Hydraulic evaluation of aquifer storage and recovery (ASR) with urban stormwater in a brackish limestone aquifer

A 5-year aquifer storage and recovery trial at Andrews Farm in South Australia involving the injection of more than 250 ML (250,000 m³) of fresh but turbid stormwater into a brackish limestone aquifer over 4 years and recovery of 150 ML in the fifth provided the opportunity to evaluate rates of clogging and unclogging and the potential to recover water suitable for irrigation supplies. Results reveal there is some clogging by injected sediment, but only to a relatively small degree considering the high suspended solid concentrations and moderate aquifer transmissivity. This clogging was offset by increased matrix porosity through calcite dissolution and by routine well redevelopments after each 40 ML of injection. No significant microbial clogging occurred. Breakthrough responses at three observation wells and the proportion of injectant in the recovered water were determined from chloride data. Temperature and caliper profiles clearly indicate the heterogeneous nature of the aquifer that is attributed, in part, to sand removal during the initial well development. The recovery efficiency was greater than 60%. The trial demonstrates that urban stormwater containing high and variable particulate levels, which receives only passive pre-treatment and is not disinfected, can be used to freshen a heterogeneous brackish aquifer to create a useful water resource.     

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).     

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.     

Aquifer Storage Recovery (ASR) of chlorinated municipal drinking water in a confined aquifer

About 1.02 × 10⁶ m³ of chlorinated municipal drinking water was injected into a confined aquifer, 94–137 m below Roseville, California, between December 2005 and April 2006. The water was stored in the aquifer for 438 days, and 2.64 × 10⁶ m³ of water were extracted between July 2007 and February 2008. On the basis of Cl⁻ data, 35% of the injected water was recovered and 65% of the injected water and associated disinfection by-products (DBPs) remained in the aquifer at the end of extraction. About 46.3 kg of total trihalomethanes (TTHM) entered the aquifer with the injected water and 37.6 kg of TTHM were extracted. As much as 44 kg of TTHMs remained in the aquifer at the end of extraction because of incomplete recovery of injected water and formation of THMs within the aquifer by reactions with free-chlorine in the injected water. Well-bore velocity log data collected from the Aquifer Storage Recovery (ASR) well show as much as 60% of the injected water entered the aquifer through a 9 m thick, high-permeability layer within the confined aquifer near the top of the screened interval. Model simulations of ground-water flow near the ASR well indicate that (1) aquifer heterogeneity allowed injected water to move rapidly through the aquifer to nearby monitoring wells, (2) aquifer heterogeneity caused injected water to move further than expected assuming uniform aquifer properties, and (3) physical clogging of high-permeability layers is the probable cause for the observed change in the distribution of borehole flow. Aquifer heterogeneity also enhanced mixing of native anoxic ground water with oxic injected water, promoting removal of THMs primarily through sorption. A 3 to 4-fold reduction in TTHM concentrations was observed in the furthest monitoring well 427 m downgradient from the ASR well, and similar magnitude reductions were observed in depth-dependent water samples collected from the upper part of the screened interval in the ASR well near the end of the extraction phase. Haloacetic acids (HAAs) were completely sorbed or degraded within 10 months of injection.     

Numerical simulation of a managed aquifer recharge system designed to supply drinking water to the city of Amsterdam,The Netherlands

Managed aquifer recharge (MAR) is increasingly used to secure drinking water supply worldwide. The city of Amsterdam (The Netherlands) depends largely on the MAR in coastal dunes for water supply. A new MAR scheme is proposed for the production of 10 × 10⁶ m³/year, as required in the next decade. The designed MAR system consists of 10 infiltration ponds in an artificially created sandbank, and 25 recovery wells placed beneath the ponds in a productive aquifer. Several criteria were met for the design, such as a minimum residence time of 60 days and maximum drawdown of 5 cm. Steady-state and transient flow models were calibrated. The flow model computed the infiltration capacity of the ponds and drawdowns caused by the MAR. A hypothetical tracer transport model was used to compute the travel times from the ponds to the wells and recovery efficiency of the wells. The results demonstrated that 98% of the infiltrated water was captured by the recovery wells which accounted for 65.3% of the total abstraction. Other sources include recharge from precipitation (6.7%), leakages from surface water (13.1%), and natural groundwater reserve (14.9%). Sensitivity analysis indicated that the pond conductance and hydraulic conductivity of the sand aquifer in between the ponds and wells are important for the infiltration capacity. The temperature simulation showed that the recovered water in the wells has a stable temperature of 9.8–12.5 °C which is beneficial for post-treatment processes. The numerical modelling approach is useful and helps to gain insights for implementation of the MAR.

