Hydrogeochemical patterns,processes and mass transfers during aquifer storage and recovery (ASR) in an anoxic sandy aquifer

The hydrogeochemical processes that took place during an aquifer storage and recovery (ASR) trial in a confined anoxic sandy aquifer (Herten, the Netherlands) were identified and quantified, using observation wells at 0.1, 8 and 25 m distance from the ASR well. Oxic drinking water was injected in 14 ASR cycles in the period 2000–2009. The main reactions consisted of the oxidation of pyrite, sedimentary organic matter, and (adsorbed) Fe(II) and Mn(II) in all aquifer layers (A–D), whereas the dissolution of carbonates (Mg-calcite and Mn-siderite) occurred mainly in aquifer layer D. Extinction of the mobilization of SO₄, Fe(II), Mn(II), As, Co, Ni, Ca and total inorganic C pointed at pyrite and calcite leaching in layer A, whereas reactions with Mn-siderite in layer D did not show a significant extinction over time. Iron(II) and Mn(II) removal during recovery was demonstrated by particle tracking and pointed at sorption to neoformed ferrihydrite. Part of the oxidants was removed by neoformed organic material in the ASR proximal zone (0 – ca. 5 m) where micro-organisms grow during injection and die away when storage exceeds about 1 month. Anoxic conditions during storage led to increased concentrations for a.o. Fe(II), Mn(II) and NH₄ as noted for the first 50–200 m³ of abstracted water during the recovery phase. With a mass balance approach the water–sediment reactions and leaching rate of the reactive solid phases were quantified. Leaching of pyrite and calcite reached completion at up to 8 m distance in layer A, but not in layer D. The mass balance approach moreover showed that Mn-siderite in layer D was probably responsible for the Mn(II) exceedances of the drinking water standard (0.9 μmol/L) in the recovered water. Leaching of the Mn-siderite up to 8 m from the ASR well would take 1600 more pore volumes of drinking water injection (on top of the realized 460).     

Fate of organic matter during aquifer storage and recovery (ASR) of reclaimed water in a carbonate aquifer

Understanding the fate of injected organic matter and the consequences of subsequent redox processes is essential to assess the viability of using reclaimed water in aquifer storage and recovery (ASR). A full-scale field trial was undertaken at Bolivar, South Australia where two ASR cycles injected approximately 3.6 × 10⁵ m³ of reclaimed water into a carbonate aquifer over a 3-a period. Organic C within reclaimed water was predominantly in the dissolved fraction, ranging from 1 to 2 mmol L⁻¹ (10–20 mg L⁻¹), markedly higher than potable supply and stormwater previously reported as source waters for ASR. Between 20% and 24% of the injected dissolved organic C (DOC) was mineralised through reaction with injected O₂ and NO₃. Furthermore, this was achieved mainly within the first 4 m of aquifer passage. Despite the presence of residual DOC, SO₄ reduction was not induced within the bulk of the injected plume. It was only near the ASR well during an extended storage phase where deeply reduced (methanogenic) conditions developed, indicating variable redox zones within the injectant plume. The quality of water recovered from the ASR well indicated that the organic C content of reclaimed water does not restrict its application as a recharge source for ASR.     

