The impact of intentional stormwater infiltration on soil and groundwater

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Pergamon

Wal. ScL Tech. Vol. 39, No.2, pp. 185-192,1999. 19991AWQ Published by Elsevier Science Ltd

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PH: S0273-1223(99)OOO22-0

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THE IMPACT OF INTENTIONAL STORMWATERINFILTRATION ON SOIL AND GROUNDWATER S. Barraud*, A. Gautier*, J. P. Bardin* and V. Riou** • URGC Hydrologie Urbaine, INSA de Lyon, 20 Avenue A. Einstein, 6962/ Villeurbanne Cedex, France •• Burgeap Sud, 156 route de Tarascon; 84000 Avignon. France

ABSTRACT Stonnwater infiltration is a principle which is more and more utilized on urban sites in France. However, given the characteristics of urban surfaces, and notably the amounts of the different pollutants that stonnwater is likely to contain, it is important to try to assess the impact of intentional stonnwater infiltration systems on the soil, and on groundwater. To try to answer that type of question, we present an experiment that was carried out in'Valence (France) on two infiltration facilities. Situated in the same street, and serving equivalent catchment areas (i.e. a road network, along with a "classical" urban type of habitat), the first one is a recent cylindrical soakaway (1994) and the second one is a rectangular chamber which is around thirty years old. After explaining the experimental protocol, we shall present a body of data that we monitored. We shall then present our conclusions concerning the role of the soil and the groundwater in the process of "clearing up" the different pollutants that are present in stonnwater, as well as the migration patterns of these pollutants. (C) 1999 IAWQ Published by Elsevier Science Ltd, All rights reserved

KEYWORDS Best management practises;experiment; groundwater quality; infiltration; stormwater; urban drainage INTRODUCTION The object of this paper is to present an experiment that was carried out in Valence (France) on two infiltration facilities. The first one is a recent soakaway, the second one is around thirty years old. The point about the experimentwas that these two instaIlations were situatedin the same street, and served equivalent types of catchment areas (i.e. a road network, along with a "classical" urban type of habitat), so that they alloweda comparison betweenshort- and long-term functioning. More exactly, the experiment involved measuring quantitative and qualitative inflow, over a period from February to November 1996, into the facilities, and then evaluating their effectiveness in capturing poIlutants and thus estimatingtheir impacton groundwater and soil. Two approaches were taken to this end: I) a first approach, measuring poIlutant transfer during a particular event, by quality control of infiltrating groundwater; and 2) a second approach, seeking to estimate longer185

S. BARRAUD et al.

186

term impact on the basis of soil accumulation of pollutants with samples taken from the bottom of the facility at the beginning and end of the measurement campaign. This metrological and analytic monitoring was conducted in close collaboration with the South Region Burgeap, the URGC-Urban Hydrology and the Valence municipal technical services (Burgeap et al., 1997). DESCRIPTION OF THE SOAKAWAYS AND THEIR ENVIRONMENT The newer soakaway was built in February 1994, and was thus two years old at the beginning of the experiment. It is cylindrical, I metre in diameter and 3 metres deep (see Figure I). The "old soakaway" is more than 30 years old, and was altered at the beginning of 1975 by the addition of interior compartments. It is a rectangular chamber of 3.2 by 1.6 metres and 3.3 metres high (see Figure 1). The site where the two soakaways are located comprises two types of surface: roads, one of which with heavy traffic (around 7,000 vehicles per day) and a partly proofed open space. The new and old facilities drain catchment areas of 290 m 2 and 1355 m 2, respectively. For the category of rainfalls measured here, they had mean runoff coefficients of 91 % and 81 % and initial losses of 1.5 mm and 0.5 mm, respectively. The groundwater over which they are situated has a low (0.3%) hydraulic gradient and insignificant piezometric variation. It also happens to have a shallow static level. so that the bottoms of both soakaways are very close to the water-table. The flow is virtually constant in all seasons, and relatively high due to the high permeability of the rough alluvia constituting the aquifer (k -10- 2 m1s). ..

S.20m----·

3m

Perforated bodies

O.60m O.90m

watertable

watertable Figure t. Diagram ofsoakaways.

Local pluyjometric

monjtorjn~

A rain gauge/recorder was installed 200 metres from the soakaways, giving rain precipitation levels with a 6 minute time step. F!owrate and pollutjon jnput measurement Flowrates were measured in the older soakaway using a regulating channel which takes all the runoff water and ends in a rectangular overflow device. In-channel water levels were measured by a piezometric gauge and recorded continuously. Runoff-water quality was measured in the newer cylindrical soakaway. Samples were taken by means of a pivoting cup receiving water from the two gully pots attached to the soakaway and linked up to the flow measurement in the rectangular chamber (see Figure 2). It comprised 24 one-litre bottles. an average sample then being reconstituted manually. Inflow to this soakaway was calculated by continuous measurement of the stored water level (piezometric gauge) and measurement of the infiltration rate.

