Background Groundwater Research Articles

Affinity of radioactive cesium and strontium for illite and smectite clay in the presence of groundwater ions

Abstract Sorption behaviour of hazardous nuclides such as 137Cs and 90Sr onto illite and smectite clay was studied in the presence of major groundwater ions (Ca2+, Mg2+, K+, Na+ and HCO3 ‐). Illite and smectite clay can selectively sorb small hydrated Cs+ ion in the presence of groundwater cations except K+ ion. In contrast, sorption of larger and stable hydrated Sr2+ ion onto illite and smectite clay is highly competed by background groundwater cations. The sorption characteristics of Cs+ and Sr2+ onto illite and smectite clay are closely related to hydration properties of ions and complex forms of ions (inner‐sphere or outer‐sphere complex) in the siloxane cavity of clay minerals. The mechanism of Cs+ and Sr2+ sorption of illite is ion exchange with an interlayer cation K+, and K+, Ca2+ and Mg2+, respectively. The effect of bicarbonate ion on Sr2+ sorption of illite is due to the change of pH and the precipitation of a solid phase (SrCO3). The zeta potentials of illite and smectite clay as a function of...

Identification of Groundwater Solute Sources Using Boron Isotopic Composition

Download Hi-Res ImageDownload to MS-PowerPointCite This:Environ. Sci. Technol. 1995, 29, 12, 2915-2922

Geochemical Indicators of Intrinsic Bioremediation

AbstractA detailed field investigation has been completed at a gasoline‐contaminated aquifer near Rocky Point, NC, to examine possible indicators of intrinsic bioremediation and identify factors that may significantly influence the rate and extent of bioremediation. The dissolved plume of benzene, toluene, ethylbenzene, and xylene (BTEX) in ground water is naturally degrading. Toluene and o‐xylene are most rapidly degraded followed by m‐, p‐xylene, and benzene. Ethylbenzene appears to degrade very slowly under anaerobic conditions present in the center of the plume. The rate and extent of biodegradation appears to be strongly influenced by the type and quantity of electron acceptors present in the aquifer. At the upgradient edge of the plume, nitrate, ferric iron, and oxygen are used as terminal electron acceptors during hydrocarbon biodegradation. The equivalent of 40 to 50 mg/I of hydrocarbon is degraded based on the increase in dissolved CO2 relative to background ground water. Immediately downgradient of the source area, sulfate and iron are the dominant electron acceptors. Toluene and o‐xylene are rapidly removed in this region. Once the available oxygen, nitrate, and sulfate are consumed, biodegradation is limited and appears to be controlled by mixing and aerobic biodegradation at the plume fringes.

Model simulations of a field experiment on cation exchange-affected multicomponent solute transport in a sandy aquifer

A large-scale and long-term field experiment on cation exchange in a sandy aquifer has been modelled by a three-dimensional geochemical transport model. The geochemical model includes cation-exchange processes using a Gaines-Thomas expression, the closed carbonate system and the effects of ionic strength. Information on geology, hydrogeology and the transient conservative solute transport behaviour was obtained from a dispersion study in the same aquifer. The geochemical input parameters were carefully examined. CEC and selectivity coefficients were determined on the actual aquifer material by batch experiments and by the composition of the cations on the exchange complex. Potassium showed a non-ideal exchange behaviour with KCa selectivity coefficients indicating dependency on equivalent fraction and K + concentration in the aqueous phase. The model simulations over a distance of 35 m and a period of 250 days described accurately the observed attenuation of Na and the expelled amounts of Ca and Mg. Also, model predictions of plateau zones, formed by interaction with the background groundwater, in general agreed satisfactorily with the observations. Transport of K was simulated over a period of 800 days due to a substantially attenuation in the aquifer. The observed and the predicted breakthrough curves showed a reasonable accordance taking the duration of the experiment into account. However, some discrepancies were observed probably caused by the revealed non-ideal exchange behaviour of K +.

APPLICATION OF STATISTICAL METHODS FOR GROUNDWATER QUALITY EVALUATION AT THE LANDFILL SITE BOUNDARIES

Ambient groundwater quality in the vicinity of landfill sites usually demonstrates areal and temporal variability. Groundwater quality in monitoring wells and piezometers down-gradient of fill areas is also variable. In order to compare regional background groundwater quality and groundwater quality at the compliance boundary, analysis of variance and tolerance interval calculations are recommended. In order to compare a fixed compliance limit with the observed concentration at the compliance boundary confidence intervals can be constructed. These statistical methods have been utilized for evaluating groundwater quality at the down-gradient boundary of a small municipal landfill site in southern Ontario.

Groundwater contamination at a landfill sited on fractured carbonate and shale

Leachate from the Upper Ottawa Street landfill site has contaminated groundwater in the underlying fractured dolomite and shale bedrock. The landfill received domestic and industrial waste from about 1954 to 1978. An extensive network of multilevel groundwater monitoring devices provided a detailed, three-dimensional view of the contaminant distributions. The leachate was lacking in reliable indicator parameters such as chlorinated solvents. Background groundwater quality was variable and often very poor. Many of the inorganic and organic species present in the leachate at high concentration were also present in some background groundwaters and often at equally high concentrations. In order to identify those groundwaters which were leachate-impacted, a number of tests were applied to the chemical data. These included a test utilizing 14 chemical characteristics of leachate, and statistical tests using descriptive principal components analysis, descriptive correspondence analysis and a fuzzy cluster analysis. The statistical tests attempted to identify a pattern in the chemical data consistent with contamination superimposed upon a mixture between a recently recharged, dilute groundwater endmember and a much older, concentrated groundwater endmember. Only sporadic leachate impact was recognized beyond 100 m from the landfill. Impact to the east indicates leachate migration towards the escarpment in the direction of the regional hydraulic gradient. Occasional impact to the southeast may indicate leachate migration across the regional gradient, perhaps in a major fracture network oriented northwest-southeast under the site. The rather minor impact of this landfill upon groundwater quality probably reflects both a low concentration of easily recognized contaminants emanating from the landfill and a very poor groundwater quality in the sedimentary bedrock.