     

Application of modified qualitative index for surveillance of water-filtration process in turbidity removal by different media

Several media have been used in treatment plants, however, their efficiency for turbidity removal, which is determined by qualitative indices, has been considered. Current qualitative indices such as turbidity and escaping particle number could not completely measure the efficiency of the filtration system; therefore defining new qualitative indices is essential. In this study, the efficiency of two different dual media filters in turbidity removal was compared in different operating condition using qualitative indices. The pilot consisted of a filter column (1-m depth) in which the filter-1 was consisted of a layer of anthracite (450-mm depth) and a layer of silica sand (350-mm depth); and filter-2 had the same media characteristics except for the first layer that was light expanded clay aggregates (LECA). Turbidities of 10, 20, and 30 NTU, coagulant concentrations of 4, 8, and 12?ppm and filtration rates of 10, 15, and 20?m/h were considered as variables. Results showed that the media of filter-2 is a suitable substitute for the media of filter-1 (P value?<?0.05). Turbidity removal efficiencies in different condition were 79.97?±?1.79 to 91.37?±?1.23% for the filter-2 and 75.12?±?2.75 to 86.82?±?1.3% for the filter-1. The LECA layer efficiency in turbidity removal was independent of filtration rates and due to its low head loss; LECA can be used as a proper medium. Results also showed that the particle index was a suitable index as a substitute for turbidity and escaping particle number as indices.     

Clogging mechanisms and preventive measures in artificial recharge systems

Groundwater which occurs in fractured rock or porous aquifers or other geological weak zones such as faults and fractures is usually extracted via boreholes, hand wells or other sources such as springs.Water scarcity has become a severe problem due to many factors, such as an alarming increase in population and per capita water consumption, over exploitation of groundwater resources and abrupt global climatic change along with its related eco-environmental geological problems. In such situation, application of artificial recharge systems(e.g. surface recharge basin and deep injection well systems) can help to effectively manage and augment the unitization of groundwater resources. However, the clogging problem,which may be caused by a complex interdependent mechanisms of physical, chemical and biological has been a challenge for the efficacy and the implementation of recharge facilities. Clogging can reduce the permeability, recharge rate and longevity of recharge facilities and increase the operational and maintenance costs. Major influencing factors associated with the occurrence of clogging include the chemical composition of groundwater(both the recharge water and native groundwater), aquifer medium and microbial diversity, together with other environmental factors such as temperature, pressure, total dissolved solids, total soluble salts, pH, Eh, nutrients, gases, carbonates and others; these factors ultimately increase the piezometric head but reduce the permeability and infiltration rates of porous/seepage media.Pretreatment of recharge water can minimize the potential clogging. In the case of clogged wells,rehabilitation methods need to be deployed. In the meantime, there is an urgent needs to understand the basic causes and developmental processes/mechanisms of clogging in order to mitigate this problem. This paper reviews the major clogging mechanisms and their possible preventive measures and redevelopments in artificial recharge systems.     

Chemical and mechanical clogging of groundwater abstraction wells at well field Heel,the Netherlands

Well field Heel, in the south east of the Netherlands, consists of a row of wells drilled in an anoxic pyrite-containing aquifer alongside a former gravel pit, which now serves as a recharge basin, where water is actively aerated. All wells are seriously affected by chemical (screen slot) and/or mechanical (well bore) clogging. The objective of this study is to explain this combined occurrence. A combination of chemical, hydraulic and well-maintenance data indicate three groundwater quality types: (1) oxic basin water, (2) anoxic iron-containing basin water after oxidation of the traversed aquifer, and (3) deeply anoxic native groundwater. Wells abstracting a mixture of oxic basin water and anoxic basin water and/or native groundwater experience chemical well clogging, whereas wells abstracting (only or partly) native groundwater are vulnerable to mechanical well clogging. In the end, after oxic basin water has completely oxidized the traversed the aquifer, only two groundwater quality types will be present. Wells abstracting only oxic basin water will show no clogging, and wells abstracting a mixture of native groundwater and oxic basin water will experience chemical and possibly also mechanical well clogging. In this reasoning, the sequence in abstracted groundwater quality types coincides with a sequence in well clogging: from mechanical to chemical to no clogging. As well field Heel is situated in sloping terrain, the interplay between regional hydraulic gradient and different water qualities results in one-sided chemical clogging in the upper part of the well screen during abstraction, and in the lower part during the resting phase.     

Benefits and hurdles of using brackish groundwater as a drinking water source in the Netherlands

The production of fresh drinking water from brackish groundwater by reverse osmosis (BWRO) is becoming more attractive, even in temperate climates. For successful application of BWRO, the following approach is advocated: (1) select brackish source groundwater with a large volume and a composition that will yield a concentrate (waste water) with low mineral saturation; (2) maintain the feed water salinity at a constant level by pumping several wells with different salinities; (3) keep the permeate-to-concentrate ratio low, to avoid supersaturation in the concentrate; (4) keep the system anoxic (to avoid oxidation reactions) and pressurized (to prevent formation of gas bubbles); and (5) select a confined aquifer for deep well injection where groundwater quality is inferior to the membrane concentrate. This approach is being tested at two BWRO pilot plants in the Netherlands. Research issues are the pumping of a stable brackish source water, the reverse osmosis system performance, membrane fouling, quality changes in the target aquifer as a result of concentrate disposal, and clogging of the injection well. First evaluations of the membrane concentrate indicate that it is crucial to understand the kinetics of mineral precipitation on the membranes, in the injection wells, and in the target aquifer.     

