Interactions between groundwater and surface water (GW-SW) strongly influence streamflow dynamics and solute transport, especially in urban catchments where discharging groundwater can be a notable source of contaminants, such as chloride (Cl-). Across the Great Lakes Basin, extensive road salt use has led to chronically high Cl- concentrations in groundwater, raising concerns about its long-term contribution to surface water salinization. Understanding the spatiotemporal dynamics of GW-SW exchange is essential for evaluating Cltransport and its impacts on urban water quality. This study examines GW-SW interactions and Cl- dynamics in an urban stream in southern Ontario, Canada, over a 2-year field campaign (May 2022 - April 2024), aiming to characterize exchange variability, assess groundwater's role in shaping in-stream Cl- concentrations, and evaluate local hydrogeologic controls. A multimethod approach was used, including streambed temperature mapping and vertical profiles, seepage meters, hydraulic gradient measurements, and geochemical sampling (delta 18O, delta 2H, and Cl-) to assess exchange dynamics and relative groundwater contributions. Results revealed spatial variability in groundwater discharge, with persistent upward flow confirmed by temperature-based methods and Darcy's Law. Concentrations of Cl- in groundwater consistently exceeded aquatic life guidelines (120-640 mg/L), particularly in the streambed drive-points, indicating persistent Clloading to surface water, even outside of winter months. These findings highlight the importance of groundwater as a long-term Cl- source in urban streams and demonstrate the value of an integrated approach for evaluating GW-SW interactions in cold-region watersheds.

Chloride (Cl) concentrations in urban surface waters and groundwaters in temperate climate regions have been rising, with this primarily attributed to the application of road salt. While this may be the case in many locations, other anthropogenic sources of Cl, such as wastewater, landfill leachate, softened water, and fertilizers, may play an important role but are often overlooked. Natural Cl sources must not be overlooked either. Proper identification of the chloride source is necessary to guide potential mitigation measures. This study demonstrates and assesses a unique suite of five techniques that have been used to characterize Cl sources affecting freshwater systems, including established and more novel methods—temporal Cl concentration patterns (including continuous specific conductance and water pressure monitoring), Cl/Na ratios, Cl/Br ratios, water isotopes, and artificial sweeteners. These were applied to an urban site with a variety of groundwater and river/stream end members, with additional groundwater—surface water interactions between them. Together, these five techniques identified influences at the site of several chloride sources, including road salt, wastewater, landfill leachate, and natural deeper groundwater. Moreover, spatial and temporal patterns in their influence were revealed. No one method was successful at clearly identifying the salt sources impacting a given water or location, especially across the entire year. The methods were further assessed on the benefits and limitations of the assessment they provide, including relative cost, to aid fellow practitioners in selection of source characterization methods.

Freshwater salinization from elevated chloride (Cl) concentrations is a major threat to aquatic ecosystems in cold climate urban areas. Rising Cl levels in urban streams in summer suggest groundwater discharge is an important and increasing contributor, yet its role is poorly quantified. This study examines the influence of groundwater discharge on spatial and seasonal variations in Cl concentrations in three urban streams. Two approaches are used: (1) analysis of Cl concentration-discharge (C-Q) data over a 24-month period, and (2) assessment of longitudinal stream Cl and radon-222 inferred groundwater discharge patterns across seasons and flow conditions. Negative C-Q relationships in summer suggest groundwater is likely the dominant source of Cl, while flatter relationships during winter indicate decreased groundwater influence. Longitudinal data reveal that local groundwater contributions and surrounding land use changes can cause high spatial and temporal variation in Cl concentrations in small streams. For instance, stream Cl concentrations increased where high groundwater discharge coincided with urban land use, in contrast to declines observed in forested areas. Dilution in larger streams lessened the impact of groundwater discharge on stream Cl levels with substantial increases observed in an urbanized high groundwater discharge area in a small stream (>800 mg/L), but only minor increases observed in a larger stream (<30 mg/L). The findings indicate that sampling only at sub-watershed outlets may miss localized hotspots, potentially underestimating Cl contamination risks. More detailed spatial and temporal monitoring is essential to properly assess and manage urban freshwater salinization.

