To calculate sewer
196
dimensions, storm water run-off in particular has been well described and is frequently used
197
in urban water models (e.g. Mitchell et al., 2001). However, some of the rain water can be
198
stored at the surface (Nehls et al., 2011) before it evaporates or infiltrates through seams,
199
pavers or pavement cracks into the subsoil (Mansell and Rollett, 2009). The portion seeping
200
into the subsoil eventually becomes isolated from the surface because of the pore
201
discontinuity, and infiltrates to the aquifer. Quantifying the flow paths of precipitation on
202
paved surfaces is critically important to understand the urban water cycle.
203
Interfaces between surface and ground water are important even where surface
204
sealing in urban areas is extensive. Though not specific, water interfaces in urban areas show
205
features that often markedly deviate from surface water-ground water interactions in forested
206
or agricultural catchments, partly because of the connection between natural and technical
207
water systems. For example, subsurface water abstraction by bank filtration leads to water 7
208
infiltration from lakes and rivers into the aquifer while suppressing the exfiltration from
209
surface waters into the aquifer. An important variable to consider when managing bank
210
filtration systems is the infiltration capacity. It depends on the well-operation mode and
211
hydraulic resistance of the bed sediments. The latter varies over time. Specifically,
212
fluctuations in temperature and lake stage lead to transient changes in the leakage coefficient
213
(Doppler et al. 2007). Numerical analyses by Wiese and Nützmann (2009) revealed that the
214
water exchange between a lake and the surrounding aquifer follows non-linear behaviour.
215
Although the relationships between surface and ground water are reasonably well understood,
216
the ecological impacts of reversing the flow path as a result of bank filtration are not well
217
known (Nützmann et al., 2011). They could include negative effects on submerged
218
macrophytes and their role in stabilizing clear-water conditions in shallow urban water bodies
219
(Hilt et al., 2011).
220
Ground water in urban aquifers is interconnected with technical water infrastructure in
221
various ways other than water extraction wells for bank filtration (Fig. 1). Leaky sewer
222
systems and water supply pipes are particularly noteworthy, in addition to infiltration ponds
223
for treated sewage and rainwater infiltration systems. Importantly, pipes not only can lose
224
significant volumes of water to the aquifer, especially pressurized drinking water pipes. The
225
hydrostatic pressure in the aquifer also leads to unknown amounts of water infiltrating into the
226
sewer network (Wittenberg and Aksoy, 2010), whereas losses are normally small because
227
sewers are not under pressure and a self-sealing effect of wastewater constituents reduces
228
seepage. Thus, large quantities of ground water can be drained by sewer pipes, especially if
229
wastewater treatment plants are located near rivers that create a positive hydrostatic pressure.
230
Unwanted consequences are wastewater dilution and increased water volumes discharged into
231
wastewater treatment plants.
232
Sewer networks themselves have several technical interfaces. An important one where
233
gas transfer occurs is the interface between wastewater and the sewer atmosphere. If volatile 8
234
hydrogen sulfide (H2S) produced by anaerobic sulfate-reducers escapes the sewer system, it
235
causes odour problems in the urban surroundings. Alternatively, H2S is oxidized within the
236
sewer system, and the product of this oxidation, SO3, which is the anhydride of sulphuric acid,
237
can thus subsequently corrode the sewer pipe construction material. Increased centralization
238
of wastewater disposal has favoured the extension of sewer networks and prolonged the
239
residence time of wastewater in the pipes. At the same time, the specific volume of
240
wastewater per capita has declined, because water saving measures in households has
241
increasingly concentrated the wastewater. This promotes unpleasant odour emission and pipe
242
corrosion by sulphuric acid. Various quantitative relationships have been established to
243
predict H2S production in sewers (e.g. Pomeroy 1970; Nielsen et al., 1998; Urban and
244
Heilmann, 2011). However, application of these empirical relationships is limited to specific
245
conditions, making them ill-suited as a general basis for taking countermeasures to prevent
246
undesirable odour and corrosion (Saračević, 2009). On the other hand, many countermeasures
247
have been developed and applied to sewer networks based on local operational experience.
