The combined use of Sr/Sr and carbon and water isotopes to study the hydrochemical interaction between groundwater and lakewater in mantled karst

The hydrochemical interaction between groundwater and lakewater influences the composition of water that percolates downward from the surficial aquifer system through the underlying intermediate confining unit and recharges the Upper Floridan aquifer along highlands in Florida. The ⁸⁷Sr/⁸⁶Sr ratio along with the stable isotopes, D, ¹⁸O, and ¹³C were used as tracers to study the interaction between groundwater, lakewater, and aquifer minerals near Lake Barco, a seepage lake in the mantled karst terrane of northern Florida. Upgradient from the lake, the ⁸⁷Sr/⁸⁶Sr ratio of groundwater decreases with depth (mean values of 0.71004, 0.70890, and 0.70852 for water from the surficial aquifer system, intermediate confining unit, and Upper Floridan aquifer, respectively), resulting from the interaction of dilute oxygenated recharge water with aquifer minerals that are less radiogenic with depth. The concentrations of Sr²⁺ generally increase with depth, and higher concentrations of Sr²⁺ in water from the Upper Floridan aquifer (20–35 μg/L), relative to water from the surficial aquifer system and the intermediate confining unit, result from the dissolution of Sr-bearing calcite and dolomite in the Eocene limestone. Dissolution of calcite [δ¹³C = −1.6 permil (%o)] is also indicated by an enriched δ¹³C_DIC(-8.8 to -11.4% o) in water from the Upper Floridan aquifer, relative to the overlying hydrogeologic units (δ¹³C_DIC < -16%o).

Groundwater downgradient from Lake Barco was enriched in ¹⁸O and D relative to groundwater upgradient from the lake, indicating mixing of lakewater leakage and groundwater. Downgradient from the lake, the ⁸⁷Sr/⁸⁶Sr ratio of groundwater and aquifer material become less radiogenic and the Sr²⁺ concentrations generally increase with depth. However, Sr²⁺ concentrations are substantially less than in upgradient groundwaters at similar depths. The lower Sr²⁺ concentrations result from the influence of anoxic lakewater leakage on the mobility of Sr²⁺ from clays. Based on results from mass-balance modeling, it is probable that cation exchange plays the dominant role in controlling the ⁸⁷Sr/⁸⁶Sr ratio of groundwater, both upgradient and downgradient from Lake Barco. Even though groundwater from the three distinct hydrogeologic units displays considerable variability in Sr concentration and isotopic composition, the dominant processes associated with the mixing of lakewater leakage with groundwater, as well as the effects of mineral-water interaction, can be ascertained by integrating the use of stable and radiogenic isotopic measurements of groundwater, lakewater, and aquifer minerals.     

Radiocarbon dating of dissolved inorganic carbon in groundwater from confined parts of the Upper Floridan aquifer, Florida, USA

Geochemical reaction models were evaluated to improve radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater from confined parts of the Upper Floridan aquifer in central and northeastern Florida, USA. The predominant geochemical reactions affecting the ¹⁴C activity of DIC include (1) dissolution of dolomite and anhydrite with calcite precipitation (dedolomitization), (2) sulfate reduction accompanying microbial degradation of organic carbon, (3) recrystallization of calcite (isotopic exchange), and (4) mixing of fresh water with as much as 7% saline water in some coastal areas. The calculated cumulative net mineral transfers are negligibly small in upgradient parts of the aquifer and increase significantly in downgradient parts of the aquifer, reflecting, at least in part, upward leakage from the Lower Floridan aquifer and circulation that contacted middle confining units in the Floridan aquifer system. The adjusted radiocarbon ages are independent of flow path and represent travel times of water from the recharge area to the sample point in the aquifer. Downgradient from Polk City (adjusted age 1.7 ka) and Keystone Heights (adjusted age 0.4 ka), 14 of the 22 waters have adjusted ¹⁴C ages of 20–30 ka, indicating that most of the fresh-water resource in the Upper Floridan aquifer today was recharged during the last glacial period. All of the paleowaters are enriched in ¹⁸O and ²H relative to modern infiltration, with maximum enrichment in δ¹⁸O of approximately 2.0‰. Electronic Publication     

