Controls on the Chemical and Isotopic Compositions of
Urban Stormwater in a Semiarid Zone
L. Asaf1, R. Nativ1*, D. Shain1, M. Hassan2 and S. Geyer3
1
Dept. of Soil and Water Sciences, The Hebrew University of Jerusalem,
P.O. Box 12, Rehovot 76100, Israel
2
Dept. of Geography, University of British Columbia, Vancouver, Canada
3
UFZ - Umweltforschungszentrum, Leipzig-Halle, Germany
* Corresponding author
1
ABSTRACT
The temporal variations in the chemical and isotopic compositions of urban stormwater
under different land uses, and their dependence on physical parameters such as
precipitation intensity, stormwater discharge, cumulative stormwater volumes and the
size of the drainage area, were investigated in the coastal city of Ashdod, Israel. During
2000/2001 and 2001/2002, 46 stormwater events were intensively monitored for
precipitation distribution and intensity at three stations across the city, and for
stormwater discharge at seven stations draining 85% of the city area. Sixty-eight and
202 precipitation samples were collected and analyzed for chemical and isotopic
compositions, respectively, as were 186 stormwater samples, collected from the drains
during 15 of the 46 events. Land use had only a minor effect on the concentrations of
major ions and trace elements. Conversely, the concentrations and variety of volatile
and semi-volatile organic compounds were significantly higher in stormwater
generated in the industrial area than in that draining from residential areas. Ion and
trace-metal concentrations were very low (below drinking-water standards) in 97% of
the stormwater samples collected from all drains. Stormwater concentrations were
higher at stations draining a larger area, thereby linking concentrations to the length of
the stormwater flow paths. A first-flush effect was documented on both a seasonal and
event basis for both ions and trace elements. The high concentrations of fecal coliform
bacteria exceeded the drinking-water standards and displayed a random pattern. The
isotopic ratios of oxygen and hydrogen in the stormwater suggest very little exposure
to the atmosphere, resulting in very limited fractionation. The presence of fecal
coliforms, ammonium in some samples, and specific ratios of oxygen and nitrogen
isotopes, suggest that although the sewer and stormwater-collection systems are
separated, wastewater, possibly from overflowing sewers, contributed to the drained
stormwater.
2
INTRODUCTION
Although people have lived in cities for thousands of years, urban development has
significantly accelerated over the past century. In 1900, only 10% of the world’s
population lived in cities. More than half are currently living in urban areas and these
areas are expected to accommodate most of the projected increases in population for
both developed and developing countries (United Nations, 1991). The expansion of
towns and cities has been generally associated with an increase in impermeable land
surface area and the quantity of surface runoff. Consequently, in arid and semiarid
areas, urbanization can result in the enhancement of stormwater generation at the
expense of groundwater recharge (Karmon and Shamir, 1997).
Changes in land use also have a significant influence on surface-water quality.
Substantial quantities of chemicals may be contributed to the water cycle by airborne
particles that are absent in natural and rural areas. Under severe circumstances,
groundwater in urban areas is impaired by both recharge quantity and quality and the
stormwater’s quality precludes its further use (Ellis, 1986; Nix, 1994).
Urban areas have been classified in the literature into main roads (including parking
lots and airports), roofs, residential areas, commercial areas, industrial areas, parks and
lawns, and open, undeveloped areas, all of which generate stormwater of different
quality. Roads, parking lots and gas stations have been known to contribute a large
variety of contaminants, directly related to vehicles (hydrocarbons, oxides of nitrogen,
sulfur and lead) or salt de-icing (halite) (Bannerman et al., 1993; Hermann et al., 1994;
Smith et al., 2000). The high level of pollution found on major arterial roads in urban
areas and highways has been correlated to traffic density (Shinya et al., 2000;
Polkowska et al., 2001).
The quality of stormwater from industrial areas is highly dependent upon the type of
industry and the conditions at the specific site. The variety of contaminants (some of
which can be related to atmospheric deposition) is large and their concentrations can be
very high. Whereas in areas of light industry stormwater quality may be similar to that
in commercial areas, in dense, heavy-industrial areas, stormwater can be highly
polluted with heavy metals and organic compounds (Mikkelsen et al., 1994).
Residential areas usually produce high-quality stormwater (Remmler and Hutter,
1997). However some contaminants, including detergents, plant-related nutritional
materials and fertilizers, herbicides and insecticides, often characterize them (Pitt,
3
1999). While some consider roofing to produce clean water (Karmon and Shamir,
1997; Remmler and Hutter, 1997), others have reported high concentrations of heavy
metals, depending on the roofing material (Hermann et al., 1994; Förster, 1999;
Gromarie-Mertz et al., 1999). A major source of lead in stormwater in both residential
and industrial areas is wash-off from old painted structures (Davis and Burns, 1999;
Davis et al., 2001).
As urban stormwater is comprised of many individual flow components draining
various areas, the “mix” at the outlet depends on the characteristics of those areas,
pollutant wash-off potentials and the features of the specific rain event (Pitt, 1999).
Elevated concentrations of nitrate, trace metals, organic compounds and total dissolved
solids (TDS), as well as specific ratios of 15N/14N and 34S/32S, are typical indicators of
anthropogenically contaminated ground- and stormwater (e.g. Barrett et al., 1999, and
references therein).
18
O/16O and 2H/1H ratios are evaporation tracers (Craig, 1961),
thereby providing qualitative information about stormwater residence time on land
surface prior to its drainage or infiltration.
