Patterns of roof runoff contamination and their potential implications on practice and regulation of treatment and local infiltration

~

Wal. Sci. Tech. Vol. 33. No.6, pp. 39-4&, 1996. Copyright CC 1996 IA wQ. Published by Elsevier Science Ltd Printed in Great Bntain. All rights reserved. 0273-1223196 $I~ 'OO + 0-00

Pergamon

PH: S0273-1223(96)00329-0

PATTERNS OF ROOF RUNOFF CONTAMINATION AND THEIR POTENTIAL IMPLICATIONS ON PRACTICE AND REGULATION OF TREATMENT AND LOCAL INFILTRATION JUrgen Forster University of Bayreuth. Depanment of Hydrology. D-95440 Bayreuth. Germany

ABSTRACf Roof runoff water was sampled from an c:xperimental roof system and from house roofs in the city of Bayreuth. Germany. Samples were analysed for organic micropollutants. heavy metals and sum parameters. The pollution level and the shape of the runoff profiles are dependent on the individual properties of the precipitation event and the roof. but patterns with high concentrations at the beginning of the event and a subsequent decrease (first flush effect) are very typical. For dissolved substances. the profile can often be well described by a negative exponential function. Metal surfaces on the roofs cause extreme runoff pollution with heavy metals (Cu. Zn) that constitutes an environmental hazard. It is concluded that there is a need for the development of flexible drainage strategies for surface runoff and that metal surfaces should be avoided on roofs. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Dry deposition; heavy metals; organic micropollutants; precipitation; roof materials; roof runoff; runoff pollution; urban hydrology; wet deposition. INTRODUCfION Urban hydrology is one of the areas where knowledge of inevitably both environmental science and environ• mental engineering are necessary when strategies for the optimum solution of environmental problems have to be developed. This becomes evident when examining a process which is central for urban hydrology: roof runoff. Roof surfaces account for about half of the total runoff volume from impermeable surfaces in urban areas of industrialised countries. As for all storm runoff, drainage strategies have to account for the high degree of discontinuity and unpredictability. However, no decision should be made by taking roof runoff as a purely volume-related problem, because the receiving environmental compartment (streams, rivers, ponds, lakes and their sediments, but also the groundwater) and organisms as final receptors living in and from them can be endangered by pollution. The net input of substances from the runoff will be dependant on the kind and 39

40

J. F6RSTER

amount of pollutants present in roof runoff. and on the drainage pathway - be it via a sewer system with treatment facilities. or be it via direct discharge. Therefore. assessment and control of pollutant input by surface (roof) runoff are as important as the technical handling of the water volume. In the literature. there is disagreement about the quality of roof runoff water. mostly due to a lack of reliable and comparable data: the assessment ranges from unpolluted (e.g. van Sluis. ten Hove and de Boer. 1989; Shinoda. 1990; Krejci. Schilling and Gujer. 1990) to severely polluted (e.g. Pratt. Harrison and Adams. 1984; Leschber. Pernak and Zimmermann. 1991). Similarly. current treatment regulations and strategies in various countries range from local re-use or infiltration (sometimes unplanned consequence of "no-care" "strategies") to as complete as possible drainage and subsequent treatment in the sewage plants. the latter requiring enormous investments for e.g. construction and maintenance of large sewage pipes or rain runoff storage basins. Any profound investigation of pollution in environmental systems requires an assessment of the influencing factors. For roof runoff. these can be listed as follows: -Roof material: chemical characteristics. roughness. surface coating. age. weatherability.... -Physical boundary conditions of the roof: size. inclination. exposure.... -Precipitation event: intensity. wind. pollutant concentrations in the rain.... -Other meteorological factors: season. air masses. duration and weather characteristics of antecedent dry time •... -Chemical properties of the substance: vapour pressure. partition coefficient. solubility in water. Henry's constant•... -Concentration of the substance in the atmospheric boundary layer: emissions. transport. half-life. phase distribution (gas/particle) •... EXPERIMENTAL Roof runoff water was sampled from an experimental roof system (Forster. 1993a) situated on top of a flat roof on the campus of the University of Bayreuth. Germany. and from ordinary house roofs at various locations inthe city of Bayreuth. The experimental roof system encompasses roofs of five different materials. each with the same size and exposition. a wet-only collector (glass). automatic runoff samplers. (micro-)me• teorological sensors. and samplers for gaseous and particulate atmospheric pollutants. Thus. it allows us to control some (roof material and physical boundary conditions) and monitor most of the other influencing factors listed above. The atmospheric pollution conditions at the location can be described as "urban background". i.e .• there are no local emitters on or near the roofs. In contrast. the roofs in the city area are exposed to local or nearby pollution sources like chimneys or traffic. Samples were taken during selected precipitation events between May 1990 and June 1994. As most relevant processes during runoff are closer related to runoff volume than to time (duration of event). the automatic samplers were programmed for fractionation by runoff volume (resolution 0.3 mm runoff height and better). allowing the representation of approximately equal runoff fractions by each sample in the resulting runoff profiles. Samples were taken to the laboratory immediately after the precipitation events. filtered and stored dark at

