Toxic Groundwater Contaminants: An Overlooked Contributor to

Article pubs.acs.org/est

Toxic Groundwater Contaminants: An Overlooked Contributor to Urban Stream Syndrome? James W. Roy* and Greg Bickerton National Water Research Institute, Environment Canada, Burlington, Ontario, Canada S Supporting Information *

ABSTRACT: Screening for common groundwater contaminants was performed along eight urban stream reaches (100s− 1000s of m) at approximately 25−75 cm below the streambeds. Four sites had known or suspected chlorinated-solvent plumes; otherwise no groundwater contamination was known previously. At each site, between 5 and 22 contaminants were detected at levels above guideline concentrations for the preservation of aquatic life, while several others were detected at lower levels, but which may still indicate some risk. Contaminants of greatest concern include numerous metals (Cd, Zn, Al, Cu, Cr, U), arsenic, various organics (chlorinated and petroleum), nitrate and ammonium, and chloride (road salt likely), with multiple types occurring at each site and often at the same sampling location. Substantial portions of the stream reaches (from 40 to 88% of locations sampled) possessed one or more contaminants above guidelines. These findings suggest that this diffuse and variable-composition urban groundwater contamination is a toxicity concern for all sites and over a large portion of each study reach. Synergistic toxicity, both for similar and disparate compounds, may also be important. We conclude that groundwater contaminants should be considered a genuine risk to urban stream aquatic ecosystems, specifically benthic organisms, and may contribute to urban stream syndrome.

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INTRODUCTION The term urban stream syndrome has been applied to the ecological degradation commonly observed in urban streams.1 Consistent symptoms of this syndrome have been identified as “a flashier hydrograph, elevated concentrations of nutrients and contaminants, altered channel morphology and stability, and reduced biotic richness, with increased dominance of tolerant species”.2 Contaminant loads to urban streams have been largely attributed to runoff from impervious areas (i.e., washoff), discharge from pipes and sewers, and direct contributions from wastewater treatment plants. One pathway that has received little attention with respect to its role in supplying toxic compounds to urban streams, though it has certainly been acknowledged,3 is groundwater discharge. Urban settings contain a multitude of different activities and sites commonly associated with groundwater contamination,4,5 such as manufacturing, dry-cleaning, sewage transport, road salting, gas stations, landfills, etc. Together these may be considered a form of diffuse urban groundwater pollution.6 Groundwater contaminants from such activities are known to discharge to surface water bodies, though studies of this occurrence have focused generally on individual groundwater plumes originating from particular sites (e.g., 7−11). Contaminant-induced microbial activity may also alter the levels of natural groundwater compounds, especially metals.12,13 Only a few studies have reported on widespread loading of groundwater contaminants to urban streams and these have Published 2011 by the American Chemical Society

tended to focus on select groups of contaminants, such as volatile organic compounds,14,15 major ions/metals,16 and road salt (e.g., 17), rather than diffuse pollution. However, Howard and Livingstone4 and Shephard et al.18 used groundwater modeling and widely spaced in-stream piezometers, respectively, to assess widespread and multicompound groundwater contamination from diffuse urban pollution at the cityscale. These plume-scale and larger-scale studies have generally taken a hydrogeological perspective, with a focus on quantifying transport of the chemicals from groundwater to surface water and understanding the controlling mechanisms; ecological repercussions are generally assumed, but rarely explored (but see 9,16). More attention has been given to the ecological consequences of inputs from leaking sewers and septic systems (e.g., 19−21), though the contribution from the groundwater pathway for these sources has not been specifically addressed. In fact, most large-scale studies of urban streamwater quality rely on sampling of surface water or shallow sediments (commonly ≤10 cm;22), and exclude many contaminants commonly associated with groundwater (e.g., 23−25). Received: Revised: Accepted: Published: 729

September 27, 2011 November 25, 2011 November 30, 2011 December 22, 2011 dx.doi.org/10.1021/es2034137 | Environ. Sci. Technol. 2012, 46, 729−736