Abstract Winter deicing salt use has increased freshwater salinization, with groundwater-surface water interactions exposing streambed sediments to large salinity fluctuations. This study examines how salinity fluctuations mobilize metals (Al, Cr, Cu, Fe, Ni, Pb, Zn), arsenic (As), and phosphorus (P) from streambed sediments. We conducted column experiments, exposing sediments to increasing (salting) and decreasing (freshening) NaCl concentrations. Across four scenarios with varying salinity changes and sediment sources, most elements were mobilized, with the greatest releases occurring during freshening. Rapid decreases in porewater salt concentration triggered significant increases in soluble reactive phosphorus (SRP) and dissolved Al, As, Cr, Cu, Fe, Ni, Pb, and Zn, with many exceeding Canadian aquatic life guidelines. Total metal concentrations exceeded dissolved concentrations by more than 3-fold, which together with turbidity and exchangeable sodium percentage (ESP) data suggest colloid dispersion was the dominant mobilization mechanism during freshening. In contrast, total P and As were similar to SRP and dissolved As concentrations, with data indicating that their mobilization may be due to pH-driven desorption. Mobilization decreased with smaller salinity changes but increased with higher sediment contamination. These findings demonstrate that porewater salinity fluctuations can generate hot moments of contaminant release, particularly during freshening, compounding the water-quality impacts of freshwater salinization.

This study provides new field-based evidence of the physical and socioeconomic watershed factors and streamflow conditions that influence effluent inputs to streams from onsite wastewater treatment systems (OWTSs), including potential differences between inputs via slow (groundwater) and more rapid (subsurface preferential, overland, direct pipe) transport pathways. Stream sampling data were compiled for 46 watersheds in Ontario, Canada, with analyses including a conservative chemical tracer (acesulfame) representing all (slow and rapid) pathways and a nonconservative human fecal bacteria tracer (HF183) representing only rapid pathways. Acesulfame stream concentrations ranged from tens to over 1000 ng/L, indicating OWTS effluent inputs to streams are widespread. Additionally, HF183 was detected in >20% of stream samples, indicating the prevalence of rapid pathways. Linear mixed-effects models indicate that the percentage of OWTS effluent reaching streams, based on acesulfame data, was higher under high flow conditions and in watersheds with older houses, more houses within 200 m of a stream, and a lower topographic wetness index. Higher human fecal contamination, based on HF183 detections, was observed in streams that drain watersheds with high OWTS density, more houses within 200 m of a stream, and a higher topographic wetness index. These findings support improved pollutant load predictions and better targeting of watersheds for OWTS management.

Study region: Canadian Laurentian Great Lakes Basin. Study focus: Study examines the potential for stream pH changes to modulate the storage of phosphorus (P) in streambed sediments across multiple temporal scales by (i) synthesizing experimental data that quantify the relationships between pH and sediment P exchange, and (ii) comparing these relationships to measured long-term, seasonal, diel and event-based pH changes observed in streams. New hydrological insights for region: Synthesis of experimental data indicate that P exchange is highly sensitive to pH changes over the pH range typically observed in Ontario streams (pH >7). Of 157 monitored streams, 84 % experienced significant pH increases over the last 35 years, averaging + 0.24 pH change, and favouring long-term P release from streambed sediments or limiting P retention. Seasonally, most streams exhibited higher pH during summer, suggesting P release from sediments at times of high-productivity. Over shorter timescales, a subset of streams demonstrated diel pH fluctuations of > 1 pH unit in summer, and rapid pH changes (up to 1.7 pH units) during precipitation and snowmelt events. pH changes of this magnitude may drive rapid and extensive P exchange from streambed sediment. Overall, our results indicate pH changes may be an under-appreciated control on the transient P storage within streams, with relevance at multiple temporal scales. This finding has implications for P load predictions by mechanistic watershed nutrient models.

