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Nitrate is now recognized as a widespread ground water contaminant, which has led to increased efforts to control and mitigate its impacts. This study reports on the long-term performance of four pilot-scale field trials in which reactive porous barriers were used to provide passive in situ treatment of nitrate in ground water. At two of the sites (Killarney and Borden), the reactive barriers were installed as horizontal layers underneath septic system infiltration beds; at a third site (Long Point), a barrier was installed as a vertical wall intercepting a horizontally migrating septic system plume; and at the fourth site (North Campus), a barrier was installed as a containerized subsurface reactor treating farm field drainage water. The reactive media consisted of 15% to 100% by volume of waste cellulose solids (wood mulch, sawdust, leaf compost), which provided a carbon source for heterotrophic denitrification. The field trials have been in semicontinuous operation for six to seven years at hydraulic loading rates ranging from six to 2000 L/day. Trials have been successful in attenuating influent NO3- (or NO3-+ NH4+ at Borden) concentrations averaging from 4.8 mg/L N at North Campus to 57 mg/L N at Killarney, by amounts averaging 80% at Killarney, 74% at Borden, 91 % at Long Point, and 58% at North Campus. Nitrate consumption rates were temperature dependent and ranged from 0.7 to 32 mg L N/day, but did not deteriorate over the monitoring period. Furthermore, mass-balance calculations indicate that carbon consumption by heterotrophic denitrification has so far used only about 2% to 3% of the initial carbon mass in each case. Results suggest that such barriers should be capable of providing NO3- treatment for at least a decade or longer without carbon replenishment. Reactive barriers have now been used to treat nitrate contamination from a variety of sources including septic systems, agricultural runoff, landfill leachate, and industrial operations. This demonstration of successful long-term operation should allow this technology to become more widely considered for nitrate remediation, particularly at sites where passive treatment requiring a minimum of maintenance is desired.

An increasing amount of attention is being focused on the effects of arsenic on health and the factors that control arsenic mobility in groundwater. The release and transport of arsenic below the water table in the current tailings impoundment at Campbell Mine in Balmertown, ON, Canada, are controlled by the reduction of ferric oxide phases within the saturated tailings. The activity of chemolithotrophic bacteria is indicated by enriched δ34S−SO4 ratios, high alkalinity, depleted δ13C−CO3 ratios, and low SO4 concentrations, all of which are consistent with bacterially mediated Fe and SO4 reduction. The concentrations of Fe and As in the tailings porewater increase with depth. This increase is attributed to the bacterially mediated reduction of As-rich hematite and maghemite. The maximum concentrations of Fe and As are 20 and 100 mg/L, respectively. At the bottom of the impoundment, As concentrations decrease sharply to 0.3 mg/L. This decrease in concentration is attributed to reprecipitation of the As as ...

Sulfate minerals participate in a variety of mineral-water reactions with waters containing elevated concentrations of dissolved solids. Reliable prediction of sulfate-mineral solubility in concentrated waters is required for a multitude of geological, hydrogeological, industrial, and meteorological applications. Precipitation of pure sulfate minerals or sulfate-bearing solid solutions can limit concentrations of dissolved metals, radionuclides, and other cations at waste-disposal sites, in marine and other naturally occurring saline waters, in industrial waters, and in smog. Formation of these solids can control the major-ion geochemistry of water and, in turn, influence the behavior of trace constituents. One of the most widely used and robust approaches for predicting mineral solubility in concentrated waters is the Pitzer ion-interaction approach as first introduced by Pitzer (1973), as applied to geological systems by Harvie and Weare (1980) and Harvie et al. (1980), and as summarized by Weare (1987). Clegg and Whitfield (1991) and Pitzer (1979a,b; 1991) provided further comprehensive discussions on the developments and application of the ion-interaction approach to predict mineral solubility in natural waters. This chapter focuses on the application of the ion-interaction approach, or its variations, for predicting sulfate-mineral solubility in a variety of geochemical settings. Included are a summary of the Pitzer approach as applied to predictions of mineral solubility, a summary of the available constants for modeling sulfate-mineral solubility, and examples of applications for natural and contaminated sites. The discussion focuses on recent additions to earlier models developed for application to near-Earth-surface temperature and pressure conditions. Applications outside this temperature and pressure range are described only briefly. ### Compositions of concentrated waters The majority of concentrated waters in nature form as a result of solar evaporation of marine and continentally-derived waters. The evaporation of seawater leads to predictable sequences of mineral formation, including precipitation of several common sulfate minerals (e.g. gypsum, …

Extremely acidic mine waters with pH values as low as −3.6, total dissolved metal concentrations as high as 200 g/L, and sulfate concentrations as high as 760 g/L, have been encountered underground in the Richmond Mine at Iron Mountain, CA. These are the most acidic waters known. The pH measurements were obtained by using the Pitzer method to define pH for calibration of glass membrane electrodes. The calibration of pH below 0.5 with glass membrane electrodes becomes strongly nonlinear but is reproducible to a pH as low as −4. Numerous efflorescent minerals were found forming from these acid waters. These extreme acid waters were formed primarily by pyrite oxidation and concentration by evaporation with minor effects from aqueous ferrous iron oxidation and efflorescent mineral formation.