Environmental Science & Technology

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maximize the likelihood and spatial extent of groundwater discharge, given that gradient direction may change temporally (as in 11), sampling was performed during summer base flow periods during times without major precipitation events. Despite these attempts, there is a chance that not all sample locations were experiencing groundwater discharge during the screening process. If these same locations received groundwater discharge at other times, then the extent of groundwater contamination in the vicinity of these streams could be slightly under-represented. The screening approach outlined by Roy and Bickerton28 was applied to these eight streams. Briefly, this involved the collection of groundwater at depths of 25−75 cm below the streambed surface using a mini-profiler system, with analyses for a wide assortment of groundwater contaminants and basic geochemistry. The sampling occurred at the stream-reach scale (100−1000s m) but with a detail-oriented spacing of between 8 and 15 m along the stream. Stream water samples were also collected to identify contaminants sourced from the stream, which could affect groundwater samples from areas under recharge conditions or with deep hyporheic exchange. Although attempts were made in the field to sample below the zone of hyporheic exchange (see Supporting Information), this was not always assured. Details on the profiler system, sample preservation, and analyses are provided in the Supporting Information. Screening-Type Risk Assessment. A simple 3-level risk assessment system was developed and applied to the groundwater data of this study. The criteria are outlined in Table 2. It is largely based on a comparison of measured concentrations to guideline water concentrations for the preservation of aquatic life within Canada. These are available for 39 of the compounds analyzed and are given in Table S3, separated into eight groups of similar type compounds: salt, metals, metalloids, petroleum compounds, chlorinated ethenes, other solvent compounds, pesticides (anionic only), and nitrogen compounds. These guidelines represent long-term (chronic) no effect levels, which is appropriate for groundwater contaminants since concentrations will commonly change slowly due to the low flushing rates. Some of these same guidelines were also applied to groundwater contaminated with chlorinated solvents by Conant et al.9 In previous investigations,28 contaminant concentrations in groundwater from these sampling depths (25−75 cm) were commonly representative of levels occurring at 5−10 cm depth somewhere in the same general area, for both conservative and reactive species. At any site, concentrations will no doubt vary substantially both areally and vertically due to flow and sorption variability,9 hyporheic mixing,9 and losses from within the biologically and geochemically active shallow sediments.29 Despite this variability, the concentrations from these widely spaced groundwater samples are considered to give a good estimate of the levels to which benthic life in both deep and shallow sediments would be exposed at these sites. It is further assumed that the guidelines for aquatic life are applicable to the deeper benthic organisms at these eight sites. However, the measured groundwater concentrations should not be applied directly to organisms residing at the sediment interface or in the overlying water, since contaminant concentrations are generally much less at the sediment interface,28 due to dilution with streamwater. Measured concentrations above guideline concentrations are identified at Risk Level 2 (red). This Risk Level 2 is analogous

The discharge of groundwater contaminants has been shown to impair aquatic ecosystems of urban streams in some case studies (e.g., 26), and groundwater invertebrates have been proposed as biomonitors for groundwater quality, including that discharging to streams.27 However, the general toxicological risk posed by diffuse urban groundwater contamination to the aquatic ecosystems of urban streams has not been specifically assessed. We address this issue here by screening groundwater along eight urban stream reaches in Canada at shallow depths, intended to be below any hyporheic zone, for a suite of common groundwater contaminants, following the protocol of Roy and Bickerton.28 Then a comparison is made between the groundwater contaminant levels detected and a set of toxicity criteria related to water quality guidelines for the protection of aquatic life. This follows a simple risk assessment approach in which both a hazard must be identified (i.e., contaminants at potentially toxic levels) and its exposure to a receptor of concern must be shown (i.e., the presence of these contaminants in groundwater by the stream, where the potential receptor most directly applicable would be deep benthic organisms, though others include shallow benthic organisms, aquatic plants, and fish). Because the focus here is on the groundwater pathway, measures were taken to exclude from the risk assessment contaminants possibly derived from the stream. Despite the use of measured, discrete concentration data from the groundwater screening, this risk assessment only provides a general evaluation of the toxicity risks posed to urban streams from contaminants sourced from groundwater. This is because the screening approach “snap-shot” cannot capture the spatial (area and depth) and temporal variations in contaminant concentrations occurring within the shallow sediments at each stream site. However, this approach is consistent with this study’s focus on uncovering general patterns that may be applicable to the management of urban stream syndrome, rather than on performing detailed sitespecific assessments.