Controls on the regional-scale salinization of the Ogallala aquifer,Southern High Plains,Texas, USA

An extensive saline plume (>250 km²) within the regionally important unconfined aquifer in the Neogene Ogallala Formation overlies the Panhandle oil and gas field in the Southern High Plains, Texas, USA. Relative to upgradient Ogallala water, the plume waters have δ¹⁸O (−6.7 to −8.8‰) and δD (−42 to −88‰) values that tend to be depleted and have higher Cl (>150 mg/l) and SO₄ (>75 mg/l) concentrations. Various end-member-mixing models suggest that the plume composition reflects the presence of paleowaters recharged during Middle to Late Wisconsinan time rather than salinization associated with petroleum production. Paleowaters probably mixed with salt-dissolution zone waters from the underlying Upper Permian formations before discharging upward into the Ogallala Formation. Cross-formational discharge is controlled primarily by the geometry of the underlying units, as influenced by the Amarillo uplift, pinch-out of the laterally adjoining confined aquifer in the Triassic Dockum Group, variations in the saturated thickness of the Ogallala aquifer and the presence of potential pathways related to salt dissolution.     

Processes affecting geochemistry and contaminant movement in the middle Claiborne aquifer of the Mississippi embayment aquifer system

Groundwater chemistry and tracer-based age data were used to assess contaminant movement and geochemical processes in the middle Claiborne aquifer (MCA) of the Mississippi embayment aquifer system. Water samples were collected from 30 drinking-water wells (mostly domestic and public supply) and analyzed for nutrients, major ions, pesticides, volatile organic compounds (VOCs), and transient age tracers (chlorofluorocarbons, tritium and helium-3, and sulfur hexafluoride). Redox conditions are highly variable throughout the MCA. However, mostly oxic groundwater with low dissolved solids is more vulnerable to nitrate contamination in the outcrop areas east of the Mississippi River in Mississippi and west Tennessee than in mostly anoxic groundwater in downgradient areas in western parts of the study area. Groundwater in the outcrop area was relatively young (apparent age of less than 40 years) with significantly (p < 0.05) higher dissolved oxygen and nitrate–N concentrations and higher detections of pesticides and VOCs compared to water samples from wells in downgradient areas. Oxygen reduction and denitrification rates were low compared to other aquifers in the United States (zero order rate constants for oxygen reduction and denitrification were 4.7 and 5–10 μmol/L/year, respectively). Elevated concentrations of nitrate–N, and detections of pesticides and VOCs in some deep public supply wells (>50 m depth) indicated contaminant movement from shallow parts of the aquifer into deeper oxic zones. Given the persistence of nitrate in young oxic groundwater that was recharged several decades ago, and the lack of a confining unit, the downward movement of young contaminated water may result in higher nitrate concentrations over time in deeper parts of the aquifer containing older oxic water.     

Using chemical and microbiological indicators to track the impacts from the land application of treated municipal wastewater and other sources on groundwater quality in a karstic springs basin