A permeable reactive barrier, designed to remove metals and generate alkalinity by promoting sulfate reduction and metal sulfide precipitation, was installed in August 1995 into an aquifer containing effluent from mine tailings. Passage of groundwater through the barrier results in striking improvement in water quality. Dramatic changes in concentrations of SO4 (decrease of 2000−3000 mg/L), Fe (decrease of 270−1300 mg/L), trace metals (e.g., Ni decreases 30 mg/L), and alkalinity (increase of 800−2700 mg/L) are observed. Populations of sulfate reducing bacteria are 10 000 times greater, and bacterial activity, as measured by dehydrogenase activity, is 10 times higher within the barrier compared to the up-gradient aquifer. Dissolved sulfide concentrations increase by 0.2−120 mg/L, and the isotope 34S is enriched relative to 32S in the dissolved phase SO42- within the barrier. Water chemistry, coupled with geochemical speciation modeling, indicates the pore water in the barrier becomes supersaturated with re...

Elevated concentrations of nutrients, resulting in prolific algal growth, were reported in some open-water ponds in one of the finest surviving wetlands on the Lower Great Lakes, Point Pelee National Park, Ontario, Canada. A study was undertaken to identify the sources of nutrients to the most impacted pond in the complex, Sanctuary Pond. Sediment chemistry and chemistry of interstitial water were examined to determine the role of benthic sediments in nutrient dynamics. The results indicate high concentrations of dissolved nutrients (about 4 mg/l of P and over 20 mg/l of N) in sediment porewater, a consequence of organic matter decomposition. No spatial differences were observed between porewater profiles, nor in the levels of dissolved constituents. Likewise, sediment P concentrations were similar at both investigated sites. Thermodynamic calculations, carried out to determine mineral equilibria of phosphate minerals potentially controlling the concentrations of dissolved P in sediment porewater, suggest that P generated from organic matter decomposition should be reacting with Fe, Ca and Al to form authigenic mineral phases vivianite, hydroxyapatite and variscite. The high levels of nutrients in porewater, the lack of spatial differences in sediment and porewater nutrient concentrations and the absence of known external nutrients sources suggest that at the Sanctuary Pond internal regeneration of nutrients from sediments is responsible for the hypereutrophic conditions in the pond.

Phosphate distribution was reviewed in 10 mature, highly monitored septic system ground water plumes in central Canada. It was shown that six plumes (primarily those on calcareous sands) of enriched P concentrations (0.5 to 5 mg/L P) exceeding 10 m in length are present. In each case, phosphate migration velocity is highly retarded (retardation factor, 20 to 100) compared to the ground water velocity, hut migration rates remain sufficiently fast (−1 m/a) to the of concern when considering long-term operation and the normal setback distance of septic systems from adjacent surface water bodies (∼ 15 m). Much smaller scale phosphate plumes (< 3 m in length) are present at the acidic sites on noncalcareous sands and on silt- and clay-rich sediments. At all of the sites, ground water concentrations are lower than effluent values by amounts ranging from 23 to 99%, suggesting that P accumulation has occurred in the vadose zone. This was confirmed by sediment analyses at four of the sites which, in each case, showed that zones of Penrichment were present within 1 m of the infiltration pipes (Wood 1993; Zanini et al. 1998). Also, observed phosphate concentrations are generally consistent with values expected based on the solubility constraints of the minerals vivianite in reducing zones (including the septic tank), and strengite and variscite in oxidizing zones, providing further evidence that min-eral precipitation reactions play a role in limiting P concentrations. Strengite and variscite have the potential to limit P to low con-centrations (co.1 mg/L) under acidic conditions, but oxidation of sewage efiluent leads to acidic conditions only in noncalcareous terrain or beneath old septic systems where calcium carbonate has been depleted. Overall, phosphate plume migration velocities in ground water appear to be controlled by sorption processes, but the phosphate concentrations that are present in the plumes appear to be strongly controlled by mineral precipitation reactions that occur in close proximity to the infiltration pipes.