Septic systems are designed to treat and release partially treated wastewater into the subsurface. In doing so they release various pollutants including nutrients, pathogens, and emerging contaminants into the environment. The objectives of this study are to (1) evaluate whether the amount of septic wastewater effluent reaching streams varies seasonally and between high and low stream flow conditions, and (2) assess the utility of using the artificial sweetener acesulfame combined with the human-specific bacterial DNA marker HF183 to infer the dominant pathways in delivering septic effluent to streams. Repeated sampling of streams in 15 subwatersheds was conducted together with detailed high-resolution longitudinal sampling along one stream. The percentage of septic effluent reaching streams was found to be highly variable between subwatersheds with septic effluent inputs greater in spring compared to other seasons, and also greater during high stream flow conditions. Stream HF183 concentrations were not correlated with acesulfame concentrations with data indicating that rapid pathways may be more important for contributing septic effluent to streams in some subwatersheds compared to others. Finally, high-resolution survey data showed that spatial trends in acesulfame and HF183 concentrations along the surveyed stream were not consistent and therefore pathways contributing effluent may vary longitudinally. The study findings are needed to improve estimates of pollutant loads to streams from septic systems and to inform septic system best management practices.

Assessments of elevated stream chloride (Cl) concentrations ([Cl]), predominantly sourced from winter application of road deicers across snow belt regions, are starting to use high-frequency data, more so in the United States (U.S.) than in Canada. Here, [Cl] was derived from high-frequency specific conductance (SC) measurements from nine streams draining urbanized subwatersheds around Hamilton, Ontario, Canada, between May 2020 and April 2021. We assess [Cl] dynamics to understand dominant transport pathways and characterize water quality guideline exceedances to assess ecological risk while comparing Canadian and U.S. methodologies. These streams exhibited an alarming extent of high [Cl] as six streams exceeded the Canadian short-term guideline >90% of both the salting and non-salting seasons. High-frequency stream [Cl] revealed Cl-impacted groundwater maintaining baseflow [Cl], while fast pathways (e.g., sewers) drive [Cl] pulses in the salting season and episodic dilutions in the non-salting season. Application of the higher U.S. guideline gave consistently lower exceedances. Its application of rolling averages to high-frequency data also obscures episodic dilutions that reduce [Cl] below guideline thresholds and may provide brief intervals of refuge to organisms. High-frequency data provided insight into Cl pathways and ecological risk, though exceedance results are sensitive to the guideline methodology.

High phosphorus (P) loads to the Laurentian Great Lakes contribute to eutrophication, harmful algal blooms, and hypoxia. Groundwater impacted by wastewater effluent from household septic systems, common in coastal communities, is a recognized P source to nearby lakes. However, the long-term impact of neighbourhood-scale septic system decommissioning (i.e., conversion to sewer connections) on this P loading is not well understood or quantified. The objective of this study was to investigate long-term P loading to Nottawasaga Bay from groundwater plumes of decommissioned septic systems in the coastal community of Wasaga Beach. Detailed groundwater sampling characterized a legacy P plume from a septic system decommissioned 35 years ago, revealing elevated soluble reactive phosphorus concentrations extending over 40 m and reaching the shoreline. Sorption and dispersion parameters required for neighbourhood-scale modeling were derived by simulating this persistent, long but thin P plume using a numerical model. Numerical simulations of P plumes from > 800 septic systems, 0.01-1.6 km from the shoreline and active < 65 years before decommissioning, revealed P mass discharge to the lake started after similar to 30 years and will continue for > 4000 years. Relatedly, the extended P transport meant the annual mass discharge rate to the lake was consistently < 1.3 % of the annual mass input rate from septic systems to the aquifer, though it varied over time according to the septic systems' distance from shore. These findings highlight the long-term view required in accounting for decommissioned septic systems in P management strategies to protect lake water quality.