248
Three main alternatives exist: (i) wastewater treatment to prevent H2S formation, (ii) H2S
249
removal from the water phase or prevention of emissions to the atmosphere, and (iii)
250
treatment of the gas and use of corrosion-resistant material (Barjenbruch et al., 2008). For
251
example, H2S can be chemically bound by iron-based agents, or its formation can be reduced
252
by dosing sewage with nitrate to favour denitrification at the expense of sulfate reduction.
253
Alternatively, moulded trap systems, biofilters and amorphous trap systems are used to treat
254
gas and limit gas emissions (Barjenbruch et al., 2008). Comparisons of the efficiency of these
255
options (e.g. Barjenbruch, 2003; Saračević, 2009; Frey, 2008) have not yielded clear
256
recommendations of which approaches to adopt as effective control measures.
257
Water capture, treatment and supply systems: Protected groundwater is a preferred
258
source of water for human consumption. However, since underground resources are
259
frequently insufficient or polluted in urban agglomerations, water demand usually exceeds the 9
260
locally renewable resources. Therefore, water must be either withdrawn from nearby rivers or
261
brought in from distant sources by man-made water conducts. Local recycling can be a
262
complementary strategy to mitigate water shortage. Two key techniques used to ensure the
263
supply of high-quality water in urban areas are bank filtration and groundwater recharge.
264
Under favourable hydrogeological conditions, these approaches have been widely adopted,
265
both in Germany and elsewhere in the world. For example, in the city of Berlin, Germany, up
266
to 70 % of the water supply occurs via bank filtration and artificial recharge (Grünheid et al.,
267
2005). The contact zone between surface and ground water is an essential interface to ensure
268
safe and sustainable water supply. The capacity of this interface to clean up polluted surface
269
waters is remarkable, offering prospects of long-term reliability and stability of water capture
270
systems based on bank filtration and artificial groundwater recharge (Wiese et al., 2011).
271
Notwithstanding, a number of critical questions remain to ensure high water security
272
for urban populations. These include heterogeneity of redox conditions, which can
273
compromise water quality (Ernst et al., 2012), and the influence of natural organic matter
274
availability on the biodegradation of contaminants (Baumgarten et al., 2011). Furthermore, to
275
enhance water quality when persistent residues are present, additional technical solutions
276
involving interface processes are usually needed after water has been naturally treated at the
277
surface water–groundwater interface. Passage of contaminants over activated carbon has
278
proved effective, taking advantage of adsorption and desorption processes at the interface
279
between the water phase and an artificial solid, although irreversibility of the processes and
280
the desorption kinetics remain incompletely known (Worch, 2012).
281
Sediment-water interfaces of urban surface waters: A key feature of urban water
282
bodies is that they often receive high loads of organic carbon and nutrients, not only from
283
their natural catchments, but also from technical systems such as wastewater treatment plants,
284
as direct run-off from roads and other paved surfaces, and from dry and wet deposition (Nehls
285
and Shaw, 2010). As a result of this abundant resource supply, urban water bodies are often 10
286
highly productive (Yu et al., 2012), unless discharge of toxic waste products limits biological
287
activity (Meyer et al., 2005), and tend to show symptoms of eutrophication exerting negative
288
impacts on water quality, sometimes in extreme ways (Kira, 1993; Yu et al., 2012). Moreover,
289
abundant resource supply fosters heterotrophic activity and oxygen depletion in the unmixed
290
deep water layer above rich organic sediments. Potent greenhouse gases such as methane
291
(CH4) and nitrous oxide (N2O) form in these conditions as products of anaerobic carbon
292
transformations (e.g. denitrification, sulfate reduction, methanogenesis). As a result,
293
significant quantities of gas can accumulate in sediment pore water and lead to strong
294
supersaturation (Casper et al., 2000). Subsequently, the gases either diffuse into the water
295
column or rise as bubbles, a process known as ebullition. It is unknown, however, to what
296
extent rates of such anaerobic processes in urban water bodies and the corresponding gas
297
fluxes across the sediment-water interface differ from those in natural aquatic ecosystems
298
(McKaina et al., 2012).