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MATERIALS AND METHODS Field Screening for Groundwater Contaminants. Details on sampling and reach characteristics for the eight stream sites (Supporting Information (SI) Figure S1) are provided in Table 1. For three of these streams, full (Amherst, Angus) or partial (Halifax Regional Municipality; HRM) contaminant concentration data were reported previously,28 but an ecological risk assessment was not performed. The selection of the chosen reaches was not without some bias. The three sites reported on previously28 each had a stream section where discharge of groundwater containing chlorinated solvents was known or near-certain, while the Greenwood, NS site had a known chlorinated solvent groundwater contamination issue in the general area. For the remaining sites, there was no known contamination. However, stream sections were generally targeted where the chance of contamination from groundwater was deemed more likely, such as older areas and those with some commercial or industrial activity. The hydraulic gradient between groundwater and the stream (i.e., indicating groundwater discharge or recharge conditions) was not measured during the screening of the eight streams. However, stream reaches were chosen where topography or the presence of springs suggested groundwater would be predominantly discharging, or where contaminant plumes were known to discharge (as indicated above). To further 730

dx.doi.org/10.1021/es2034137 | Environ. Sci. Technol. 2012, 46, 729−736


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Table 1. Characteristics of the Eight Stream Sites at the Time of Sampling and Screening Details stream width (m)

sampled locations

Angus, ON Amherst, NS

10−15 1−2

44 15

HRM, NS Green., NS

12−20 3−6

76 133

2−4 2−5 1−4 2−4

19 21 99 43

site

Burl-SC, ON Burl-TC, ON Barrie-DC, ON Barrie-HC, ON

stream side sampled

predominant urban land use

known contaminants

both sides in same area middle (because so narrow) east side irregularly alternating

residential and commercial residential and industrial

chlorinated chlorinated wells chlorinated chlorinated

commercial residential, commerical and military base commercial commercial and residential residential, commercial, light-industrial residential (septic and sewer)

north/west side east side irregularly alternating irregularly alternating

solvent plume at stream compounds in adjacent solvent plume at stream solvent site in area

none none none none

Table 2. Outline of Criteria for Assessing the Level of Risk Posed by the Various Contaminant Groups (Eight Toxic Contaminant, Three Indicator) for Each Groundwater Sample Location

ξ

Multiplication of concentrations by 0.9 and 1.1 is to account for analytical uncertainty, described in Supporting Information.

to a hazard quotient >1,30 if the aquatic guideline concentrations (Table S3) are used as the toxicological benchmark concentrations for each compound. This does not imply that harm to aquatic organisms will be occurring, as guideline values generally incorporate a safety factor and sensitivity may differ between those benthic organisms on site and those used for guideline development. Conditions should be considered hazardous to the stream organisms, however. For this work, a second level (Risk Level 1 − yellow; Table 2) was introduced, in part, to accommodate some of the uncertainty associated with the limited spatial detail of the screening methodology, in comparison to the substantial spatial variability, both areally (submeter scale) and with depth (cm scale), that may occur in the field (e.g., 9,28). In addition, even low concentrations of a strictly anthropogenic contaminant may indicate the arrival of a larger problem. This additional level also allowed for the inclusion of three groups of nontoxic (at least no guidelines yet available) compounds phosphates, artificial sweeteners and perchloratethat may be useful indicators for toxic compounds not included in our analytical suite, such as pharmaceuticals, dyes, and other chemicals, which could arise from common sources (e.g., leaking sewers,

septic systems, landfills). Criteria for these indicator groups are given and explained in Table 2. Risk Level 0 (blue; Table 2) indicates that the contaminants are not at levels of concern (i.e., the groundwater is deemed clean), or that the stream itself is a potential source of the contaminants in groundwater (i.e., levels are higher in the stream). This latter condition (which rarely influenced the study findings) is deemed conservative here, as the groundwater concentration could be independent despite being lower than that of the stream.

