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.

Leachate-contaminated groundwater from historical municipal landfills, typically lacking engineered liners and leachate collection systems, poses a threat to nearby urban streams, particularly to benthic ecosystems. Effective monitoring and assessment of such sites requires understanding of the spatial patterns (i.e., two-dimensional footprint) of contaminated groundwater discharge and associated controlling factors. However, discharges from groundwater contaminated by modern wastewater can complicate site assessments. The objectives of this study were to (1) demonstrate the use of artificial sweeteners (AS): saccharin (SAC), cyclamate (CYC), acesulfame (ACE), and sucralose (SUC), to distinguish groundwater discharge areas influenced by historic landfill leachate (elevated SAC and sometimes CYC; low ACE and SUC concentrations) from those influenced by wastewater (high ACE and SUC concentrations), and (2) investigate contaminant discharge patterns for two gaining urban stream reaches adjacent historic landfills at base flows. Contaminant discharge patterns revealed by the AS were strongly controlled by hyporheic flow (low AS concentrations), particularly for the straight reach, and stream sinuosity, particularly for the meandering reach. These patterns were different and the contaminant footprint coverage (<25% of streambed area) much less than most past studies (typically >50% coverage), likely due to the homogeneous streambed-aquifer conditions and shallow, narrow landfill plume in this setting.

Many types of contaminants of emerging concern (CECs), including per- and poly-fluoroalkyl substances (PFAS), have been found in leachate of operating municipal landfills. However, there is only limited information on CECs presence in leachate of historic landfills (≥3 decades since closure, often lacking engineered liners or leachate collection systems) at concentrations that may pose a risk to nearby wells and surface water ecosystems. In this study, 48 samples of leachate-impacted groundwater were collected from 20 historic landfills in Ontario, Canada. The CECs measured included artificial sweeteners (ASs), PFAS, organophosphate esters (OPE), pharmaceuticals, bisphenols, sulfamic acid, perchlorate, and substituted phenols. The common presence of the AS saccharin, a known indicator of old landfill leachate, combined with mostly negligible levels of the AS acesulfame, an indicator of modern wastewater, revealed that most samples were strongly influenced by leachate and not cross-contaminated by wastewater (which can contain these same CECs). Several landfills, including ones closed in the 1960s, had total PFAS concentrations similar to those previously measured at modern landfills, with a maximum observed here of 12.7 μg/L. Notably elevated concentrations of several OPE, sulfamic acid, cotinine, and bisphenols A and S were found at many 30-60 year-old landfills. There was little indication of declining concentrations with landfill age, suggesting historic landfills can be long-term sources of CECs to groundwater and that certain CECs may be useful tracers for historic landfill leachate. These findings provide guidance on which CECs may require monitoring at historic landfill sites and wastewater treatment plants receiving their effluent.

The use of ethanol in alternative fuels has led to contamination of groundwater with high concentrations of this easily biodegradable organic compound. Previous laboratory and field studies have shown vigorous biodegradation of ethanol plumes, with prevalence of reducing conditions and methanogenesis. The objective of this study was to further our understanding of the dynamic biogeochemistry processes, especially dissolved gas production, that may occur in developing and aging plume cores at sites with ethanol or other organic contamination of groundwater. The experiment performed involved highly-detailed spatial and temporal monitoring of ethanol biodegradation in a 2-dimensional (175cm high×525cm long) sand aquifer tank for 330days, with a vertical shift in plume position and increased nutrient inputs occurring at ~Day 100. Rapid onset of fermentation, denitrification, sulphate-reduction and iron(III)-reduction occurred following dissolved ethanol addition, with the eventual widespread development of methanogenesis. The detailed observations also demonstrate a redox zonation that supports the plume fringe concept, secondary reactions resulting from a changing/moving plume, and time lags for the various biodegradation processes. Additional highlights include: i) the highest dissolved H2 concentrations yet reported for groundwater, possibly linked to vigorous fermentation in the absence of common terminal electron-acceptors (i.e., dissolved oxygen, nitrate, and sulphate, and iron(III)-minerals) and methanogenesis; ii) evidence of phosphorus nutrient limitation, which stalled ethanol biodegradation and perhaps delayed the onset of methanogenesis; and iii) the occurrence of dissimilatory nitrate reduction to ammonium, which has not been reported for ethanol biodegradation to date.