299
Finally, water surfaces of lakes and running waters also acts as exchange zones for
300
water, energy, particles, solutes and gases (Gualtieri and PulciDoria, 2013). In the case of
301
methane, one of the most important greenhouse gases, only a fraction of the gas released from
302
the sediment is emitted to the atmosphere. This is because methane is efficiently oxidized
303
when reaching the interface between anoxic deep and oxic surface water layers in stratified
304
water bodies. This so-called oxycline, defined by a steep vertical gradient in oxygen
305
concentration, thus functions as a biological methane trap (Duc et al., 2010; Casper et al.,
306
2009). In rising gas bubbles, methane oxidation at the oxycline and in the overlying
307
oxygenated surface water may be effectively circumvented, especially in shallow water
308
bodies. However, whether these fluxes from urban water surfaces are significant at local,
309
regional and possibly global scales, and how they compare with fluxes from technical
310
compartments of urban water systems, is unknown. The temperature rise projected by climate
311
change scenarios will affect such fluxes across water interfaces in the future, influencing not 11
312
only greenhouse gas dynamics but also urban micro-climate and surface water dynamics, not
313
least through shifts in the timing and duration of ice cover on urban lakes.
314 315 316
4. Selected research challenges
317
Understanding the dominant controls of processes is a prerequisite to understand,
318
predict and influence the overall performance of any system (Schulz et al., 2006; Cadenasso
319
et al., 2007). In urban water systems, hydrological and biogeochemical processes have been
320
found to be strongly influenced by physical and chemical characteristics of the subsurface
321
(Paola et al., 2006) as well as by small-scale land-use patterns (Haase, 2009), which generate
322
a multitude of interfaces. It appears, therefore, that understanding interface processes could be
323
particularly important to manage urban water resources efficiently (Aronica and Lanza, 2005).
324
This suggests that concepts, models and data derived from studies in either natural or rural
325
catchments, or from technical systems alone, cannot be readily applied to urban settings,
326
calling for investigations into patterns and processes specifically at interfaces of the urban
327
water system. This includes, but is not limited to, processes at technical-natural interfaces. An
328
emerging research priority to improve urban water resource management is, therefore, to
329
describe and quantify the fluxes and transformations of matter and energy as water flows
330
across interfaces characteristic for urban areas.
331
Meeting this objective not only requires improved analytical techniques to characterize
332
steep physical and biogeochemical gradients, but also advanced computational methods,
333
including numerical modeling. Numerous biogeochemical transformations alter the
334
composition of ground water moving through the aquifer, especially where ground water
335
reaches the sediment-water interface of surface water bodies (Eiswirth et al., 2004).
336
Temperature, pressure, and chemical concentrations of dissolved substances can change
337
substantially as water flows across such interfaces. A difficulty for accurate quantitative 12
338
descriptions of the dynamics beyond the local scale is that locations of high activity are
339
patchily distributed. They also often occur as pulsed events, driven by hydrological conditions
340
that are dictated by local precipitation and run off patterns and, at larger scale, by river flow
341
regimes including severe floods and droughts. Drivers of surface-subsurface water exchange
342
are not restricted to hydraulic gradients, however; temperature, oxygen concentration and
343
biological activity at the sediment-water interfaces can also affect rates of biogeochemical
344
transformations and need to be considered (Greskowiak et al., 2005).
345
Patchiness of the processes and high dynamics imply that measurements at high
346
temporal and spatial resolution are required to identify where and when the processes occur.
347
A range of new and increasingly affordable technologies, both ground-based sensors and air-
348
borne instruments (Lewandowski et al., 2013), are now available to record ecohydrological
349
data in real-time and thus monitor rapid flow alterations or changes in physico-chemical water
350
quality (e.g. during pulse events) as well as detect early indications when and critical
351
thresholds where urban water systems might switch between states (Battin et al., 2003;
352
Argerich et al., 2011). This potential of the new technologies has been insufficiently exploited
353
to date.
354
Even basic hydrological processes in urban areas are not well understood. For
355
example, rainwater partitioning after light rain events is not well investigated. Process-based
356
models embracing the whole range of rain intensities are lacking to date, and important
357
features such as non-linear dynamic filling of surface stores in porous media or the extent of
358
relief storage (puddles, lane grooves) are not currently embodied in such models. Importantly,
359
both run off and evaporation must be represented in the models at sufficiently fine resolution,
360
both in terms of volumes and time, to address questions such as the cooling power of paved
361
surfaces, daily rainwater availability (for toilet flushing, urban greening, and urban gardening)
362
or first-flush toxicity assessments. Thus, an important research need is to investigate the
363
fundamental dynamics of rainwater partitioning, including evaporation from overheated 13
364
pavers into saturated atmospheres, and factors influencing such processes (e.g. hydrophobicity
365
caused by dust accumulation). Coupling of process-oriented rainwater partitioning equations
366
into urban water simulation models could help to get an integrated view of the water
367
dynamics on paved urban surfaces (Rim, 2011).