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RESULTS AND DISCUSSION Contaminants of Concern. Groundwater contaminants were detected at levels above the guidelines for aquatic life, thus achieving a Risk Level 2 (probable risk), at all eight streams (Table 3). The number of these per stream reach ranged from 5 (Barrie-HC) to 22 (HRM, though 3 were known previously), out of a maximum of 39 tested (Table S2). These contaminants span all eight contaminant groups (Tables 2 and S3), with all sites represented by two or more groups. In total, 29 compounds surpassed aquatic life guidelines, in some cases by several orders of magnitude (see Table S4 for maximum concentration data). An additional nine compounds were 731



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Table 3. Groundwater Contaminants with Concentrations at One or More Sampling Locations at Risk Level 2 (i.e., above Aquatic Life Guidelines, Table S3; Probable Risk), or at Several Sampling Locations at Risk Level 1 (Table 2; Potential Risk)a contaminants at Risk Level 2

others only at Risk Level 1 t + c-DCE, TCE, PCE 1,1-DCA, 1,4-dioxane, t + c-DCE phosphorus, nitrite, Cr, Pb, MTBE

Greenwood

As, Al, Cu, Cd, Zn, nitrate As, Al, Cd, Cr, Cu, U, Zn, 1,1,1-TCA, TCE Cl, nitrate, As, Se, Ag, Al, Cd, Cu, Mo, V, U, Zn, benzene, toluene, ethyl-benzene, xylene, TMB, naphthalene, t + c-DCE, TCE, PCE, TCM Cl, nitrite, nitrate, ammonium, As, Ag, Al, Cd, Cu, Ni, Pb, U, V, Zn, toluene, PCE

Burlington-SC Burlington-TC Barrie-DC

Cl, As, Cd, U, Zn, 1,1,1-TCA As, Al, Cd, Cu, Zn, toluene Cl, nitrite, nitrate, ammonium, Cd, Cr, Zn, VC, t+c-DCE, TCE, toluene, xylene, TMB, naphthalene

Barrie-HC

nitrite, nitrate, ammonium, Cd, Zn

Angus Amherst HRM

a

Ag, Ni, Pb, perchlorate, glyphosate, VC, t + c-DCE, TCE TCE, glyphosate, 2,4-D glyphosate As, phosphorus, benzene, saccharin, PCE, 1,1-DCA, perchlorate U, acesulfame, perchlorate, glyphosate, 2,4-D

Contaminants in bold were known (or suspected in the case of Greenwood) prior to screening.

Figure 1. Risk assessment results (0 − no apparent risk (blue); 1 − potential risk (yellow); 2 − probable risk (red); Table 2) for the HRM site, determined for 7 of the 8 toxic contaminant groups (Pet − petroleum; C eth − chlorinated ethenes; O C − other chlorinated compounds; M’l − metals; M’d − metalloids; S − salt; N − nitrogen), 1 indicator group (P − phosphorus), and the combined risk level (Co; all screened compounds; also indicated on the site air photo).

identified at Risk Level 1 only, including four indicator compounds (perchlorate, phosphorus, and the artificial sweeteners acesulfame and saccharin). The chlorinated compounds that were known or suspected at four of the sites are highlighted in Table 3; they are important but not dominant contributors to this list. In addition, previously unknown

chlorinated ethenes or other solvents were detected at four sites. These findings suggest that there is a risk to the benthic community of these urban streams from groundwater contaminants and that the risk is posed by a variety of toxic compounds, likely related to multiple sources of groundwater contamination 732