The presence of surfactants in the unsaturated zone is one of the scenarios where the flow depends on the solute concentration. The simulation of flow and transport of surfactants requires the coupling of flow and transport, where the surfactant concentration-dependent changes to water retention (i.e. changes in surface tension and/or contact angle) and hydraulic conductivity need to be considered. This research presents a numerical experiment that compares surfactant flow and transport under intermittent boundary conditions to that of a conservative tracer. A modified version of Hydrus 2D, with the concentration-dependent surface tension and viscosity effects, is used. The numerical experiment examines the effect of surfactant infiltration, redistribution and flushing in a low-water content unsaturated porous medium with a deep water table. The numerical experiment encompasses not only the hysteresis as a result of transient localised drainage and rewetting associated with the surfactant-induced pressure gradients, and the intermittent boundary conditions. The comparison of surfactant flow to that of a conservative tracer indicates significant differences for the same boundary conditions. The results from this experiment will be beneficial for the design and optimisation of infiltration galleries employing surfactants for soil and ground water remediation, as well as for the understanding the flow and transport of grey water in the unsaturated zone.

Groundwater contamination from ethanol (e.g., alternative fuels) can support vigorous biodegradation, with many possible reactions producing dissolved gases. The objective of this study was to improve the understanding of the development and evolution of trapped gas phase changes occurring within an ethanol plume undergoing biodegradation. The experiment performed involved highly detailed spatial and temporal monitoring of gas phase saturations using Time Domain Reflectometry probes embedded in a 2-dimensional (175 cm high × 525 cm long) synthetic aquifer (homogeneous sand tank with horizontal groundwater flow). Ethanol injection immediately promoted gas-producing reactions, including: fermentation, denitrification, sulphate-reduction and iron(III)-reduction, with methanogenesis developing between 69 and 109 days. Substantial in situ increases in trapped gas were observed over ~330 days, with maximum gas saturations reaching 27% of the pore volume. Despite sustained gas production, this maximum was never exceeded, likely due to the onset of gas phase mobilization (i.e., ebullition) upon reaching a buoyancy-capillarity threshold. Reductions in the quasi-saturated hydraulic conductivity, resulting from the gas phase accumulation, were restricted by ebullition to a factor of ≤2; but still appeared to alter the groundwater flow field. Overall, trapped gas saturations exhibited high spatial and temporal variability, including declines within the plume and increases outside of the plume. Influential factors included vertically-shifting ethanol inputs and resultant secondary redox reactions, microbial controls on redox zonation, ebullition, and altered groundwater flows. These observations have implications for the transport of gases and volatile compounds within plumes and above the water table at sites with groundwater contamination from ethanol or other highly degradable organics.

The total dissolved gas pressure (P-TDG) probe has been used in groundwater studies for over a decade, but rarely in assessing contaminant degradation, despite the many degradation reactions that produce or consume dissolved gases. Here we present three studies to demonstrate the application of P-TDG measurements to groundwater experiencing contaminant degradation, with discussion of its benefits and limitations. The first study is a pilot-scale laboratory experiment simulating dissolved ethanol contamination of an anaerobic sand aquifer. Continuous monitoring of P-TDG showed the rapid onset of microbial hydrocarbon degradation via denitrification and fermentation. The subsequent formation of a gas phase was revealed when P-TDG began mimicking the bubbling pressure (P-G*; sum of hydrostatic and atmospheric pressure), fluctuating with atmospheric pressure. Some deviations of P-TDG above P-G* occurred also, which may hold promise for signalling substantial changes in the rate or type of degradation process (here, the onset of methanogenesis). In the second study, synoptic field measurements at a petroleum plume site demonstrated how elevated P-TDG could identify wells with evidence of hydrocarbon degradation (denitrification and/or methanogenesis). And finally, combined field measurements of dissolved oxygen (DO) and P-TDG in monitoring wells of a nitrate-contaminated aquifer (Abbottsford-Sumas) revealed areas where denitrification was likely occurring. Limitations to P-TDG use identified in these studies included the masking of degradation processes by the presence of a gas phase, as when trapped following water table fluctuations or formed from rigorous degradation reactions, and confounded assessment of P-TDG patterns from other natural or anthropogenic processes that can also influence groundwater P-TDG.