Characterization of accumulated precipitates during subsurface iron removal

The principle of subsurface iron removal for drinking water supply is that aerated water is periodically injected into the aquifer through a tube well. On its way into the aquifer, the injected O₂-rich water oxidizes adsorbed Fe²⁺, creating a subsurface oxidation zone. When groundwater abstraction is resumed, the soluble Fe²⁺ is adsorbed and water with reduced Fe concentrations is abstracted for multiple volumes of the injection water. In this article, Fe accumulation deposits in the aquifer near subsurface treatment wells were identified and characterized to assess the sustainability of subsurface iron removal regarding clogging of the aquifer and the potential co-accumulation of other groundwater constituents, such as As. Chemical extraction of soil samples, with Acid-Oxalate and HNO₃, showed that Fe had accumulated at specific depths near subsurface iron removal wells after 12 years of operation. Whether it was due to preferred flow paths or geochemical mineralogy conditions; subsurface iron removal clearly favoured certain soil layers. The total Fe content increased between 11.5 and 390.8 mmol/kg ds in the affected soil layers, and the accumulated Fe was found to be 56-100% crystalline. These results suggest that precipitated amorphous Fe hydroxides have transformed to Fe hydroxides of higher crystallinity. These crystalline, compact Fe hydroxides have not noticeably clogged the investigated well and/or aquifer between 1996 and 2008. The subsurface iron removal wells even need less frequent rehabilitation, as drawdown increases more slowly than in normal production wells. Other groundwater constituents, such as Mn, As and Sr were found to co-accumulate with Fe. Acid extraction and ESEM-EDX showed that Ca occurred together with Fe and by X-ray Powder Diffraction it was identified as calcite.     

Twenty years of groundwater evolution in the Triassic sandstone aquifer of Lorraine: Impacts on baseline water quality

The Lorraine Triassic Sandstone Aquifer (LTSA), which has already been the subject of a chemical and radioisotopic study (1979), is used to investigate the impacts of 20 a of large scale pumping on baseline water quality. In parallel, new sampling of the aquifer (2001) provides new inorganic geochemical data (including trace elements) that allow improving the knowledge of baseline conditions and hydrochemical functioning of a major sandstone aquifer. The good correlation between ¹⁴C activities, temperature and depth along the main flow line indicate regular downgradient trends and possible water stratification. Unreactive tracers, mainly stable isotope ratios ¹⁸O and ²H, as well as C isotopes are used to define a timescale for the aquifer, showing two groups of groundwater, namely of modern and Holocene age, and late Pleistocene age, with a mixing zone. Baseline quality is then represented by a wide range of concentrations, mainly the result of time-dependent water–rock interaction, as already observed elsewhere in Triassic sandstone aquifers. Some trace elements such as Li, Rb, Cs, which are not limited by solubility constraints, show linear trends. During saturated flow downgradient, the chemistry is also specifically characterised by a regular increase in Na and Cl (and locally SO₄) as a result of evaporite dissolution related to overlying or basement limits. The aquifer is mostly oxidising with a redox boundary marked by U decrease, some 40 km from outcrop.     

A comparison of the geochemical response to different managed aquifer recharge operations for injection of urban stormwater in a carbonate aquifer

Managed aquifer recharge (MAR) is increasingly being considered as a means of reusing urban stormwater and wastewater to supplement the available water resources. Subsurface storage is advantageous as it does not impact on the area available for urban development, while the aquifer also provides natural treatment. However, subsurface storage can also have deleterious effects on the recovered water quality through water–rock interactions which can also impact on the integrity of the aquifer matrix. A recent investigation into the potential for stormwater recycling via Aquifer Storage Transfer and Recovery (ASTR) in a carbonate aquifer is used to determine the important hydrogeochemical processes that impact on the recovered water quality. An extensive period of aquifer flushing allows observation of water quality changes under two operating scenarios: (1) separate wells for injection and recovery, representing ASTR; and (2) a single well for injection and recovery, representing Aquifer Storage and Recovery (ASR).     

Arsenic mobility and impact on recovered water quality during aquifer storage and recovery using reclaimed water in a carbonate aquifer