The major ion and trace metal geochemistry of a septic system plume in a shallow sand aquifer was characterized to assess geochemical processes controlling the transport of nutrients and their release to a nearby wetland. The plume was generated from a 16-year-old tile bed, and is more than 60 m long, 40 m wide and 7 m thick. The groundwater pH at the site is near neutral, but up to 0.4 units lower in the plume core as a result of H+ generated from NH3 and DOC oxidation in the unsaturated zone. The plume can be divided into distinct redox zones, which show differences in nutrient mobility. Proximal to the tile bed, there is a shallow suboxic zone, with intermediate Eh values (>400 mV), low concentrations of dissolved oxygen (

Characterization of phosphorus (P) enriched solids was undertaken in the sediments below four mature septic system infiltration beds, where previous monitoring of phosphate (PO4) concentrations in the groundwater had indicated that substantial retention of P was occurring in the vadose zone. At each site, zones of sediment P enrichment were identified by an acid extraction procedure. Acid extractable sediment P concentrations were found to be 2–5 times higher than background values, within narrow discrete zones generally 10–30 cm in thickness, located within one meter of the infiltration pipes. Back scattered electron images of the P enriched zones indicated that the P solids occurred as distinct authigenic grains (up to 300 μm diameter) and as grain coatings. Microprobe analyses indicated predominantly Fe–P in calcareous sediments (Cambridge and Langton) and Al–Fe–P in non-calcareous sediments (Muskoka and Harp Lake). Porewater analyses indicated that the zones of P accumulation were closely associated with zones of redox change characterized by the conversion of effluent NH4+ to NO3−. The data suggests that a substantial amount of the septic derived P is being attenuated by mineral precipitation reactions that occur rapidly after the effluent encounters subsurface sediments. Reductive dissolution of ferric (oxy)hydroxide minerals as a consequence of reducing environments near the infiltrations pipes, the release of Fe2+ in solution and subsequent conversion of Fe2+ to Fe3+ may promote the precipitation of ferric or ferrosoferric PO4 minerals. In sediments with limited buffering capacity (calcite deficient), the decrease in pH resulting from effluent oxidation may cause Al (oxy)hydroxide dissolution and subsequent precipitation of Al–P rich phases. These precipitation reactions appear to have the capacity to immobilize a substantial amount of septic derived P (25–99% at these sites) for a considerable period of time.

Laboratory batch and column studies were conducted to develop permeable reactive mixtures to remove phosphorus from the effluent of onsite wastewater disposal systems. Mixtures can be placed in situ, as horizontal or vertical reactive barriers in sediments receiving wastewater discharge, or can be used in single pass, self-contained treatment modules in alternative treatment systems. Reactive mixtures composed of silica sand, high calcium crushed limestone, and readily available metal oxides were tested to evaluate phosphorus attenuation. Iron/calcium oxides, produced from steel manufacturing, and fine-grained activated aluminum oxide outperformed other oxides tested during batch experiments. These materials removed greater than 99% of PO4 from a 10 mg/L PO4−P solution within 1 h of contact. Long-term attenuation capacities of the mixtures were assessed by continually loading bench-scale columns with a 3.3 mg/L PO4−P solution, at representative groundwater flow rates. A column containing 50 wt % silica sa...

Porous, permeable, geochemically reactive walls, installed in situ in the path of migrating groundwater, present a promising passive treatment alternative for remediating metal-contaminated groundw...

Laboratory tests and field-scale demonstrations indicate that permeable reactive walls, designed to induce bacterially mediated sulfate reduction within aquifers, have the potential to prevent the discharge of acidic, metal-rich waters. Laboratory batch studies were conducted to determine optimal mixtures of organic materials. Column studies were conducted to evaluate the potential for sulfate reduction and metal sulfide precipitation under dynamic flow conditions at groundwater velocities similar to those observed in the field. These laboratory studies established that sulfate reduction and metal sulfide precipitation mechanisms result in decreases in the concentrations of sulfate and iron and other metals. In the column experiments, sulfate and Fe were removed from synthetic mine drainage water at rates of 500-800 mmol/day/m3. In a pilot-scale field study, test cells installed into an aquifer containing a plume of mine waste-impacted groundwater, induced sulfate reduction and metal-sulfide prec...

Permeable-reactive redox walls, placed below the ground surface in the path of flowing groundwater, provide an alternative remediation approach for removing electroactive chemicals from contaminated groundwater. Four types of Fe-bearing solids, siderite [FeCO3], pyrite [FeS2], coarse-grained elemental iron [Fe0], and fine-grained Fe0, were assessed for their ability to remove dissolved Cr(VI) from solution at flow rates typical of those encountered at sites of remediation. Batch studies show that the rate of Cr(VI) removal by fine-grained Fe0 is greater than that for pyrite and coarse-grained Fe0. Results from column studies suggest that partial removal of Cr(VI) by pyrite and coarse-grained Fe0 and quantitative removal of Cr(VI) by fine-grained Fe0 occur at rapid groundwater flow velocities. The removal mechanism for Cr(VI) by fine-grained Fe0 and coarse-grained Fe0 is through the reduction of Cr(VI) to Cr(III), coupled with the oxidation of Fe0 to Fe(II) and Fe(III), and the subsequent precipitation of ...