Phosphorus (P) that accumulates in agricultural riparian zones can be released under certain hydrological and biogeochemical conditions, thereby limiting the effectiveness of these zones in reducing P loads from field to stream. The study objective was to explore factors that may be contributing to, or limiting, high soluble reactive phosphorus (SRP) concentrations in the shallow aquifer of an alluvial upland riparian zone located in a continental climate. Field investigations including porewater sampling from six vertical nests, soil sampling, and continuous soil moisture, groundwater table, and redox measurements were conducted over 19 months. Porewater SRP concentrations were generally low in the aquifer considering all sampling times (median = 14.7 µg/L; interquartile range [IQR] = 11.1 µg/L, 287 samples). The overall low SRP may be due to low reducible labile soil P (median = 21.1 µgP/g dw, IQR = 10.9 µgP/g dw, 21 samples). However, high SRP concentrations (>52 µg/L, 95% quartile) did occur intermittently in space and time with no clear spatial or temporal patterns. Analyses indicate that most high concentrations were likely not associated with factors previously reported to influence SRP release in riparian aquifers, including redox conditions, pH, and soil drying and wetting. Further, data indicate that internally released or externally supplied SRP may undergo rapid (re-) sequestration within the aquifer, limiting its vertical or horizontal transport. The study findings highlight the complexity of P behavior in riparian zones and the need for caution when assessing the effectiveness of conservation practices and in interpreting potential impacts of subsurface water quality on stream water quality when monitoring locations are distant from the stream.

Groundwater transport of chloride (Cl) containing road salt deicers is an important contributor to salinization of fresh surface waters in temperate climates. While mass loading of salt to streams via groundwater has received greater recognition lately, only a few studies have demonstrated the unique risk posed by the direct discharge of salt-laden groundwater to aquatic life residing in the benthic zone (e.g., macroinvertebrates, mussels, plant roots, fish eggs). These studies revealed high Cl concentrations in stream porewater but provided limited information on its spatio-temporal variability and factors influencing it. To address this gap, this study conducted a detailed 2-year field investigation on 100-m scale reaches of two streams adjacent to salt-receiving roads in London, Ontario, applying year-round streambed porewater sampling, streambed surface electrical conductivity mapping, and electrical imaging (geophysics) techniques. Areas of probable groundwater upwelling and surface water downwelling were identified using streambed temperature mapping and hydraulic head gradients. Benthic porewater Cl concentrations were commonly above water quality guidelines, but also varied substantially over small spatial scales, ranging from <100 to >5000 mg/L over ≤10 m distances. Generally, porewater Cl concentrations were relatively stable seasonally compared to the stream water, leading to benthic areas with year-round exposure to elevated Cl and limiting prospects for benthic zone refugia. Key factors affecting this spatial and temporal variability included proximity to roads, impacts from point sources (snow storage locations), and surface water downwelling (hyporheic flows). These observations have direct implications for the health of urban stream benthic communities and for designing management and remediation strategies.

While in situ toxicity testing with caged organisms has been used to assess surface water and sediment contamination, no successful application to benthic organisms exposed to highly contaminated groundwater plumes discharging to surface waters has been reported. The objective of this study was to demonstrate and evaluate this application using four sets of tests performed at three previously reported contaminated groundwater sites, which include one river site affected by volatile organic contaminant plumes, and two sites, one pond and one small urban stream, impacted by landfill plumes. The study examined multiple cage designs and orientations and two test organisms: an amphipod (Hyalella azteca) and midge larvae (Chironomus riparius; only one study). Cages were deployed for between 5 and 28 days and assessed for organism survival and growth. At all sites and for some deployment conditions, cages exposed to high contaminant concentrations in the plume footprint had greater mortality compared to those exposed to lower or background concentrations. Organism growth was less clear as a metric of toxicity. Vertically oriented cages typically showed high mortality to plume contaminants, but some were also affected by other non-target groundwater conditions (e.g., low dissolved oxygen, other contaminant sources), while horizontally oriented cages were rarely responsive to either groundwater influence. A hybrid cage design showed much promise in its single study. Useful observations on the test organisms and on potentially problematic site conditions were also made. The informed use of in situ toxicity cages could be an additional beneficial tool for groundwater contaminated site assessments.