Past studies of entrapped air dissolution have focused on one-dimensional laboratory columns. Here the multidimensional nature of entrapped air dissolution was investigated using an indoor tank (180x240x600cm(3)) simulating an unconfined sand aquifer with horizontal flow. Time domain reflectometry (TDR) probes directly measured entrapped air contents, while dissolved gas conditions were monitored with total dissolved gas pressure (P-TDG) probes. Dissolution occurred as a diffuse wedge-shaped front from the inlet downgradient, with preferential dissolution at depth. This pattern was mainly attributed to increased gas solubility, as shown by P-TDG measurements. However, compression of entrapped air at greater depths, captured by TDR and leading to lower quasi-saturated hydraulic conductivities and thus greater velocities, also played a small role. Linear propagation of the dissolution front downgradient was observed at each depth, with both TDR and P-TDG, with increasing rates with depth (e.g, 4.1 to 5.7x slower at 15cm vs. 165cm depth). P-TDG values revealed equilibrium with the entrapped gas initially, being higher at greater depth and fluctuating with the barometric pressure, before declining concurrently with entrapped air contents to the lower P-TDG of the source water. The observed dissolution pattern has long-term implications for a wide variety of groundwater management issues, from recharge to contaminant transport and remediation strategies, due to the persistence of entrapped air near the water table (potential timescale of years). This study also demonstrated the utility of P-TDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.

Elevated levels of nutrients, especially phosphorus, in urban streams can lead to eutrophication and general degradation of stream water quality. Contributions of phosphorus from groundwater have typically been assumed minor, though elevated concentrations have been associated with riparian areas and urban settings. The objective of this study was to investigate the importance of groundwater as a pathway for phosphorus and nitrogen input to a gaining urban stream. The stream at the 28-m study reach was 3-5 m wide and straight, flowing generally eastward, with a relatively smooth bottom of predominantly sand, with some areas of finer sediments and a few boulders. Temperature-based methods were used to estimate the groundwater flux distribution. Detailed concentration distributions in discharging groundwater were mapped using in-stream piezometers and diffusion-based peepers, and showed elevated levels of soluble reactive phosphorus (SRP) and ammonium compared to the stream (while nitrate levels were lower), especially along the south bank, where groundwater fluxes were lower and geochemically reducing conditions dominated. Field evidence suggests the ammonium may originate from nearby land-fills, but that local sediments likely contribute the SRP. Ammonium and SRP mass discharges with groundwater were then estimated as the product of the respective concentration distributions and the groundwater flux distribution. These were determined as approximately 9 and 200 g d(-1) for SRP and ammonium, respectively, which compares to stream mass discharges over the observed range of base flows of 20-1100 and 270-7600 g d(-1), respectively. This suggests that groundwater from this small reach, and any similar areas along Dyment's Creek, has the potential to contribute substantially to the stream nutrient concentrations. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.