Volatile Aromatic and Chlorinated Organic Contaminants in Groundwater at Six Ontario Landfills

Abstract Studies of the migration of organic contaminants in shallow aquifers impacted by landfill leachate at six sites in Ontario are reported. Three sites are located on very permeable sand deposits, one on less permeable sand till and two on fractured sedimentary bedrock. The migration rate and persistence of volatile, one-and-two carbon, halogenated hydrocarbons (halocarbons) and volatile aromatic hydrocarbons are emphasized. These compounds are ubiquitous in sanitary landfill leachates and are quite mobile in groundwater. They are at very low concentrations (less than 5 ppb each) at the Borden landfill site, where most waste was burned before landfilling. At the Woolwich site, volatile halocarbons are found at very low concentrations (less than 0.5 ppb each) up to one kilometer from the site, indicating that they may be very mobile and persistent in this aquifer. Attenuation, probably due mainly to dispersion, has resulted in only sub-ppb concentrations persisting beyond two hundred meters of the site. The contaminant plume at North Bay has been discharging to the surface about eight hundred meters from the site for a number of years. Some mobile volatile organics, therefore, are found throughout the plume. Halo-carbons do not persist and some aromatics appear to be undergoing biodegradation as well. For these and other contaminants, dramatic attenuation is observed within the eight hundred meter plume, probably as the result of dispersion. Groundwater velocities in the less-permeable sand and sand till at the new Borden site are much lower than in the other aquifers, so contaminants have only migrated perhaps two hundred meters laterally. Volatile halocarbons may be migrating at the groundwater velocity, while some retardation of aromatics may be occurring. However, the erratic contaminant distribution complicates the consideration of contaminant migration. Contaminant distributions are irregular in fractured bedrock at the Bay-view and Hamilton sites. The irregular and generally low concentration of halocarbons, coupled with the generally-poor background water quality in these bedrock flow systems, makes the definition of the zone of contamination at these sites very difficult. Although these low-porosity carbonate/ shale bedrock systems could distribute leachate contamination through a large volume of rock, it is encouraging to note the rather restricted zone of clearly-impacted groundwater. The major, mobile organic contaminants at the Hamilton site are the volatile aromatic hydrocarbons. Recognition of only-slightly-impacted groundwater at this site is complicated by the occurrence of these organics at ppb levels in apparently uncontaminated, background groundwater. Temporal variations, over weeks and years, are found for all contaminants at these sites. Input from the landfill appears to be temporally variable and so is a major cause of subsequent variations within the leachate plume. The processes of dispersion, which smoothes such variations at some sites (Borden), does not appear to be effective at damping temporal variability along the plume at North Bay nor in the fractured-bedrock systems.

Migration of contaminants in groundwater at a landfill: A case study: 5. Cation migration in the dispersion test

A shallow natural-gradient tracer test was conducted in an unconfined sandy aquifer at the Borden landfill site in June 1978. The tracer injected was a chloride solution containing approximately equimolar concentrations of the major cations at levels significantly above those in the background groundwater. Samples collected during monitoring of the tracer were analysed for calcium, magnesium, sodium and potassium. The results showed an order of attenuation for major cations of K > Mg > Na. Ca, which was the dominant species on the exchange sites before injection of the tracer solution, was liberated during flow of the solution. This resulted in the development of a calcium halo at the leading edge of the tracer solution during flow. A one-dimensional mixing-cell model which includes a chemical equilibria subroutine to appraise the effects of major-ion complexing, calcite dissolution and precipitation, and cation exchange was used to simulate the results of the injection experiment. Both the field and model results show that the chemical composition of the background groundwater is changed due to exchange reactions as it flows in the path of the injection solution. These influences continue until all the exchange sites along the flow path re-equilibrate to their former pre-injection values by the passage of background groundwater. The mixing-cell model was calibrated to the dispersive characteristics of the aquifer by varying average groundwater velocity, mixing-cell size and dispersivity to match the chloride distribution. Model results provide simulated cation distributions which are in close agreement with the field results including a “Ca-halo” produced due to the liberation of Ca 2+ from exchange sites. The migration of Na + in the flow system is observed to closely parallel the movement of Cl − although the maximum concentrations are decreased over the concentration at injection. Mg 2+ migrates downflow at a rate slower than Cl −, and at a much reduced concentration reflecting a strong tendency for sorption onto exchange sites. Both model and field results show pronounced K + attenuation during flow of the injection solution. Concentrations throughout the flow system during the monitoring program were always very close to background values. The bulk of the injected potassium was sorbed very close to the injection wells and is slowly liberated by interaction of background groundwater following the passage of the injection solution.