The generation and release of acidic drainage containing high concentrations of dissolved metals from decommissioned mine wastes is an environmental problem of international scale. A potential solution to many acid drainage problems is the installation of permeable reactive walls into aquifers affected by drainage water derived from mine waste materials. A permeable reactive wall installed into an aquifer impacted by low-quality mine drainage waters was installed in August 1995 at the Nickel Rim mine site near Sudbury, Ontario. The reactive mixture, containing organic matter, was designed to promote bacterially mediated sulfate reduction and subsequent metal sulfide precipitation. The reactive wall is installed to an average depth of 12 feet (3.6 m) and is 49 feet (15 m) long perpendicular to ground water flow. The wall thickness (flow path length) is 13 feet (4 m). Initial results, collected nine months after installation, indicate that sulfate reduction and metal sulfide precipitation is occurring. Comparing water entering the wall to treated water exiting the wall, sulfate concentrations decrease from 2400 to 4600 mg/L to 200 to 3600 mg/L; Fe concentrations decrease from 250 to 1300 mg/L to 1.0 to 40 mg/L; pH increases from 5.8 to 7.0; and alkalinity (as CaCO3) increases from 0 to 50 mg/L to 600 to 2000 mg/L. The reactive wall has effectively removed the capacity of the ground water to generate acidity on discharge to the surface. Calculations based on comparison to previously run laboratory column experiments indicate that the reactive wall has potential to remain effective for at least 15 years.

Recent studies have shown promising results for subsurface remediation of dissolved chromate using permeable-reactive redox walls. Chromate reduction in the presence of iron filings and quartz grains was studied to determine the fate of reduced chromium in proposed wall material. Using a flow-through column apparatus, iron filings mixed with quartz grains were reacted with solutions that contained about 20 mg/L dissolved Cr(VI). Reacted iron filings developed coatings comprised of goethite with chromium concentrated in the outermost edges. Surface analysis showed all detectable chromium occurred as Cr(III) species. In addition, in regions of increased chromium concentration, goethite acquired chemical and structural characteristics similar to Fe2O3 and Cr2O3. Results of the study show that complete reduction of Cr(VI) to Cr(III) occurred and that Cr(III) was incorporated into sparingly soluble solid species.

A modified cellulase enzyme assay was developed to monitor organic matter degradation in passive treatment systems for mine drainage. This fluorogenic substrate method facilitates assessment of exo-(1,4)-β-D-glucanase, endo-(1,4)-β-D-glucanase, and β-glucosidase, which compose an important cellulase enzyme system. The modified method was developed and refined using samples of organic carbon-amended mine tailings from field experiments where sulfate reduction was induced as a strategy for managing water quality. Sample masses (3 g) and the number of replicates ( ≥ 3) were optimized. Matrix interferences within these metal-rich samples were found to be insignificant. Application of this modified cellulase assay method provided insight into the availability and degradation of organic carbon within the amended tailings. Results of this study indicate that cellulase enzyme assays can be applied to passive treatment systems for mine drainage, which commonly contain elevated concentrations of metals.

A versatile numerical model that couples oxygen diffusion and sulfide-mineral oxidation (PYROX) has been developed to simulate the oxidation of pyrite in the vadose zone of mine tailings. A shrinking-core oxidation model and a finite element numerical scheme are used to simulate the transport of oxygen and oxidation of pyrite grains. The rate of pyrite oxidation is assumed to be limited by the transport of oxygen to the reaction site. The model determines the spatially variable bulk diffusion coefficient for oxygen on the basis of moisture content, porosity, and temperature, all of which are variable input parameters. The model PYROX has been coupled to an existing reactive transport model (MINTRAN), which uses a finite element scheme for transport of contaminants and MINTEQA2 to solve for the equilibrium geochemistry. The reactions described by MINTRAN are subject to the local equilibrium assumption. The resulting model, MINTOX, is capable of simulating tailings impoundments where the oxidation of pyrite or pyrrhotite is causing acidic drainage and where acid neutralization and attenuation of dissolved metals can be attributed to equilibrium reactions. Because MINTOX uses realistic boundary conditions and hydrogeological properties, the potential benefits of various remediation schemes, such as moisture-retaining covers, can be quantitatively evaluated.