Dynamic soil water repellency is a pending challenge in water repellency research. The dynamic change or temporal dependence of repellency is commonly expressed as the persistence of repellency. Persistence, or dynamic changes in contact angle, are however, difficult to directly measure and incorporate into mechanistic conceptual and numerical models. To provide insight into the mechanistic nature of infiltration in variably repellent porous media over larger spatial and temporal scales than afforded by commonly applied characterization approaches (i.e. drop tests), this study reports upon observations made during in situ 3D tension infiltration experiments conducted at a post-wildfire site. Tension infiltration tests have proven to be uniquely sensitive to changes in repellency over time. Tension infiltration experiments using mini-disk infiltrometers were conducted. Drop tests provided initial measures of repellency. Tension infiltration experiments were used to generate insights on longer term infiltration behaviours using water, ethanol, and aqueous-ethanol solutions. Molarity of Ethanol Drop (MED) - derived aqueous ethanol solutions (of 5%, 25% and 50% ethanol concentration) were used as intermediate infiltration fluids to generate greater insight into the transitional behaviours between repellent and apparently wettable infiltration. Early time infiltration rates are not reliable indicators of longer term infiltration rates. However, relating the two measures was informative in characterising repellency across materials and at different sites, while preserving temporal differences in fluid behaviours. Comparison of the late-time infiltration rates of aqueous solutions of varying ethanol concentrations proved a useful indicator of repellency and fractional wettability effects. (C) 2014 Elsevier B.V. All rights reserved.

The changing nature of soil water repellency and its apparent affect on hydrologic processes in the vadose zone are regularly reported upon, yet the fundamental mechanisms governing these changes over time and how they are manifested during infiltration are poorly understood. To address this knowledge gap, we investigate dynamic soil water repellency and report upon the concerted roles of contact angle dynamics and fractional wettability in post-wildfire materials collected approximately 1.5years post-fire. A term commonly used in the multiphase flow literature, fractional wettability describes proportional spatial variability in contact angles in bulk media. Twenty two laboratory column experiments were used to measure 1D infiltration rates and moisture contents over time. Tension infiltrometers were used to mechanically maintain a negative pressure throughout water repellent layers. Those data were analysed in conjunction with advancing contact angle data to better understand infiltration processes in these materials and the propensity for them to change with time. The hydrophobic materials expressing low fractional wettability experienced the longest delays in infiltration which we attribute to slow contact angle dynamics. Materials with greater fractional wettability expressed faster rates of contact angle change through much shorter delays in infiltration and unique infiltration rate behaviours. These findings were used to develop a conceptual model that integrates contact angle dynamics and fractional wettability and explains their combined effect on soil-water processes in complex water repellent media.

Water table fluctuations continuously introduce entrapped air bubbles into the otherwise saturated capillary fringe and groundwater zone, which reduces the effective (quasi-saturated) hydraulic conductivity, Kquasi, thus impacting groundwater flow, aquifer recharge and solute and contaminant transport. These entrapped gases will be susceptible to compression or expansion with changes in water pressure, as would be expected with water table (and barometric pressure) fluctuations. Here we undertake laboratory experiments using sand-packed columns to quantify the effect of water table changes of up to 250 cm on the entrapped gas content and the quasi-saturated hydraulic conductivity, and discuss our ability to account for these mechanisms in ground water models. Initial entrapped air contents ranged between 0.080 and 0.158, with a corresponding Kquasi ranging between 2 and 6 times lower compared to the Ks value. The application of 250 cm of water pressure caused an 18% to 26% reduction in the entrapped air content, resulting in an increase in Kquasi by 1.16 to 1.57 times compared to its initial (0 cm water pressure) value. The change in entrapped air content measured at pressure step intervals of 50 cm, was essentially linear, and could be modeled according to the ideal gas law. Meanwhile, the changes in Kquasi with compressionexpansion of the bubbles because of pressure changes could be adequately captured with several current hydraulic conductivity models.

The effect of ionic strength of soil water on surfactant-induced unsaturated flow is investigated for the anionic surfactant DOWFAX 8390. Instrumented infiltration experiments were performed to study the impact of surfactant and ionic strength of the soil-water solution on unsaturated flow. A solution having a measured surface tension of 68 mN/m was prepared with 0.014% w/w DOWFAX using distilled water and applied to the soil column instrumented with time domain reflectometry probes and pressure transducer equipped tensiometers. Results of the experiment were compared with those of pure water to investigate surfactant-induced flow and the corresponding influence of the ionic strength of soil water. The DOWFAX solution behaved like a solution having a surface tension of 49 mN/m, which was directly attributed to the interaction between the surfactant and soil water. Soil-moisture characteristic curves for soil wetted with water and DOWFAX were also measured, and a scaling relationship as a function of reduced surface tension is proposed.