Multiple chemical constituents (nutrients; N, O, H, C stable isotopes; 64 organic wastewater compounds, 16 pharmaceutical compounds) and microbiological indicators were used to assess the impact on groundwater quality from the land application of approximately 9.5 million liters per day of treated municipal sewage effluent to a sprayfield in the 960-km² Ichetucknee Springs basin, northern Florida. Enriched stable isotope signatures (δ¹⁸O and δ²H) were found in water from the effluent reservoir and a sprayfield monitoring well (MW-7) due to evaporation; however, groundwater samples downgradient from the sprayfield have δ¹⁸O and δ²H concentrations that represented recharge of meteoric water. Boron and chloride concentrations also were elevated in water from the sprayfield effluent reservoir and MW-7, but concentrations in groundwater decreased substantially with distance downgradient to background levels in the springs (about 12 km) and indicated at least a tenfold dilution factor. Nitrate-nitrogen isotope (δ¹⁵N–NO₃) values above 10 ‰ in most water samples were indicative of organic nitrogen sources except Blue Hole Spring (δ¹⁵N–NO₃ = 4.6–4.9 ‰), which indicated an inorganic source of nitrogen (fertilizers). The detection of low concentrations the insect repellent N,N-diethyl-metatoluamide (DEET), and other organic compounds associated with domestic wastewater in Devil’s Eye Spring indicated that leakage from a nearby septic tank drainfield likely has occurred. Elevated levels of fecal coliforms and enterococci were found in Blue Hole Spring during higher flow conditions, which likely resulted from hydraulic connections to upgradient sinkholes and are consistent with previoius dye-trace studies. Enteroviruses were not detected in the sprayfield effluent reservoir, but were found in low concentrations in water samples from a downgradient well and Blue Hole Spring during high-flow conditions indicating a human wastewater source. The Upper Floridan aquifer in the Ichetucknee Springs basin is highly vulnerable to contamination from multiple anthropogenic sources throughout the springs basin.     

Geochemical and stable isotopic evolution of the Guarani Aquifer System in the state of São Paulo, Brazil

The purpose of this report is to explain geochemical and stable isotopes trends in the Brazilian unit of the Guarani Aquifer System (Botucatu and Piramboia aquifers) in S?o Paulo State, Brazil. Trends of dissolved species concentrations and geochemical modeling indicated a significant role of cation exchange and dissolution of carbonates in downgradient evolution of groundwater chemistry. Loss of calcium by the exchange for sodium drives dissolution of carbonates and results in Na–HCO₃ type of groundwater. The cation-exchange front moves downgradient at probably much slower rate compared to the velocity of groundwater flow and at present is located near to the cities of Sert?ozinho and águas de Santa Barbara (wells PZ-34 and PZ-148, respectively) in a shallow confined area, 50–70 km from the recharge zone. Part of the sodium probably enters the Guarani Aquifer System. together with chloride and sulfate from the underlying Piramboia Formation by diffusion related to the dissolution of evaporates like halite and gypsum. High concentrations of fluorine (up to 13.3 mg/L) can be explained by dissolution of mineral fluoride also driven by cation exchange. However, it is unclear if the dissolution takes place directly in the Guarani Aquifer System or in the overlying basaltic Serra Geral Formation. There is depletion in δ ²H and δ ¹⁸O values in groundwater downgradient. Values of δ ¹³C(DIC) are enriched downgradient, indicating dissolution of calcite under closed system conditions. Values of δ ¹³C(DIC) in deep geothermal wells are very high (>–6.0‰) and probably indicate isotopic exchange with carbonates with δ ¹³C about –3.0‰. Future work should be based on evaluation of vertical fluxes and potential for penetration of contamination to the Guarani Aquifer System. Electronic Publication     

Chemical evolution of groundwater in the Wilcox aquifer of the northern Gulf Coastal Plain,USA

The Wilcox aquifer is a major groundwater resource in the northern Gulf Coastal Plain (lower Mississippi Valley) of the USA, yet the processes controlling water chemistry in this clastic aquifer have received relatively little attention. The current study combines analyses of solutes and stable isotopes in groundwater, petrography of core samples, and geochemical modeling to identify plausible reactions along a regional flow path ～300 km long. The hydrochemical facies evolves from Ca-HCO₃ upgradient to Na-HCO₃ downgradient, with a sequential zonation of terminal electron-accepting processes from Fe(III) reduction through SO₄ ^2? reduction to methanogenesis. In particular, decreasing SO₄ ^2? and increasing δ³⁴S of SO₄ ^2? along the flow path, as well as observations of authigenic pyrite in core samples, provide evidence of SO₄ ^2? reduction. Values of δ¹³C in groundwater suggest that dissolved inorganic carbon is contributed both by oxidation of sedimentary organic matter and calcite dissolution. Inverse modeling identified multiple plausible sets of reactions between sampled wells, which typically involved cation exchange, pyrite precipitation, CH₂O oxidation, and dissolution of amorphous Fe(OH)₃, calcite, or siderite. These reactions are consistent with processes identified in previous studies of Atlantic Coastal Plain aquifers. Contrasts in groundwater chemistry between the Wilcox and the underlying McNairy and overlying Claiborne aquifers indicate that confining units are relatively effective in limiting cross-formational flow, but localized cross-formational mixing could occur via fault zones. Consequently, increased pumping in the vicinity of fault zones could facilitate upward movement of saline water into the Wilcox.     

