James W. Roy

Toxic Groundwater Contaminants: An Overlooked Contributor to Urban Stream Syndrome?

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.

Proactive screening approach for detecting groundwater contaminants along urban streams at the reach-scale.

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.

Elevated Dissolved Phosphorus in Riparian Groundwater along Gaining Urban Streams

Findings of low concentrations of dissolved phosphorus in groundwater in large surveys [e.g., United States Geological Surveys National Water-Quality Assessment (NAWQA) Program ( Dubrovsky, N. M.; et al. The Quality of Our Nations Water: Nutrients in the Nations Streams and Groundwater, 1992-2004 . U.S. Geological Survey Circular 1350; USGS : Reston, VA , 2010 . ); >5000 wells] support the common perception that groundwater is generally of little importance for transporting phosphorus. Here, we address whether this applies to urban riparian settings, where discharging groundwater may potentially contribute to urban stream syndrome and downstream eutrophication problems. This survey study includes 665 samples of groundwater collected along gaining stream reaches at six urban sites. Considering the combined sample set, 27% had soluble reactive phosphorus (SRP) concentrations >0.1 mg L(-1), which is more than double that determined in the NAWQA Program (12%), while for individual sites the range was 12-52%, excluding one site with consistently low SRP (0%). None of the sites showed significant correlation between SRP and the artificial sweetener acesulfame, a promising wastewater indicator, including two with known wastewater contamination (but the lowest SRP). Rather, high SRP concentrations were associated with geochemically reducing conditions. This could mean that natural aquifer or stream sediment materials were a primary contributor of the elevated SRP observed in this study.

Nutrients and toxic contaminants in shallow groundwater along Lake Simcoe urban shorelines

Abstract Poor water quality associated with eutrophication, attributed largely to phosphorus loading, and toxic contaminants has impaired Lake Simcoe’s ecological health. We investigated the potential influence of urban groundwater contamination on lake water quality using a screening approach. In 2010, shallow groundwater (25–120 cm below the lake bed) was collected at 10–15 m spacing along 2 sections of shoreline (about 1 km each) in Barrie, Ontario. Occurrences of elevated groundwater nutrients were widespread, with 24% of samples having soluble reactive phosphorus (SRP) concentrations >0.1 mg L−1 (hypereutrophic water and ≥10 times lake water SRP). Groundwater nitrate or ammonium concentrations were above that of the lake along most of the shoreline sections. Along the north and south shores, 37 and 65%, respectively, of the samples had at least one contaminant over its Canadian aquatic life toxicity guideline, while 11 and 17%, respectively, had 2 or more over guidelines. Exceedances detected at multiple locations included ammonium, nitrate, chloride, chlorinated solvent and petroleum compounds, cadmium, and selenium. In 2011, samples were collected at the same depth range as in 2010 and, additionally, at 5 cm depth (benthic zone) at 22 locations along these same sections. Concentrations of the key contaminants were mostly similar between the 2 sampling depths, suggesting the 2010 results are generally applicable to benthic organism toxicity and nutrient loading to the sediment interface, although fluxes were not determined. Concentrations of SRP, ammonium, and various other contaminants were likely controlled by groundwater redox conditions. Potential sources of key contaminants are also discussed.