368
While process-oriented models are important, an additional challenge is to develop a
369
synoptic understanding of the essential processes that characterize urban water systems, with
370
special emphasis on interfaces. One way forward to meet this goal is to engage in building
371
complex models, either comprehensive single models or a suite of coupled models designed
372
for urban water management (Blöschl, 2006). In coping with the task of representing small-
373
scale spatial structures and dynamic processes in these numerical models, it is vital to capture
374
complex information in a small number of meaningful quantities (Schulz et al., 2006). This
375
requires advanced strategies for integrating flow and reactive transport linking two different
376
models on either side of the interface (Dogan, 2011). This problem is similar to modelling
377
concepts developed for porous media and free flow (Khalili et al., 1999). In addition, the new
378
models require laboratory studies to estimate model parameters before tests are run under
379
controlled conditions for initial validation (Hinkelmann, 2005). Next, the models need be
380
subjected to tests in real-world field settings in urban areas to explore the application range
381
for predictions, and to identify significant limitations.
382
Much research effort has been directed towards urban water systems over the past few
383
decades, especially with a view to improve water treatment technology and quantitative urban
384
water management (Aronica and Lanza, 2005). At present, urban water management is
385
generally centralized, based on a small number of drinking and wastewater treatment plants.
386
By and large this approach has proved effective to assure human water security in cities.
387
However, it also suffers from notable weaknesses. In particular, the fact that centralized water
388
supply and wastewater treatment result in extensive water diversions from the original flow
389
systems, contributing to the degradation of urban aquatic ecosystems which often serve as 14
390
signature landscape features of urban areas (Kareiva et al., 2007). Extensive pipe networks
391
also involve important operational costs, particularly for energy consumption of drinking
392
water supply systems, as well as high long-term maintenance costs. Therefore, a shift from a
393
few large treatment plants to a system of small, spatially distributed facilities has been
394
advocated to manage water resources in urban areas (Langeveld et al., 2013). Elements of
395
such a decentralized approach include small-scale engineered systems, but also constructed
396
wetlands for treated wastewater management designed to mimic natural ecosystems, reduce
397
environmental impact (Potter, 2006), and even deliver direct environmental benefits (e.g.
398
habitat for wetland species). Concepts to implement these principles have been proposed
399
(Rodriguez-Iturbe and Porporato, 2004) but they need refinement with built-in flexibility to
400
accommodate a wide variety of site-specific natural, technical and socio-economic variation.
401 402 403
5. Conclusions
404
To conclude, natural, technical and hybrid interfaces play a key role in the urban water
405
cycle. Our central tenet is that analyzing, understanding and modelling processes and fluxes at
406
urban water interface at various spatial scales is key to improve sustainable water resource
407
management in urban areas. Recognizing that natural and technical system components of the
408
urban water cycle need to be viewed in an integrative fashion is an important prerequisite to
409
achieve this goal. Its implementation requires close collaboration of engineers and scientists
410
grounded on a common conceptual basis. Graduate education can serve to develop this basis
411
and catalyse the transfer of the approach from research to urban water management and
412
engineering practice.
413
Beyond the integration of water engineering and science, it is important for success to
414
involve the multitude of stakeholders with vested interests in urban water flows and water
415
bodies, ranging from the water industry and public authorities to taxpayers and a wide range 15
416
of scientists, including social scientists. For example, in catchment management studies, role
417
plays are becoming an important instrument to investigate the behaviour of stakeholders and
418
the consequences thereof (Endlicher et al., 2011). Similarly, integrated system models might
419
be developed not necessarily to provide the best possible representation of a system’s
420
behaviour, but to serve as a communication tool that facilitates understanding complex facts,
421
evaluating their repercussions, and thus reaching consensus among stakeholders as a basis for
422
evidence-based and widely accepted management decisions (Blöschl, 2006).