Contaminant concentrations can vary temporally during individual storm events. As
expected, directly connected impervious areas account for most of the runoff and
pollutants during small rains. However, as the rain depth increases, non-paved areas
can become significant contributors (Pitt, 1999). Both Ellis (1986) and Ben-Othman et
al. (1997) identified a temporal pattern of pollutant load in urban stormwater sewers
that could be described as flushing and dilution effects. The timing of the concentration
and flow peaks corresponds to the size of the contributing area and the amount of
impervious zones within it (Lee and Bang, 2000; Lee et al., 2002). Brezonik and
Stadelmann (2002) and Vaze and Chiew (2002) reported a positive correlation between
pollutant load and the length of time between rain events, thereby confirming the rapid
pollutant buildup during dry periods. Conversely, other studies carried out in humid
regions (e.g. Deletic, 1998) reported the lack of such a first flush and concluded that
this phenomenon is complex and site-specific.
The study reported here was triggered by the rapid deterioration of the Coastal Plain
Aquifer, considered to be the most important aquifer in Israel. Its water volume and
quality have been depleted and deteriorated, respectively, over the past 50 years.
Enhanced urbanization, taking place during the past decade in the Coastal Plain, is
likely to enhance these processes. The reduced recharge water percolating through
urban areas is expected to be of lesser quality, thereby hurting both the water balance
4
of the aquifer and its water quality. The increased volumes of stormwater are currently
being channeled to the sea via expensive drainage systems. Israel’s severe water
shortage requires a thorough examination of stormwater quality and an evaluation of
its reuse, rather than its drainage.
This study focused on the temporal variations in the chemical and isotopic
compositions of stormwater under different urban land uses, and their dependence on
physical parameters such as stormwater discharge, cumulative stormwater volumes or
the size of the draining area. In this framework, the natural interaction between
precipitation and stormwater was also investigated, although the detailed features of
the precipitation are discussed elsewhere (Asaf et al., submitted).
The following questions were addressed in our study:
* What are the chemical and isotopic compositions of urban stormwater draining from
areas characterized by different sizes and land uses?
* What is the change in quality of stormwater derived from urban areas under different
time scales?
* Can urban stormwater be reused, thereby forming an additional water resource?
The study is unique in the sense that it attempts to address these questions using both
quantitative and qualitative field measurements of both urban precipitation and
stormwater (rather than focusing separately on temporal and spatial variations of flow
and chemical composition), was carried out on a full city scale (rather than on a street
or neighborhood scale), and during an entire rainy season (rather than sampling
isolated individual storms). A relatively large suite of chemical and isotopic parameters
were determined in the precipitation and stormwater samples. Consequently, the
aforementioned questions can be addressed both temporally and spatially and the
interaction between rain and stormwater and between stormwater flow and quality can
be assessed.
METHODS
Study Site
The study was carried out in the city of Ashdod, situated in the central Coastal Plain of
Israel and hosting its largest port (Fig. 1). The climate is Mediterranean, with a dry
summer and a rainy season from October to April. The mean annual precipitation is
5
522 mm. The city’s area has been constantly growing, encompassing 16.2 km2 in 2002.
The population has increased from 200 people in the mid `50s (when the port and its
first residential area were constructed on the coastal sand dunes), to 6000 people in the
mid `60s, to its current 192,000 residents (Municipality of Ashdod, 2002).
Rainfall Measurement and Sampling
Automatic tipping-bucket rainfall recorders with a resolution of 0.1 mm were installed
at three stations (Fig. 1) to accurately represent the spatial and temporal distribution of
precipitation in the city. Two of the stations were located in residential areas, and the
third in the industrial zone. A rainfall sampler was installed adjacent to the rain
recorder in the industrial zone. Rainwater samples were collected using a customized
rain sampler (Adar et al., 1991), calibrated to collect a sample every 3 mm of rainfall to
account for the isotopic and chemical changes in the course of the rain event.
Rainwater samples from 46 rain events during the winter seasons of 2000/2001,
2001/2002 and 2002/2003 collected by the rainfall sampler, were analyzed to define
background compositions prior to interaction with land surface.
Stormwater Measurement and Sampling
Seven runoff stations were installed at the outlets of five stormwater drains and in one
manhole (Fig. 1). Each station included a pressure transducer (with a resolution of 10
mm) and a data logger. This sampling network gauged 85% of the city area, as well as
a smaller-scale sub-basin (three small streets, amounting to an area of 0.05 km2). The
area drained to each one of the stations was estimated using an ESRI geographical
information system (ArcInfo8), aerial orthophoto map, and topographic and
infrastructure data (including elevation, roads, buildings and drainage network)
provided by the municipality of Ashdod (Fig. 1). Because the drains channeled water
from areas of different sizes and land uses (Table 1), the impact of these characteristics
on the quantity and quality of stormwater could be evaluated.
Stormwater samples were collected both manually and automatically (Table 1).
Whereas automated samplers (ISCO) were installed next to the stormwater gauges at
the three stations draining relatively large areas (the Park, Tel-Ashdod and Ad-Halom
drains, Fig. 1), samples were collected manually in the other drains. The runoff gauges
provided information about the discharge and cumulative runoff that could be matched
for each sample (Fig. 2). The sampler was programmed to start sampling at the
6
beginning of each flow event, then at 15-min intervals during the first hour, followed
by 30-min sampling intervals thereafter (Fig. 2).