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Wenger et al.,31 in their review of the urban effects on streams, emphasized the need to identify those compounds that pose the greatest risk to urban stream ecosystems (key question #14). In this study, the most common contaminants surpassing aquatic life guidelines include a number of metals, especially Cd and Zn, which were culprits at all the sites, but also Al, Cu, Cr and U; the metalloid As; chloride, likely from road salt given the spatial pattern of its detections (not shown); nitrate and ammonium (not screened on Angus, Amherst, HRM); toluene (alone at low levels or with other petroleum hydrocarbons at higher levels); and the chlorinated ethenes (Table 3). These compounds have many ubiquitous sources within the urban setting, but they also tend to be the ones with relatively low guideline concentrations. Glyphosate was also detected frequently, though not at concentrations above the guideline. Each indicator group was only flagged at Risk Level 1 at 2−3 sites (Table 3), which seems appropriate given knowledge of the sites and comparisons with other detected contaminants (see Supporting Information for more details). This suggests that the hazard criteria concentrations for the indicator compounds (Table 2) were likely appropriate Only a few of these contaminants, the metals and pesticides predominantly, are routinely included in assessments of urban stream ecological health.23 The other contaminants may have been overlooked because, while toxic, most do not meet the other two criteria for chemicals of ecological concern: persistence and bioaccumulation. However, these groundwater contaminants could be considered pseudopersistent,32 due to a combination of long-lived sources and slow transport (i.e., flushing) with groundwater, as has been applied to the continual release of pollutants with sewage treatment and septic systems. Thus, the broader spectrum of common groundwater contaminants, fulfilling two of the three requirements, should receive greater consideration in urban stream aquatic ecosystem assessments. Site-Wide Contamination Patterns. A more detailed look at the risk assessment reveals further information on the nature of the contamination risk posed to the streams. The results of only one of the eight streams, HRM, are illustrated here (Figure 1); the rest are provided as Supporting Information (Tables S5−11). Given the large suite of compounds screened, the results are tabulated according to the eight contaminant groups and three indicator groups, plus the combined results for all contaminants. If any compound in a group achieved a Risk Level 2 (red), the group is reported as a 2. A group rating of Risk Level 1 (yellow) applies if at least one group compound was rated at that level, but none were at Risk Level 2. Thus, multiple detections (level 2 or 1) within a group are not captured in the displayed risk assessment results (Figure 1, S5−11). A Risk Level 0 (blue) rating indicates all in the group were at level 0, which is the desired outcome for ecosystem health with respect to the groundwater pathway. The HRM site has 76 groundwater sampling locations (48 from 2008, as described previously28; 28 from 2010; Figure 1). Pesticides, sweeteners, and perchlorate were not analyzed for this site. A large proportion of these locations are at Risk Level 2 for salt, metals, and metalloids. The salt is likely due to winter salting activities, as a major street runs parallel to this stream. At the other sites, sampling locations with elevated salt often occur around road crossings. Metal and metalloid compounds have a variety of potential sources, both localized and diffuse, within an urban environment. Some, especially As in this setting, may be derived from the geological materials, though these levels

may be enhanced due to changing redox conditions associated with biodegradation of other contaminants (as in petroleum contamination here28). There is some chance that deeper sediments may release previously sorbed metals/metalloids to discharging groundwater as well. Thus, their widespread occurrence here is not surprising; they are commonly reported in urban stream toxicity studies.31 These three groups generally have few locations at level 1 (for all sites), likely because background values are not appreciably lower than guideline levels, and thus, it is difficult to meet Risk Level 1 criteria (Table 2). The petroleum and chlorinated compounds tend to occur in zones, likely indicating plumes emanating from point sources, though isolated detections do occur. The known chlorinated ethene (PCE and daughter products) plume is evident in samples 6−12, though only one location reached Risk Level 2. At this same location, other chlorinated compounds, likely additives or contaminants of the PCE source, have achieved Risk Level 2. This highlights the importance of wide-spectrum compound screening. There were only a few locations where nitrogen compounds were not at Risk Level 0. These did not coincide with the several areas where the indicator phosphorus group was at Risk Level 1, which suggests different sources for these two nutrients.28 The cumulative assault by groundwater contaminants on the stream ecosystem can be assessed by considering all the contaminants together (Figure 1). For the HRM site, it is apparent that groundwater contamination poses a probable risk to some portion of the benthic aquatic life along the majority of this stream section, as there is some contaminant above its guideline (Risk Level 2, red) at nearly every location, with most of the remainder at Risk Level 1 (yellow), indicating some concern. Only 2 locations had no apparent contaminant hazard (Risk Level 0). Contaminants from all the disparate groups contributed to this overall picture. The zones of elevated risk for different contaminant groups sometimes coincide, which may indicate a common source (e.g., leaking storm sewers for metals, organics, chloride, nutrients) and which raises the potential for synergistic toxicity. But often the contaminant zones do not overlap, as befits diffuse urban groundwater contamination, thus leading to greater coverage of the stream reach. A summary of the results of the site-scale risk assessment is provided for all eight streams in Table 4. Ideally, the majority of Table 4. Summary of the Risk Assessment Applied to the Eight Urban Streams, Indicating the Percentage of Sampling Locations at Each Risk Level (0 − No Apparent Risk, 1 − Potential Risk, 2 − Probable Risk; Table 2)