While it is recognized that groundwater contaminant plumes can impact surface waters, there remains little information on the magnitude, spatial extent, and especially temporal variability of the resulting exposure to the variety of aquatic organisms, particularly for stagnant surface waters (e.g., ponds). The present study of a historic landfill plume discharging to a pond investigated contaminant exposure to multiple aquatic zones (endobenthic, epibenthic, pelagic) over approximately 1 year within a temperate climate. Landfill tracers included the artificial sweetener saccharin, ammonium, chloride, and specific conductance. Sampling of pond sediment porewater (upwelling groundwater) and continuous geophysical imaging of the subsurface showed a relatively stable plume footprint covering approximately 26% of the pond, although with spatially varying leachate composition, revealing year‐round exposure to endobenthic (within sediments) organisms. Substantial and variable contaminant exposure to epibenthic organisms within the plume footprint was shown by elevated specific conductance measured directly above the sediment interface. Exposure varied daily at times and increased through winter to values representing undiluted plume groundwater. Exposure to pelagic organisms (overlying water) covered a larger area (~50%) due to in‐pond circulation. The stream outlet concentrations were stable at approximately 10 times dilution for chloride and saccharin, but were substantially less in summer for ammonium due to in‐pond processes. Whereas groundwater contaminants are typically assumed elevated at base flows, the outlet stream contaminant mass discharges to downstream receptors were notably higher in winter than summer, following stream flow patterns. Insights from the present study into the timings and locations of contaminant plume exposure to multiple ecological zones of a pond can provide guidance to contaminated site and aquatic ecosystem managers on improved monitoring, assessment, and remediation protocols. Environ Toxicol Chem 2023;42:1667–1684. © 2023 His Majesty the King in Right of Canada and The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. Reproduced with the permission of the Minister of Environment and Climate Change Canada.

Salinization of inland fresh surface waters in temperate climates is a growing concern due to increasing salt inputs from sources including chloride (Cl)-containing road salt de-icers, industrial waste, and landfill leachate. Groundwater pathways play an important role in the year-round delivery of Cl to streams, but quantifying this pathway, including spatiotemporal variability and amount of Cl mass stored in the subsurface, is challenging. The objective of this study was to demonstrate, evaluate, and compare the potential applications of the geoelectrical techniques - electromagnetics (EM) and direct current (DC) resistivity - for mapping salt contamination in shallow urban groundwater and characterizing the groundwater pathways delivering Cl to urban streams. EM and DC surveys were conducted (3D mapping and 2D time-lapse) across a 20 m salt-impacted stream section and surrounding riparian zone that is located near an arterial road and parking lot. Groundwater samples and soil cores were also collected to validate the geoelectrical results. Both the EM and DC surveys detected high salt concentrations in the shallow subsurface (up to 3 m depth) near the road, parking lot, and stream; however, DC more accurately represented groundwater Cl concentrations. DC results were used to calculate the total Cl mass in the subsurface, with the spatial mass distribution used to infer the temporal variability in the subsurface salt plume. Finally, time-lapse DC showed that the highest groundwater salt concentrations existed near the stream between June and October - this is expected to contribute to the elevated salt concentrations in the stream during summer months. This study has shown that EM and DC can be useful for identifying groundwater salt concentration, storage, and transport in a non-intrusive and efficient manner, making them valuable field tools for characterizing and quantifying groundwater salt pathways to urban streams.

Stream communities and processes are known to differ among reaches and habitat types in accordance with environmental variation. However, groundwater input has not been well-explored as a driver of ecological heterogeneity among stream reaches and habitats. We assessed stream biofilm communities (biomass and diatom assemblage composition) and cellulose decomposition in run and riffle habitats across three stream reaches with high, moderate, and low groundwater input in Kintore Creek, Ontario, Canada. Algal biomass, as well as density and composition of diatom assemblages, differed between runs and riffles in reaches with moderate and high groundwater inputs, but not in the low groundwater reach. Reaches with moderate and high groundwater input had faster streambed cellulose decomposition in riffles than in runs, whereas the reach with low groundwater input had no difference in streambed cellulose decomposition. Subsurface cellulose decomposition in riffles and runs was fastest in the high groundwater reach. We found that measured environmental variables did not explain the apparent effects of groundwater inputs. Findings from this study highlight the covarying influence of groundwater input and habitat type in altering in stream ecological response in enriched streams.