The samples collected automatically in the large drains or manually in the smaller
drains were analyzed for chemical and isotopic composition (major ions, trace
elements, and the stable isotope ratios of oxygen, deuterium, nitrogen and sulfur).
Whereas most samples collected for trace elements were filtered through a 0.45-µm
filter and acidified with 5 ml of HNO3-, approximately one-third of them were collected
in pairs, including both filtered and unfiltered samples. Samples for organic
compounds were collected manually from all drains in preheated (400oC) borosilicate
bottles, stored on ice and treated in the laboratory within 5 days of sampling, to
minimize volatilization. Electrical conductivity (EC), pH and temperature were
measured in the field in all samples using an Extech field kit (±0.01 ms/cm, ±0.03 pH
units and ±0.1oC, respectively). Due to financial constraints, only 186 (of the 262)
samples, collected at six drains and from 15 events, were ultimately analyzed.
In addition, a detailed sampling of stormwater was carried out in one drain during the
Feb 11, 2002 event. Aside from the routine sampling, 14 samples were also collected
for microbiological indicators (general count, coliform bacteria and fecal coliform
bacteria) during this event. These samples were treated in the laboratory on the day of
sampling to avoid bacterial growth.
Finally, an EC electrode was added in the third year of measurements to the
stormwater recorder already installed in the Industrial Zone drain, to continuously
monitor the temporal variations in EC and temperature during flow events.
Analysis of the Water Samples
The analysis of major ions was carried out at the laboratory of the Institute for Desert
Research, Ben Gurion University of the Negev. Following the determination of
bicarbonate (by acid-base titration, ± 5 mg/L), all samples were filtered through a 0.45µm filter. Calcium, magnesium, sodium and potassium were measured using AA
(Perkin Elmer, ±1% ). Chloride, sulfate, nitrate and bromide were measured using IC
(Dionex, ±1%). Ammonia was measured using a spectrophotometer (Hitachi-U2000,
±0.05 mg/L with a detection limit of 0.03 mg/L).
Analyses of trace elements and organic compounds were carried out at the
Interdepartmental Laboratory at the Faculty of Agricultural, Food and Environmental
Quality Sciences of The Hebrew University of Jerusalem in Rehovot. Analyses for
7
trace elements were carried out using inductively coupled plasma-atomic emission
spectrometry (ICP-AES). Two instruments were used, model "Spectroflame" with a
radial torch and model "Spectroflame Modula E" with an axial end-on-plasma torch,
for the macro- and micro-elements, respectively (both from Spectro). The samples
were prepared for analysis following EPA method #3015 for dissolved and total trace
elements, using an MLS 1200 mega microwave digestion unit (Milestone Sorisole
(BG)). A blank was processed in parallel for each sample. Detection limit varied from
0.3 to 5.4 µg/L, depending on the element.
Analyses for volatile organic compounds were carried out using gas chromatographymass spectrometry (GC-MS, Varian Saturn 2000) equipped with a Tekmar 3000 purge
and trap system, according to EPA 5030B methods. Analyses for semi-volatile organic
compounds were performed according to EPA 3535, using solid-phase extraction disks
(Supelco
–ENVI
TM
–18
DSK).
Organic
compounds
were
extracted
in
dichloromethane. Detection limit varied, depending on the response factor of each
compound and the volume of the water samples. Consequently, the precision varied
from 0.1 to 0.01 µg/L.
The isotopic analyses were carried out at the Umweltforschungszentrum LeipzigHalle’s (UFZ) Department of Hydrogeology. The stable isotopic composition of the
water samples was determined by the chromium technique (2H) with an analytical
precision of ±0.8‰, and the standard H2O-CO2 equilibration method (18O) (Epstein
and Mayeda, 1953) with an analytical precision of ±0.1‰ in an IRMS delta S
(Finnigan MAT). The results of the hydrogen and oxygen isotope measurements are
expressed as delta notations (δ18O, δ2H), relative to the Vienna Standard Mean of
Ocean Water (VSMOW). Prior to the analyses of
15
N/14N and
34
S/32S, sulfate and
nitrate were concentrated on an anion-exchange resin (BIORAD AG2), which
quantitatively retained dissolved nitrate and sulfate and was subsequently rinsed with 3
M HCl. Sulfate was precipitated with BaCl2 to form BaSO4. Sulfur isotopic
compositions were measured by the continuous flow combustion technique in an IRMS
delta S with a reproducibility of ±0.3‰ and reported in delta notations (δ34S) as part
per thousand (‰) variations relative to the Canon Diablo Troilite (CDT) standard.
Nitrate was either converted to AgNO3 if nitrate concentration was above 10 mg/L or
converted to (NH4)2SO4 by Kjeldahl-distillation. Nitrogen isotopic compositions were
measured in an IRMS delta C (Finnigan MAT) using the Carlo Erba/Continous Flow
8
technique with a usual reproducibility of 0.2 and 1.5‰ in samples with only minor
nitrate concentrations. The values were normalized against international and internal
standards and δ15N values were reported with respect to AIRx.
Microbial analyses for general count, coliform bacteria and fecal coliform bacteria
were carried out at the Bacto-Chem laboratory in Ness- Ziona using standard methods
(U.S. Geological Survey, 2000). Units for fecal coliform are reported in colonyforming units (cfu) per 100 ml.