site Angus Amherst HRM Greenwood Burl. SC Burl. TC Barrie-DC Barrie-HC

total sample locations

locations at Risk Level 0 (%)



locations with ≥2 groups at Risk Level 2 (%)

44 15 76 133 19 21 99 43

23 0 3 13 11 10 15 21

14 60 9 32 21 10 14 37

64 40 88 55 68 81 71 42

5 33 61 12 11 43 30 2

the sample locations would be at Risk Level 0, with no apparent risk due to groundwater contaminants. However, the 733



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(especially for organic contaminant degradation products that are also toxic, e.g., vinyl chloride), will likely be occurring at shallower depths in some areas around the sample locations. Sorbed or precipitated contaminants in shallow sediments may also be released to the open water column following sediment suspension.41 The discussion above illustrates the challenges (e.g., spatial and temporal variability, multiple receptor sites, mixed contaminant toxicity, broad spectrum contaminant analyses) in assessing the risk posed to these eight urban stream reaches at the scale of 100s−1000s of m, even with the large data set obtained in this study. However, based on the risk assessment criteria applied heredetection of toxic contaminants at levels above aquatic guidelines or other levels of concern (Table 2) in proximity to potential receptorsit seems apparent that groundwater contaminants do pose a substantial toxicity risk to the aquatic ecosystems of all eight stream reaches. These findings are obviously not representative of all sections of all urban streams (of similar size: < 20 m width; we will not attempt to extrapolate the results to larger rivers). Strictly losing streams or reaches (i.e., those not receiving groundwater discharge) will be exempt. For newly urbanized areas lacking developed diffuse urban groundwater pollution, any contamination would likely be less variable and sparser. In contrast, highly industrialized areas may have reaches in worse condition. As noted already, there was some bias in the choice of stream reaches for this study. At four sites known or suspected chlorinated compounds were present; although, these compounds do not constitute a large fraction of the contaminants found at hazardous levels at these sites (Table 3). In addition, these eight stream sections were targeted based on (limited) knowledge of local land use and groundwater flow patterns; thus, other streams or reaches in these same cities are likely to be less inundated with groundwater contaminants. However, we suspect that screening of similarly targeted reaches in urban areas across Canada and other developed nations would result in similar results. Urban pollution of groundwater is a common global occurrence;4,5,18 thus, these results should be generally relevant. In addition, the contamination issues for these eight streams occurred across a spectrum of urban land uses residential, commercial and light industrial (Table 1)though the study was not designed to investigate the role of land use. This then suggests that toxicity associated with groundwatersourced contaminants could be an important contributor to the urban stream syndrome, which itself is a global problem.2 This would have important implications for stream restoration activities,42 especially those involving hyporheic restoration;43 zoning of urban areas (including set-back for sewer and septic systems); policy development with respect to use and handling of hazardous materials; and even litigation or management for outfall/runoff sources of contamination (i.e., fingering the appropriate transgressor).

