Wastewater filtration and re-use: An alternative water source for London

Science of the Total Environment 437 (2012) 173–184

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Wastewater filtration and re-use: An alternative water source for London Jonathan D. Paul ⁎, Martin J. Blunt Department of Earth Science and Engineering, Royal School of Mines, Imperial College, Prince Consort Road, London SW7 2BP, UK

a r t i c l e

i n f o

Article history: Received 3 April 2012 Received in revised form 2 August 2012 Accepted 2 August 2012 Available online xxxx Keywords: Chalk Contaminant hydrology London Wastewater Alternative water source

a b s t r a c t The rapid growth and climate of the Greater London region have contributed towards large deficits in water supply. Inexpensive, energy-efficient and sustainable water resource schemes are increasingly sought as a means to boost supply. Here, we propose a small-scale recycling scheme whereby tertiary-treated wastewater is pumped to the Cretaceous chalk of the London Basin. By taking advantage of the natural filtration properties of the underlying chalk, contaminants can be effectively attenuated over relatively short length scales to result in pure water. The problem is approached from four different scales. First, we define two localities in London where such a pumping scheme might operate; regions which combine a thick unsaturated zone and high chalk transmissivity, both essential to ensure maximum contaminant removal and minimum environmental impact. Secondly, the effects of pumping fluid into the Chalk at the two localities are quantified using a finite-difference groundwater flow model. We show that rivers impose a regular groundwater flow regime, whereas pre-existing abstraction wells will lead to less predictable results. Thirdly, we consider the effect of fractures on channelling rapid fluid flow within the rock mass. By digitising a fracture map based upon outcrop measurements from chalk exposed on the Kent coast similar to that beneath London, we quantify transport patterns of wastewater after injection. Imbibition to the chalk matrix (and therefore filtration) will occur where fluid pressure gradients are highest, for instance around disconnected fracture tips. Finally we demonstrate the efficacy of chalk in contaminant removal by injecting an analogue ‘effluent’ through a chalk core. ICP-AES analysis on the recovered solution shows the contaminants (viz. a suite of heavy metals) are arrested or removed over relatively small time- and length-scales. Numerical and analytical solutions fit the data poorly, shedding some light on the importance of hydrodynamic dispersion of aqueous contaminants within the chalk. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The inhabitants of the London Basin live in a relatively arid environment: the average annual precipitation of ~600 mm is lower than the equivalent totals in Rome, Dallas, and Istanbul. While the water supply–demand deficit is predicted to reach over 200 Ml d− 1 by 2019 as Available Water For Use (AWFU) decreases (Fig. 1), many of the mitigation strategies presently in place across London are expensive, unsustainable, and could lead to deleterious environmental consequences: indeed, sea importation and construction of new desalination plants in east London will take precedence to 2015 (Thames Water, 2009). Small-scale and sustainable water production schemes are increasingly sought after as a means to complement existing water sources (e.g. Al-Jayyousi, 2003; Jefferson et al., 1999). Wastewater re-use has the potential to greatly increase AWFU, while groundwater injection often dissociates reclaimed water from its source in the minds of the public (NRC, 2012).

The physiography of south-east England is dominated by Upper Cretaceous chalk strata, folded into a series of synclines and anticlines during the Alpine Orogeny (Blundell, 2002). The London Basin itself is one such syncline whose axis trends roughly NE–SW. Chalk outcrops are common across SE London, but across central and northern London the Chalk is overlain by Cenozoic sediments up to 70 m in thickness (Fry and Kelly, 2008). These more recent sediments include the Eocene-aged London clay, through which much of the London Underground network was bored, and more sandy and unconsolidated underlying strata collectively termed the Lower London Tertiary deposits. The London clay is highly impermeable (k = ~10− 16 m2: Chandler et al., 1990), giving rise to a confined chalk aquifer under much of northern London from which relatively pure water is sourced. The mechanics of fluid flow within this chalk aquifer have been treated to comprehensive numerical and analogue simulations (e.g. Allen, 1995; Barraclough et al., 1994; Little et al., 1996). The most important realisations are outlined below.

⁎ Corresponding author at: Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, UK. Tel.: +44 7784 772818. E-mail address: [email protected] (J.D. Paul).

• Chalk is a highly fractured medium with two sets of porosity: fractures are largely responsible for fluid transmission; pores tend to account for storage and do not drain readily due to their small throats (Fry and Kelly, 2008). Belayneh et al. (2007) describe the

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Fig. 1. Supply–demand balance in the London Water Resource Zone to 2040. AWFU = Available Water For Use (Thames Water, 2009).

Upper Chalk as an unusually high-porosity and low-permeability aquifer, with ~ 80% of primary transmissivity derived from cm- to m-scale fractures. • Only the top 5–20 m of chalk below the water table transmits water freely (Headworth, 2004). • There exists a threshold matric potential above which fluid flow within fractures will be initiated, thus by-passing the chalk matrix

entirely (Reeves, 1979). Indeed, Barraclough et al. (1994) found that ‘by-pass flow’ only occurred if the Chalk were fully saturated, the hydraulic conductivity being insufficient to accept incoming meteoric water (Fig. 2). Besides the complexities of flow mechanisms, chalk is recognised as an effective medium for the removal of excess nitrogen, pathogens,

Fig. 2. Idealised fracture networks in Upper Cretaceous chalk (after Allen, 1995; Reeves, 1979). a: Profile of fracture density as a function of depth. b: The initiation of ‘by-pass flow’ in response to a high hydraulic stress (Lloyd et al., 1981).