A topo-geochemical sequence study of groundwater in Asa drainage basin, Kwara State, Nigeria

 A strong geochemical gradient was observed in the thick overburden aquifer of the Asa drainage basin. Different types of groundwater occur at different (downslope) locations and groundwater table depths. The following sequence was noticed with increasing distance downslope or with increasing groundwater table depth: 1. Ca–Mg–HCO₃ water at about 390-m groundwater table elevations or upslope locations. 2. Ca–Mg–HCO₃–Cl water at middle-slope locations or groundwater table elevations of about 350 m above sea level; 3. Ca–Mg–SO₄–Cl water at downslope locations or groundwater table elevations of about 300 m above sea level. In this basin, changes in the type of water are expected at about every 40–50 m depth from the surface. Statistical analysis via the determination of the correlation coefficient (r) and regression analysis shows that about 80–99% of the variation in groundwater chemistry is accounted for by the topography, using the model presented in this paper. The rate of change in the sequence will depend on the permeability of the aquifer, which determines the rate of groundwater flow and the residence time, and the nature of recharge. Received: 4 February 1997 · Accepted: 22 July 1997     

Hydrochemical characteristics of groundwater in the Kingston Basin,Kingston, Jamaica

The Kingston Basin in Jamaica is an important hydrologic basin in terms of both domestic and industrial sector. The Kingston hydrologic basin covers an area of approximately 258 km² of which 111 km² underlain by an alluvium aquifer, 34 km² by a limestone aquifer and the remainder underlain by low permeability rocks with insignificant groundwater resources. Rapid development in recent years has led to an increased demand for water, which is increasingly being fulfilled by groundwater abstraction. A detailed knowledge of the water quality can enhance understanding of the hydrochemical system, promoting sustainable development and effective management of groundwater resources. To achieve this, a hydrochemical investigation was carried out in the Kingston Basin. Results showed that the water is Na–Ca–Cl–HCO₃ and Na–Ca–HCO₃ type with higher concentrations of nitrate, sodium and chloride as the leading causes of contamination in most of the wells. High concentrations of nitrate correlate with wells from areas of high population density and could be attributed to anthropogenic causes, mainly involving improper sewage treatment methodologies or leaking sewer lines. Jamaica, owing to its island nature, has the continuous problem of saline water intrusion, and this is reflected in the higher levels of chloride, sodium and conductivity in the water samples collected from the wells. The wells studied show higher concentrations of chloride ranging from around 10.2 mg/l in wells located approximately (4931.45 m) from the coast to around 234 mg/l in the well located near to the coast. The conductivity values also closely correlate with the chloride levels found in the wells.     

Delineation of recharge patterns and contaminant transport using 3H–3He in a shallow aquifer contaminated by chlorinated solvents in South Korea

Stable isotopes of water and ³H–³He were used to delineate recharge patterns and contaminant transport for a granitic regolith aquifer in an industrial complex in Wonju, South Korea, that has historically been contaminated with chlorinated solvents including trichloroethene (TCE) and carbon tetrachloride (CT). Groundwater recharge mainly occurred in upgradient forested areas while little recharge occurred in the downgradient industrial areas covered with extensive sections of impermeable pavement and paddy fields. δ¹⁸O and δD data indicated that groundwater was mainly derived from summer precipitation. The apparent groundwater ages using ³H–³He ranged from 1 to 4 yrs in the upgradient area and from 9 to 10 yrs in the downgradient area. Comparison of groundwater flow velocities based on Darcy’s law and those calculated with simple mass balance models and groundwater age supported the presence of preferential pathways for TCE movement in the study area. Measureable TCE was observed in groundwater irrespective of groundwater age. Considering the 3-yr duration of the TCE spill, 14 yrs before sampling, this indicates that TCE plumes were continuously fed from sources in the unsaturated zone after the spill ended and moved downgradient without significant degradation in the aquifer.     