187

Intentional stonnwater infiltration on soil and groundwater

h~Q

cylindrical Iloakaway h

rectangular Iloakaway

----..---..----. -------t-.· -

piezometer •

h

h

piezometer.

__ groundwater

piezometer

manual sampling

I

automatic sampling

h

measurement of water level

Figure 2. Recapitulation of measurement instruments installed on site.

Infiltration rate measurement Infiltration rate was measured at the beginning and at the end of the campaign by loading the newer soakaway by means of a fire-hydrant. It was also in effect estimated during rain events by measuring the level inside the soakaway. Groundwater

monitorin~

The hydraulic impact of infiltrated water was assessed by continuous measurement of water level in three piezometers (piezometric gauges). The first was placed 10 metres upstream from the cylindrical soakaway. the second I metre downstream. and the third 1.5 metres downstream of the older soakaway (see Figure 2). Groundwater was sampled by hand under rainy and dry conditions. Measurement of slud~e Quality in soakaways and in undedyin~ sojl. Prior to the star! of the measurement programme. soil samples were taken. firstly in the piezometers used for groundwater monitoring. seven analyses being made of soil extracted at between 1.7 m and 6 m deep when the piezometers were being sunk. Samples were also taken at between 0 and I m deep under the bottom of each soakaway and seven analyses made. A 50 em high. 100 mm diameter perforated PVC tube was filled with fine sand and planted into the bottom of each soakaway; the sand was analysed at the end of the experiment. Table I. Rain features sampled during campaign Dates Duration (hours: minutes) Total precipitation (mm) Max mean intensity over 6 ron (mm/h) Dry time before event (hours)

01/06 2:32 9.00

20/06

1:32 4.40 6

8

168

313

17/05

30/07

05/08

16.80

3:12 8.60

10/08 3:07 12.8

21108

2:06 2.00

4:37 13.8

01110 4:12 18.2

18

46

10

18

30

30

22

425

8

51

68

73

241

223

2:06 6.80

05107 2:57

188

S. BARRAUD et al.

INPUT PEATURES StQODwater characteristics as studied

durin~

measurement

campaj~n

Over the measurement period, 59 rain events were measured, all with a return period of less than I year. Of the events recorded, 9 were sampled for quality. The stormwater characteristics are shown in Table 1. RunQff-water pQllution at sQakaway entry Average concentrations of the various pollutants were in the usual ranges for runoff-water in this type of area (see Table 2). However, certain pollutants (SS, Pb and Zn) varied greatly in concentration from event to event, sometimes (for Pb and Zn) even exceeding the usual range. Table 2. Runoff-water quality at soakaway entry Parameter Conductivity (l1s/cm) pH Nitrates N03 (mgll) N total (mg/l) Zn (11g/1) Pb (11g/1) Cd (11g/1) COD (mg/I) TOC (mg/I) SS (mg/l) Mineral oil (11g/1)

Min

Max

Mean

Standard deviation

54 6.7 2 \.2 64 < <5 15 0.7 4 <

110 7.7 4 3.6 5700 692 <5 123 45 130 270

74.4

17.4

7.1 2.75

0.3 0.66 0.86 1851 225 <5 29.83 15.2 39.76

2.23 802.4 97.75 <5 77.13 20.46 44.62 110.12

72

INFILTRATION IMPACT ON GROUNDWATER DURING A RAINFALL Groundwater cQnditjQns in dry weather Dry weather groundwater conditions were analysed from 2 series of samples taken from the 3 piezometers. Groundwater was highly mineralised (HT between 38.5° and 45° fr) with fairly constant conductivity (650690 f.1SIcm). Zn traces were found in 3 of the 6 samples, Pb traces in one only. As regards organic pollution, COD, organic and TOC concentrations were low. Nitrate concentrations were high but relatively constant at around 60 mgll. Micropollutants were not found, except for traces of long-chain heavy hydrocarbons, i.e., this groundwater was virtuaIly free of mineral and organic pollutants except for nitrates. Such lack of background noise facilitated the study of soakaway infiltration impact.