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and heavy metals from solution (e.g. Baxter, 1985; Baxter and Clark, 1984; Mühlherr et al., 1998). The principal processes of contaminant retardation (via sorption and cation exchange) and elimination (for instance, by precipitation and hydrolysis) occur in the unsaturated zone of the Chalk (Fig. 3 and Foster et al., 1994). Most of these reactions require the absence of oxygen (i.e. the complete confinement of chalk). All non-conservative contaminants are then subject to dilution with groundwater in the saturated zone. The reliability of contaminant attenuation (e.g. multiple engineered barriers) and monitoring will affect any future considerations towards retention (e.g. surface or aquifer storage) or blending with freshwater in distribution systems post-treatment (NRC, 2012). Small-scale wastewater recycling schemes which exploit these natural filtration mechanisms in porous rock are common outside the UK. The first direct injection scheme, of treated reclaimed water into Miocene-aged sandstone, was initiated in Orange County, California, in 1976 (NRC, 2012). This scheme served the dual purposes of forming a barrier to saline intrusion, and augmenting potable supply. Parizek (2007) invokes the concept of a ‘living filter’ to describe the tertiary biopurification of wastewater as it passes through the Carboniferousaged limestones of the Appalachian Mountains, USA; other successful carbonate-filtration systems are well documented at the larger scale, in Limassol, Cyprus (Papiacovou, 2001) and in Campania, Italy (De Feo et al., 2007). ~42% of the UK's daily 100 Ml wastewater is discharged to chalk aquifers, yet only 23% of London's water supply is derived from groundwater resources (Thames Water, 2009) — the lowest regional value across England. In the UK, wastewater recharge schemes have mostly been restricted to a series of trials, including a localised chalkinjection undertaking under the auspices of Southern Water in Hampshire (Baxter et al., 1981; Headworth, 2004). Within London, freshwater is currently pumped into the chalk aquifer. O'Shea et al. (1995) focus on one such scheme: the Enfield–Haringey artificial recharge operation in north London. This work aims to increase drought

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yield, but it does not provide new freshwater (as water must be treated before percolating into the Chalk), nor is it sustainable, depending on surplus treated water from periods of higher-than-average precipitation. Sustainable recharge and recycling using wastewater are therefore feasible possibilities. However, many authors (Foster, 1998; Headworth, 2004; Parizek, 2007) stress the need to devise innovative environmental protection management plans, especially given that the physical effects of pumping fluid into the Chalk can vary widely (e.g. Baxter and Clark, 1984; Jones and Cooper, 1998; Reeves, 1979; Robins, 1998). Similarly, NRC (2012) note that the degree of public health protection provided by such natural systems is often not well defined, with the performance of soiland rock-aquifer treatment systems across the USA and elsewhere being poorly understood outside of specific hydrogeological conditions. The uncertainties of fluid dynamics within chalk, as well as its capacity for contaminant attenuation, provide the motivation behind this study. We employ a multi-disciplinary approach to address four questions, each aimed towards elucidating optimal conditions for the safe and effective application of a small-scale wastewater recycling scheme in London. First, in Section 2.1, we identify two locations for the operation of such a scheme. The significant hydraulic heterogeneity of the Chalk, as noted earlier, predicates the need for excluding locations which are unsuitable to accept wastewater. Following the definition of these two locations, we then analyse the effects of pumping fluid into the Chalk upon the local groundwater flow regime and water table levels (Section 2.2). Since fractures are widely understood (see e.g. Macdonald et al., 1998, and Owen and Robinson, 1978) to act as conduits for rapid flow within chalk, their effect is quantified in Section 2.3, by simulating a waterflood using an observed fracture network. Finally, in Section 2.4, we analyse the efficacy of chalk as a ‘natural filter’ capable of removing contaminants. This element involves a core-scale pumping experiment, using an input suite of aqueous heavy metals analogous to contaminants within wastewater, which are subsequently analysed using ICP-AES (Inductively Coupled Plasma

Fig. 3. Summary of major processes promoting contaminant attenuation in chalk groundwater systems. Thickness of the dark blue regions corresponds to the relative importance of dilution, retardation, and elimination effects (after Allen, 1995; Baxter, 1985; Foster et al., 1994). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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W W

W

30 W

20 10

W

W W

0

W

W

LEGEND

W

2

Chalk transmissivity (m /d)

W

Under 20 20 - 99 100 - 499 500 - 1000 Over 1000

W

Unsaturated zone thickness 10

10m contours

Miscellanea

W

10 20 30 40 50

0

5mAOD height contours River (open) River (buried/covered) Well, abstraction greater 3 5 than 10 m /yr (2007)

? London clay absent (Chalk is unconfined)

W W

N

W

?

1 : 20 000 W

0

1000m

Fig. 4. Comparison of Upper Chalk transmissivity with unsaturated zone thickness across the centre of the London Basin. Topographic contours, paths of major water courses, and abstraction wells are also shown. Faded position of map in SE London = insufficient data coverage. Two potentially feasible localities for the proposed scheme combine high primary transmissivity with a thick unsaturated zone: Marylebone (central London: 51°31′N 0°9′W) and Streatham (south London: 51°27′N 0°7′W).

Atomic Emission Spectroscopy). The results from each section are then discussed within the framework of other similar recycling schemes worldwide, and within the broader theme of single-phase fluid transport in porous media.

1889). Fig. 4 is the resulting map of feasibility for the pumping scheme, which collates over five hundred data points from three separate datasets.