Arsenic release from aquifers can be a major issue for aquifer storage and recovery (ASR) schemes and understanding the processes that release and attenuate As during ASR is the first step towards managing this issue. This study utilised the first and fourth cycles of a full scale field trial to examine the fate of As within the injectant plume during all stages of the ASR cycle, and the resultant water quality. The average recovered As concentration was greater than the source concentration; by 0.19 μmol/L (14 μg As/L) in cycle 1 and by 0.34 μmol/L (25 μg As/L) in cycle 4, indicating that As was being released from the aquifer sediments during ASR and the extent of As mobilisation did not decline with subsequent cycles. In the injection phase, As mobilisation due to oxidation of reduced minerals was limited to an oxic zone in close proximity to the ASR well, while desorption from Fe oxyhydroxide or oxide surfaces by injected P occurred further in the near well zone (0–4 m from the ASR well). With further aquifer passage during injection and greater availability of sorption sites there was evidence of attenuation via adsorption to Fe oxyhydroxides which reduced concentrations on the outer fringes of the injectant plume. During the period of aquifer storage, microbial activity resulting from the injection of organic matter resulted in increased As mobility due to reductive Fe oxyhydroxide dissolution and the subsequent loss of sorption sites and partial reduction of As(V) to the more mobile As(III). A reduced zone directly around the ASR well produced the greatest As concentration and illustrated the importance of Fe oxyhydroxides for controlling As concentrations. Given the small spatial extent of this zone, this process had little effect on the overall recovered water quality.     

A multi-period optimization model for conjunctive surface water–ground water use via aquifer storage and recovery in Corpus Christi,Texas

The present study develops and evaluates a decision support system for the conjunctive management of the current surface and proposed aquifer storage and recovery (ASR) facility of the city of Corpus Christi, TX using a simulation–optimization approach. The objective of the model is to maximize water storage in the surface and subsurface storage units while meeting (1) the freshwater inflow requirements to the Corpus Christi estuary and (2) the water demands of the city and its service area. The model is parameterized using streamflow data from the U. S. Geological Survey gauging stations on the Nueces River and its tributaries as well as long-term climatic data and regional hydrogeologic information. Results indicate that a single-well field ASR facility is capable of storing approximately 925 ha-m (7,500 ac-ft) of water over a 5-year period in the Evangeline Aquifer with a total potential storage of about 2,715 ha-m (22,000 ac-ft) of water over the jurisdictional area of the Corpus Christi Aquifer Storage and Recovery Conservation District. Surplus surface water sources are seen to contribute approximately 49–96 % of the water stored in the ASR during the simulation period. The remaining storage came from either Choke Canyon Reservoir or Lake Corpus Christi, which also resulted in a slight reduction in evapotranspiration in both reservoirs. The analysis indicates that the proposed ASR system is not limited on the supply side but multiple well fields may be required to increase the storage capacity within the aquifer.     

Elevated arsenic in deeper groundwater of the western Bengal basin, India: Extent and controls from regional to local scale

The deeper groundwater (depending on definition) of the Bengal basin (Ganges-Brahmaputra delta) has long been considered as an alternate, safe drinking-water source in areas with As-enrichment in near-surface groundwater. The present study provides the first collective discussion on extent and controls of elevated As in deeper groundwater of a regional study area in the western part of the Bengal basin. Deeper groundwater is defined here as non-brackish, potable (Cl⁻ ? 250 mg/L) groundwater available at the maximum accessed depth (∼80-300 m). The extent of elevated As in deeper groundwater in the study area seems to be largely controlled by the aquifer-aquitard framework. Arsenic-enriched deeper groundwater is mostly encountered north of 22.75°N latitude, where an unconfined to semi-confined aquifer consisting of Holocene- to early Neogene-age gray sand dominates the hydrostratigraphy to 300 m depth below land surface. Aquifer sediments are not abnormally enriched in As at any depth, but sediment and water chemistry are conducive to As mobilization in both shallow and deeper parts of the aquifer(s). The biogeochemical triggers are influenced by complex redox disequilibria. Results of numerical modeling and profiles of environmental tracers at a local-scale study site suggest that deeper groundwater abstraction can draw As-enriched water to 150 m depth within a few decades, synchronous with the advent of wide-scale irrigational pumping in West Bengal (India).     