Quality assurance and quality control
The sample control and documentation procedures used in this study allowed sample
tracking through all steps from collection to analysis, and a display of the results. EPA
standard operating procedures were employed during sample collection, preparation
and analysis for each method. Instrument calibration was performed with certified
standards to account for bias and precision of the results. Samples with charge-balance
errors exceeding 5% were excluded from the major-ion database. For trace elements,
duplicates, reagent blanks, surrogate standards (e.g. Sc) and control standards were
used with about every 10th sample.
RESULTS AND DISCUSSION
Major Ions and Trace Elements
General characteristics
The ranges and mean values (with standard deviations) of the EC, pH, major ions and
trace elements measured in 68 and 186 samples of rain and stormwater, respectively,
are presented in tables 2 through 4. When the minimum value of a specific element in a
specific analysis was below its detection limit, the detection-limit value itself was
assigned to that element in that analysis for the calculation of its mean value and
standard deviation.
The pH values of the stormwater samples ranged from 6.18 to 8.13 and averaged 6.87.
These values are slightly less acidic than those recorded in the rainwater, ranging from
5.83 to 7.26 and averaging 6.44. The EC ranged from 0.07 to 1.13 ms/cm and averaged
0.31 ms/cm. Whereas the range of the EC values is similar to that of the rain samples
(0.05 to 0.69 ms/cm), the mean value of the former is almost fourfold that of the latter
(0.074 ms/cm). TDS ranged from 49 to 756 mg/L and averaged 210 mg/L. Typically,
9
major ion concentrations in the stormwater samples did not exceed those of the Israel
Drinking Water Standards (IDWS; Israel Ministry of Health, 2000). Trace-element
concentrations in filtered samples were mostly below IDWS.
In most of the stormwater samples, HCO3>Cl>SO4>NO3 and Ca>Na>Mg>K
(equivalent concentrations), reflecting input of HCO3 and Ca from terrestrial dust
(Nativ and Mazor, 1987; Singer, 1994). This anion distribution differed from that in
rainwater, where in 70% of the events, SO4 and Cl equivalent concentrations exceeded
that of HCO3 (Asaf et al., submitted).
Concentrations of most ions in the stormwater samples (Tables 3 and 4) were higher
than those in the precipitation samples (Table 2). The ratios between the mean
concentrations of the individual constituents in the stormwater and in the rainwater
(hereafter referred to as enrichment factor A) amounted to 3.35, 2.41, 1.80, 3.81, 4.50
and 3.66 for Ca, Mg, Na, K, NH4 and Si, and to 1.55, 4.06 and 1.41 for Cl, HCO3 and
NO3, respectively (Table 3 and Fig. 3). for Cl, HCO3 and NO3, respectively (Table 3
and Fig 3). SO4 had a negative enrichment factor (0.91), and its loss during the
evolution of rainwater to stormwater is still an unresolved question. Whereas the
enrichment factors of Ca, Mg and HCO3 support the contribution of limestone- and
dolomite-related dust, that of Si suggests the contribution of clay-related dust: both
types of dust particles have been recorded in Israel (Ganor et al., 1991). The large
enrichment factor for NH4 is probably related to wastewater overflow from the sewers
(separated from the stormwater collecting system) during storm events and to the use
of fertilizers in private yards and city parks. The enrichment factors for K, NH4 and
NO3 also suggest the contribution of fertilizers and probably dust related to the potash
exported through the Ashdod port.
Good correlation was observed between TDS and various ions, e.g. Ca, HCO3, Na and
Cl, suggesting the contribution of both halite (sea spray) and carbonates (terrestrial
dust) to the chemical composition of the stormwater. The Na/Cl ratio in the stormwater
samples ranged from 0.47 to 2.87, averaging 1.04, and slightly exceeding the values
observed in rain (0.98, Table 2), seawater (0.86) and dissolved halite (1.0). Values
exceeding 1 indicate input of additional sodium salts.
Large enrichment factors were calculated for most trace elements. The highest mean
values (for all samples) were 16.17, 8.04, 5.68 and 3.23 for lithium, iron, phosphorus
and aluminum, respectively, reflecting the large anthropogenic input of trace elments
in the stormwater (Table 4). A comparison between the analyses of filtered and
10
unfiltered sub-samples (Fig. 4) indicated that whereas titanium, aluminum, iron and
lead were mostly (>90%) sorbed onto the solid phase (suspended particles), nickel,
lithium phosphorus, molybdenum, tin and strontium were mostly (>50%) found in the
aqueous phase.
The impact of land use
The 186 stormwater samples were collected in stations draining residential, industrial
and mixed (i.e. commercial, industrial and residential) urban areas (Fig. 1, Table 1).
The results presented and discussed from hereon in focus on those obtained primarily
from the Tel-Ashdod, Quarter A, Industrial Zone and Park stations, representing the
different land uses.