Soil water repellency is a near surface phenomenon that has received much attention in recent years, most notably for its adverse effects on vadose zone processes. In this paper we evaluated the wettability of fire-affected soil materials collected 1.5years post-fire using contact angles. Contact angles were collected using Axisymmetric Drop Shape Analysis (ADSA) which facilitated the capture of a large number of measures. This relatively large dataset provided insight into the nature of contact angle dynamics and fractional wettability in these materials. While both fractional wettability and contact angle dynamics are important properties in water repellent soils, most studies do not directly report upon them, particularly in natural materials. ADSA proved to be a valuable tool in the reliable characterization of advancing contact angles and wetting behaviour at relatively small scales (mm). The majority of sample materials exhibited systematic contact angle dynamics over the duration of testing (approx. 3min). Over this interval, these materials commonly expressed substantial changes in wettability (i.e. from non-wetting to wetting). Advancing contact angles varied between 156° and 30°. Distinctive differences between surface (0cm depth) and near surface (2cm depth) materials were also observed. While the materials investigated here are highly complex in nature, ADSA was able to deconvolute competing variables and/or mechanisms such that a more complete and representative measure of hydrophobicity was determined.

The spontaneous expansion and mobilization of discontinuous gas above dense non-aqueous-phase liquid (DNAPL) pools can affect the aqueous-phase concentrations of the DNAPL constituents above the pool. The results of an intermediate-scale, two-dimensional flow cell experiment showed that the discontinuous gas flow produced by spontaneous expansion, driven by the partitioning of 1,1,1-TCA from the surface of a DNAPL pool, resulted in detectable aqueous-phase concentrations of 1,1,1-TCA well above the pool surface. In comparison to a conventional model for DNAPL pool dissolution in the absence of a discontinuous gas phase, these concentrations were greater than expected, and were present at greater than expected elevations. Additionally, this study showed that the discontinuous gas flow produced transient behavior in the aqueous-phase concentrations, where the elevated concentrations occurred as short-term, pulse-like events. These results suggest that the spontaneous expansion and mobilization of discontinuous gas in DNAPL source zones could lead to the misdiagnosis of source zone architecture using aqueous concentration data, and that the transient nature of the elevated concentrations could further complicate the difficult task of source zone characterization.

The partitioning of volatile non-aqueous phase liquid (NAPL) compounds to a discontinuous gas phase can result in the expansion of that gas phase, and the resulting gas flow can significantly affect the mass transfer from NAPL source zones. This recently reported gas flow generated by the spontaneous expansion of a discontinuous gas phase has not been extensively characterized in the literature. This study measured the expansion rate of a single gas cluster in a 1.1mm sand above a pool of trans-1,2-dichloroethene (tDCE) in small-scale flow cell experiments. To characterize the gas flow, gas injection experiments in three sizes of sand were conducted at very slow injection rates typical of gas flow rates produced by gas expansion due to NAPL partitioning. Gas cluster spontaneous expansion rates above a tDCE pool were found to be 0.34±0.02 and 0.29±0.01mL/day in duplicate experiments, which is sufficiently slow to result in discontinuous gas flow in porous media with a grain size diameter greater than 0.02mm. Measured capillary pressures during gas injection showed patterns consistent with discontinuous gas flow, and identified multiple fragmentation events and expansion by coalescence with trapped clusters. The combination of pressure data and light transmission images were used to identify fragmentation and obtain direct measurements of the critical cluster length (i.e. the length at which withdrawal of the gas phase from a pore space occurs) in quasi-two-dimensional porous media for the first time. The measured critical cluster lengths were 1.4–3.6, 3.2–6.0 and 2.8–6.5cm in 1.1, 0.7 and 0.5mm sands, respectively. These values agreed well with estimates of the critical cluster length made using previously reported equations, and parameters derived from the medium’s capillary pressure-saturation relationship.