Proactive Screening Approach for Detecting Groundwater

Jul 9, 2010 - at the Reach-Scale. JAMES W. ROY* AND GREG BICKERTON. National Water Research Institute, Environment Canada,. Burlington, Ontario ...
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Environ. Sci. Technol. 2010, 44, 6088–6094

Proactive Screening Approach for Detecting Groundwater Contaminants along Urban Streams at the Reach-Scale JAMES W. ROY* AND GREG BICKERTON National Water Research Institute, Environment Canada, Burlington, Ontario, Canada

Received April 29, 2010. Revised manuscript received June 29, 2010. Accepted June 30, 2010.

Here we outline and demonstrate a screening approach for the detection of groundwater contaminants along urban streams within unconsolidated beds. It involves the rapid acquisition of groundwater samples along urban stream reaches at a spacing of about 10 m and from depths of about 25-75 cm below the streambed, with analyses for a suite of potential contaminants. This screening approach may serve two functions: a) providing information for assessing and mitigating the toxicity and eutrophication risks to aquatic ecosystems posed by groundwater contaminants and b) detecting and identifying groundwater contamination in urban settings more rapidly and inexpensively compared to land-based well installations. The screening approach was tested at three urban streams, each affected by a known chlorinated-solvent plume. All three known groundwater plumes were detected and roughly delineated. Multiple, previously unknown, areas or types of groundwater contamination were also identified at each stream. The newly identified contaminants and plumes included petroleum hydrocarbons (BTEX, naphthalene, MTBE), 1,4-dioxane, nitrate and phosphate, road salt, and various metals (including arsenic, cadmium, chromium, copper, lead) at elevated concentrations compared to background values and relevant Canadian water quality guidelines. These findings suggest that this screening approach may be a useful tool for both ecologists performing ecological assessments and stream restorations and for hydrogeologists undertaking groundwater protection activities. Given the numerous contaminants detected, it may be appropriate to apply this technique proactively to better determine the pervasiveness of urban groundwater contaminants, especially along urban streams.

Introduction Urban settings accommodate a host of activities and sites commonly associated with groundwater contamination (1, 2) including manufacturing, dry-cleaning, sewage transport, septic systems, road salting, gas stations, landfills, etc.; which together may be considered a form of diffuse urban pollution (3). Groundwater contamination has the potential to affect human health through wells supplying drinking water and, for volatile contaminants, through vapor intrusion into buildings. Also, urban-sourced groundwater contaminants are known to discharge to surface water bodies (4, 5), where they may adversely affect aquatic ecosystems through * Corresponding author e-mail: [email protected]. 6088 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010

toxicological and eutrophication effects. Note that the majority of burrowing aquatic organisms resides within the first meter of the streambed surface (6), where relatively undiluted groundwater contaminants may occur before discharging to surface water. Subsurface microbial activity resulting from the presence of contaminants may also cause deleterious changes to the conditions of the ambient groundwater, such as its pH, dissolved oxygen (DO), and the concentrations of iron and other metals (7). Detection of groundwater contaminants in urban areas tends to be sporadic and reactionary; for example, in response to the discovery of contamination in a drinking supply well, vapor intrusion into a building or issues identified during a site-specific investigation (e.g., as part of a land transfer or development process). These investigations tend to be costly and time-consuming due to the conventional practices and/ or requirements for installing multiple monitoring wells and tend to have a local focus considering a limited number of contaminants based on past land uses. Broader-scale studies can be completed using existing wells with routine sampling (8) or integral pumping tests (9), but the distribution and numbers of available wells may be inadequate, especially in urban areas. For studies addressing impacts of toxic substances on aquatic ecosystems, groundwater information from wells will be hindered by the availability and access to existing wells and ability to install new ones along urban streams. Such data that are available will not account for attenuation (e.g., biodegradation, sorption) of groundwater contaminants prior to reaching the stream. An additional concern is that these studies tend not to capture many of the contaminants that are commonly transported with groundwater. These studies generally rely on sediment samples taken to a very shallow depth (commonly e10 cm (10)) and focus on persistent compounds associated with wastewater treatment plants or industry outflows (e.g., PAHs, PCBs, metals such as mercury) (11). Monitoring for groundwater-derived contaminants using surface water samples may be hindered by dilution or volatilization from the surface water (4, 12) and attenuation processes such as sorption and biodegradation, especially in the biologically and geochemically active shallow sediments (13, 14). The challenge of detecting contaminants at these lowered levels is compounded by the generally wide spacing of surface water sample locations. Given these monitoring issues, the extent of groundwater contamination in urban areas and the risk it poses to aquatic ecosystems may be underappreciated. Here we describe a novel screening approach that focuses on the pro-active detection and crude delineation of groundwater contaminants discharging to urban streams. It is suitable for areas with unconsolidated geologic materials and for streams with unconsolidated sediments over bedrock. It may also be applied to other surface water bodies (perhaps with some modifications), but this study is restricted to stream applications. This screening approach serves a dual function: a) it provides information useful for assessing the risk to aquatic ecosystems posed by toxic chemicals in groundwater and for mitigating this risk; and b) it has the potential to detect and identify sources of urban groundwater contamination more rapidly and inexpensively in comparison to installing monitoring wells. This screening approach was applied at three urban-stream reaches, each affected by a known groundwater chlorinated solvent plume. The detections of the known plume and of any previously unknown groundwater contamination were considered as a two-part test of the approach. 10.1021/es101492x