Development of a new model tool for evaluating groundwater resources within the Floridan Aquifer System in Southern Florida,USA

The US Army Corps of Engineers (USACE) and the South Florida Water Management District (SFWMD) are partners in an ambitious plan to restore water flows throughout the Everglades ecosystem. An important component of the restoration plan involves storing excess stormwater deep underground in the Floridan Aquifer System using aquifer, storage and recovery (ASR) wells. In order to determine the optimal ASR system and to assess environmental impacts, USACE spent over 11 years and significant resources to develop a three-dimensional groundwater model of the Floridan Aquifer System covering a large portion of the Florida peninsula. This SEAWAT model is capable of evaluating changes in aquifer pressures and density-dependent flows in the entire study area. The model has been used to evaluate the Everglades ASR system already but could also be used by water managers for other important water resources studies in Florida including water supply estimates and adaptation to climate change. As part of an effort to make the model more readily available for other important studies, this study documents and summarizes the overall development of the SEAWAT model including a discussion regarding the intensive calibration and validation efforts undertaken during model development. The paper then demonstrates the use of the model using Everglades ASR project alternatives. Lastly, the paper outlines potential future uses of the model along with its overall limitations. Supplementary online resources are also included that provide researchers with further detail regarding the model development effort beyond the scope of this summary article as well as model development databases.     

Exploring sustainability of aquifers based on predictive modeling of sorption characteristics of arsenic enriched Holocene sediments in Bangladesh

The importance of accessing safe aquifers in areas with high As is being increasingly recognized. The present study aims to investigate the sorption and mobility of As at the sediment-groundwater interface to identify a likely safe aquifer in the Holocene deposit in southwestern Bangladesh. The upper, shallow aquifer at around 18 m depth, which is composed mainly of very fine, grey, reduced sand and contains 24.3 μg/g As, was found to produce highly enriched groundwater (190 μg/L As). In contrast, deeper sediments are composed of partly oxidized, brownish, medium sand with natural adsorbents like Fe- and Al-oxides; they contain 0.76 μg/g As and impart low As concentrations to the water (4 μg/L). These observations were supported by spectroscopic studies with SEM, TEM, XRD and XRF, and by adsorption, leaching, column tests and sequential extraction. A relatively high in-situ dissolution rate (R_r) of 1.42 × 10⁻¹⁶ mol/m²/s was derived for the shallower aquifer from the inverse mass-balance model. The high R_r may enhance As release processes in the upper sediment. The field-based reaction rate (K_r) was extrapolated to be roughly 1.23 × 10⁻¹³ s⁻¹ and 6.24 × 10⁻¹⁴ s⁻¹ for the shallower and deeper aquifer, respectively, from the laboratory-obtained adsorption/desorption data. This implies that As is more reactive in the shallower aquifer. The partition coefficient for the distribution of As at the sediment-water interface (K_d-As) was found to range from 5 to 235 L/kg based on in-situ, batch adsorption, and flow-through column techniques. Additionally, a parametric equation for K_d-As (R² = 0.67) was obtained from the groundwater pH and the logarithm of the leachable Fe and Al concentrations in sediment. A one-dimensional finite-difference numerical model incorporating K_d and K_r showed that the shallow, leached As can be immobilized and prevented from reaching the deeper aquifer (∼150 m) after 100 year by a natural filter of oxidizing sand and adsorbent minerals like Fe and Al oxides; in this scenario, 99% of the As in groundwater is reduced. The deeper aquifer appears to be an adequate source of sustainable, safe water.     

Groundwater flow in an arsenic-contaminated aquifer,Mekong Delta,Cambodia

To advance understanding of hydrological influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50 km² area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer (K ≈ 10⁻⁴ ms⁻¹) and overlying clay aquitard (K ≈ 10⁻⁸ ms⁻¹) were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04–0.4 ma⁻¹ downward through the clay and 1–13 ma⁻¹ horizontally within the sand aquifer, producing aquifer residence times on the order 100–1000 a. The results of numerical modeling support this conceptual model of the flow system and, when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2–10 m of fine-grained material) are an important source of As to the underlying aquifer.     