4·C. Further pre-treatment (extraction or acidification) followed usually within one week. Samples to be analysed for concentrations of heavy metals were taken in pre-conditioned polyethylene bottles. filtered and acidified; the particulate phase was extracted by acid digestion. Concentration measurements were made by means of atomic absorption spectrometry (AAS). Samples to be analysed for organic trace pollutants were taken in glass bottles (solvent-cleaned and heated before use). While the liquid phase was extracted by solid• phase extraction. particles trapped on the filters were extracted by organic solvents in an ultrasonic bath. fol• lowed by clean-up with column chromatography. Gas chromatography with electron capture detection (GClECD) was used for the measurement of chlorinated pesticides. while polycyclic aromatic hydrocarbons (PAH) and (nitro-)phenols were quantified by high performance liquid chromatography (HPLC) with fluorescence and UV detection. respectively. The analysis procedures are described in detail by Ft~rster (1993a).

Patterns of roof runoff contamination

41

RESULTS AND DISCUSSION Organic trace pollutants and sum parameters. Some general characteristics of pollutant behaviour during roof runoff are illustrated by figure I which shows average concentrations (loads) of some organic micropol• lutants with respect to rain for the beginning of three events (first mm of runoff): every column larger than unity (by definition: input by rain) indicates additional pollution of the runoff, either by washoff of substances dry deposited to the roof surface before the precipitation event (e.g. nitrophenols), or by dissolution of the roof material itself due to weathering. While there are only few cases where the roof acts as a sink during the phase of the rain event shown (y-HCH, event 516 Aug. 1990), the roofs' source function dominates the plot and exhibits a high degree of variability with respect to almost every factor considered in the investigation: different roof surfaces during the same event, different rain events on the same roof, different parameters characterising pollution, even different substances within the same group of pollutants (here: nitrophenols). The picture of large variability is imilar when more events and other pollutants are plotted, with the addition of a seasonal influence on PAH concentrations (cf. Forster, 1993b). Like for most other trends, the general tendence can be explained through knowledge about the system under investigation: sources of PAH (e.g. residential heating) are stronger in winter, and the potential for photodecomposition then is smaller due to lower irradiation intensities. The source factors of 2,4-dinitro-6-methylphenol (also known as dinitrorothocresol, DNOC) and 2,4-dinitrophenol are higher than that of, e.g., 3-methyl-4nitrophenol, owing to their use as herbicide and pesticide for wood protection, respectively, thus creating sources for additional emissions to the atmosphere and gas-phase dry deposition to the roofs.

Roofs as Sources and Sinks of Pollutants Nitrophenols, 'Y.HCH and Sum Parameters

25

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~~

-._- ....

....... -.....

20 ur c:

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.§. 15

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III

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==c:0

::3

a:

E

~

.,;

li

~

20

E li

III

...... -.....

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....... _-- .. -

c:

'iii

10 a:

lI:

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0

c:

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5

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a:

Figure I.

J. FORSTER

42

Smaller differences, however, remain attributed to individual properties of the event or the roof. Better un· derstanding of the processes can often be achieved by considering high-resolution runoff profiles ("pollutographs") instead of mean values (Fig. 2 - II). Like all other runoff profiles presented in this paper, pH values in runoff (Fig. 2) are plotted against runoff height (see also experimental section) in order to avoid veiling of the physico-cbemical processes during runoff which is likely, owing to the highly variable rain intensities, when runoff properties are plotted against time. Samples are represented by markers which are centred to the middle of the respective sampling interval; the markers are connected by lines when samples were taken continuously.

pH-values in Rain and Roof Runoff 04. October 1990 . . . Raln lIE-lIE Concrete Tilel ~-~ Fibrous Cement (N) +-+ Fibrous Cement (S) x-x Panlile. (N) 6-6 PanUies (S) 0-0 Zinc Sheet (N)

<>-<> Zinc Sheet (S) v-v Tar Felt (N)

5

.----------.--~

4L.~~~~~~~~~~~~~~~~

0.5

a

1

1.5

Runoff Height [L

2.5

2

m·2]

Figure 2.