Published 2010 by the American Chemical Society

Published on Web 07/09/2010

TABLE 1. Length of Stream Reach, Number of Groundwater Samples Collected and Contaminants Detected (At Elevated Levels) in Groundwater below the Stream during Screening

reach length (m) no. of samples chlorinated solvents petroleum hydrocarbons and fuel oxygenates nutrients metals (elevated compared to background) other a

Angus ON

Amherst NS

HRM NS

∼450 43 PCEa

175 15 TCEa, 1,1,1-TCAa

∼650 50 PCEa BTEX, naphthalene, TMB, MTBE, DIPE nitrate, phosphate arsenic, iron, cadmium, cobalt, lead, silver, uranium (road) salt (NaCl)

MTBE, DIPE aluminum, arsenic, copper, cadmium, zinc

arsenic, chromium, copper 1,4-dioxane

Metabolites of these compounds also detected.

Outline of the Screening Approach The key components of the screening approach for urban streams are that 1. groundwater is collected below the stream (i.e., from stream sediments), preferably (as explained below) below the hyporheic zone (streamwater flows into subsurface sediments and then returns (15)); 2. collection is performed at the stream reach scale -100s-1000s of m; 3. it uses relatively fine scale spacing between samples about 10 m; 4. groundwater is collected without permanent installations, heavy equipment, or major cleaning between samples (which allows for rapid execution) and from the stream bank or within the stream itself; 5. samples are analyzed for a broad range of potential groundwater contaminants. Sampling below the hyporheic zone rather than at the sediment-water interface avoids issues with attenuation processes in the shallow sediments. However, it may also capture contaminants sourced from deeper stream sediments rather than just those transported via groundwater. This may be especially important for metals, which can accumulate in sediments and be released with changes in groundwater redox conditions (e.g., for arsenic (16)). Thus, sediment sources should be considered during interpretation of the screening results. Few studies have reported on reach-scale loading of multiple groundwater contaminants to surface water bodies in urban or industrialized settings. Rather, most have focused on individual groundwater plumes (17, 4, 5, 18, 19). However, a few studies have addressed more widespread impacts but for a limited set of contaminants (volatile organic contaminants (20, 21); road salt (e.g., ref 22) and generally with wider sample spacing. Most large-scale studies of water quality rely on surface water or sediment samples and do not focus on contaminants commonly associated with groundwater (e.g., ref 23). Some studies on diffuse metal loading to streams in mining areas incorporate available groundwater seeps and springs into their detailed stream sampling network (e.g., ref 24). Spacing between samples for the screening approach was based on an informal examination of the widths of documented plumes (e.g., refs 7 and 4). Ten meter spacing was deemed to provide an appropriate balance between plume detail and stream coverage, though the spacing can be adjusted based on individual site conditions and objectives. It must be emphasized that this screening is not designed to fully characterize individual plumes. Rather, further detailed field investigation could follow and be guided by the screening results. Also, sample concentrations should only be considered as semiquantitative indicators, given the

smaller scale of natural variation (e.g., Figure S2) and hyporheic attenuation-mixing processes.