423 424 425
Acknowledgements
426
We would like to thank our colleagues Peter Casper, Sabine Hilt, Michael Hupfer,
427
Birgit Kleinschmit, Anke Putschew, Ulrich Szewzyk and Gerd Wessolek for their helpful and
428
constructive input during the preparation of this manuscript as a part of a graduate PhD school
429
proposal.
430 431 432 433
References
434
Alberti M. 2005. The effects of urban patterns on ecosystem function. International Regional
435 436 437
Science Review 28: 168-192. Alley WM, Healy RW, LaBaugh JW, Reilly TE. 2002. Flow and storage in groundwater systems. Science 296: 1985-1990.
438
Argerich A, Marti E, Sabater F, Ribot M. 2011. Temporal variation of hydrological exchange
439
and hyporheic biogeochemistry in a headwater stream during autumn. Journal of the North
440
American Benthological Society 30: 635–652.
16
441 442
Aronica G, Lanza L. 2005. Hydrology in the urban environment. Hydrological Processes 44: 1005-1006.
443
Barjenbruch M, Hinkelmann R, Hüttl R, Huhnt W, Krämer T, Nehrig M, Rühmland S, Röben
444
R. 2008. An Online-Monitoring and Operation System to Prevent Odour and Corrosion in
445
Sewer Networks - Feasibilty Study. Report, Kompetenzzentrum Wasserg GmbH.
446 447
Barjenbruch M. 2003. Prevention of odour emergence in sewage networks. Water Science & Technology 47: 357-363.
448
Battin TJ, Kaplan LA, Newbold JD, Hendricks SP. 2003. A mixing model analysis of stream
449
solute dynamics and the contribution of a hyporheic zone to ecosystem function. Freshwater
450
Biology 48: 995–1014.
451
Baumgarten B, Jährig J, Reemtsma T, Jekel M. 2011. Long term laboratory column
452
experiments to simulate bank filtration: Factors controlling removal of sulfamethoxazole.
453
Water Research 45: p. 211-220.
454 455 456 457
Blöschl G. 2006: Hydrologic synthesis: Across processes, places, and scales. Water Resources Research 42: W03S02, doi:10.1029/2005WR004319. Braud I, Fletcher TD, Andrieu H. 2013. Hydrology of peri-urban catchments: processes and modelling. Journal of Hydrology 485: 1-4.
458
Brion GM, Neelakantan TR, Lingireddy S. 2001. Using neuronal networks to predict peak
459
Cryptosporidium concentrations. Journal of the American Water Works Association 93: 99-
460
105.
461 462 463
Broto VC, Allan A, Rapoport, E. 2012. Interdisciplinary perspectives on urban metabolism. Journal of Industrial Ecology 16: 851-861. Cadenasso ML, Pickett STA, Schwarz K. 2007. Spatial heterogeneity in urban ecosystems.
464
Reconceptualizing land cover and a framework for classification. Frontiers in Ecology and
465
the Environment 5: 80-88.
17
466 467
Casper P, Maberly SC, Hall GH, Finlay BJ. 2000. Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere. Biogeochemistry 49, 1-19.
468
Casper P, Albino MF and Adams DD. 2009. Diffusive fluxes of CH4 and CO2 across the
469
water-air interface in the eutrophic lake Dagow, NE Germany. Verhandlungen der
470
Internationalen Vereinigung für Limnologie 30: 874-877.
471
Decker EH, Elliott S, Smith FA, Blake DR, Rowland FS. 2000. Energy and material flow
472
through the urban ecosystem. Annual Review of Energy and the Environment 25: 685-740.
473
Dogan MO. 2011.Coupling of porous media flow with pipe flow. Dissertation, Mitteilungen
474 475
Heft 199, Institut für Wasserbau, Universität Stuttgart Doppler T, Franssen HJH, Kaiser HP, Stauffer F. 2007. Field-evidence of a dynamic
476
leakage coefficient for modeling river-aquifer interactions. Journal of Hydrology 347: 177-
477
187.