Electric Conductivity in Rain and Roof Runoff 18 August 1990 300r-----------------------------~

l1-li Rain

:- 250 'E

lII-lI(

Concrete Tile Roof

~-~

Fibrous Cement Roof

1.5

2

o-c Zinc Sheel Roof

u

~ 200

>:~ 150

g

'g 100 o

(J

Q;

50 0.5

1

Runoff Height [L m·2] Figure 3.

2.5

Patterns of roof runoff contamination

43

The pH values' stable differences between the various roof materials under investigation (Fig. 2) clearly indicate that the shift towards more alkaline values is basically due to the influence (dissolution) of the roof material itself and not to dissolution of deposited aerosol particles. This finding is complemented by measurements of the electric conductivity (Fig. 3), a parameter representing the samples' total ion content: again, there is a clear difference from the values measured in the rain, and the highest values are found in the fibrous cement runoff, indicating its weatherability. However, the data exhibit a distinct trend: starting with the respective maximum value, the conductivity runoff profiles decrease steeply, and between 0.5 to I nun runoff height, they approximate a pattern of differences to the conductivity input by rain that remains constant for the rest of the event. While the latter phase is dominated by continuous dissolution of the material, the peaks at the beginning can be attributed to a combination of two factors: quick dissolution of both deposited aerosol particles and constituents of the roof material weathered during the dry time antecedent to the event.

2,4·Dinitrophenol, Pantile Roof Runoff 14 May 1991

60r---------------------------------, Fitting of an Exponential Curve

-c 50

'0 E 40

C • 0.6045 + 56.968 • e(·2.7383'.)

.s

-

c:

o 30

R2.O.9999

~8 20 c:

o

o 10 1.5

0.5

1

2

2.5

2

Runoff Height [L m· Figure 4.

This type of runoff profile is very typical for roof runoff; it can be found for many substances and the majority of rain events. For pollutants with a good solubility in water, the runoff profile can in most cases be well described by an exponential curve. Fig. 4 shows an example (2A-dinitrophenol) where the fit is almost perfect. The constant term in fitting equations of the type shown represents continuous input by wet deposition. The transport of particles - and hence also of pollutants adsorbed to them- is dependent on the hydraulic conditions (shear stress), governed by rain intensity and surface roughness. However, the typical runoff profile still starts with high pollutant load and generally shows a decreasing trend (Fig. 5), while a clear modification of the runoff profile will only be found when rain intensities are very low (Fig. 6). These two examples of PAH concentrations in runoff (see also FBrster, 1993b) show that particles are easily washed off the very smooth surface of the zinc sheet roof even when rain intensities are low, while the rough surface of the tar felt roof leads to much lower PAH concentrations at the start (differences in dry deposition can be neglected) and clearly hinders particle transport when the rain intensity is low. As one consequence, the amount of PAH left over on the roof at the end of the runoff phase shown is greater on the tar felt roof. Although it is coated with slate particles, a minor fraction of the less water-soluble pollutants will also adsorb due to the hydrophobic nature of the tar felt. In both examples, the parallelism of the runoff profiles of the various P AH is remarkable; it indirectly indicates that the different substances of this group of pollutants have identical sources and are adsorbed to particles of the same size.

44

J.FORSTER

Heavy Metals. While deposition of heavy metals to roof surfaces via atmospheric pathways is dependent on the proximity of strong sources, metal surfaces in contact with the water running off will dominate the runoff pollution patterns. The contamination often is of such an extent that differences among profiles with lower concentrations of heavy metals only become visible when a logarithmic scale is used (Fig. 7-11).

PAH in Runoff from the Zinc Sheet Roof (N) 26 October 1990

....

1000

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fluoranthene x-x pyrene lJ.-lJ. benzo[aJpyrene 0-0 chtysene

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~lJ.

200

__

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a

0

~a==-"

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0.5

1.5

Runoff Height [L

m·2)

Figure 5.

PAH in Runoff from the Tar Felt Roof (N) 26 October 1990

100

....

~

+-+ phenanthrene 0-0 fluoranthene x-x pyrene lJ.-lJ. benzo{a)anlhracene 0-0 chrysene A-A precipitation Inlenshy

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'0 E

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0.2

0.4

0.6

0.8

Co "" '0

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Co

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Runoff Height [I m-2] Figure 6.