Methodology Field Sites. The three urban streams examined by this study are located in Angus, Ontario (4); Amherst, Nova Scotia; and the Halifax Regional Municipality (HRM), Nova Scotia. The stream in Amherst was small and shallow enough (1-2 m wide; e30 cm deep) that sampling (September 16, 2008) was performed along the midline of the stream. Sampling at Angus was performed along both sides of the stream (10-15 m wide; e1.5 m deep), over three periods in 2008: August 15; September 30-October 2; October 22-23. Primary sampling (screening; September 10-14, 2008) was focused along the east shoreline of the HRM stream (10 m wide; e3 m deep), while vertical profile sampling was performed on July 15, 2009, at six locations, both near-shore and middle of the stream, in the area of the known solvent plume. The approximate profiling depths were 0, 12.5, 50, and 100 cm below the streambed surface. Sampling Protocol. In this study, spacing between sampling locations ranged from 8 and 15 m. Groundwater from below the streambed was sampled using a mini-profiler system (see the Supporting Information). It consisted of one or more coupled hollow steel-rods (5/8” diameter) attached to a stainless steel drive-point (Figure S1) with sampling ports internally connected to 1/4” polyethylene tubing. The rods were driven into the sediment using a hand-held hammerdrill (Figure S2), while streamwater was pumped down the tubing using a peristaltic pump to minimize clogging of the ports. At the desired sampling depth, flow was reversed. Chemical properties (dissolved oxygen (DO), pH, and electrical conductivity (EC)) of the stream and sampled subsurface water were monitored using hand-held meters to determine when purging of surface water was completed. These readings also provided an indication of mixing of surface water with groundwater, whether from flow of streamwater through cobbles or along the profiler rod during sampling (i.e., short-circuiting) or as a result of natural hyporheic mixing. If mixing was suspected or pumping was difficult, the mini-profiler was advanced deeper into the sediments, and the pumped water was re-examined. In a few cases, the EC and DO parameters remained constant with depth or further penetration was restricted, and, thus, a sample was taken regardless. Samples were generally obtained 25-75 cm below the stream bottom. Stream samples were also collected to identify contaminants likely sourced from the stream (i.e., with higher stream than groundwater concentrations). Collected samples were filtered and preserved as appropriate and analyzed for a suite of common groundwater contaminants and basic geochemistry (see the Supporting Information for full details and a list of analytes). The number of samples collected and reach length screened for each stream are listed in Table 1. All sampling locations VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Observed concentrations for select contaminants in groundwater collected below the Angus stream, with values from two stream samples presented to the right. The double-dashed line indicates the position of the bridge, with positive sample stations in the North direction. Conant et al. (4) found chlorinated ethenes over a 60-m stretch north of the bridge (Figure S2). were georeferenced using a survey-grade GPS system (Magellan ProMark3).

Results and Discussion Detection of Contaminants. The known chlorinated-solvent plumes were detected and approximately delineated in each of the three streams. In each case the contaminants were found in multiple samples, giving apparent plume widths of 30-60 m. Previously unknown zones of contamination and/ or unidentified contaminants were also detected in each of the three streams (Table 1). Detailed findings for each site are discussed in the following sections. Screening at Angus, Ontario. Groundwater was sampled at about 20 locations on each side of the stream at the Angus site (>450 m total). This included the known discharge area of the chlorinated solvent plume mapped out by Conant et al. (4). Tetrachloroethene (PCE) and its degradation products, for example trichloroethene (TCE) and cis-dichloroethene (c-DCE), were detected in samples across an approximately 50-m stretch of stream along the east bank immediately north of the bridge (Figure 1a; total chlorinated ethenes), which is coincident with the approximately 60-m plume detailed by Conant et al. (4) (Figure S2). Chlorinated solvents were not detected in samples collected along the west bank, where Conant et al. (4) reported contaminants only at very low concentrations. A number of previously undocumented contaminants were detected in addition to the chlorinated hydrocarbons 6090

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(Table 1). The solvent plume area had BTEX (benzene, toluene, ethylbenzene, xylene) and naphthalene, though at low concentrations (20 times (e.g., Al peak 650 µg L-1; Al background 860 mg L-1; the US Criteria Maximum Concentration for acute toxicity (32)) and corresponding sodium concentrations (Figure 3). The proximity of a major road running parallel to the stream suggests that road salt is the likely source. The chloride concentrations likely reflect long-term groundwater contamination, rather than recent