percentage of sample locations at this level per site ranged from 0% for the Amherst site to 23% for the Angus site, with no apparent trend with sample numbers. Rather, at nearly all sites, the largest fraction of locations was at Risk Level 2, ranging from 40% for Amherst to 88% for HRM. Generally, the metals group was the most dominant across all sites, perhaps due to the wide array of urban sources or the relatively low guideline levels (Table S3), but the dominant group(s) varied between sites (see Supporting Information for further details). In addition, a substantial proportion of the sampling locations, between 2% and 61%, had two or more different groups at Risk Level 2. Note that this does not include the many additional locations where two or more compounds within the same group (e.g., Cd and Zn for metals) were at Risk Level 2. This extra information could be easily captured from the data, but the further detail was not deemed necessary to illustrate the ubiquitous nature of the groundwater contamination detected along these urban streams. Implications. Groundwater screening results from these eight urban streams indicate widespread contamination composed of a variety of different toxic compounds of several different types (e.g., metals, organics, salts, etc.) along all streams. The least contaminated site, Barrie-HC, still had 42% of the samples with at least one contaminant above aquatic guidelines, and only 21% of the samples considered “clean”. It should be noted that the coverage of contaminant analyses was not exhaustive. Key compounds not analyzed include phenols, pharmaceuticals and other xenobiotics,33 and a greater selection of pesticides (only four analyzed here; many others such as atrazine, simazine, and prometon are found commonly in urban streams34). In addition, contaminants may have nonadditive or synergistic toxic effects, which are generally poorly understood at this time.31 This can lead to an underestimation of toxicity for assessments based solely on individual compounds,31,34 as was done here. Thus, the contaminant risk posed by groundwater at these eight sites may be higher than portrayed by these results (Table 4). A valid question is how applicable these groundwater concentrations, sampled at about 25−75 cm depth, are to aquatic life. They are directly applicable to those burrowing aquatic organisms, particularly benthic invertebrates, which live all or part of their lives in this zone, generally up to 1 m depth.35,36 However, invertebrate life tends to be concentrated in the few cm nearest the sediment−water interface, and this is also where many fish species (e.g., salmonids) deposit their eggs and many aquatic plants root themselves. Furthermore, loading of groundwater contaminants to surface waters is still a concern for protecting organisms that reside within the open water column (e.g., fish, many invertebrates, algae). The contaminant concentrations within the shallow sediments and the contaminant flux to surface water will be dependent on flow processes, such as flow direction (e.g., upwelling versus downwelling11,37) and hydraulic conductivity heterogeneity,38 and will also be affected by geochemical and biological attenuation processes, which tend to be elevated in organicrich shallow sediments.8,9,11,39,40 Concentrations can be reduced substantially in the overlying surface water by rapid dilution9,37 and volatilization to the atmosphere10 in some cases. Thus, the groundwater concentrations determined in this study should not be considered ubiquitous throughout the benthic zone, or representative of the average flux concentration to the streams. However, previous work9,28 suggests that similar concentrations, or potentially even higher values

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ASSOCIATED CONTENT

S Supporting Information *

Additional details and tables relating to methods, maximum contaminant concentrations, and risk assessment results for the remaining 7 streams. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. 734