478 479 480 481 482
Duc N, Crill P, Bastviken D. 2010. Implications of temperature and sediment characteristics on methane formation and oxidation in lake sediments. Biogeochemistry 100, 185-196. Eiswirth M, Wolf L, Hötzl H. 2004. Balancing the contaminant input into urban water resources. Environmental Geology 46: 246-256. Endlicher W, Hostert P, Kowarik I, Kulke E, Lossau J, Marzluff J, van der Meer E, Mieg H,
483
Nützmann G, Schulz M, Wessolek G (Eds.). 2011. Perspectives in Urban Ecology: Studies
484
of Ecosystems and Interactions between Humans and Nature in the Metropolis of Berlin,
485
Germany. Springer, Heidelberg, pp. 351.
486
Ernst M, Hein A, Asmin J, Krauss M, Fink G, Hollender J, Ternes T, Jorgensen C, Jekel M,
487
McArdell CS. 2012. Water quality analysis: Detection, fate, and behaviour, of selected trace
488
organic pollutants at managed aquifer recharge sites, Chapter 12. IWA Publishing - Water
489
Reclamation Technologies
490
9781843393443): p. 197-224.
491
for
Safe
Managed
Aquifer
Recharge,
2012
(ISBN
Fletcher TD, Andrieu H, Hamel P. 2013. Understanding, management and modeling of urban 18
492
hydrology and its consequences for receiving waters: a state of the art. Advances in Water
493
Resources 51: 261-279.
494
Frey M. 2008. Untersuchungen zur Sulfidbildung und zur Effizienz der Geruchsminimierung
495
durch Zugabe von Additiven in Abwasserkanalisationen. Kassel University Press, ISBN
496
978-3-89958-453-0.
497
Greskowiak J, Prommer H, Massmann G, Johnston CD, Nützmann G, Pekdeger A. 2005.The
498
impact of variably saturated conditions on hydrogeochemical changes during artificial
499
recharge of groundwater. Applied Geochemistry 20:1409–1426.
500 501 502
Grimm NB, Faeth SH, Golubiewski NE, Redman CL, Wu J, Bai X, Briggs JM. 2008. Global change and the ecology of cities. Science 319: 756-760. Grünheid S, Amy G and Jekel M. 2005.Removal of bulk dissolved organic carbon (DOC) and
503
trace organic compounds by bank filtration and artificial recharge. Water Research 39:
504
3219-3228.
505 506
Gualtieri C, Mihailovic DT (Eds.). 2013. Fluid Mechanics of Environmental Interfaces, Second Edition. CRC Press/Balkema, Leiden, The Netherlands, 480 pp.
507
Gualtieri C, PulciDoria G. 2013. Gas-transfer at unsheared free surfaces. In: Gualtieri C and
508
Mihailovic DT (Eds.). 2013. Fluid Mechanics of Environmental Interfaces, Second Edition.
509
CRC Press/Balkema, Leiden, The Netherlands, 145-180.
510 511
Haase D. 2009. Effects of urbanization on the water balance – A long-term trajectory. Environmental Impact Assessment Review 29: 211-219.
512
Harremoës P. 2002. Integrated urban drainage: status and perspectives. Water Sciences
513
&Technology 45: 1-10.
514
Hilt S, Köhler J, Kozerski HP, Scheffer M, Van Nes E. 2011. Abrupt regime shifts in space
515
and time along rivers and connected lake systems. Oikos 120: 766-775.
19
516
Hinkelmann R. 2005. Efficient Numerical Methods and Information-Processing Techniques
517
for Modeling Hydro- and Environmental Systems. Lecture Notes in Applied and
518
Computational Mechanics, Vol. 21, Springer, Berlin, Heidelberg
519 520 521 522 523 524 525 526 527
Kareiva P, Watts S, McDonald R, Boucher T. 2007. Domesticated nature: shaping landscapes and ecosystems for human welfare. Science 316: 1866-1869. Khalili A, Basu AJ, Pietrzyk U, Jörgensen BB. 1999. Advective transport through permeable sediments: a numerical and experimental approach. Acta Mechanica 32: 221-227. Kira T. 1993. Major environmental problems in world lakes. Memorie dell'Istituto Italiano di Idrobiologia 52: 1-7. Langeveld JG, Schilperoort RPS, Weijers SR. 2013. Climate change and urban wastewater infrastructure: There is more to explore. Journal of Hydrology 476: 112-119. Laub BG, Baker DW, Bledsoe BP, Palmer MA. 2012. Range of variability of channel
528
complexity in urban, restored and forested reference streams. Freshwater Biology 57: 1076–
529
1095.