Geochemical controls on sediment reactivity and buffering processes in a heterogeneous aquifer

The injection and recovery of oxic water into deep anoxic aquifers may help to alleviate short- and long-term imbalance between freshwater supply and demand. The extent and structure of physical and geochemical heterogeneity of the aquifer will impact the water quality evolution during injection, storage and recovery. Water–sediment interactions within the most permeable parts of the aquifer, where the bulk of the injectant will penetrate, may dominate, however, water quality may also be impacted by interactions within the finer-grained, less permeable but potentially highly reactive media. In this study, the heterogeneity of the reductive capacity of an aquifer selected for water reuse projects was characterised, the amount, type and reactivity of the sedimentary reductants present determined, and the relationship between reductive capacity and sedimentary lithologies quantified. The average potential reductive capacities (PRC_TOT), based on total organic C and pyrite concentrations of the sediment, were quantified for sands (382 μmol O₂ g⁻¹), clays (1522 μmol O₂ g⁻¹), and silts (1957 μmol O₂ g⁻¹). Twenty-seven samples, spanning the three different lithologies, were then incubated for 50 days and the measured reductive capacities (MRC) determined for the sands (29.2 μmol O₂ g⁻¹), silts (136 μmol O₂ g⁻¹), and clays (143 μmol O₂ g⁻¹). On average, the MRC were 10% of the PRC_TOT. The main consumers of O₂ were pyrite (20–100%), sedimentary organic matter (SOM; 3–56%), siderite (3–28%) and Fe(II)-aluminosilicates (8–55%). The incubation data plus hydrogeochemical modelling, indicated that pH-buffering was controlled firstly by dissolution of trace level carbonates, followed by dissolution of feldspars. Zinc, Co, Ni, Cd and Pb were readily mobilized during incubation.     

Reactive transport impacts on recovered freshwater quality during multiple partially penetrating wells (MPPW-)ASR in a brackish heterogeneous aquifer

The use of multiple partially penetrating wells (MPPW) during aquifer storage and recovery (ASR) in brackish aquifers can significantly improve the recovery efficiency (RE) of unmixed injected water. The water quality changes by reactive transport processes in a field MPPW-ASR system and their impact on RE were analyzed. The oxic freshwater injected in the deepest of four wells was continuously enriched with sodium (Na⁺) and other dominant cations from the brackish groundwater due to cation exchange by repeating cycles of ‘freshening’. During recovery periods, the breakthrough of Na⁺ was retarded in the deeper and central parts of the aquifer by ‘salinization’. Cation exchange can therefore either increase or decrease the RE of MPPW-ASR compared to the RE based on conservative Cl⁻, depending on the maximum limits set for Na⁺, the aquifer’s cation exchange capacity, and the native groundwater and injected water composition. Dissolution of Fe and Mn-containing carbonates was stimulated by acidifying oxidation reactions, involving adsorbed Fe²⁺ and Mn²⁺ and pyrite in the pyrite-rich deeper aquifer sections. Fe²⁺ and Mn²⁺ remained mobile in anoxic water upon approaching the recovery proximal zone, where Fe²⁺ precipitated via MnO₂ reduction, resulting in a dominating Mn²⁺ contamination. Recovery of Mn²⁺ and Fe²⁺ was counteracted by frequent injections of oxygen-rich water via the recovering well to form Fe and Mn-precipitates and increase sorption. The MPPW-ASR strategy exposes a much larger part of the injected water to the deeper geochemical units first, which may therefore control the mobilization of undesired elements during MPPW-ASR, rather than the average geochemical composition of the target aquifer.     