Redox evolution in glacial drift aquifers: role of diamicton units in reduction of Fe(III)

High iron concentrations create water quality problems for municipal use in glacial drift aquifer units. The chemical evolution of oxic groundwater in shallow aquifer units to anoxic groundwater in deeper aquifer units, in which soluble Fe(II) is stable, is attributed to coupled reduction of Fe(III) on aquifer solids with oxidation of organic carbon. The objective of this study was to characterize the distribution of organic carbon in aquifer and aquitard sediments to determine the availability of potential electron donors to drive these reactions. To do this, four complete rotasonic cores in a glacial aquifer/aquitard system were sampled at close intervals for analyses of grain-size distribution and organic carbon content. The results indicate significantly higher organic carbon concentrations in diamicton (till) units that function as aquitards, relative to coarse-grained aquifer units. In addition, readily reducible iron content in the diamicton units and lower aquifer unit materials is sufficient to produce far more dissolved iron than is present in the aquifer. Groundwater evolves to the level of iron reduction as a terminal electron-accepting process as it moves downward through aquitard units along flow paths from upland recharge areas to downgradient discharge areas. Deeper aquifer units are therefore unlikely to contain groundwater with low iron concentration.     

Urban impacts analysis on hydrochemical and hydrogeological evolution of groundwater in shallow aquifer Linares,Mexico

In northeast Mexico is Linares City, which has an extensive agricultural area and many industrial activities. Near this city is the Cerro-Prieto Dam (~12 km NE direction); this drinking water reservoir captures the water of the Pablillo River catchment area and constitutes an important source of potable water for the metropolitan area of Monterrey, the largest urban center of this region. Groundwater sources in this area provide drinking water to Linares inhabitants. A hydrogeological and hydrochemical study was conducted on the shallow aquifers surrounding the urban centers (Linares and Hualahuises) to determine the evolution of the water quality between 1981 and 2009. The hydrochemistry was assessed upgradient and downgradient from the potential contamination sources in Linares city. Groundwater showed a chemical evolution from calcium-bicarbonate type to calcium–sodium-sulfate type. The water qualities in the downstream area after Linares are inferior compared to the upstream area before the city. Nitrate concentrations in groundwater increased significantly after 28 years indicating an important pollutant process in this period of time over the study area. The possible pollution sources could be the agricultural and farm activities, industrial development, landfills leachate, septic tanks and wastewater of municipal and domestic consumption. If the present scenario continues, an aquifer vulnerability assessment would be important for the sustainable water management.     

Hydrogeology and hydrogeochemistry of Günyüzü semi-arid basin (Eski?ehir, Central Anatolia)

Groundwater is often the only water source in semi-arid regions of Turkey. Günyüzü Basin, located in the Sakarya River basin, SW of Eskişehir, exhibits semi-arid conditions. The study area is composed of Paleozoic metamorphic rocks, Eocene granitic rocks, Neogene sedimentary rocks, and Quaternary alluvium. In the basin, Paleozoic Marbles are the main reservoir rocks for hot and cold water, bordered by impermeable diabases dykes at the sides and by impermeable granites and schists. Neogene-aged limestones, conglomerates and alluvium represent the other significant aquifers. Water samples chosen to exemplify the aquifer characteristics, were collected from springs and wells in both the dry and the wet seasons. The cation and anion permutation of the samples show that carbonates are the dominant lithology in the formation of chemical composition. δ¹⁸O (−11.2 to −8.9‰) and δ²H (−79 to −60‰) isotopic values show that all waters (thermal and cold) are meteoric in origin. The hydrological, hydrochemical, and isotopic properties of the waters reveal that there exist two main groups of groundwater systems; one of these is deep circulating, while the other one is shallow. Tritium values, 0–4 TU (Tritium Unit) indicate the presence of old, static water in these aquifer systems.     