2.2. Groundwater flow modelling 2. Material and methods 2.1. Feasibility map The first stage involves the identification of localities across London which would couple low environmental impacts with the requisite geological conditions. These conditions must involve a thick unsaturated zone in the Upper Chalk – maximising the length scales over which contaminants will be attenuated – and a high chalk transmissivity, such that wastewater-matrix imbibition and filtration will take place. First, a map of central London was generated and geo-referenced to a coordinate system using ArcGIS. Transmissivity data were obtained from the archives of the BGS Hydrology Group, Wallingford, UK (Allen et al., 1997; MacDonald and Allen, 2001; Price, 1987). Across the central London Basin, the upper bound on the unsaturated zone is taken as the base of the London clay aquiclude (Headworth, 2004). A grid of unsaturated zone thickness was derived via subtraction of water table from basal London clay structure contours (Fry and Kelly, 2008; Whitaker,

The effects of injecting fluid into the Chalk at two candidate localities are now examined. While three abstraction boreholes are present in the Marylebone area (Fig. 4), the River Effra and its tributaries flow northward in culvert across the Chalk and Lower London Tertiary deposits which characterise and define the physiography of south London. The MODFLOW finite difference groundwater flow modelling code was utilised to explicitly interrogate the effects drainage and abstraction might have upon fluid injection to the Chalk (see Appendix B for further details about the code and finite difference method). Local stratigraphy down to Upper Cretaceous chalk is approximated as a series of layers (using borehole data from Whitaker, 1889), each of 2 km × 2 km area: Fig. 5 illustrates the block sections used for each scenario. Parameters used in the model are given in Appendix A. Dirichlet boundary conditions include constant head at the left- and right-hand model boundary, which pertain to present water tables at each locality. No-flow boundaries are also imposed at the top and the bottom, while abstraction/injection and river flow rates are taken from Fry and Kelly (2008) and Thames Water (2009), respectively.

Fig. 5. Block stratigraphic sections beneath the two candidate localities mentioned in Fig. 4, together with results from the MODFLOW groundwater flow modelling at each. An injection well is placed within each locality and is assumed to discharge to the Upper Chalk at a constant rate of 1 Ml/yr, a figure of the same order of magnitude as similar re-use schemes globally (e.g. Al-Jayyousi, 2003; Papiacovou, 2001; Parizek, 2007). The 2D plane is a section taken across the top of the Upper Chalk showing the effect of discharging fluid upon the antecedent flow regime; contours = water table level; arrows = direction of flow.

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2.3. Simulation of fluid flow within fractures As discussed in Section 1, the chalk aquifer is unusual for many reasons: it is the de facto most important (in terms of storativity) and complex in the UK, with a relatively high porosity and low permeability. The presence of fissures and fractures as conduits for channelling flow may impair and complicate recharge calculations (Lloyd et al., 1981; Reeves, 1979). In this section we adapt an object-oriented code, CSP (Complex Systems Platform: see e.g. Belayneh et al., 2007) to model in greater detail the effect of a waterflood on a fractured chalk rock mass. We photographed a well exposed outcrop of Upper Chalk at Dumpton Gap, Ramsgate, Kent, in November 2009 — see Fig. 6 for location. Following Downing et al. (1979), Little et al. (1996), and Mortimore (2009), we assume the attitude of the fracture network remains ~constant in the Chalk across the London Basin, from outcrop at the Kent Coast and North Downs, to its synclinal hinge 70–80 m below central London. Fracture density at outcrop is therefore a fair representation of the situation at the two localities proposed earlier. Three distinct fracture sets were defined (Fig. 7) and 61 measurements of dip, dip direction, and aperture were taken in the field. Next, fractures within each set were picked and digitised using the Rhino CAD-drawing package onto a 2D plane representing the model space. A spatially adaptive mesh, consisting of elements and nodes, was then generated. Input parameters are given in Appendix A. The porosity and permeability values assigned to the chalk matrix were derived from laboratory testing of a core collected at Dumpton Gap (see succeeding section for details); equivalent values assigned to each fracture set were derived from field measurements using Bear's (1972) empirical

relationship between permeability k and mean fracture aperture within each set b, k¼

b2 : 12

ð1Þ

Eq. (1) assumes that the fractures maintain their orientation and aperture across their length and that the fluid flow is laminar. For the duration of the model run, wastewater saturation= 1 and freshwater saturation = 0 at the left boundary; an arbitrary right-lateral fluid pressure gradient, Δp = 4 × 105 Pa is applied, the largest difference for which the model would retain stability. These boundary conditions simulate a flooding of pressurised wastewater from the left of the 40× 23 m model space into a 2D fractured chalk slice, based on observed data, already saturated with freshwater. The model is allowed to run until breakthrough of wastewater at the right side, 4 × 10 6 s (~6.5 weeks) after the commencement of flooding. 2.4. Chalk core pumping experiments We now turn our attention towards testing a chalk core obtained at Dumpton Gap (Fig. 7) for efficacy of contaminant attenuation. A dilute ICP-AES stock solution ‘effluent’ was prepared to contain ten metals (viz. La, Cu, Ni, Zn, Cd, As, Co, Mo, Pb and Hg) at concentrations of 10 ppm. These metals were chosen as they all sorb to varying degrees, and are often found in wastewater and considered hazardous (e.g. Ganji et al., 2005; Wan Ngah and Hanafiah, 2008). Other organic contaminants such as nitrate, bacteria and viruses were not included due to the difficulty in detection using the ICP-AES method; it was for this reason

Fig. 6. Map of localities used for sampling Upper Chalk outcrops along the Kent coastline.

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72.3 / 079.7

Stage 1. Cursory fracture tracing, and scaling, using Adobe Illustrator.

Stage 2. Precise tracing on a surface using Rhino modelling software.

Stage 3. Generation of the mesh; detail focused on fracture tips & intersections.