One of the roofs under investigation in the city of Bayreuth (Graserstrasse) was newly tiled and fitted with copper sheets along the sides, next to the chimney and around the roof windows. Afterwards, the weathering of the copper sheets was extremely well documented in the runoff profiles when compared to rain and runoff

Patterns of roof runoff contamination

45

from other roofs (Fig. 7 and 8): the concentration increase as compared to the falling rain was 2-3 and 3-4 orders of magnitude for dissolved and particulate copper, respectively. In phases where the other roofs also show copper pollution with respect to rain in their runoff, this can be attributed to dry deposition and the in• fluence of copper as a minor constituent of the zinc gutters fitted to all roofs. For most of the time and all roofs except "Gagemstrasse" which had the highest pH (6.8 - 7.0 as compared to 6.0 - 6.3 for the other two roofs and 4.9-5.1 for the rain), the phase distribution is dominated by the dissolved phase. Although some• what obscured by the logarithmic scale of the plot, the general trend of the copper pollution of the "Graser• strasse" pantile roof runoff again is a decrease within the course of the event.

Dissolved Copper in Rain and Roof Runoff 07108 December 1993

;I o

-Rain Panlile Roof - Gagernslrasse x-x Pantlle Roof - Graserstrasse .-. Pantile Roof - Keuperstrasse

E

c: c:

0-0

~

o

-

~ c:

Q)

o

c:

o

o

o

0.2

0.4

0.6

0.8

Runoff Height [L 1m2] Figure 7.

Particulate Copper in Rain and Roof Runoff 07/08 December 1993

;I o E oS c: o

Pantlle Roof· Gagamstrasse x-x Pantlle Roof· Graserstrasse .-. Pantlle Roof· Keuperstrasse 0-0

.-~~

~

C CI) o

c:

o

o

10°

o~~--~----~------~----~----~ 0.2 0.4 0.6 0.8 Runoff Height [L 1m2]

oIIIST ]]·6·[

Figure 8.

46

J.FORSTER

What is the situation like when the whole roof is made of metal? Figures 9 and 10 show zinc concentrations in rain and runoff from three roofs of the experimental roof system, including the zinc sheet roof (Zn alloyed with 0.025% Cu and Ti each): the pattern is similar to that of the Cu contamination shown in Figs. 7 and 8, with a difference of about three orders of magnitude between the zinc sheet roof runoff and less contaminated samples (rain, pantile and concrete tile roof runoff), but the Zn concentrations are more than ten times higher due to the greater surface area and the less noble character of Zn as compared to Cu. The use of zinc gutters is much more widespread (the majority of gutters in Germany, for instance, are made of Zn) than zinc sheet roofs. As Fig. II indicates, Zn contamination of runoff from roofs with zinc gutters (Konigsallee, Keuperstrasse, Graserstrasse) takes an intermediate position between the highly contaminated zinc sheet roof and the tar felt roof which had a plastic gutter. CONCLUSIONS AND RECOMMENDAnONS All conclusions are based on the two main results of the investigation that can be generalised: the first flush effect observable under most circumstances, and the extreme heavy metal contamination of runoff in contact with metal surfaces on the roof.

Dissolved Zinc in Rain and Roof Runoff 30 August 1993

103 ::i' ::. 0

E

.:; c:

102 -Rain

101

0-0

~ cCI):

-...

Pantile Roof

x-x Concrete Tile. . - Zinc Sheet Roof

0

10°

0

c:

0

0

10.1 10.2

0

2

3

4

5

Runoff Height [L 1m2] Figure 9.

Looking at the treatment alternatives under consideration, it should be advisable to direct the first, more pol• luted fraction of runoff towards the sewage treatment system in case the great bulk of the runoff volume is to be infiltrated locally or "re-used" after storage. From this recommendation results an immediate challenge for environmental engineers: construction of simple, smaIl and inexpensive instruments (valves, relays) to switch automatically between the runoff pathways after the first flush has passed. This strategy will take away the main pollutant load from the local systems and the main volume load from the sewer and treatment system! However, as the concentrations of most pollutants as well as their phase distributions - important for removal techniques - are dependent on the individual properties of the roof and the precipitation event (cf. listing in the introductory section), effort should be placed on the development of more advanced, intelligent and flexible drainage strategies (real-time management).

47

Patterns of roof runoff contamination

Particulate Zinc in Rain and Roof Runoff 30 August 1993

2

10 ~---------------------------------,

:J' ::::. o E

.:!r c:

o ~ ....

c: CI>

o c: o

-Rain Pantlle Roof x-x Concrete Tiles

o

0-0

10.3

.............~.......-'-.......__~-'-_................J._.......-__ -...z..Jln_c~s_he ...e_t_R~o0..Jf

o

2

3

Runoff Height [L /

4

5

m2]

Figure 10.