FIGURE 4. Observed concentrations of phosphate, copper, and aluminum in groundwater collected below the HRM stream. melting of salt-contaminated snow at the shoreline, since the samples were collected in late summer. Elevated concentrations of the nutrients phosphate and nitrate were also detected (Figures 4a and 5a), though in distinctly different locations. Other compounds present in similar patterns to these (i.e., a rudimentary environmental fingerprinting approach) may help provide insight into the possible sources. In the two areas with elevated phosphate, the groundwater had lower levels of sodium and chloride (Figure 3) and strontium (not shown) and elevated concentrations of copper (Figure 4b), lead and zinc (not shown), and aluminum (Figure 4c). These relationships are consistent with water derived from municipal distribution systems (e.g., refs 33–37) supplied by surface waters rather than groundwater. Thus, leaking sanitary sewers are suggested as a possible source of this phosphate. In contrast, the elevated nitrate in samples coincided with elevated levels of silver and cadmium (Figure 5) and low levels of chloroform (not shown). Elevated cadmium is associated with both fertilizers VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Observed concentrations of nitrate, cadmium, and silver in groundwater collected below the HRM stream. and sewage (38), silver is used in fungicides and water treatment devices as an antimicrobial (39), and chloroform is a component of some fumigants and insecticides (40). Thus, lawn-care products are suspected as a possible source of the nitrate, though another form of leaking sewage infrastructure cannot be dismissed. Additional sampling for nitrogen isotopes, pesticides, or pharmaceuticals could help resolve the nutrient sources. Representative Samples. Dilution of sampled groundwater by streamwater (short-circuiting or hyporheic mixing) was assessed by comparing sampled water to streamwater using: a) field (DO and EC) measurements, b) the presence of anthropogenic contaminants, c) water chemistry, and/or d) water isotopes. Details of this assessment are reported in the Supporting Information. However, none of the approximately 110 samples collected were deemed to be completely or even predominantly composed of streamwater. To gain insight into how representative the collected samples were of groundwater conditions, vertical profiling was conducted in the HRM stream. This consisted of sample collection at 4 depths (approximately 0, 12.5, 50, and 100 cm below the streambed) at 6 locations within the discharge area of the chlorinated solvent plume. Results for benzene (a biodegradable species) and acesulfame (an artificial sweetener considered to behave rather conservatively and likely indicating the influence of leaking sewers (41)) are shown in Figure 6 (additional species presented in Figure S5). The lower concentrations in samples from the sedimentwater interface (0-cm depth) illustrates that significant dilution occurred even at the streambed surface. A range of concentrations was found over the remaining three depths, with no substantial decline in concentrations with reduced depths (i.e., approaching the streambed). This suggests that biodegradation, sorption, and hyporheic mixing were not dominating the fate of these contaminants over this interval (12.5-100 cm). Thus, the groundwater samples collected during screening (generally 25-75 cm below streambed) are likely representative of relatively consistent groundwater conditions within the stream sediments (up to 10 cm) for this section of the HRM stream. Benthic organisms that 6092

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FIGURE 6. Observed concentrations with depth below the streambed from the 6 profiling locations in the HRM stream for a) benzene and b) artificial sweetener acesulfame. resided below (and perhaps within) this 10-cm surface layer would have been exposed to the measured contaminants. Rapid Screening. For this initial developmental stage the average daily productivity (even from the boat) was between 120 and 150 m of stream length with a 3-person crew (10-15 samples day-1). Actual daily productivity varied as each sampling location presented its own particular set of challenges. It is anticipated that further experience and optimization of the field approach (e.g., deploying a pair of 2-person crews) will improve productivity. Review of the analytical results showed some evidence of minor carry-over (e1 µg L-1) of some volatile compounds from samples with high concentrations to field blanks and potentially subsequent samples. A strict quality control and assurance protocol (e.g., new tubing for each location) should remedy this issue; however, it would considerably reduce the sampling rate and increase the amount of waste. Given that sampling speed was one of the primary considerations in the development of this screening approach and that detection rather than quantification was the aim, some lowlevel carry-over was deemed acceptable.

Study Implications This study provided a successful demonstration of the developed screening approach, which fills a monitoring/ surveillance gap not covered by conventional sampling of wells, streamwater, or stream sediments, and would be a

complementary tool to benthic surveys for stream ecosystem health. It can be applied proactively, in a relatively rapid manner, and may provide easier access to areas of interest (including avoiding land-ownership issues in some cases), more detailed coverage of contaminant distributions, and reduced field expenses compared to land-based investigations. Thus, the screening approach developed and tested here may be a useful tool for providing information to two distinct, yet complementary groups: i) those assessing the potential risks posed by contaminants on urban aquatic ecosystems and developing remediation strategies for them, and ii) those attempting to detect and identify sources of groundwater contamination in urban environments. These findings also suggest that diffuse groundwater contamination may be more prevalent in urban stream ecosystems than currently understood.

Acknowledgments The authors thank Rita Mroz & Dave MacArthur (Environment Canada-Dartmouth); Nicole Perry & Gordon Check (Nova Scotia Environment); Ron Patterson & Jason MacDonald (Town of Amherst, Nova Scotia); Robin Barnes, Susan Brown, Pam Collins, Melissa Hollingham, Jerry Rajkumar, Steve Smith, John Voralek, and Drs. Lee Grapentine, John Spoelstra, Dale Van Stempvoort (Environment CanadaBurlington).

Supporting Information Available Additional data, tables, and figures relating to methods and distinguishing streamwater influences on sampling. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ES101492X