Characterization of the hydrogeologic environment at a petroleum hydrocarbon contaminated site in Korea

The hydrogeologic environment of a petroleum hydrocarbon contaminated site in Korea is characterized by hydrogeologic field work and chemical analyses of groundwater. Quaternary alluvium is the main aquifer contaminated by petroleum hydrocarbons, mainly TEX (toluene, ethylbenzene and xylenes). Contamination at this site was derived from the leaking of petroleum storage tanks. The aquifer is highly permeable, but vertically heterogeneous. Water-table fluctuations reach up to 2 m during heavy rains. Contaminants migrated to the northwest along the main groundwater-flow direction. The concentration of hydrocarbons in groundwater is particularly high downgradient from the source area. The ubiquitous distribution of TEX was caused by the heterogeneity of the aquifer material and the significant fluctuation of the water-table. Chemical properties of the contaminated groundwater and field parameters indicate that intrinsic biodegradation, including aerobic respiration, nitrate reduction, iron reduction, manganese reduction, and sulfate reduction, occurs at this site. The dilution and mixing due to new groundwater recharge from rainfall is also identified as one of the major attenuation processes of TEX.     

Geochemical and isotopic composition of groundwater in the Complex Terminal aquifer in southwestern Tunisia,with emphasis on the mixing by vertical leakage

The Complex Terminal (CT) confined aquifer of the Djerid basin, southwestern Tunisia, was studied using major ion concentrations and stable isotope contents in order to (1) investigate the changes on its hydrodynamic functioning due to the long-term over-pumping and the large-scale flood irrigation practices, (2) determine the principal mineralization processes of its fossil groundwater, and (3) examine the mode of recharge of this aquifer and whether it contains part of modern hydrological regime. The observed geochemical patterns indicated that the main mineralization processes affecting the CT groundwater water/rock interactions and mixing. The native Na > Cl and Cl > SO₄ > Ca > Na waters, resulting from the dissolution of halite and gypsum and from pyrite oxidation, interacted with those of the underlying and the overlying aquifers without changing their chemical facies. Stable isotope data provided evidences about upward and downward leakage into the CT aquifer and their relationships with anthropogenic activities. They demonstrated that the long-term over-pumping of the CT aquifer, which contributed to the loss of its potentiometric pressure, favored the upward leakage of the artesian deep groundwater along parts of the major faults. Moreover, the large-scale flood irrigation practices in the oases domain, which ensured the recharge of the shallow water table by return flow, enhanced the downward leakage toward the CT aquifer.     

Geochemistry analysis and evolution of a bolson aquifer,basin and range province in the southwestern united states

This paper expands significantly on the major-ion geochemical characterization, evolution, and differentiation of groundwater in the Presidio-Redford Bolson (PRB) Aquifer of Texas as presented in Chowdhury et al. (2008). For 19 groundwater samples from the PRB Aquifer, the author calculated major cation–anion balance errors, equilibrium carbon dioxide partial pressure values and saturation indices for selected minerals. Comparison of major-ion analyses for groundwater from basin margin wells with those for basin center wells is documented and illustrated with ion-concentration maps and Piper and Stiff diagrams and reveals significant increases in concentrations of chloride, sulfate and sodium coupled with notable decrease of calcium in bolson-center well samples. These geochemical changes suggest dissolution of aquifer minerals and cation exchange as groundwater migrates downgradient to the bolson center. The US Geological Survey (USGS) computer code, NETPATH, was used to interpret probable net geochemical mass-balance reactions that potentially have occurred within the PRB Aquifer along groundwater flowpaths from bolson margin to bolson center. For all four upgradient–downgradient well pairs studied, at least three NETPATH models contain cation exchange values; calcium is being exchanged for sodium. The Rio Grande Alluvium Aquifer and Rio Grande River are notably minor sources of recharge to the PRB Aquifer, based on Chowdhury et al. (2008) and geochemical evaluations of this study.     