Fig. 7. Photograph of Upper Chalk outcrop at Dumpton Gap, Kent, digitised for the CSP model (Section 2.3). See Fig. 6 for location. Also included are surface traces and characteristics (viz. dip, dip direction, aperture b, and calculated permeability k, of three major fracture sets across the outcrop); 61 measurements of these quantities were taken. The attitude of the fractures remains ~ constant in the Chalk across the entire London Basin (Downing et al., 1979; Mortimore, 2009). Snapshots of the two initial stages in the modelling process are shown as windows across the outcrop: (i) fracture tracing and digitising using the Rhino suite; and (ii) generation of a spatially adaptive mesh.

that chloride – a useful conservative tracer – was also omitted from the input effluent solution. Laboratory set-up and basic measurements and techniques are outlined in Fig. 10. A 76 × 38 mm chalk core was first obtained from rock samples collected at Dumpton Gap, Kent; this core was then saturated, using a vacuum pump, with de-ionised water which had reached prior equilibrium with the chalk. At the same time, ground chalk was mixed with the input effluent to allow an equilibrium to be attained, negating the risk of dissolution reactions in the core during subsequent pumping. After a 24 hour equilibration time, the solution was centrifuged to separate the ground chalk from the effluent. The porosity and permeability of the core were then measured (see Fig. 10 for method). Afterwards, the saturated chalk core was placed in a core-holder and the pump connected as per Fig. 10. The pump was then loaded with equilibrated effluent before pumping commenced at Q = 1 ml/min — this figure having been determined as the average velocity of wastewater through the chalk matrix in the CSP model (Section 2.3). Sampling of the output solution took place at 5 min (and 5 ml) intervals in ICP-AES vials, which were immediately acidified with 5 ml 2 M HCl such that each sample became amenable to spectroscopic analysis. Sampling continued for ~ 4 pore volumes, 150 min or 150 ml, this figure being the total volume of the input effluent. The ICP-AES method analysed each of the 30 × 5 ml effluent vials for a total of 20 elements, including the 10 which were present in the original solution. For any given element, the output concentration was then normalised against the mean input concentration. The breakthrough curve for Mo was compared to an approximate numerical

solution, using an ad hoc advection–diffusion solver (see Appendix B for further details). Furthermore, the analytical (error function) solution for advection–diffusion of Mo cations within the chalk core was also derived and plotted in order to validate the numerical and laboratory-based results. Appendix B details the specific transport equation which is solved numerically, together with the boundary conditions and analytical solution.

3. Results Two areas within central London (Fig. 4) satisfy the criteria mentioned in Section 2.1. The London clay, up to 40 m thick under west London, thins to 5–10 m in a region approximately delineated by the Marylebone area in north-west central London (Whitaker, 1889). Combined with high recorded chalk transmissivity values (viz. >500 m2 d− 1), the resulting thick unsaturated zone makes for a possible locality. Similar values of high chalk transmissivity and permeability have been noted for a region in south London centred around Camberwell, Brixton and Streatham. However the limiting factor in this instance is the degree of confinement of the Chalk: east of Brixton the Upper Chalk crops out at the surface, the overburden having been removed by erosion (Ellison, 1983). This lack of confinement would result in aerobic conditions within chalk pore space, unsuitable for biochemical transformation and elimination of contaminants in the wastewater (e.g. Allen, 1995; Foster et al., 1994). Fig. 5 compares the post-pumping groundwater flow regime between the two proposed localities (Fig. 4) once equilibrium has been attained. The model space is a 2D slice through each block geological section taken at the

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injection (Chalk) level, showing the effect of pumping upon the local water table and flow vectors. Results of the simulated waterflood through a fractured chalk rock mass are presented in Figs. 8 and 9, which illustrate snapshots of wastewater saturation and fluid pressure, respectively, at ~1 day and ~ 46 days following commencement of flooding. In the core-scale pumping experiment, breakthrough curves for certain elements which, when normalised, display similar characteristics have been grouped together (Fig. 11). The greatest recovery of any metal fed into the core was for molybdenum. However, neither the approximate numerical solution nor the analytical (advection–diffusion) solution fits the breakthrough curve well; complete recovery of the input is not achieved — see Fig. 11b.

a

4. Discussion This study has addressed the feasibility and potential effects of a small-scale wastewater recycling scheme on the London Chalk by means of four research areas divided on the basis of scale. In this section, the results from each will be drawn together. The GIS-based scheme suitability map (Fig. 4) defines large areas which are clearly unsuitable for wastewater injection: either the Chalk is unconfined, sub-London clay strata are fully saturated, or chalk transmissivity is too low (b100 m2 d− 1) for effective transmission of injected fluid. Interestingly, the ~NE–SW trend of unsuitable land across the London Basin correlates almost exactly with the strike of its synclinal axis (e.g. Fry and Kelly, 2008; Lucas and Robinson, 1995): this match could be a function of the accommodation space created for sedimentation during Cenozoic times. Two localities have been specified which combine a thick unsaturated zone in the Chalk and Lower London

a

b

Fig. 8. Wastewater saturation (blue) across a digitised Upper Chalk section (Fig. 7) at a: 1 × 105 s and b: 4 × 106 s after pumping commences from the left boundary. Model dimensions are 40 m × 23 m. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

b

Fig. 9. 2.5D exaggerated representation of fluid pressure across a digitised Upper Chalk section (Fig. 7) at a: 1 × 105 s and b: 4 × 106 s after pumping commences from the left boundary. Model dimensions are 40 m × 23 m.