530
Lewandowski J, Meinikmann K, Ruhtz T, Pösche, F, Kirillin, G. 2013. Localization of
531
lacrustine groundwater discharge (LGD) by airborne measurement of thermal infrared
532
radiation. Remote Sensing of Environment 138: 119-125.
533 534
Mansell M, Rollet F. 2009. The effect of surface texture on evaporation, infiltration and storage properties of paved surfaces. Water Science and Technology 60: 71-76.
535
McClain, ME, EW Boyer, CL Dent, SE Gergel, NB Grimm, PM Groffman, SC Hart, JW
536
Harvey, CA Johnston, E Mayorga, WH McDowell, G Pinay. 2003. Biogeochemical hot
537
spots and hot moments at the interface of terrestrial and aquatic ecosystems. Ecosystems 6:
538
301-312.
539
McKaina K, Wofsy SC, Nehrkorn T, Eluszkiewicz J, Ehleringer JR, Stephens BB. 2012.
540
Assessment of ground-based atmospheric observations for verification of greenhouse gas
20
541
emissions from an urban region. Proceedings of the National Academy of Science of the
542
USA 109: 8423-8428.
543 544 545 546 547 548 549 550 551
Meyer JL, Paul MJ, Taulbee WK. 2005. Stream ecosystem function in urbanizing landscapes. Journal of the North American Benthological Society 24: 602-612. Mitchell VG, Mein RG, McMahon TA. 2001. Modelling the urban water cycle. Environmental Modelling & Software 16: 615-629. Nehls T, Jozefaciuk G, Sokolowska Z, Hajnos M, Wessolek G. 2008. Filter properties of seam material of paved urban soils. Hydrology and Earth System Sciences 12: 691-702. Nehls T, Shaw R. 2010. Black carbon in soils - relevance, analysis, distribution. Soil Science Society of America. Soil Survey Horizons 51: 79-84 Nehls T, Rim YN, Wessolek G 2011. Technical note on measuring run-off dynamics from
552
pavements using a new device: the weighable tipping bucket. Hydrology and Earth System
553
Sciences 15: 1379-1386.
554 555
Nielsen PH, Raunkjaer K, Hvitved-Jacobsen T. 1998. Sulfide Production and Wastewater Quality in Pressure Mains. Water Science & Technology 37: 97-104.
556
Nützmann G, Wiegand C, Contardo-Jara C, Hamann E, Burmester V, Gerstenberg K. 2011.
557
Contamination of Urban Surface and Ground Water Resources and Impact on Aquatic
558
Species. In: Endlicher et al. (Eds.) Perspectives in Urban Ecology, Springer, Heidelberg, 43-
559
88.
560 561 562
Palmer MA, Bernhardt E. 2006. Hydroecology and river restoration: Rip for research and synthesis. Water Resources Research 42: W03S07, doi:10.1029/2005WR004354. Paola C, Foufoula-Georgiou E, Dietrich WE, Hondzo M, Mohrig D, Parker G, Power ME,
563
Rodriguez-Iturbe I, Voller V, Wilcock P. 2006. Toward a unified science of the Earth’s
564
surface: Opportunities for synthesis among hydrology, geomorphology, geochemistry, and
565
ecology, Water Resources Research 42: W03S10, doi:10.1029/2005WR004336.
566
Pickett STA, Cadenasso ML. 2008. Linking ecology and built components of urban mosaics: 21
567 568 569
an open cycle of ecological design. Journal of Ecology 96: 8-12. PomeroyRD.1970. Sanitary sewer design for hydrogen sulphide control. Public Works 101 (10): 93-96.
570
Potter KW. 2006. Small-scale, spatially distributed water management practices: Implications
571
for research in the hydrologic sciences. Water Resources Research 42: W03S08,
572
doi:10.1029/2005WR004295.