EC values in the samples from the four drains varied from 0.07 to 1.13 ms/cm and the
mean values were 0.34, 0.32, 0.29, and 0.16 ms/cm in the Tel-Ashdod, Park, Industrial
Zone and Quarter A drains, respectively. These numbers suggest that land use has only
a minor influence on the salinity of the stormwater samples. Moreover, the mean
salinity measured in stormwater in the Industrial Zone drain was slightly lower than
that at the Tel-Ashdod station, draining only residential areas. This increased salinity
reflects the higher mean concentrations of all ions in the stormwater draining the
residential and mixed areas with respect to those from the Industrial Zone. Moreover,
concentrations of several trace elements (e.g. Al, Mn, Ni and Sr) in samples collected
in the Industrial Zone were also lower than those in the residential and mixed areas
(Table 4). Although data from an EC electrode, recently installed in the Industrial Zone
drain, indicated higher salinity at the beginning of different flow events (Fig. 5),
typically missed by the manual sampling protocol in this drain, the average and median
EC values based on 1206 measurements were low, amounting to 0.206 and 0.160
ms/cm, respectively. These values confirmed the aforementioned lower EC values
measured in manually collected samples from the Industrial Zone drain with respect to
those measured in samples from the large residential drains. This is an unexpected
finding, because usually stormwater drained from industrial zones contains a higher
pollution load than that derived from residential areas (e.g. Mikkelsen et al., 1994). It
should be noted, however, that the industrial zone monitored in our study hosts only
carpentries, automotive garages, and small businesses, as opposed to heavy industrial
zones reported in the literature, known to form point-pollution sources.
11
No major differences in salinity, ion or trace-element concentrations were observed
between stormwater samples collected in the Park and Tel-Ashdod stations,
representing mixed and residential land uses, respectively.
The impact of basin size
The area of the sub-basins channeled to the drains varied over three orders of
magnitude (from 5.09 to 0.05 km2; Table 1) and allowed an assessment of the impact
of basin size on stormwater quality. As expected, the lowest values for EC, major ions,
trace elements and enrichment factors were observed in the stormwater from the
smallest basin in Quarter A (Tables 3 and 4). These values increased in the mediumscale basin (the Industrial Zone station) and were highest in the large basins (the TelAshdod and Park stations). The longer flow path in larger basins, which allows for the
dissolution of a larger amount of accumulated soluble salts on land surface, could
explain this observation. The range of these parameters and the standard deviation
from their mean values increased with basin size. A possible reason for this
observation is the more uniform coverage of rain in the small basins, as opposed to the
more complicated rain distribution pattern in larger basins, resulting in stormwater
contributions from variable areas to the drain throughout the rain event.
Temporal variations
Where the first runoff of individual storm events could be collected and compared to
later runoff (in the Tel-Ashdod, Park and Ad-Halom stations, equipped with automatic
stormwater samplers, and in the Industrial Zone station, equipped with an EC electrode),
a first-flush effect (Lee et al., 2002) could be observed. Samples collected during the
first event in the season or at the beginning of each storm event had higher
concentrations than those in the following events or in the following samples within the
same event, respectively (Figs. 3, 5 and 6). For instance, whereas the mean TDS
concentrations of samples collected at the Tel-Ashdod and Park stations throughout the
rainy season were 2.4- and 2.3-fold the mean TDS concentrations in the rain (enrichment
factor A in Table 3), the mean TDS concentrations of samples collected at these stations
during the first storm event were 6.3- and 4.2-fold the mean TDS concentrations in the
rain (enrichment factor B in Table 3). Highest enrichment in the first runoff was
measured for NH4 (up to 11.7-fold at the Tel-Ashdod station), suggesting the flushing of
fertilizer dust and pet feces in the streets. Whereas a clear decrease in EC with
12
cumulative flow was observed (Fig. 6), the relationships between EC and flow discharge
(representing the dilution capacity of the salt load by the stormwater) did not display a
clear pattern (Fig. 5).
Rain and flood intermissions during a storm event resulted in a similar first-flush effect.
Although the first stormwater sample in the renewed storm was more dilute than the first
sample representing the beginning of the storm, it was more saline than the last sample
prior to runoff intermission or subsequent samples (Figs. 5 and 7). This observation
suggests rapid renewal of salt/dust on land surface, probably as a result of particle
suspension by gusting winds (typical of many storm events), which has not been
documented in more humid regions (Deletic, 1998). Where a sufficient number of
samples for trace elements were collected within the same event (e.g. Fig. 7), trace
elements did not seem to build up between flows within the same event. Whereas the
first-flush effect was well documented for major ions, biological indicators like fecal
coliform bacteria displayed erratic patterns (Table 5 and Fig. 7). Transient contributions
from the separate but sometimes overflowing sewage system probably account for this
observation.
Organic Compounds
Because of the stormwater’s exposure to air, the concentrations of detected volatile
organic compounds (VOCs; 61) were low (ppb) in most samples. Higher
concentrations were observed in stormwater drained from the Industrial Zone. The
highest concentrations (up to hundreds of ppb) were observed at this station during the
season's first flow of stormwater (in December 2002), and during a second event,
following a contaminant spill event that occurred in January 2003.
Table 6 presents maximum and mean concentrations of VOCs found in stormwater
collected in more than one residential station (i.e. the Tel-Ashdod, Quarter A, MeiAmi and Ad-Halom stations), along with their respective concentrations in stormwater
collected from stations draining the industrial and mixed-land-use areas. When the
minimum value of a specific compound in a specific analysis was below its detection
limit, the detection-limit value itself (0.01 µg/L) was assigned to that compound in that
analysis for the calculation of its mean value. The most abundant compounds included
fuel and its derivatives (hydrocarbons, xylene, ethylbenzene, toluene and methyl isobutyl ketone [MIBK]), solvents (methylene chloride, chloroform) and d-limonene. In
13
the residential areas, concentrations were below IDWS for most compounds. Except
for d-limonene and MIBK, VOC concentrations in the industrial zone were typically
two to fourfold those of the residential area, and for hydrocarbons and methylene
chloride up to 32- and 27-fold, respectively (Table 6), observations which are in
agreement with those of Lopes and Bender (1998). Figure 8a displays the frequency of
the occurrence of these VOCs at all stations. Hydrocarbons were the most abundant
VOCs in all land uses, followed by xylene and toluene. Hydrocarbons, xylene,
methylene chloride and ethylbenzene were detected at higher frequencies in the
Industrial Zone station. Conversely, d-limonene, chloroform and toluene were found at
a higher frequency in the Ad-Halom station, draining a residential area (Fig. 8a).