Tertiaries with a high chalk transmissivity. These two factors are firstorder constraints on other similar wastewater recycling schemes (e.g. Al-Jayyousi, 2003; Baxter and Clark, 1984; Papiacovou, 2001), which require steady fluid flow through unsaturated rock, such that the requisite anaerobic biological reactions should remove or transform contaminants in the wastewater (Fig. 3), leaving potable water for later abstraction. At Marylebone, injection of fluid into the Upper Chalk, where the unsaturated zone is at most 5 m thick, raises the level of the water table by as much as 14 m (Fig. 5). The resulting pattern of groundwater flow reveals the profound effect of the three nearby abstraction wells, where all of the injected material ends up being captured. Conversely, at Streatham, the unsaturated zone is thicker and more uniform. The effect of fluid injection has negligible effect upon local water table levels and groundwater flow, which are both governed by the course of the overlying River Effra. On the strength of the two flow simulations, the search for a potential locality may be further refined: indeed, many workers have emphasised the importance of understanding antecedent groundwater flow conditions prior to injection (Baxter et al., 1981; Baxter, 1985; Foster et al., 1994; Papiacovou, 2001). In this instance, wastewater would ideally be injected into a thick unsaturated zone, possibly close to a river, which imposes relatively predictable groundwater conditions. Setting the scheme in the vicinity of pre-existing abstraction boreholes must be avoided as wastewater could be extracted before impurities and contaminants have been fully attenuated (c.f. Baxter and Clark, 1984). Having defined the feasibility conditions upon location, the next important step is to get some purchase on the mechanisms of fluid flow within the Chalk. Fig. 8 illustrates primary transmissivity in chalk being provided by fractures (e.g. Reeves, 1979; Robins, 1998). During a theoretical waterflood across a fractured chalk mass, fluid

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flow is channelled by fractures, the magnitude of this effect scaling with fracture permeability. Breakthrough of wastewater on the right-hand model boundary is achieved around 3 weeks following the commencement of the model run, giving a mean Darcy velocity of 2 × 10− 5 m s− 1. However, this value is only an average lateral velocity: in reality, the velocity vectors would vary according to the activation of successive fractures (Fig. 8). Fluid flows through the model (fractured outcrop) domain via two means: • ‘By-pass flow’ initially (see Barraclough et al., 1994; Reeves, 1979), characterised by high-magnitude flow velocities, high fluid pressure gradients (Fig. 9a), and fast channelling of wastewater along the most permeable fractures; • Steady-state flow, achieved relatively rapidly (after ~ 10 h), after which time both fluid pressure (Fig. 9b) and flow vectors remain constant until the end of the simulation (~6.5 weeks). After the steady-state is reached, wastewater progressively begins to imbibe into the chalk matrix. But where exactly in the rock mass does this all-important flow into the chalk – and filtration – take place? By comparing areas saturated with wastewater in Fig. 8b with the steady-state fluid pressure profile in Fig. 9b, high fluid pressure gradients force imbibition to the matrix. Therefore, regions within the chalk mass which are subject to a rapid pressure drop – in this case, where fractures are poorly connected, or at the intersection of high-k and low-k fractures – will become rapidly saturated with wastewater. This finding highlights the importance of fracture connectivity upon the design of any potential scheme which injects fluid into the Chalk, to maximise the benefits of chalk as a natural filter. Such filtration properties were examined via a core-scale pumping experiment. Of the ten heavy metals in the input ‘effluent’ solution, two (Pb and La) were completely adsorbed by chalk. Fifteen out of the twentyseven elements analysed were present, but below detectable limits for ICP-AES analysis. Fig. 11a plots four elements whose normalised concentration increases greatly to a maximum just before one pore volume (~37 ml) of effluent had passed through the core. The maximum recorded concentration of Na was 11,710 ppm, similar to typical concentrations of Na in seawater (e.g. Culkin and Cox, 1976); this result suggests that these elements were leached from the core itself, none having been present in the input solution. The four elements in Fig. 11b, however, were injected into the core initially. The related breakthrough curves are of conventional sigmoidal form (e.g. Mahmood-ul Hassan et al., 2007): • Arsenic. Only ~70% of the input is recovered; this phenomenon could be attributed to arsenic speciation in the brine. • Nickel and Cobalt. Both elements sorb far more than As and Mo. If the retardation coefficient, R, was constant everywhere, the concentration of nickel is expected to remain ~0 until breakthrough begins. However, small increases in recovered concentration during the early stages of pumping are suggestive of faster breakthrough, aqueous Ni travelling via some faster flow path. Thus, R varies spatially. • Molybdenum. The lowest retardation of all four metals: R = 1.35. Neither an approximate numerical solution, nor the element-specific

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analytical solution, fit the laboratory data well: a simple advection– diffusion formulation (Appendix B) cannot describe fluid transport in the chalk core. Both the higher-than-predicted initial gradient of the Mo breakthrough curve, and the systematic failure to obtain complete recovery, suggest the presence of pore-scale heterogeneities within the chalk, leading to complex preferential flow pathways (c.f. Little et al., 1996; Mahmood-ul Hassan et al., 2007), and highlighting the intricacies of hydrodynamic dispersion within the core. Therefore while this sample of Upper Cretaceous chalk demonstrates a strong capacity for contaminant attenuation and elimination of certain heavy metals (analogous to the content of wastewater), flow mechanisms and matrix diffusion are just as complicated at this smaller scale as at fracture/outcrop scale. 5. Conclusions 1. A theoretical wastewater recycling scheme, which uses the Upper Chalk of the London Basin as a natural filter to arrest contaminants and impurities, has been outlined. 2. The best location for the scheme must combine a thick unsaturated layer (where most of the reactions which remove contaminants will take place) with a reasonable chalk transmissivity. A location close to a river is preferable: groundwater flow conditions can more easily be predicted here. Locations south of the River Thames such as at Streatham are favoured. 3. A full characterisation of the fractured chalk mass must take place prior to trial pumping: fractures act as fast flow paths; at high water saturation, wastewater imbibition into the chalk matrix may not occur (by-pass flow). 4. Matrix imbibition (and effective filtration) of wastewater takes place after flux steady-state, and is greatest in areas of high fluid pressure gradients, such as poorly connected fracture tips. 5. A chalk core is capable of completely adsorbing certain heavy metals – analogues for typical contaminants found in wastewater – and impeding the passage of others. A chalk rock mass on a much larger scale will effectively attenuate similar contaminants, leading to a comparatively pure, potable water output. 6. There exist preferential flow pathways within the Chalk at two scales: cm- to m-scale fractures, and pore-scale ‘fast routes’. Microscale fluid transport within the Chalk is complex and hydrodynamic dispersion plays an important role. The poor fit of numerical and analytical solutions to laboratory data suggests that fluid flow cannot be modelled as a simple advection–diffusion process as this scale. Acknowledgements We thank Andrew Berry and Barry Coles for fruitful discussions about the ICP-AES work, Yukie Tanino and Stefan Iglauer for their help with the experimental setup, and Stephan Matthäi for troubleshooting problems with the CSP code. Figures were prepared using Adobe Illustrator, ArcGIS9.3, and GMT4.2.0.