Remediation programs proposed for decommissioned sulphide tailings may include the addition of a cover layer rich in organic-carbon material such as sewage sludge or composted municipal waste. These covers are designed to consume oxygen and prevent the oxidation of underlying sulphide minerals. The aerobic and anaerobic degradation of such organic-carbon-rich waste can release soluble organic compounds to infiltrating precipitation water. In laboratory experiments, and in natural settings, biotic and abiotic interactions between similar dissolved organic compounds and ferric-bearing secondary minerals have been observed to result in the reductive dissolution of ferric (oxy)hydroxides and the release of ferrous iron to pore waters. In weathered tailings, oxidation of sulphide minerals typically results in the formation of abundant ferric-bearing secondary precipitates near the tailings surface. These secondary precipitates may contain high concentrations of potentially toxic metals, either coprecipitated with or adsorbed onto ferric (oxy)hydroxides. Reductive dissolution reactions, resulting from the addition of the organic-carbon covers, may remobilize metals previously attenuated near the tailings surface. To assess the potential for metal release to tailings pore water by reductive dissolution reactions, a laboratory study was conducted on weathered tailings collected from the Nickel Rim mine tailings impoundment near Sudbury, Ontario, Canada. This site was selected for study because it is representative of many tailings sites. Mineralogical study indicates that sulphide minerals originally present in the vadose zone at the time of tailings deposition have been replaced by a series of secondary precipitates. The most abundant secondary minerals are goethite, gypsum and jarosite. Scanning electron microscopy, coupled with elemental analyses by X-ray energy dispersion analysis, and electron microprobe analysis indicate that trace metals including Ni, Cr and Cu are associated with these secondary minerals. To assess the masses of trace metals associated with each of the dominant secondary mineral phases, a series of extraction procedures was used. The masses of metals determined in three fractions (water soluble, reducible and residual) suggest that the greatest accumulation of metals is in the reducible fraction. These measurements indicate that high concentrations of metals are potentially available for release by reductive dissolution of the ferric-bearing secondary minerals. The actual mass of metals that can be released by this mechanism will depend on a number of site-specific characteristics, particularly the intensity of the reducing conditions established near the tailings surface.

Chromium isotopes are potentially useful indicators of Cr(VI) reduction reactions in groundwater flow systems; however, the influence of transport on Cr isotope fractionation has not been fully examined. Laboratory batch and column experiments were conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction under both static and controlled flow conditions. Organic carbon was used to reduce Cr(VI) in simulated groundwater containing 20 mg L(-1) Cr(VI) in both batch and column experiments. Isotope measurements were performed on dissolved Cr on samples from the batch experiments, and on effluent and profile samples from the column experiment. Analysis of the residual solid-phase materials by scanning electron microscopy (SEM) and by X-ray absorption near edge structure (XANES) spectroscopy confirmed association of Cr(III) with organic carbon in the column solids. Decreases in dissolved Cr(VI) concentrations were coupled with increases in δ(53)Cr, indicating that Cr isotope enrichment occurred during reduction of Cr(VI). The δ(53)Cr data from the column experiment was fit by linear regression yielding a fractionation factor (α) of 0.9979, whereas the batch experiments exhibited Rayleigh-type isotope fractionation (α = 0.9965). The linear characteristic of the column δ(53)Cr data may reflect the contribution of transport on Cr isotope fractionation.

MINTRAN is a new model for simulating transport of multiple thermodynamically reacting chemical substances in groundwater systems. It consists of two main modules, a finite element transport module (PLUME2D), and an equilibrium geochemistry module (MINTEQA2). Making use of the local equilibrium assumption, the inherent chemical nonlinearity is confined to the chemical domain. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient two-step sequential solution algorithm. The advantages of the coupled model include access to the comprehensive geochemical database of MINTEQA2 and the ability to simulate hydrogeological systems with realistic aquifer properties and boundary conditions under complex geochemical conditions. The model is primarily targeted toward groundwater contamination due to acidic mine tailings efiiuents but is potentially also applicable to the full range of geochemical scenarios covered by MINTEQA2. The model is tested with respect to ion exchange chemistry and with respect to precipitation/dissolution chemistry involving multiple sharp fronts. The companion paper presents two-dimensional simulations of heavy metal transport in an acidic mine tailings environment, focusing on environmental implications.

The mobility of potentially toxic dissolved metals discharged from a mine tailings source into an aquifer is investigated by using the multicomponent reactive transport model MINTRAN. A generic aquifer resembling a site in northern Ontario is used as a basis, and a scenario analysis is performed to determine the effect of terminating the source input of acidity and dissolved metals after a finite time. It is found that for most metals the buffering reactions in the carbonate aquifer lower the dissolved metal concentrations substantially, often by several orders of magnitude. In the case of a 12-year source duration, most metals are effectively immobilized by precipitation, and the concentrations of metals remaining in solution are well below drinking water limits. The risk of contamination of water resources by toxic metals can thus be controlled.

Two 200-L fixed-bed bioreactors, containing porous-medium material of coarse sand and organic carbon (tree bark, wood chips and leaf compost), were used to treat NO 3 contamination from agricultural runoff. Flow from a farm-field drainage tile containing NO 3 -N concentrations of 3–6 mg L −1 was successfully treated in the reactors (NO 3 -N −1 ) at a rate of 10–60 L day −1 over a 1-yr period. Treatment occurs by anaerobic denitrification promoted by the added solid-phase organic carbon. Because the reactor design is simple, economical to construct and maintenance free, it may provide a practical solution to the problem of treating redox-sensitive contaminants, such as NO 3 , in agricultural runoff.