Semi-volatile compounds (SVOCs) were found in a larger variety (up to 128
compounds) and at higher concentrations than VOCs in the stormwater. The measured
compounds can be classified into (1) decomposition products of natural organic matter
(e.g. terpinol, borneol, menthol, limonin), (2) food derivatives (e.g. caffeine,
cholesterol), (3) pesticides (e.g. symazine, atrazine, ametryne, linuron), (4) fuel and its
derivatives (e.g. hydrocarbons, tetrahydronaphthalene), (5) common cleansers (various
phenols), and (6) industrial raw materials (e.g. bisphenol A). It should be noted that
whereas the hydrocarbons in the volatile group represent primarily light fuel
components and its derivatives, those in the semi-volatile group represent heavier
compounds related to asphalt, etc. Their concentrations were low (up to several ppb) in
most samples from most of the drains and were below IDWS in the samples from
residential areas. Stormwater from the industrial area included a larger variety of
SVOCs (63 of the 128 detected compounds were found exclusively there), and their
concentrations amounted in some samples to thousands of ppb of bisphenol A,
hundreds of ppb of methylene chloride and hydrocarbons, and tens of ppb of
pesticides.
Table 7 presents concentrations of SVOCs found in stormwater collected in more than
one residential station, along with their respective concentrations in water collected
from the industrial and mixed-land-use drains. Figure 8b displays the frequency of
occurrence of these SVOCs at all stations. Hydrocarbons had the highest frequency of
semi-volatile compounds (up to 69% in the Ad-Halom station and averaging 42%),
observed in stormwater samples from the residential stations and were followed by
hexa (methoxymethyl) melamine (39% mean value). Some of the SVOCs abundant in
the residential areas were either below detection limits or had low concentrations in the
14
stormwater from the industrial zone (e.g. cholesterol, cholestanol and ethanol, 2butoxy phosphate [3:1]). However, bisphenol A concentrations in the stormwater from
the Industrial Zone station were up to three orders of magnitude higher than those from
the residential areas (Table 7). Figure 8b displays the frequency of occurrence of these
SVOCs at all stations.
The frequency of occurrence of the SVOCs was generally lower than that of the VOCs
(Figs. 8a, 8b), suggesting different sources. No correlation was found between the
occurrence of samples contaminated with organic compounds and the precipitation
amount or flow discharge. In addition, no correlation was observed between the total
concentration of all SVOCs measured in the samples and their salinity. In 12 out of 17
cases where contamination was observed in the five stations draining the residential
areas (and when more than one sample was collected for SVOCs), the first sample was
less contaminated than those collected later on in the event. Because samples for
organic compounds were collected manually (to comply with the sampling protocols for
organic compounds), their number and collection times varied, thereby prohibiting a
more straightforward correlation between the contamination occurrence and its timing
within the event. Consequently, an effort was made to collect a large number of
stormwater samples per event for organic compounds from the onset of the flow during
one event at the Tel-Ashdod station (Feb 11-12, 2002, 14 samples) and three events at
the Industrial Zone station (Dec 10 and 24, 2002 and Jan 14, 2003; five, three, and three
samples, respectively). The results indicated fast flushing in the residential Tel-Ashdod
station, as only the first two samples contained traces of organic compounds. In the
Industrial Zone station, concentrations decreased as the flow proceeded, as illustrated by
the concentration of three pesticides in the Dec 10, 2002 flow event (26.3, 109.9, 16.2,
4 and 10.1 ppb of atrazine, 24.8, 121.8, 9.8, 3.1 and 8.6 ppb of ametryne and 70.7,
206.9, 171.3, 14.9 and 16.1 ppb of simazine). On the basis of our observations during
these events, and considering the limitations of the remaining samples, it is suggested
that a first-flush effect is also applicable to organic compounds.
Isotopic Composition
Values of δ18O and δD in the stormwater ranged from -7.61 to +2.27‰ and from
-45.8 to +30.1‰, respectively, and averaged -3.92 and –13.07‰, respectively (Table
8, Fig. 9). The lowest and highest values were measured on stormwater samples
collected during the Jan 19, 2001 and Feb 11, 2002 flow events, respectively, reflecting
15
the relatively low and high temperatures prevailing during these events. The minimum
and maximum values of δ18O and δD in the stormwater samples fell within the
respective range of precipitation, which exhibited a lower minimum (-10.26 and
-61.2‰, respectively) and a higher maximum (+1.83 and +28.4‰, respectively, Table
2). The mean values of δ18O and δD in the stormwater are somewhat higher than those
of the precipitation (-5.14 and -21.29‰, respectively) (Tables 2 and 8). This pattern
was also typically observed on an event basis, as the δ18O and δD values in the
stormwater were similar or slightly higher with respect to the corresponding
precipitation (Fig. 10). However, in some cases, the stormwater samples had lower
values with respect to those of the relevant precipitation samples, thereby suggesting
that the flow must have been triggered by a short, intensive shower, with values lower
than those of the composite 3-mm precipitation samples collected in our rain sampler
(Fig. 10b).