Occurrence of hydrogen sulfide in the ground water of Kuwait

Hydrogen sulfide occurs in high concentration (10–200 mg/l) in different parts of Kuwait City and its suburbs at relatively shallow depths (15–40 m from the surface). This was revealed by drilling through the aquifer system underlying the city and sampling and analyzing the ground water at the drilled locations. The near-absence of coliform bacteria in the sulfide-rich zones, the presence of sulfur-reducing bacteria in the deep (>80 m) Dammam Formation aquifer and a linear positive relation between the concentration of hydrogen sulfide and the total dissolved solids content suggested non-anthropogenic origin of the sulfide in the ground water of Kuwait. The upward movement of sulfide-rich water from depth and its differential flushing by surface recharge through outcrops of the aquifers appear to have given rise to the present distribution of hydrogen sulfide in the aquifers underlying the Kuwait City.     

Distribution of terminal electron-accepting processes in an aquifer having multiple contaminant sources

Concentrations of electron acceptors, electron donors, and H₂ in groundwater were measured to determine the distribution of terminal electron-accepting processes (TEAPs) in an alluvial aquifer having multiple contaminant sources. Upgradient contaminant sources included two separate hydrocarbon point sources, one of which contained the fuel oxygenate methyl tertbutyl ether (MTBE). Infiltrating river water was a source of dissolved NO₃, SO₄ and organic carbon (DOC) to the downgradient part of the aquifer. Groundwater downgradient from the MTBE source had larger concentrations of electron acceptors (dissolved O₂ and SO₄) and smaller concentrations of TEAP end products (dissolved inorganic C, Fe²⁺ and CH₄) than groundwater downgradient from the other hydrocarbon source, suggesting that MTBE was not as suitable for supporting TEAPs as the other hydrocarbons. Measurements of dissolved H₂ indicated that SO₄ reduction predominated in the aquifer during a period of high water levels in the aquifer and river. The predominant TEAP shifted to Fe³⁺ reduction in upgradient areas after water levels receded but remained SO₄ reducing downgradient near the river. This distribution of TEAPs is the opposite of what is commonly observed in aquifers having a single contaminant point source and probably reflects the input of DOC and SO₄ to the aquifer from the river. Results of this study indicate that the distribution of TEAPs in aquifers having multiple contaminant sources depends on the composition and location of the contaminants and on the availability of electron acceptors.     

Tritium�Chelium groundwater age used to constrain a groundwater flow model of a valley-fill aquifer contaminated with trichloroethylene (Quebec, Canada)

Tritium?Chelium groundwater dating was carried out in a trichloroethylene (TCE)-contaminated valley-fill aquifer system in Quebec, Canada, where a numerical groundwater flow model was developed. Forty seven discrete groundwater and dissolved gas samples were obtained along two flow paths originating from known TCE source zones whose related plumes converge down gradient to form a single plume. Sampling points in monitoring wells were projected onto vertical sections showing particle tracks along the two flow paths. At these points, simulated advective ages obtained from particle tracking were matched to tritium?Chelium ages using different porosity values; the best match was 0.35. Ages were also obtained above and below a prodeltaic silty aquitard in a portion of the aquifer where some source zones are located, which provide groundwater and TCE transit times through the aquitard as well as a mean vertical hydraulic conductivity that agrees with previous estimates used in the model. In certain locations, anomalously old ages associated with high terrigenic ⁴He indicate areas where groundwater from the underlying proglacial unit flows upward into the deltaic sand aquifer through aquitard windows. Upflow locations correspond with increased TCE concentrations, suggesting significant TCE provenance through the proglacial unit originating from a previously unrecognized TCE source zone.