573 574 575 576 577 578 579 580 581
Rim Y-N. 2011. Analyzing Runoff Dynamics of Paved Soil Surface Using Weighable Lysimeters.PhD Thesis, Technische Universitaet Berlin, urn:nbn:de:kobv:83-opus-30471 Rodriguez-Iturbe I, Porporato A. 2004. Ecohydrology of water-controlled ecosystems: soil moisture and plant dynamics. 1st ed. Cambridge University Press. Roy JW, Bickerton G. 2012. Toxic Groundwater Contaminants: An Overlooked Contributor to Urban Stream Syndrome? Environmental Science & Technology 46, 729-736. Saračević E. 2009. Zur Kenntnis der Schwefelwasserstoffbildung und -vermeidung in Abwasserdruckleitungen. Wiener Mitteilungen 211, ISBN 978-3-85234-103-3. Schulz K, Seppelt R, Zehe E, Vogel HJ, Attinger S. 2006. Importance of spatial structures in
582
advancing
583
doi:10.1029/2005WR004301.
584 585
hydrological
sciences.
Water
Resources
Research
42:
W03S08,
Shuster WD, Bonta J, Thurston H, Warnemuende E, Smith DR. 2005. Impacts of impervious surface on watershed hydrology: A review. Urban Water Journal 2: 263-275.
586
Urban U, Heilmann A. 2011. Möglichkeiten der Belüftung von Druckleitungen zur
587
Minderung von Geruch und Korrosion. 4. OWL Abwassertag, Section of Environmental
588
Engineering Department of Life Sciences, Aalborg University; http://www.sewer.dk/.
589
Walsh CJ, Roy AH, Feminella JW, Cottingham PD, Groffman PM, Morgan RP. 2005. The
590
urban stream syndrome: current knowledge and the search for a cure. Journal of the North
591
American Benthological Society 24: 706-723.
22
592 593 594
Wiese B, Nützmann G. 2009. Transient leakage and infiltration characteristics during lake bank filtration. Ground Water 47: 57-68. Wiese B, Massmann G, Jekel M, Heberer T, Dünnbier U, Orlikowski D, Grützmacher G.
595
2011. Removal kinetics of organic compounds and sum parameters under field conditions
596
for managed aquifer recharge. Water Research 45: 4939-4950.
597 598 599 600 601
Wittenberg H, Aksoy H. 2010. Groundwater intrusion into leaky sewer systems. Water Science & Technology 62: 92-98. Worch E. 2012. Adsorption Technology in Water Treatment: Fundamentals, Processes, and Modeling, De Gruyter. Yu S, Yu GB, Liu Y, Li GL, Feng S, Wu SC, Wong MH. 2012. Urbanization impairs surface
602
water quality: Eutrophication and metal stress in the Grand Canal of China. River Research
603
and Applications 28: 1135-1148.
604
23
605 606 607 608 609 610 611
FIGURE CAPTIONS
Figure 1:
Scheme of an urban water cycle
Table 1:
Examples of urban water interfaces, major processes and key questions
612 613 614
24
615 616 617
Table 1: Examples of urban water interfaces, major processes and key questions
618 Interfaces
Processes
Key questions
Surface water - atmosphere
Release of greenhouse gases
Gas production processes and flux rates are poorly known and differ from those in natural systems
Soil surface - atmosphere
Vapor and heat transport across the interface
Rainwater partitioning, especially from small rain events, storage and evaporation dynamics of paved urban soils
Surface water - sediment groundwater
Bank filtration / exchange between aquifers and urban streams and rivers
Fate of nutrients and pollutants at the interface between surface water – aquifer; Reactivity of this interface
Sewer pipe: wastewater gas space
Transport of sulfurous compounds in sewers
Transport and kinetics of sulfuric acids in sewers are poorly known, especially under variable temperatures
619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647
Surface water - treated wastewater
Interaction of trace substances originating from treated wastewater and freshwater biota
To which extent is the treated wastewater diluted in the surface water and how does it affect the water quality of the receiving water?
648 649 650 651 652 653 654 655 656 657 658 659 25
Figure
660
Highlights
661 662
We identify boundaries involving compartments of the urban water system as urban water interfaces.
663
These urban water interfaces are characterized by steep physical and biogeochemical gradients.
664
We regard these interfaces as hotspots of environmentally relevant fluxes.
665
We discuss a framework to improve our fundamental understanding of aquatic interface processes.
666 667 668
26