Laboratory batch experiments were conducted to examine mechanisms of Hg(II) removal by reactive materials proposed for groundwater treatment. These materials included granular iron filings (GIF), 1:1 (w/w) mixtures of metallurgical granular Fe powder + elemental S (MGI+S) and elemental Cu + elemental S (Cu+S), granular activated carbon (GAC), attapulgite clay (ATP), ATP treated with 2-amino-5-thiol-1,3,4-thiadiazole (ATP-a), and ATP treated with 2,5-dimercapto-1,3,4-thiadiazole (ATP-d). Following treatment of simulated groundwater containing 4 mg L(-1) Hg for 8 or 16 days, the solution pH values ranged from 6.8 to 8.8 and Eh values ranged from +400 to -400 mV. Large decreases in aqueous Hg concentrations were observed for ATP-d (>99%), GIF (95%), MGI+S (94%), and Cu+S (90%). Treatment of Hg was less effective using ATP (29%), ATP-a (69%), and GAC (78%). Extended X-ray absorption fine structure (EXAFS) spectra of Hg on GIF, MGI+S, and GAC indicated the presence of an Hg-O bond at 2.04-2.07 A, suggesting that Hg was bound to GIF corrosion products or to oxygen complexes associated with water sorbed to activated carbon. In contrast, bond lengths ranging from 2.35 to 2.48 A were observed for Hg in Cu+S, ATP-a, and ATP-d treatments, suggesting the formation of Hg-S bonds.

A laboratory-scale column experiment was conducted to evaluate the effect of organic carbon amendments on the mobility of As, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Tl and Zn in mine tailings. Three columns were packed with sulfide- and carbonate-rich tailings, which were amended with a 1:1 (vol.) mixture of peat and spent brewing grain at proportions of 0, 2 and 5 vol. %. A simulated input solution characterized by circumneutral pH and elevated concentrations of SO 4 and S 2 O 3 was passed through the columns for 540 days. The input solution contained low concentrations of metal(loid)s during the initial 300 days and elevated concentrations thereafter. Decreases in mass transport of S 2 O 3 were observed in all columns; with increased attenuation observed at 5 vol. % organic carbon content. Removal of Mn, Ni, Cu, Sb and Mo was observed in all columns during the initial 300 days. However, during this time, mobilization of Fe, As, Zn and Pb was observed, with the greatest increases in concentration observed at the higher organic carbon content. During the final 240 days, S 2 O 3 removal was enhanced in columns containing organic carbon, and Fe, Mn, Ni, Tl, As and Sb removal also was observed. This study demonstrates the influence of organic carbon amendments on metal(loid) mobility in mine tailings. Decreases in mass discharge of metal(loid)s may be achieved using this technique; however, site-specific geochemical conditions must be considered before field-scale implementation.

Development of practical techniques to predict the mobility of halogenated hydrocarbons in aquifers is an important step toward protecting the quality of groundwater resources. This study involved the use of laboratory-column and field-column experiments to estimate retardation coefficients of five halogenated hydrocarbons (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2-dichlorobenzene and hexachloroethane) in the Borden aquifer. These compounds were selected because they were used in the highly-monitored, natural-gradient field-tracer experiment conducted in the unconfined sand aquifer at Canadian Forces Base Borden in southern Ontario, Canada. The field-column experiments were conducted in the vicinity of the natural-gradient tracer experiment, and the laboratory experiments were conducted using aquifer material collected from the field site. The retardation coefficients were found to increase as the average pore-water velocity in the columns decreased, suggesting the presence of non-equilibrium conditions in the reactive transport process. Analysis of the data with a two-site reactive-solute transport model, that allows for time-dependent interaction between solutes and a fraction of reaction sites, provided significant improvement over analysis with a reactive-solute transport model that assumes local equilibrium. For experiments conducted at similar velocities, the mobilities were in close agreement for both the laboratory-and field-column experiments. Retardation factors calculated from batch experiments also agreed closely with the column results and were consistent with those obtained from previous studies at the Borden site. While the retardation factors calculated from the present small-scale methods were in close agreement with those observed during the first month of the large-scale natural-gradient experiment, large deviations were observed for longer travel times.

Three high volume septic systems in Ontario, Canada, were examined to assess the potential for onsite wastewatertreatment systems to release pharmaceutical compounds to the environment and to evaluate the mobility of these compounds in receiving aquifers. Wastewater samples were collected from the septic tanks, and groundwater samples were collected below and down gradient of the infiltration beds and analyzed for a suite of commonly used pharmaceutical and trace organic compounds. The septic tank samples contained elevated concentrations of several pharmaceutical compounds. Ten of the 12 compounds analyzed were detected in groundwater at one or more sites at concentrations in the low ng L(-1) to low microg L(-1) range. Large differences among the sites were observed in both the number of detections and the concentrations of the pharmaceutical compounds. Of the compounds analyzed, ibuprofen, gemfibrozil, and naproxen were observed to be transported atthe highest concentrations and greatest distances from the infiltration source areas, particularly in anoxic zones of the plumes.