Irrigation produces elevated arsenic in the underlying groundwater of a semi-arid basin in Southwestern Idaho

The shallow aquifer beneath the Western Snake River Plain (Idaho, USA) exhibits widespread elevated arsenic concentrations (up to 120 μg L⁻¹). While semi-arid, crop irrigation has increased annual recharge to the aquifer from approximately 1 cm prior to a current rate of >50 cm year⁻¹. The highest aqueous arsenic concentrations are found in proximity to the water table (all values >50 μg L⁻¹ within 50 m) and concentrations decline with depth. Despite strong vertical redox stratification within the aquifer, spatial distribution of aqueous species indicates that redox processes are not primary drivers of arsenic mobilization. Arsenic release and transport occur under oxidizing conditions; groundwater wells containing dissolved arsenic at >50 μg L⁻¹ exhibit elevated concentrations of O₂ (average 4 mg L⁻¹) and NO₃ (average 8 mg L⁻¹) and low concentrations of dissolved Fe (<20 μg L⁻¹). Sequential extractions and spectroscopic analysis of surficial soils and sediments indicate solid phase arsenic is primarily arsenate and is present at elevated concentrations (4–45 mg kg⁻¹, average: 17 mg kg⁻¹) relative to global sedimentary abundances. The highest concentrations of easily mobilized arsenic (up to 7 mg kg⁻¹) are associated with surficial soils and sediments visibly stained with iron oxides. Batch leaching experiments on these materials using irrigation waters produce pore water arsenic concentrations approximating those observed in the shallow aquifer (up to 152 μg L⁻¹). While As:Cl aqueous phase relationships suggest minor evaporative enrichment, this appears to be a relic of the pre-irrigation environment. Collectively, these data indicate that infiltrating irrigation waters leach arsenic from surficial sediments to the underlying aquifer.     

Comparison of 4He ages and 14C ages in simple aquifer systems: implications for groundwater flow and chronologies

⁴He concentrations in excess of the solubility equilibrium with the atmosphere by up to two to three orders of magnitude are observed in the Carrizo Aquifer in Texas, the Ojo Alamo and Nacimiento aquifers in the San Juan Basin, New Mexico, and the Auob Sandstone Aquifer in Namibia. A simple ⁴He accumulation model is applied to explain these excess ⁴He concentrations in terms of both in situ production and a crustal flux across the bottom layer of the aquifer. Results from the model simulations suggest variability in the ⁴He fluxes, ranging from 6×10⁻⁶ cm³ STP cm⁻² yr⁻¹ for the Auob Sandstone Aquifer to 3.6×10⁻⁷ cm³ STP cm⁻² yr⁻¹ for the Carrizo aquifer. For the Ojo Alamo and Nacimiento aquifers an intermediate value of 3×10⁻⁶ cm³ STP cm⁻² yr⁻¹ was estimated. The contribution of in-situ produced ⁴He to the measured concentrations was also estimated. This contribution is negligible for the Auob Sandstone Aquifer as compared with both the concentrations measured at the top and bottom of the aquifer for most of the pathway. In the Carrizo aquifer, in-situ produced ⁴He contributes 27.5% and 15.4%, to the total ⁴He observed at the top and bottom of the aquifer, respectively. For both aquifers of the San Juan Basin in-situ production almost entirely dominates the ⁴He concentrations at the top of the aquifer for most of the pathway. In contrast, the internal production is negligible as compared with the measured concentrations at the bottom of these aquifers, reaching, at most, 1.1%. The model simulations require an exponential decrease in the horizontal velocity of the water with increasing recharge distance to reproduce the distribution of ⁴He in these aquifers. For the Auob Sandstone Aquifer the highest range in the velocity values is obtained (25 to 0.4 m yr⁻¹). The simulations for the Carrizo aquifer and both aquifers located in the San Juan Basin require velocities varying from 4 to 0.1 m yr⁻¹, and from 2 to 0.3 m yr⁻¹, respectively. For each aquifer, average permeability values were also estimated. They are generally in agreement with results obtained from pumping tests, hydrodynamic modeling and previous ¹⁴C measurements. On the basis of the results obtained by calibrating the model with the measured ⁴He concentrations, the mean water residence times were estimated. They agree reasonably well with ¹⁴C ages. When applied as chronologies for noble gas temperatures in the same aquifers, the calculated ⁴He ages allow the identification of three different climate periods similar to those previously identified using ¹⁴C ages: (1) the Holocene period (0–10 Ka BP), (2) the Last Glacial Maximum (≈18 Ka BP), and (3) the preceeding period (30–150 Ka BP).     