Dissolved Zinc in Rain and Roof Runoff 04 May 1994

103

"-.

::J ::::. 0

E

.:;

102

AAA

C

~

-= C CI>

10

AA"

n-" y

0

---------.-.~ Cl..a

_

_~_)(_x-x-X-"'-x

1

x-x

A.ln Tar Fek Roof P.nUIe Aoof- Gtu.,IIr....

--- Zinc Sh ••t Aoof A-A P.ntll. Aoof- Kiinlg..lle. 0-0 Pantlle Roof - Keupe,atra•••

0

C

0 (.)

0-0

100 0

O~

1~

2

Runoff Height [L 1m2] Figure II.

Heavy metals constitute a special environmental problem due to their combination of persistence and toxicity which is especially high for the aquatic fauna. Zinc and copper concentrations as measured in roof runoff are so high that they constitute an environmental hazard even after strong dilution: the typical eu concentrations from the roof (Graserstrasse) with copper side fittings - not an uncommon construction - are, for example, 30 times the LC50 for daphnia magna (Arambasic, Bjelic and Subakov, 1995), and the maxi• mum Zn concentrations measured are 2500 times the EC recommendation for drinking water. Hence, catch• ments with noticeable portions of metal roofs constitute an environmental threat and a problem even for good treatment plants. As it is most desirable to avoid additional exposure of unprotected metal surfaces to

J. FORSTER

48

the atmosphere. architects and civil engineers have to be enlightened about the hazard in order to overcome the current architectural trend which is unfortunately favouring the construction of metal roofs. Traditional pantile roofs are good alternatives to metal roofs. but more ecological advantages come with the planted ("green") roofs that are capable of storing considerable amounts of the precipitation. resulting in lower and flatter runoff volume peaks. and of filtering out many pollutants deposited with the rain. While concentrations in rain have frequently been taken in this paper as a reference for "clean water". it should nevertheless be kept in mind that also the rain is polluted in many regions of the world. Hence. reduction of anthropogenic emissions to the atmosphere is also in the interest of - in all aspects - better urban drainage. ACKNOWLEDGEMENTS This work was partly funded by the German Research Council (DFG) under contract He 482116. I also wish to thank J. Kranz. S. Kretzschmar and H. Zier for technical assistance and careful analyses in the laboratory. REFERENCES Arambasic. M.B .• Bjelic. S .• Subakov. G. (1995): Acute toxicity of heavy metals (copper. lead. zinc). phenol and sodium on Allium Cepa L.. Lepidum Sativum L. and Daphnia Magna SI.: comparative investigations and the practical applications. Water Research 29.497-503. Forster. J. (1993a). Dachfliichen als Interface zwischen atmosphiirischer Grenzschicht und Kanalsystem: Untersuchungen zum Transportverhalten ausgewahlter organischer Umweltchemikalien an einem Experimentaldachsystem. Dissertation. University of Bayreuth. 229pp. FOrster. I. (I993b). The influence of atmospheric conditions and stann characteristics on roof runoff pollution: studies with an experimental roof system. In: Marsalek. 1.. Torno. H. (eds.): Proceedings of the sixth international conference on urban storm drainage. Niagara Falls. 1. 411-416. Seapoint. Victoria Krejci. V .• Schilling. W .• Gujer. W. (1990). Ziele und Aufgaben der Siedlungsentwasserung. Milleilungen der EA WAG. 29. 1-10. Zurich. Leschber. R.. Pemak. K.-D .• Zimmermann. U. (1991). Untersuchung des Verhaltens anorganischer und organischer Stoffe bei konzentrierter Regenwasserversickerung. Stadtentwdsserung und GewiJsurschutz. 4. 169-188. Pratt. C.J .. Harrison. 1.1 .• Adams. J.R.W. (1984). Storm runoff simulation In runoff quality investigations. In: Balmer. P .• Malmquist. P.A.. Sjoberg. A. (eds.): Analysis and deSign of stormwater systems. Chalmers University of Technology. Gothenburg. 285-294. Shinoda. T. (1990). Comparative study on surface runoff by stormwater infiltration facilities. In: Iwasa, Y .• Sueishi. T. (cds.): Drainage systems and runoff reduction. Proceedings of the fifth international conference on urban storm drainage. Osaka, 2. 783-788. Van Sluis. I.W .• Ten Hove. D .• De Boer. B. (1989). Final report of the 198211989 NWRW research programme. Conclusions and recommendations