An anaerobic plume of process-affected groundwater was characterized in a shallow sand aquifer adjacent to an oil sands tailings impoundment. Based on biological oxygen demand measurements, the reductive capacity of the plume is considered minimal. Major dissolved components associated with the plume include HCO(3), Na, Cl, SO(4), and naphthenic acids (NAs). Quantitative and qualitative NA analyses were performed on groundwater samples to investigate NA fate and transport in the subsurface. Despite subsurface residence times exceeding 20 years, significant attenuation of NAs by biodegradation was not observed based on screening techniques developed at the time of the investigation. Relative to conservative tracers (i.e., Cl), overall NA attenuation in the subsurface is limited, which is consistent with batch sorption and microcosm studies performed by other authors. Insignificant biological oxygen demand and low concentrations of dissolved As (

The retardation factor for dissolved chemicals in ground water is an important parameter in predicting the rate of migration of contaminants and in the design of remediation schemes for contaminated aquifers. Conventional methods for evaluating retardation factors include laboratory batch and column tests, and in the case of nonpolar organic contaminants, correlations with the organic carbon content of the aquifer material. In these procedures it is difficult to insure the geochemical integrity of the sediment material and the solutions used in the tests, leading to uncertainties concerning the applicability of the results. Alternatively, large-scale tracer tests can be conducted; however, because of the time and cost requirements, these procedures have generally been limited to research applications. This paper describes an in situ column procedure for measuring retardation factors. The column is installed in advance of the cutting head of hollow-stem augers, minimizing geochemical disturbance of the sediment, and the sediment within the column is exposed only to natural ground water from the immediate vicinity of the test device. By maintaining geochemical integrity, the method has considerable advantages over laboratory procedures, while being substantially less costly than conventional field tracer tests. Two tests are described in which retardation factors for strontium and for five halogenated organic compounds, obtained using the in situ device, were found to be in good agreement with results obtained by conventional methods.

[1] Multicomponent reactive transport modeling was conducted to analyze and quantify the acid neutralization reactions observed in a column experiment. Experimental results and the experimental procedures have been previously published. The pore water geochemistry was described by dissolution and precipitation reactions involving primary and secondary mineral phases. The initial amounts of the primary phases ankerite-dolomite, siderite, chlorite, and gypsum were constrained by mineralogical analyses of the tailings sample used in the experiment. Secondary gibbsite was incorporated into the model to adequately explain the changes in pH and concentration changes of Al in the column effluent water. The results of the reactive transport modeling show that the pH of the column effluent water can be explained by dissolution reactions of ankerite-dolomite, siderite, chlorite, and secondary gibbsite. The modeling results also show that changes in Eh can be explained by dissolution of ferrihydrite during the experiment. In addition, the modeling results show that the kinetically limited dissolution of chlorite contributes the largest mass of dissolved Mg and Fe (II) in the effluent water, followed by ankerite-dolomite, which contributes substantially less. In summary, reactive transport modeling based on detailed geochemical and mineralogical data was successful to quantitatively describe the changes in pH and major ions in the column effluent.

Reactive transport modeling was used to evaluate the performance of two similar column experiments. The experiments were designed to simulate the treatment of acid mine drainage through microbially mediated sulfate reduction and subsequent sulfide mineral precipitation by means of an organic carbon permeable reactive barrier. Principal reactions considered in the simulations include microbially mediated reduction of sulfate by organic matter, mineral dissolution/precipitation reactions, and aqueous complexation/hydrolysis reactions. Simulations of column 1, which contained composted leaf mulch, wood chips, sawdust, and sewage sludge as an organic carbon source, accurately predicted sulfate concentrations in the column effluent throughout the duration of the experiment using a single fixed rate constant for sulfate reduction of 6.9 x 10(-9) mol L(-1) s(-1). Using the same reduction rate for column 2, which contained only composted leaf mulch and sawdust as an organic carbon source, sulfate concentrations at the column outlet were overpredicted at late times, suggesting that sulfate reduction rates increased over the duration of the column experiment and that microbial growth kinetics may have played an important role. These modeling results suggest that the reactivity of the organic carbon treatment material with respect to sulfate reduction does not significantly decrease over the duration of the 14-month experiments. The ability of the columns to remove ferrous iron appears to be strongly influenced by the precipitation of siderite, which is enhanced by the dissolution of calcite. The simulations indicate that while calcite was available in the column, up to 0.02 mol L(-1) of ferrous iron was removed from solution as siderite and mackinawite. Later in the experiments after approximately 300 d, when calcite was depleted from the columns, mackinawite became the predominant iron sink. The ability of the column to remove ferrous iron as mackinawite was estimated to be approximately 0.005 mol L(-1) for column 1. As the precipitation of mackinawite is sulfide limited at later times, the amount of iron removed will ultimately depend on the reactivity of the organic mixture and the amount of sulfate reduced.