Tracing dissolved O2 and dissolved inorganic carbon stable isotope dynamics in the Nyack aquifer: Middle Fork Flathead River, Montana, USA

The geochemistry and microbiology of shallow groundwater aquifers is greatly influenced by the concentration of dissolved oxygen gas (DO); however, the mechanisms that consume DO in groundwater (e.g., biotic or abiotic) are often ambiguous. The use of stable isotopes of molecular O₂ (δ¹⁸O-DO), in conjunction with stable isotopes of dissolved inorganic carbon (δ¹³C-DIC), has potential to discriminate between the various mechanisms causing DO depletion in subsurface waters.Here we report the results of spatial and seasonal changes in δ¹⁸O-DO and δ¹³C-DIC at the Nyack floodplain aquifer along the Middle Fork of the Flathead River near West Glacier, Montana, USA. Over a short, well constrained flow path (∼100 m) near a main recharge zone of the floodplain, the δ¹⁸O-DO consistently increased as DO concentrations decreased with distance from the recharge source. Concurrently, DIC concentrations increased and δ¹³C-DIC values decreased. These observations are explained by community respiration coupled with dissolution of calcite from cobbles in the aquifer matrix. When these results are compared to data from wells distributed over the entire floodplain (several km) a much less predictable relationship was observed between DO concentration and δ¹⁸O-DO. Many wells with low DO concentrations (e.g., <125 μmol L⁻¹ or 4 mg L⁻¹) had anomalously low δ¹⁸O-DO values (e.g., <20‰). Mass balance calculations show that approximately equal amounts of O₂ may be contributed to the aquifer by diffusion from the vadose zone and by advection from the river recharge. Calculations presented here suggest that diffusion across a narrow air-water interface can contribute isotopically light δ¹⁸O-DO to the saturated zone. Possible contributions of light δ¹⁸O-DO from other processes, such as isotopic exchange and radial oxygen loss from plant roots in or near the water table, are compared and evaluated.     

Geochemistry of natural chromium occurrence in a sandstone aquifer in Bauru Basin, São Paulo State, Brazil

Anomalous concentrations of Cr(VI) occur in groundwaters of the Adamantina Aquifer, in a large region in the western state of São Paulo, sometimes exceeding the potability limit (0.05 mg L⁻¹). To identify the possible geochemical reactions responsible for the occurrence of Cr in groundwater in Urânia, borehole rock samples were collected in order to carry out mineralogical and chemical analyses. In addition, multilevel monitoring wells were installed and groundwater samples were analyzed. Analyses of the borehole rock samples show the occurrence of a geochemical anomaly of Cr in the quartzose sandstones (average concentrations of 221 ppm). Chrome-diopside is one of the main minerals contributing to this anomaly, having an average Cr content of 1505 ppm. Sequential extraction experiments indicated weakly adsorbed Cr in the order of 0.54 ppm, and this quantity is enough to provide the Cr concentrations observed in groundwater. Groundwaters from the monitoring wells proved to be stratified, with the highest concentrations of Cr(VI) (0.13 mg L⁻¹) being associated with high redox and pH values (over 10) and high concentrations of Na. Geochemical reactions that may explain the release of Cr from the solid phase to groundwater involve the release of Cr(III) from minerals (like chrome-diopside and Cr-Fe hydroxide), followed by oxidation of Cr(III) to Cr(VI), probably related to the reduction of Mn oxides present in the aquifer. Then cation exchange occurs and dissolution of carbonates which increases the pH of groundwater, resulting in the desorption and mobilization of Cr(VI) into groundwater.     