The δ18O and δD values of the stormwater samples are aligned along a line
(δD = 7.2818O + 15.69) similar in slope and in deuterium access to that of the
precipitation samples (δD = 7.06 δ18O + 15.01) (Fig. 9). This similarity and the lack of
correlation between salinity and δ18O and δD values (typically documented when
evaporation affects both ion concentrations and isotope fractionation) suggest very
little evaporation of the stormwater, as expected from its short exposure to the
atmosphere before it is channeled into the city drains. The deuterium access of both
lines is larger than that of the Global Meteoric Water Line (δD = 8δ18O + 10; Craig,
1961) but smaller than that of the Mediterranean Meteoric Water Line
(D = 8δ18O + 20; Gat and Dansgaard, 1972).
Figure 11 displays the isotopic compositions of the stormwater samples by stations.
Whereas samples collected from the stations draining large areas (e.g. the Park and
Tel-Ashdod stations, channeling 3.89 and 5.09 km2, respectively) displayed a wide
range of isotopic compositions, those collected from the small drain (Quarter A,
draining 0.05 km2) appeared to have a narrower, more distinctive range with a lower
standard deviation. This difference can be explained by the variations in duration,
intensity, and spatial distribution of rain typical of larger areas, resulting in a larger
heterogeneity of the isotopic compositions.
Measured values of δ34S in 84 stormwater samples ranged from 4.4 to 12.5‰ and
averaged 7.65‰. Lower values were observed in 21 rain samples, ranging from 2.9‰
to +8 ‰, averaging +4.6‰. These values of δ34S are consistent with those measured in
16
sulfur derived from the combustion of both petroleum and coal (slightly negative to
~10‰; Krouse, 1980), rather than those derived from modern sea spray (21‰) (Rees
et al., 1978). δ18O in the sulfate was measured in 81 of these 84 samples, ranging from
5.5 to 18.5‰ and averaging 13.6‰. These values are consistent with (1) multi-step
(industrial-related) conversion of SO2 to sulfate, ranging from +7 to +17‰, or with (2)
non-recent evaporites ranging from +7 to +20‰. Clearly, these values are not related
to modern marine-derived sulfate (+9.5‰) (Krouse and Mayer, 1998). Consequently,
although the sulfate in some stormwater samples was affected by soil sulfate and sea
spray, atmospheric-derived sulfate appears to dominate most stormwater samples and
is heavily affected by anthropogenic input from the nearby oil refineries and coal-fed
power station (Fig. 12 and Table 8).
The measured values of δ15N in 47 stormwater samples ranged from –3.73 to 14.3‰
and averaged 3.18‰ (Fig. 13 and Table 8). Values of δ15N ranging from negative
values up to +8‰ generally indicate contributions of atmospheric nitrogen (rain),
fertilizers, or soil organic nitrogen (Heaton, 1986; Kendall, 1998). Where fertilizers are
the nitrogen source, the nitrate-N concentration is >2 mg/L and often over 10 mg/L
(drinking-water limit), and the δ15N signature is between +0 and +8‰. Human and
animal waste signatures are generally >+8‰ (Barrett et al., 1999; Heaton, 1986;
Kendall, 1998). The observed values in 5 of the 47 samples suggest the input of
wastewater into the stormwater, thereby supporting the aforementioned conclusion
based on nitrate and ammonium concentrations. This input is further confirmed by the
δ18O vs. δ15N relationships in the nitrate of 10 stormwater samples (Fig. 13). On the
basis of known fields of this combination (Kendall, 1998), the origin of the nitrate is
divided between fertilizers and human and animal waste.
CONCLUSIONS
During the winter seasons of 2000/2001, 2001/2002 and 2002/2003, urban
precipitation and stormwater were intensively monitored in the rapidly growing city of
Ashdod, Israel. The monitoring program included the recording of 46 rain events for
precipitation (temporal and spatial distribution) and the measurement of its chemical
and isotopic compositions, as well as stormwater discharge at seven stations draining
85% of the city area. One hundred and eighty-six stormwater samples were collected
from these drains during 15 events and analyzed for chemical and isotopic
17
composition. The combined analysis of these datasets led us to the following
observations:
* Land use had only a minor effect on the stormwater concentrations of major ions and
trace elements. Conversely, the concentrations and variety of organic compounds (both
volatile and semi-volatile) were significantly higher in stormwater generated in the
industrial area than in that draining from residential areas.
* Stormwater concentrations were higher in stations draining a larger area, thereby
linking concentrations to the length of the stormwater flow paths. In these stations, the
range and standard deviation from the mean values of major ions, trace elements and
isotopic ratios of oxygen and hydrogen were also larger. It is suggested that the more
complicated rain distribution pattern in larger basins (as opposed to the more uniform
coverage of rain in the small basins) result in runoff contribution from variable areas to
the drain throughout the rain event, thereby increasing stormwater heterogeneity.
* A first-flush effect was documented on both a seasonal and event basis.