Appendix A. Modelling parameters Table 1. Parameters used in the MODFLOW groundwater flow model. M = Marylebone; S = Streatham. kH, kV, SS and Sy = coefficients of permeability in the horizontal and vertical directions, specific storage, and specific yield, respectively. a: Mavroulidou and Woods (1998); b: Allen (1995); c: O'Shea et al. (1995); d: This study. Depths to strata taken from Whitaker (1889).

Layer

kH (m s− 1)

kV (m s− 1)

SS (m− 1)

ϕ

Sy

London clay Reading and Woolwich Fms. Thanet Sand Fm. Upper Chalk

1.16 × 10− 9a 5.79 × 10− 6a 2.08 × 10− 5a 1.16 × 10− 4a

1.16 × 10− 10a 1.16 × 10− 8a 1.16 × 10− 5a 8.10 × 10− 5a

10− 4b 0.1b 0.1b 10− 3c

0.45a 0.45a 0.30a 0.43d

0.02b 0.14b 0.25b 0.02c

Top (mAOD)

Base (mAOD)

M

S

M

S

+ 32 −4 − 14 − 15

+ 53 +45 0 − 14

−4 − 14 − 15 ~− 70

+ 45 0 − 14 ~− 70

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Laboratory work procedure

De-ionised water + excess ground chalk Mix overnight Separate with centrifuge (15min, 1500rpm), then double filtration Saturate core with remaining liquid in vacuum chamber for ~1 hour (150psi)

Core saturation & measurements Average length L = 75.93mm Average diameter d = 38.00mm Bulk volume Vb = 86.117cm3 Weight (dry) w(d) = 131.20g Weight (saturated) w(s) = 168.21g Pore volume Vp = 37.057cm3

Experimental set-up 1 Pump is loaded - 4 pore volumes’ worth of effluent solution

Vacuum pump (diesel)

1ml / min

“Brine” prep

Vacuum chamber 2

Vacuum set up in the cell; vacuum pump disconnected

“Effluent” prep

Pump and control console

Calculations

Mix overnight

Permeability from Darcy’s Law, viz.,

No filtration used: risk of heavy metal ions adsorbing onto paper

0 psi

Porosity o = Vp / Vb = 0.430

Saturated chalk core

Q = Ak . ( p / L) Taking Q = 1cm 3/min, A = 11.34cm2 , and p = 150psi, then k = 1.079 x 10-12 m2 (~1D)

Pressurised nitrogen cylinder 150 psi

Cell (core holder) 1L “effluent” solution + excess ground chalk

Separate with centrifuge (2 x 15min, 1000rpm)

Equilibrated “effluent”

“Brine” added here initially

3

Core pressurised (70psi confining pressure) Pressure difference applied across cell (150psi)

4 Sampling Collection Valve is opened; let any trapped air escape vial

Sample continuously every 5ml (=5 minutes) Duration = ~2.5h (total 30 vials) Acidify immediately after sampling with 5ml 2M HCl

Fig. 10. Experimental setup for core-scale pumping experiment, details of preparation and procedures followed, and brief summary of results obtained.

Table 2. Parameters used in the CSP model (Figs. 8 and 9). a: This study: laboratory testing of chalk core obtained at Dumpton Gap, Kent (see Fig. 10). b: Values averaged across 61 measurements taken at the corresponding outcrop at Dumpton Gap (see Figs. 6 and 7).

Global material properties Matrix permeabilitya Matrix porositya System compressibility Wastewater density Wastewater viscosity Freshwater density Freshwater viscosity Brooks–Corey parameter Residual water saturation Entry pressure Hydrostatic pressure

Local material properties (fractures) k = 1 × 10− 12 m2 ϕ = 0.43 β = 1 × 10− 9 Pa− 1 ρ = 1000 kg m− 3 μ = 1 × 10− 3 Pa s ρ = 1000 kg m− 3 μ = 1 × 10− 3 Pa s α = 2 m s2 kg− 1 Swc = 0 p = 1000 Pa pH = 0

Appendix B. Basis for MODFLOW and laboratory-scale numerical solution The suite of MODFLOW flow codes and the numerical solution used in Fig. 11b are predicated upon a simple Advection–Diffusion Equation [ADE]. This equation, and variants, are well-known for modelling fluid transport in porous media. Based on simple mass conservations, there are nevertheless several important assumptions: that Darcy's Law (see Bear, 1972) is applicable; flow is steady; and permeability is constant