For the 1/10/96 event, downstream groundwater was monitored for the newer soakaway. During the rainfall, groundwater conductivity was continuously monitored in order to identify the moments of maximum infiltration impact. The conductivity parameter was chosen because of its good dry-time stability (around 600 Ils/cm) and the large difference between groundwater and runoff values (around 100 us/em for the latter). Two samples were taken, at 1.1S p.m, and at 3.20 p.m. (see Table 3). Regarding organic pollution, the COD, TOC and TKN levels were high, sometimes even above entry concentration levels, suggested lack of retention and perhaps even organic matter washdown at the

Intentional stormwater infiltration on soil andgroundwater

189

beginning of infiltration. This is hardly surprising: mainly soluble organic matter cannot be purified out except via a non-saturated area, more-or-less non-existent in these installations. Between the two samples, Pb and Zn concentrations increased, suggesting that some fraction of the transported or previously retained metals crossed the soakaway floor barrier; this would influence other experimental find ing (Malqu ist et al. 1981; Nightingale , 1987; Mikkelsen et al., 1997; HUtter et al., 1996). This is particularly true in the case of Zn, although it is also true that Zn concentrations were especially high in this event. Even so, heavy metal levels were lower than at soakaway entry and well below drinking water standard thresholds. For Zn and Pb, mean abalement with infiltration was 74% and 98.5% , respectively, as estimated on the basis of a dilution rate for runoff-water in groundwater taking nitrates as a dilution indicator, since they are highly soluble and little adsorbed by the soil. Table 3. Groundwater analyses during the rainfall Groundwater under dry weather conditions (mean value)

54

675

179

123

7 3 1.2 5700 45

7.4 59 <1

7.4

Conductivity (~S/cm)

pH Nitrates N03 (mgll) TKN (mgll) Zn (~gII) Ph (~gIl) Cd (Ilgll) COD (mg/l) TOC (mg/l)

..,. I

I

•

' 0 '0

0

63 8.7

'.

,

,

.

•

. ft.t

0

~

<5 42 21.2

27 3.5

. J ,..

M'dd). pl.H., te,

I-:-~~ __ 'b

__

1 ",. 0

'0

to

I .. to to

:"I . .:.. 1 .......... . . . l--Z. ~ ~

,

l

,

d.,lh (JR)

_ _ 'b

____ w i.. nloil

. .• • . . . Cd

. . . .. . . Cd

. I ..

J.t

~ "

~Mit.,.lo.

d.plll (M)


,oo

:",

:=-:--"':oc:::::::::::

<5 0 0,9

1.2 90

,

,I "

~

'0

'"


",

"

8

8.2 4 1.4 50 13

13

Oto 8 Ot06

Uplift •• pltlO.... r ' - -'

Groundwater during the rainfall downstream from newer soakway 3.20 pm 1.15 pm

Runoff-water at soakway entry (01/10/96)

D •••• tn •• pi•••••• ,

~ . . ,

,

'"Ii (. )

,..

I

:'.I l

•

:.• e

I":;':'

h __ n _ N "'Ala i

.... ... c.

Fisure 3. Analyses of .oil at the besinning of the campaign (soilextractedfromthe piezometers).

190

S. BARRAUD et al.

INFILTRATION IMPACT ON SOIL (ESTIMATION OF LONG TERM IMPACf) Surroundin~ soil conditions

at start ofcampai~n

Figure 3 shows the results of analysis of soil samples taken at the time of piezometer sinking. Only heavy metal and mineral oils concentrations are shown. The other parameters (total nitrogen. total organic carbon. nitrates, hydrocarbon) are below the detection threshold. SOakawa;y soil conditions at start of campaien Soakaway-bottom soil samplestaken at the start of the measurement campaign (see Figure 4) showeda high level of heavy metal concentration in the first 10cm. falling off sharply thereafter with depth. although in the older soakaway pollution persisted down to about I metrebelow the floor and. below20 em, was greater than in the newer facility (see Figure 4). Soakaway-bottom soil was also contaminated by hydrocarbons. mainly from mineral oils (1.4 glkg on surface in the 2 year-old facility). Toluene was detected in the sample taken from inside the central compartment of the older soakaway, in almost direct contact with the groundwater. Older loak....y (",elllnc"'''

...1600

,u _ _ __ _ _ __ _ _ _ _ _ ___ ____ _ I

1«Xl

i

I

IlDO

1000 100

600 600

lDO 0 0

Nt""r .o.k....' (tyUDdric:.1)

Figure 4. Analyses of soakaway-bottom samples.

Results (estimated amounts trapped in soakaway durine measurement campajen) Purification performance of the facilities and of the surrounding soil Was calculated on the basis of two quantities: firstly. the amountsof pollutant broughtinto the soakaway, calculated from the volumeof intlowing runoff and the concentration for each pollutant(taken as constantand equal to the mean entry values foundduring the campaign);