Controls on δ34S and δ18O of dissolved sulfate in aquifers of the Murray Basin,Australia and their use as indicators of flow processes

The usefulness of stable isotopes of dissolved SO₄ (δ³⁴S and δ¹⁸O) to study recharge processes and to identify areas of significant inter-aquifer mixing was evaluated in a large, semi-arid groundwater basin in south-eastern Australia (the Murray Basin). The distinct isotopic signatures in the oxidizing unconfined Murray Group Aquifer and the deeper reducing Renmark Group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections. δ³⁴S values in the unconfined Murray Group Aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20.8 to 0.3‰. The concomitant increasing SO₄/Cl ratios, as well as relatively low δ¹⁸O_SO4 values, suggest that vertical input of biogenically derived SO₄ via diffuse recharge is the predominant source of dissolved SO₄ to the aquifer. Further along the hydraulic gradient towards the discharge area near the River Murray, δ³⁴S values in the unconfined Murray Group Aquifer increase, and SO₄/Cl ratios decrease, due to upward leakage of waters from the confined Renmark Group Aquifer which has a distinctly low SO₄/Cl and high δ³⁴S (14.9–56.4‰). Relatively positive δ³⁴S and δ¹⁸O_SO4 values, and low SO₄/Cl in the Renmark Group Aquifer is typical of SO₄ removal by bacterial reduction. The S isotope fractionation between SO₄ and HS⁻ of ∼24‰ estimated for the confined aquifer is similar to the experimentally determined chemical fractionation factor for the reduction process but much lower than the equilibrium fractionation (∼70‰) even though the confined groundwater residence time is >300 Ka years. Mapping the spatial distribution of δ³⁴S and SO₄/Cl of the unconfined Murray Group Aquifer provides an indicative tool for identifying the approximate extent of mixing, however the poorly defined end-member isotopic signatures precludes quantitative estimates of mixing fractions.     

Origin and evolution of formation water at the Jujo–Tecominoacán oil reservoir,Gulf of Mexico. Part 2: Isotopic and field-production evidence for fluid connectivity

The chemical and isotopic characterization of formation water from 18 oil production wells, extracted from 5200 to 6100 m b.s.l. at the Jujo–Tecominoacán carbonate reservoir in SE-Mexico, and interpretations of historical production records, were undertaken to determine the origin and hydraulic behavior of deep groundwater systems. The infiltration of surface water during Late Pleistocene to Early Holocene time is suggested by ¹⁴C-concentrations from 2.15 to 31.86 pmC, and by ⁸⁷Sr/⁸⁶Sr-ratios for high-salinity formation water (0.70923–0.70927) that are close to the composition of Holocene to modern seawater. Prior to infiltration, the super-evaporation of seawater reached maximum TDS concentrations of 385 g/L, with lowest δ¹⁸O values characterizing the most hypersaline samples. Minor deviations of formation water and dolomite host rocks from modern and Jurassic ⁸⁷Sr/⁸⁶Sr-seawater composition, respectively, suggest ongoing water–rock interaction, and partial isotopic equilibration between both phases. The abundance of ¹⁴C in all sampled formation water, ⁸⁷Sr/⁸⁶Sr-ratios for high-salinity water close to Holocene – present seawater composition, a water salinity distribution that is independent of historic water-cut, and a total water extraction volume of 2.037 MMm³ (1/83–4/07) excludes a connate, oil-leg origin for the produced water of the Jurassic–Cretaceous mudstone-dolomite sequence. Temporal fluctuations of water chemistry in production intervals, the accelerated migration of water fronts from the reservoir flanks, and isotopic mixing trends between sampled wells confirms the existence of free aquifer water below oil horizons. Vertical and lateral hydraulic mobility has probably been accelerated by petroleum extraction.