Set 1 permeabilityb Set 1 porosityb Set 2 permeabilityb Set 2 porosityb Set 3 permeabilityb Set 3 porosityb Dirichlet boundary conditions Left: fluid pressure Right: fluid pressure Left: wastewater saturation Left: freshwater saturation

k = 8.50 × 10− 10 m2 ϕ = 1.0 × 10− 4 k = 3.09 × 10− 7 m2 ϕ = 1.9 × 10− 3 k = 7.03 × 10− 8 m2 ϕ = 9.2 × 10− 4 p = 5 × 10− 5 Pa p = 1 × 10− 5 Pa S=1 S=0

everywhere. For a case of 1D transport, several methods are possible for deriving an analytical solution given certain initial and boundary conditions; for instance, the length of the theoretical porous medium column must be semi-infinite. Considering the simplest form of the ADE, and assuming no chemical/radioactive decay or sorption:

ϕ

∂C ðx; t Þ ∂2 C ðx; t Þ ∂C ðx; t Þ −q ¼D ; ∂t ∂x ∂x2

ðB:1Þ

J.D. Paul, M.J. Blunt / Science of the Total Environment 437 (2012) 173–184

Normalised concentration

a

Normalised concentration

b

The basic discretised version of Eq. (B.1), as solved in MODFLOW, may be written

100 90 80 70 60 50 40 30 20 10 0

Na Mg S Fe

ϕ

C nþ1 −C nj j Δt

! ¼D

nþ1 C nþ1 þ C nþ1 jþ1 −2C j j−1

ðΔxÞ2

! −q

! C nþ1 −C nþ1 j j−1 ; Δx

ðB:7Þ

where j denotes a lengthstep and n is a timestep. For further information on implicit and explicit finite differencing schemes, see Press et al. (1992). References

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Mo Ni As Co

0

20

40

60

80

100

120

140

Time, minutes and cumulative volume of effluent produced, ml Fig. 11. Breakthrough curves for a: Fe, Mg, Na, S — all elements which were not injected into the chalk core and have most likely been leached by the effluent flow; b: As, Ni, Mo, Co — four contaminants present in the input effluent solution. Also presented are the analytical (thin grey line: error function) and numerical (thin black line: advection– diffusion finite difference model) solutions using Mo-specific parameters.

subject to the following Dirichlet boundary conditions: C ðx; 0Þ ¼ 0

ðB:2Þ

C ð0; t≥0Þ ¼ C 0

ðB:3Þ

C ðx→∞; t Þ ¼ 0;

ðB:4Þ

where ϕ = porosity; D = the coefficient of molecular diffusion [L 2T − 1]; q = Darcy velocity (volume flowing per unit area per unit time) [LT− 1]; and C0 = the initial (spike) contaminant concentration [ML− 3]. In Eq. (B.1) we solve for C(x,t). The middle term is diffusive (i.e. transport is dependent on ∇F, the rate of change of concentration gradient); while the right-hand term is advective. In this solution, and indeed in the MODFLOW programs, the entirety of the model space is assumed to be initially devoid of a contaminant (Eq. (B.2)), and that at t = 0, the concentration C0 at one end is held constant (Eq. (B.3)). The analytical boundary conditions deviate from the numerical conditions in Eq. (B.4). While an analytical solution can only be calculated assuming a semiinfinite model space (i.e. the chalk column), the numerical solution assumes that the other end (x = L) is purged such that no contaminant accumulates there. The porous medium can therefore be considered a perfect sink for the program to model. Via substitution into a moving co-ordinate system, we derive the analytical solution to Eq. (B.1) (shown in Fig. 11b) thus: C ðx; t Þ ¼

183

     hqxi C0 x−qt x þ qt er f c pffiffiffiffiffiffi þ exp er f c pffiffiffiffiffiffi ; 2 D 2 Dt 2 Dt

ðB:5Þ

with erfc(x) as the complementary error function, 2 x −y2 er f cðxÞ ¼ 1− ∫0 e dy: π