Streambed sediment and groundwater represent important stores of soluble reactive phosphorus (SRP) in freshwater catchments yet their contributions to watershed SRP loads remain unclear. This study evaluates the spatio-temporal variability and geochemical and hydrologic controls on porewater SRP in the streambed along an agricultural reach, including the influence of groundwater-surface water interactions. High porewater SRP (>1000 mu g/L) and high available sediment-bound SRP were observed in low groundwater discharge zones, specifically in the shallow (<= 0.1 m) streambed sediments. Porewater SRP concentrations were linked to iron and possibly manganese redox cycling with more-reducing conditions consistently coinciding with high SRP. Porewater SRP varied temporally but distinct seasonal changes across the stream reach were not observed. A shallow oxic cap that may limit SRP release to the stream was observed in the very shallow streambed but its presence was spatially and temporally discontinuous. Finally, data indicate P-rich shallow streambed sediments are the likely source of the observed high SRP porewater concentrations in the streambed rather than incoming groundwater from the adjacent fields. Overall, the findings highlight the complexity of SRP dynamics in agricultural streambeds and provide insight into processes governing the timing and location of SRP loading streams.

The role of groundwater as a driver of spatial and temporal heterogeneity of ecological conditions in streams is not well understood, particularly at larger spatial scales. To evaluate the association between groundwater and ecological communities and processes, we assessed stream biofilm communities (biomass and diatom assemblage composition) and cellulose decomposition over four seasons in 19 reaches with varying amounts of groundwater input in a headwater stream network in southern Ontario, Canada. Seasonal variation was found to drive diatom assemblage composition and cellulose decomposition among seasons. Moreover, there were clear seasonal patterns in taxa succession in the diatom assemblages where the dominant taxa shifted in accordance with ecological preferences of the taxa. Within season assessments of stream biofilms among individual reaches showed that biomass, diatom assemblage composition, and cellulose decomposition were not associated with amount of groundwater input. The lack of concordance between groundwater influence and stream biofilm condition suggests that the groundwater signal may have been overwhelmed by surface water influences, such as nutrients and thermal effects, at the reach scale. Our findings also demonstrate the importance of assessing stream biofilm communities and cellulose decomposition in multiple seasons to identify and understand discrete shifts in ecosystem conditions of temperate streams.

Quantifying the contribution of septic systems to contaminant, including nutrient, loading to streams is needed in many watersheds to inform water quality management programs. However, this quantification is challenging due to the distributed locations of septic systems and uncertainties regarding the pathways delivering effluent from septic systems (functioning and failing) to a stream. The objectives of this study were firstly to evaluate how septic effluent inputs to streams vary with stream discharge conditions for multiple subwatersheds with different characteristics (i.e., geology, septic system density, and typical age), and secondly to examine new approaches for distinguishing the pathways and the contributing areas delivering septic effluent to streams. These approaches use the artificial sweetener acesulfame as a conservative tracer for septic effluent in applications of: (i) stream concentration-discharge (C-Q) relationships using low frequency sampling data, (ii) hysteresis behavior in event-based C-Q relationships, and (iii) longitudinal stream sampling. For all nine subwatersheds studied, the amount of septic effluent reaching the subwatershed outlets was considerably higher during high stream discharge (event) conditions compared to low discharge (baseflow) conditions, suggesting pathways other than groundwater may also be important. Generally, the percentage of septic effluent reaching the outlets was less for subwatersheds with newer households compared to those with older households. The combined interpretation of low frequency and event-based C-Q relationships indicate that complex pathways control the delivery of septic effluent to the subwatershed outlets. The interpretations suggest that groundwater pathways may dominate in some subwatersheds, while more rapid pathways associated with failing septic systems (e.g., overland runoff) may be important in others. Finally, longitudinal stream sampling illustrate the potential of acesulfame data to identify key areas contributing septic effluent to the stream. The novel approaches used here can be applied to guide future investigations aiming to quantify and manage water quality impairment from septic systems.