Publisher Summary This chapter provides two case studies of permeable reactive barriers (PRBs) installed to intercept and treat groundwater affected by acid drainage and heavy metals. The Nickel Rim permeable barrier was designed to treat effluent containing potential acidity (as dissolved ferrous iron) from decommissioned mine wastes, and the Vancouver barrier was designed to treat groundwater impacted by elevated concentrations of heavy metals, including cadmium, copper, nickel, and zinc. Both reactive barriers treated the impacted groundwater to acceptable concentration levels while decreasing the net acidity within the groundwater. Sulfate reduction rates within both barriers decreased with time, however, rates were sufficient to attenuate heavy metals below regulatory guidelines. In order to design and install an effective PRB, the geology, hydrogeology, and geochemistry of the aquifer system must be well characterized and understood. Design specifications should account for heterogeneities within the aquifer and barrier as well as the effects of fluctuating water tables and the possibility of reductive dissolution of ferric iron (oxy)hydroxide phases within the aquifer downgradient of the reactive barrier. Permeable reactive barriers provide a cost-effective strategy for the long-term treatment of acid mine drainage and heavy metals in both urban and remote settings.

Review of 10 mature septic system plumes in Ontario, revealed that phosphorus (P) attenuation commonly occurred close to the infiltration pipes, resulting in discrete narrow intervals enriched in P by a factor of 2-4 (. MSc thesis, Dept. Earth Sci., University of Waterloo, Waterloo, Ont.; Ground Water 36 (1995) 1000; J. Contam. Hydrol. 33 (1998) 405). Although these attenuation reactions appeared to be sustainable under present conditions, the potential for remobilization of this P mass, should geochemical conditions change, is unknown. To test the stability of these P solids, dynamic flow column tests were carried out using sediments from three of the previously studied sites (Cambridge, Langton and Muskoka) focusing on sediments from the 'High-P' and underlying (Below) zones. Tests were continued for 166-266 pore volumes (PVs), during which time varying degrees of water saturation were maintained. During saturated flow conditions, relatively high concentrations of PO4 were eluted from the Cambridge and Langton High-P zones (up to 4 and 9 mg/l P, respectively), accompanied by elevated concentrations of Fe (up to 1.4 mg/l) and Mn (up to 4 mg/l) and lower values of Eh (150 mV). The Below zones from Cambridge and Langton, however, maintained lower concentrations of P (generally2 mg/l), Fe (0.2 mg/l) and Mn (1 mg/l) and maintained higher Eh (250 mV) during saturated flow conditions. During unsaturated flow, P and Fe declined dramatically in the High-P zones (P1 mg/l, Fe0.2 mg/l), whereas concentrations remained about the same during saturated and unsaturated flow in the Below zones. This behavior is at least partly attributed to the development of reducing conditions during saturated flow in the High-P zones, leading to reductive dissolution of Fe (III)-P solids present in the sediments. Reducing conditions did not develop in the Below zones apparently because of lower sediment organic carbon (OC) contents (0.03-0.04 wt.%) compared to the High-P zones (0.2-0.65 wt.%). At the Muskoka site, where the sediments were noncalcareous, low values of P (0.2 mg/l) were maintained in both the High-P and Below columns and reducing conditions did not develop. Results indicate the possibility of remobilizing P accumulated below septic system infiltration beds should conditions become more reducing. This could occur if sewage loading patterns change, for example when a seasonal use, lakeshore cottage is converted to a permanent dwelling.

As an immiscible phase, a liquid petroleum product from a spill or leak generally does not travel far in the subsurface. However, soluble organic compounds derived from the immiscible phase can slowly cause widespread contamination of aquifers as a result of groundwater transport. In unconfined sand and gravel aquifers of the type commonly threatened by petroleum-product contamination, groundwater generally moves laterally at a velocity of about 0.05 to 5 m per day. Thus, years or even decades may have to pass for dissolved contaminants from the location of a spill or leak to reach off-site wells or surface water bodies. In this paper, emphasis is placed on benzene, toluene and xylenes (BTX) because these compounds are common causes of groundwater contamination. Several mechanisms control the fate of contaminants after they enter a groundwater flow regime. Dispersion causes the fringes of zones of BTX contamination to spread more extensively than would be predicted on the basis of advection alone. Biodegradation in aerobic zones sometimes causes the severity of BTX contamination to diminish, and sorption causes these compounds to move more slowly than groundwater flow. A case study of the retardation of BTX in a sand aquifer which involved two laboratory methods and one field method of measurement was conducted. Results of this study were compared to a long-term field tracer experiment in the aquifer. All experiments showed the sorption coefficients to be inversely related to the aqueous solubility of the hydrocarbons. However, BTX were all found to be very mobile under the conditions studied. A high mobility is possible in aquifers that contain very little solid-phase orgnic matter, a condition common to sand and gravel aquifers.