ðB:6Þ

Al-Jayyousi O. Greywater reuse: towards sustainable water management. Desalination 2003;156(1–3):181–92. Allen D. Chalk aquifer study: permeability and fractures in the English Chalk: a review of hydrogeological literature. British Geological Survey Technical Report; 1995. WD/95/43. Allen D, Brewerton L, Coleby L, Gibbs B, Lewis M, MacDonald A, et al. The physical properties of the major aquifers in England and Wales. British Geological Survey Technical Report; 1997. WD/97/34. Barraclough D, Gardner C, Wellings S, Cooper J. A tracer investigation into the importance of fissure flow in the unsaturated zone of the British Upper Chalk. J Hydrol 1994;156: 459–69. Baxter K. The effects on groundwater quality of secondary sewage treatment to an effluent recharge site on the Chalk of Southern England. J Hydrol 1985;77:333–59. Baxter K, Clark L. Effluent recharge: the effects of effluent recharge on groundwater quality. Water Research Centre Technical Report; 1984. TR 199. Baxter K, Edworthy K, Beard M, Montgomery H. Effects of discharging sewage to the Chalk. Sci Total Environ 1981;21:77–83. Bear J. Dynamics of fluids in porous media. New York: Elsevier; 1972. Belayneh M, Matthäi S, Cosgrove J. The implications of fracture swarms in the Chalk of SE England on the tectonic history of the basin and their impact on fluid flow in high-porosity, low-permeability rocks. In: Ries AC, Butler RWH, Graham RH, editors. Deformation of the continental crust: the legacy of Mike Coward, 272. Spec Pub Geol Soc London; 2007. p. 499–517. Blundell D. Cenozoic inversion and uplift of southern Britain. Spec Pub Geol Soc London 2002;196:85-101. Chandler R, Leroueil S, Trenter N. Measurements of the permeability of London clay using a self boring permeameter. Géotechnique 1990;40(1):113–24. Culkin F, Cox R. Sodium, potassium, magnesium, calcium and strontium in seawater. Deep Sea Res Oceanogr Abstr 1976;13(5):789–804. De Feo G, Galasso M, Belgiorno V. Groundwater recharge in an endoreic basin with reclaimed municipal wastewater. Water Sci Technol 2007;55(1–2):449–57. Downing R, Pearson F, Smith D. The flow mechanism in the Chalk based on radio-isotope analyses of groundwater in the London Basin. J Hydrol 1979;40(1–2):67–83. Ellison R. Facies distribution in the Woolwich and Reading Beds of the London Basin, England. Proc Geol Assoc 1983;94(4):311–9. Foster S. Groundwater recharge and pollution vulnerability of British aquifers: a critical overview. In: Robins NS, editor. Groundwater pollution, aquifer recharge and vulnerability, 130. Spec Pub Geol Soc London; 1998. p. 7-22. Foster S, Gale I, Hespanhol I. Impacts of wastewater use and disposal on groundwater. British Geological Survey Technical Report; 1994. WD/94/55. Fry V, Kelly T. Management of the London Basin Chalk aquifer. Environment Agency Report; 2008. EA/br/e/std/v1. Ganji M, Khosravi M, Rakhsaee R. Biosorption of Pb, Cd, Cu and Zn from wastewater by treated Azolia filiculoides with H2O2/MgCl2. Int J Environ Sci Technol 2005;1: 265–71. Headworth H. Recollections of a Golden Age: the groundwater schemes of Southern Water 1970–1990. Spec Pub Geol Soc London 2004;225:339–62. Jefferson B, Laine A, Parsons S, Stephenson T, Judd S. Technologies for domestic wastewater recycling. Urban Water 1999;1:285–92. Jones H, Cooper J. Water transport through the unsaturated zone of the Middle Chalk: a case study from the Fleam Dyke lysimeter. In: Robins NS, editor. Groundwater pollution, aquifer recharge and vulnerability, 130. Spec Pub Geol Soc London; 1998. p. 117–28. Little R, Muller E, Mackay R. Modelling of contaminant migration in a chalk aquifer. J Hydrol 1996;175:473–509. Lloyd J, Harker D, Baxendale R. Recharge mechanisms and groundwater flow in the Chalk and drift deposits of southern East Anglia. Q J Eng Geol 1981;14:87–96. Lucas H, Robinson V. Modelling of rising groundwater levels in the Chalk aquifer of the London Basin. Q J Eng Geol Hydrogeol 1995;28:551–62. MacDonald A, Allen D. Aquifer properties of the Chalk of England. Q J Eng Geol Hydrogeol 2001;34:371–84. Macdonald A, Brewerton L, Allen D. Evidence for rapid groundwater flow and karst-type behaviour in the Chalk of southern England. In: Robins NS, editor. Groundwater pollution, aquifer recharge and vulnerability, 130. Spec Pub Geol Soc London; 1998. p. 95-106. Mahmood-ul Hassan M, Akhtar M, Gregory P. Solute movement through intact columns of cryoturbated Upper Chalk. Hydrol Process 2007;22:2086–93. Mavroulidou M, Woods RI. Modelling of pumping from heterogeneous unsaturated-saturated porous media. Proceedings of the 7th International Conference on Hydraulic Engineering Software, vol. 4; 1998. p. 499–508. Mortimore R. Emeritus Professor, Brighton University. Personal communication; 2009.

184

J.D. Paul, M.J. Blunt / Science of the Total Environment 437 (2012) 173–184

Mühlherr I, Hiscock K, Dennis P, Feast N. Changes in groundwater chemistry due to rising groundwater levels in the London Basin between 1963 and 1994. In: Robins NS, editor. Groundwater pollution, aquifer recharge and vulnerability, 130. Spec Pub Geol Soc London; 1998. p. 47–62. National Research Council. Water reuse: potential for expanding the nation's water supply through reuse of municipal wastewater. Washington, DC: The National Academies Press; 2012. O'Shea M, Baxter K, Charalambous A. The hydrogeology of the Enfield–Haringey artificial recharge scheme, north London. Q J Eng Geol 1995;28:115–29. Owen M, Robinson V. Characteristics and yield in fissured chalk. Proceedings of the Conference on the Thames Groundwater Scheme London, 1. Institution of Civil Engineers; 1978. p. 33–49. Papiacovou I. Case study — wastewater reuse in Limassol as an alternative water source. Desalination 2001;138:55–9. Parizek R. Opportunities to enhance management of karstic aquifers. Environ Geol 2007;51:731–5.

Press W, Teukolsky S, Vetterling W, Flannery B. Numerical recipes in FORTRAN: the art of scientific computing. 2nd ed. Cambridge: Cambridge University Press; 1992. Price M. Fluid flow in the Chalk of England. Spec Pub Geol Soc London 1987;34:141–56. Reeves M. Recharge and pollution of the English Chalk: some possible mechanisms. Eng Geol 1979;14:231–40. Robins N. Recharge: the key to groundwater pollution and aquifer vulnerability. In: Robins NS, editor. Groundwater pollution, aquifer recharge and vulnerability, 130. Spec Pub Geol Soc London; 1998. p. 1–5. Thames Water. Revised draft water resources management plan. Reading: Thames Water reports; 2009. Wan Ngah W, Hanafiah M. Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: a review. Bioresour Technol 2008;99(10):3935–48. Whitaker W. The geology of London and part of the Thames Valley, vol. II. London: Memoir of the Geological Survey of London; 1889.