Common approaches for characterizing streambed architecture, and its influence on groundwater-surface water (GW-SW) exchanges, are generally limited by their invasiveness and low spatial sampling density, which is a particular issue in streambeds that typically have high spatial heterogeneity. Combined DC resistivity and induced polarization (DC-IP) imaging can provide rapid, non-invasive and continuous information on streambed lithology; however, its full potential remains unrealized, leading to its underutilization for streambed in-vestigations. The objective of this study is to demonstrate the value of DC-IP imaging, in both 3D and high -resolution, for characterizing streambed architecture and interpretating GW-SW exchange patterns. The study focused on a 50 m long stream reach located in Kintore, Ontario, Canada. Traditional methods - streambed temperature mapping, vertical head gradient measurements, streambed porewater quality, and sediment cores - were used to qualitatively identify spatial GW-SW exchanges. Underwater 3D DC-IP surveying was then con-ducted across the stream reach to obtain high-resolution distributions of resistivity and chargeability. Resistivity first identified three distinct zones along the stream reach: Zone 1 (0-12 m) and Zone 3 (38-50 m) exhibits high resistivity (>100 ohm-m), while Zone 2 (12-38 m) exhibits relatively low resistivity (<40 ohm-m). Chargeability highly complements resistivity by confirming that the shallow streambed contains only non-clayey materials; therefore, the more resistive Zones 1 and 3 is attributed to more permeable coarse sand and gravel, while the more conductive Zone 2 is attributed to less permeable finer sand, and consequently, increased porewater EC due to longer residence times and hyporheic exchanges. This geoelectrical interpretation is well-supported by in-formation from traditional methods (e.g., higher temperature and hydraulic gradients correspond to the more permeable Zone 1 and Zone 3). This study demonstrates the unrealized value of spatially continuous, high -resolution DC-IP information for mapping streambed architecture and its control on GW-SW exchange patterns.

The extraction of surface mined bitumen from oil sands deposits in northern Alberta, Canada produces large quantities of liquid tailings waste, termed oil sands process-affected water (OSPW), which are stored in large tailings ponds. OSPW-derived chemicals from several tailings ponds migrating past containment structures and through groundwater systems pose a concern for surface water contamination. The present study investigated the toxicity of groundwater from near-field sites adjacent to a tailings pond with OPSW influence and far-field sites with only natural oil sands bitumen influence. The acute toxicity of unfractionated groundwater and isolated organic fractions was assessed using a suite of aquatic organisms (Pimephales promelas, Oryzias latipes, Daphnia magna, Hyalella azteca, Lampsilis spp., Ceriodaphnia dubia, Hexagenia spp., and Vibrio fischeri). Assessment of unfractionated groundwater demonstrated toxicity towards all invertebrates in at least one far-field sample, with both near-field and far-field samples with bitumen influence toxic towards P. promelas, while no toxicity was observed for O. latipes. When assessing the unfractionated groundwater and isolated organic fractions from near-field and far-field groundwater sites, P. promelas and H. azteca were the most sensitive to organic components, while D. magna and L. cardium were most sensitive to the inorganic components. Groundwater containing appreciable amounts of dissolved organics exhibited similar toxicities to sensitive species regardless of an OSPW or natural bitumen source. The lack of a clear distinction in relative acute toxicities between near-field and far-field samples indicates that the water-soluble chemicals associated with bitumen are acutely toxic to several aquatic organisms. This result, combined with the similarities in chemical profiles between bitumen-influenced groundwater originating from OSPW and/or natural sources, suggests that the industrial bitumen extraction processes corresponding to the tailings pond in this study are not contributing unique toxic substances to groundwater, relative to natural bitumen compounds present in groundwater flow systems.