Concentrations of ions, trace elements and organic compounds were higher (up to
fourfold for NH4+) in stormwater from the first flow event of the season than in that
collected during later events. Similarly, during individual events, ion and trace-element
concentrations in the first stormwater samples were always higher than those in later
samples (up to one order of magnitude). Increased concentrations were also observed
in the first samples collected following flow intermissions within specific events,
suggesting rapid salt accumulation. Conversely, biological indicators, such as fecal
coliform bacteria, displayed random patterns, and may be controlled by sewage
overflow.
* Ion and trace-metal concentrations were very low (below drinking-water standards)
in stormwater collected from all drains in 97% of the samples (including the first
samples collected during specific events or during the first event of the season),
regardless of land use. Concentrations of volatile and semi-volatile organic compounds
were very low in stormwater drained from residential areas and originated from natural
substances and fuel. A larger variety and higher concentrations (up to hundreds of ppb)
of volatile and semi-volatile organic compounds were measured in stormwater draining
18
the industrial area. Fecal coliform bacteria concentrations were very high and exceeded
the drinking-water standards.
* The isotopic ratios of oxygen and hydrogen in the stormwater suggest very little
exposure to the atmosphere, resulting in very limited fractionation. The isotopic ratios
of sulfate and oxygen in the sulfate suggest that anthropogenic-related atmospheric
sulfate is the main source of sulfate in the stormwater. The isotopic ratios of nitrogen
and oxygen in the nitrate suggest contributions of wastewater, perhaps by overflowing
sewers.
* The low concentrations of ions, trace metals and organic compounds in stormwater
samples collected from residential areas in the city of Ashdod suggest that this water
can be reused with little treatment (e.g. filtering and chlorination). If used for artificial
recharge of the Coastal Plain Aquifer, its low salt content will help dilute the
increasing salinity of the groundwater in the aquifer.
ACKNOWLEDGEMENTS
The study was funded by the BMBF-MOS program between the Ministries of Science
in Germany and Israel, under the Water Technology framework. We thank Miron
Kubov from the Municipality of Ashdod for providing us with the GIS database for all
infrastructure layers of the city. We thank Eng. Arie Reisser for his enthusiasm in
advocating the study of urban stormwater and for providing us with the literature and
case studies that paved the way for our study. Many thanks go to Adi Ben Nun from
the GIS center at the Hebrew University of Jerusalem for his important help.
19
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FIGURE CAPTIONS
Figure 1: The study area and location of the various monitoring and sampling stations.
Figure 2: Stormwater discharge and cumulative flow in the Tel-Ashdod drain during
the Jan 23-26, 2001 flow event. The triangles and circles identify the automatic and
manual sampling times, respectively.
Figure 3: The ratio between the mean values of various parameters in the stormwater
samples collected during the first flow event and during all events and their
corresponding values in the precipitation (enrichment factor B and enrichment factor
A), respectively.
Figure 4: Distribution of trace elements between the solid and aqueous phases in
Ashdod stormwater.
Figre 5: Stormwater discharge (line) and temporal variations in EC (open diamonds)
during the stormwater event of Dec 20, 2002 in the Industrial Zone station .
Figure 6: Cumulative flow volume vs. EC in all stormwater samples from the Tel
Ashdod station.
Figure 7: Stormwater discharge (line) and temporal variations in fecal coliforms
(cfu/100 ml), EC (µs/cm), and Cl, P and Cu (mg/L) during the storm event of Feb 11,
2002 in the Tel-Ashdod station.
Figure 8: Frequency of the occurrence of VOCs (a) and SVOCs (b) found in
stormwater at more than one station draining a residential area. The calculated average
refers to samples collected from stations draining only residential areas and excludes
those from the Industrial Zone station.
Figure 9: δD vs. δ18O in both stormwater and precipitation samples.
Figure 10: δ18O in precipitation (open triangles) and stormwater (filled squares)
samples and precipitation intensity (rectangles) and stormwater discharge (line)
during the January 19, 2001 (a,c) and the Dec 19-20, 2000 (b,d) events. The circles
represent the sampling time of the stormwater sampling with respect to the stormwater
discharge.
Figure 11: δD vs. δ18O in samples collected at different stormwater stations.
18
32
18
15
Figure 12: δ O vs. δ S in the sulfate in precipitation and stormwater samples with
respect to potential sulfate sources (after Krouse and Mayer, 1998).
Figure 13: δ O vs. δ N in the stormwater nitrate with respect to potential nitrogen
sources (after Kendall, 1998).
23
TABLE CAPTIONS
Table 1: Specifications of the stormwater stations.
Table 2: Ranges and mean values of major ions (mg/L), trace elements (mg/L),
isotopic values (‰) and ion ratios in precipitation samples collected in the city of
Ashdod.
Table 3: Ranges, mean values and enrichment factors of pH, EC (ms/cm), TDS (mg/L)
and major ions concentrations (mg/L) in 186 stormwater samples.
Table 4: Ranges, mean concentrations and enrichment factors of trace elements
(mg/L) in 186 stormwater samples.
Table 5: EC (ms/cm), ions and trace elements (mg/L) and fecal coliforms (cfu/100 ml)
in stormwater samples collected during the Feb 11-12, 2002 event at the Tel-Ashdod
station.
Table 6: Maximum and mean concentrations (µg/L) of volatile organic compounds
found in stormwater samples from more than one station draining residential areas.
Table 7: Maximum and mean concentrations (µg/L) of semi-volatile organic
compounds found in stormwater samples from more than one station draining
residential areas.
Table 8: Ranges and means of isotopic values (‰)in stormwater collected in the city
of Ashdod.
24