Intentional stormwater infiltration on soil and groundwater

191

secondly. the amounts of pollutant accumulated in the cores of sand planted at the beginning and analysed at the end of the experiment (i.e. 10 months later). The methodology here is open to criticism: the tube of sand may have functioned as a drain, filtering a greater quantity of effluent. the filtering power of the sand probably not being the same as that of the soakaway floor. For most pollutants (Pb, aromatic HC and mineral oils). input mass was lower than the mass trapped (see Tables 4 and 5). There are several plausible. probably concomitant, explanations for this. Rain-events show very variable concentrations, and those taken into account in these calculations did not represent the mean values for events as a whole during the measurement campaign. There are also possible input sampling problems. e.g, for mineral oils. where the sampler may fail to take up the floating phase. There may have been "uncontrolled" discharges into the facilities (car-engine oil-change. etc.). Zn retention (54% to 87.5%) was good in the newer soakaway and apparently less so in the older. This difference was probably due to the structure and operation of the older facility. which makes punctual measurement of floor concentrations relatively unrepresentative of surface levels as a whole. particularly for the central compartmentalised part. Table 4. Purification performance, newer soakaway element

Zn Pb Cd Aromatic hydrocarbons Mineral oils

concentration in sand-cores (mglkg) DIS cm 51lOcm mean 8S7

trapped mass (g)

input mass (g)

% retention

S4·88 98- ? ? ?

917 212

887 204.S

70 16

2.S

2.3S

0.18

80-130 10-16.3 <

0.29

0.1

0.19

0.02

<

1290

1600

144S

113

10

197 2.2

? ? : not ca/r:ulable

< : not calculable (concentratJon below the thrasholdlevel of deter:tJon)

Table 5. Purification performance, older soakaway element

Zn Pb Cd Aromatic hydrocarbons Mineral oils

concentration in sand-cores (mglkg) 0110 cm

trapped mass (g)

input mass (g)

% retention

143 82 0.9

118 67 0.7

380

31

46 2.37

0 866

0 710

SS

? 29.S ? ?

< : not calculable (conr:entratlon below the thrasholdlevel of deter:tion)

<

? : not calr:ulable

CONCLUSION Regarding runoff impact on soil, we found metal and hydrocarbon concentrations to be very high in the first centimetres from the surface, but falling off rapidly thereafter to a low level after a few decimetres down (below Dutch standards for non-polluted soil). Even so it would seem that, on a long term (30 years), heavy metals and mineral oils can contaminate the soil over a radius of at least one metre around installations, with concentrations significantly higher than for control soil. Thus, over time, there is a spread of pollution. even if the concentrations of various pollutants always remain below Dutch standard base A values. We further found low groundwater impact. An increase in heavy metal concentration was. however, detected, which may indicate that retention is less than total when the depth of non-saturated soil is around 40 em, In published experiments no such effect has been disclosed. but this may be because infiltration conditions were more favourable to purification (finer soil and deeper water table).

192

S. BARRAUD et al.

The difficulty in conducting this kind of experiment lies in the fact that measurement is carried out in the field, in an uncontrolled environment subject to quantitatively and qualitatively highly variable interferences. Over the long term, it therefore would seem to be necessary. in order to draw any definite conclusions. to prolong such experimentation over longer periods, so as to have more representative values; measurement design needs to be refined and made more reliable. and the study team needs to be much more multidisciplinary, incorporating, in particular, chemists and biologists. Such is the object of the OTHU (Urban Hydrology Field Observatory) which has recently been launched. A long-term experiment (10 years) will be carried out on an infiltration basin. especially designed for measurement and for operational drainage issues. (OrnU). 1997. REFERENCES Burgeap Sud Est • INSA de Lyon (1997) . Impact de I'injiltration dts talU pluviales sur /a nappe itutU expirlmentale. 35 p. rapport nORlAv.498. France. Gautier. A. (1998). Contribution d /a connaissance du fonctionnemen: d'ouvrages d'injillralion d'eau de ruissellemen: pluvial urbain: These de doctorat. lNSA de Lyon. 5 fl!vrier 1998, 248 p. France. HUtter, U. and Remmler, F. (1996). Stormwater injiltration at a site with critical subsoil conditions: invesligations ofsoil, seepage waler and groundwater. Preprint Seventh International Conference on urban storm drainage. Hannover. 1996. pp. 713718 . Malquist, P. A. and Hard. S. (1981). Ground quality changes caused by stormwater injiltration. Second International Conference on urban storm drainage, Urbana Illinois, 1981, pp. 89·97 . Mikkelsen. P. S •• Hafliger, M.• Ochs, M.• Jacobsen. P.• Tjell.J. C. and Boller. M. (1997). Pollution of soil and groundwater from infiltration of highly contaminated stormwater - a case study. Wal. Sci. Tech.. 36(8·9). 325.330. Nightingale. H. I. (1987) Water quality benealh urban runoff water management basins. Water Resources Bulletin. 13(2). 197·205. OTHU (1997). Mise en place d'un observatoire de terrain en hydrologie Urbaine, GRAlE Lyon. 1997. 125 p. France . »