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(17) Williams, D. D.; Williams, N. E.; Cao, Y. Road salt contamination of groundwater in a major metropolitan area and development of a biological index to monitor its impact. Water Res. 1999, 34 (1), 127−138. (18) Shepherd, K. A.; Ellis, P. A.; Rivett, M. O. Integrated understanding of urban land, groundwater, baseflow and surfacewater quality  the City of Birmingham, UK. Sci. Total Environ. 2006, 360, 180−195. (19) Duda, A. M.; Lenat, D. R.; Penrose, D. L. Water Quality in Urban Streams: What We Can Expect. J. Water Pollut. Control Fed. 1982, 54 (7), 1139−1147. (20) Hatt, B. E.; Fletcher, T. D.; Walsh, C. J.; Taylor, S. L. The influence of urban density and drainage infrastructure on the concentrations and loads of pollutants in small streams. Environ. Manage. 2004, 34, 112−124. (21) Walsh, C. J.; Kunapo, J. The importance of upland flow paths in determining urban impacts on stream ecosystems. J. N. Am. Benthol. Soc. 2009, 28, 977−990. (22) Chapman, P. M.; Anderson, J. A decision-making framework for sediment contamination. Integr. Environ. Assess. Manage. 2005, 1, 163−73. (23) U.S. EPA. A Guidance Manual to Support the Assessment of Contaminated Sediments in Freshwater Ecosystems: Vol. I − An Ecosystem-Based Framework for Assessing and Managing Contaminated Sediments; 905-B02-001-A; Great Lakes National Program Office: Chicago, IL, 2002; p 2. (24) Navarro-Ortega, A.; Tauler, R.; Lacorte, S.; Barcelo, D. Occurrence and transport of PAHs, pesticides and alkylphenols in sediment samples along the Ebro River Basin. J. Hydrol. 2009, DOI: 10.1016/j.jhydrol.2009.12.031. (25) Navarro, A.; Lacorte, S.; Barceló, D. Occurrence and transport of pesticides and alkylphenols in water samples along the Ebro River basin. J. Hydrol. 2009, DOI: 10.1016/j.jhydrol.2009.06.039. (26) Dickman, M.; Rygiel, G. Municipal landfill impacts on a natural stream located in an urban wetland in Regional Niagara, Ontario. Can. Field Nat. 1998, 112 (4), 619−630. (27) Malard, F.; Plenet, S.; Gibert, J. The use of invertebrates in ground water monitoring: A rising research field. Ground Water Monit. Rem. 1996, Spring, 103−113. (28) Roy, J. W.; Bickerton, G. A proactive screening approach for detecting groundwater contaminants along urban streams at the reachscale. Environ. Sci. Technol. 2010, 44, 6088−6094. (29) Jones, J. B., Mulholland, P. J., Eds. Streams and Groundwater; Academic Press: San Diego, CA, 2002; 425 p. (30) Burns, L. A. PIRANHA: Pesticide and Industrial Chemical Risk Analysis and Hazard Assessment, Version 2.0; U.S. EPA, Environmental Research Laboratory, Office of Research and Development: Athens, GA, 1991. (31) Wenger, S. J.; Roy, A. H.; Jackson, C. R.; Bernhardt, E. S.; Carter, T. L.; Filosos, S.; Gibson, C. A.; Hession, W. C.; Kaushal, S. S.; Marti, E.; Meyer, J. L.; Palmers, M. A.; Paul, M. J.; Purcelli, A. H.; Ramírez, A.; Rosemond, A. D.; Schofield, K. A.; Sudduth, E. B.; Walsh, C. J. Twenty-six key research questions in urban stream ecology: An assessment of the state of the science. J. N. Am. Benthol. Soc. 2009, 28 (4), 1080−1098. (32) Daughton, C. G. Emerging chemicals as pollutant in the environment: A 21st century perspective. Renew. Resour. J. 2005, 23 (4), 6−23. (33) Reinstorf, F.; Strauch, G.; Schirmer, K.; Gläser, H.-R.; Möder, M.; Wennrich, R.; Osenbrück, K.; Schirmer, M. Mass fluxes and spatial trends of xenobiotics in the waters of the city of Halle, Germany. Environ. Pollut. 2008, 152 (2), 452−460. (34) Gilliom, R. J. Pesticides in U.S. streams and groundwater. Environ. Sci. Technol. 2007, 41, 3409−3414. (35) Williams, D. D.; Hynes, H. B. H. The occurrence of benthos deep in the substratum of a stream. Freshwater Biol. 1974, 4, 233−256. (36) Hynes, H. B. N. Groundwater and stream ecology. Hydrobiologia 1983, 100, 93−99.

ACKNOWLEDGMENTS We thank Rita Mroz and Dave MacArthur (Environment Canada-Dartmouth); Nicole Perry, Gordon Check, Jason Dauphinee-Muise, and Jeff Garnhum (Nova Scotia Environment); Allan Pearson (National Defence − Canadian Forces Base Greenwood); Ron Patterson and Jason MacDonald (Town of Amherst, NS); Lorran Cooney (Town of Barrie, ON); Robin Barnes, Susan Brown, Pam Collins, Melissa Hollingham, Jerry Rajkumar, Steve Smith, Harris Switzman, John Voralek, and Drs. Lee Grapentine, John Spoelstra, and Dale Van Stempvoort (Environment Canada-Burlington).

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