Mobilisation of Iron from rocks in a fractured aquifer: Lithological and geochemical controls

Iron mobilisation from aquifer rocks in an important fractured aquifer system in South Africa is resulting in clogging of boreholes by Fe oxide minerals. Leach experiments using natural waters were conducted to determine the effects of redox conditions, pH lithology and presence of organic acids on the rate and extent of Fe dissolution from aquifer rocks, with the aim of clarifying the association of Fe clogging with geological formations that show Fe staining on weathering. The results indicate that the greatest amount of Fe (>30 mmol/kg rock) is leached from arenaceous rocks with low total Fe contents (49.0–75.0 mmol/kg) under anoxic conditions. Rocks with the highest Fe contents (>800 mmol/kg) generated low concentrations of Fe (<10 mmol/kg) even under favourable conditions of 0 mg/L DO and pH 3. The extent of Fe dissolution from the rocks was found to be most strongly dependent on the redox conditions, and the form of Fe present in the rock, with ascorbate-extracted amorphous Fe being the most mobile. The rate of dissolution is affected by pH and the presence of natural organic acids in the leachate. However, the effect of organic acids was only noticeable on arenaceous rocks.     

TECHNICAL NOTE An experimental investigation of air flow patterns in saturated soils during air sparging

Air sparging is an emerging method used to remediate saturated soils and groundwater that have been contaminated with volatile organic compounds (VOCs). During air sparging, air is injected into the subsurface below the lowest known depth of contamination. Due to buoyancy, the injected air will rise through the zone of contamination. Through a variety of mechanisms, including volatilization and biodegradation, the air will serve to remove or help degrade the contaminants. The contaminant-laden air will continue to rise towards the ground surface, eventually reaching the vadose zone, where the vapours are collected and treated using a soil vapour extraction (SVE) system.Air sparging performance and ultimately contaminant removal efficiency is highly dependent on the pattern and type of subsurface air flow. This paper presents the results of a laboratory experimental study which investigated the injected air flow pattern development within an aquifer simulation apparatus. The test apparatus consisted of a tank measuring 61 cm long by 25.4 cm wide by 38.1 cm high. The apparatus was equipped with one air injection well and two vapour extracton wells. Three different soils were used to simulate different aquifer conditions, including a sand, a fine gravel and a medium gravel. Experiments were performed with different injected air pressures combined with different vacuum and groundwater flow conditions. Experiments were also conducted by injecting air into simulated shallow aquifers with different thicknesses. The air flow patterns observed were found to depend significantly on the soil type, groundwater flow conditions and system controls, including injected air pressure, flow rate and applied vacuum. © Rapid Science Ltd. 1998     

Investigation of the geochemical impact of CO2 on shallow groundwater: design and implementation of a CO2 injection test in Northeast Germany

A small scale and temporally limited CO₂ injection test was performed in a shallow aquifer to investigate the geochemical impact of CO₂ upon such aquifers and to apply and verify different monitoring methods. Detailed site investigation coupled with multiphase simulations were necessary to design the injection experiment and to set up the monitoring network, before CO₂ was injected over a ten-day period at three injection wells, at a depth of 18?m below surface level into a quaternary sand aquifer located close to the town of Wittstock in Northeast Germany. Monitoring methods comprised groundwater sampling and standard analyses, as well as trace element analyses and isotope analyses; geoelectrical borehole monitoring; passive samplers to analyse temporally integrated for cations and multi-parameter probes that can measure continuously for dissolved CO₂, pH and electrical conductivity. Due to CO₂ injection, total inorganic carbon concentrations increased and pH decreased down to a level of 5.1. Associated reactions comprised the release of major cations and trace elements. Geoelectrical monitoring, as well as isotope analyses and multi-parameter probes proved to be suitable methods for monitoring injected CO₂ and/or the alteration of groundwater.     

Hydrogeology of deep-well disposal of liquid wastes in southwestern Florida, USA

 Deep-well injection has been used to dispose of municipal liquid wastes in southwestern Florida since 1988. The liquid wastes are injected into an extremely high-transmissivity zone of fractured dolomite in the Early Eocene Oldsmar Formation of the Floridan aquifer system; this zone is commonly referred to as the Boulder Zone. Data collected during the drilling and operational testing of southwestern Florida injection wells provide insights into the nature of the injection zone and overlying confining beds. The location of high-transmissivity zones that are capable of accepting large quantities of waste water is vertically and horizontally variable and cannot be predicted with certainty. A 40.9-m thick high-permeability interval in one injection well, for example, was absent in a well drilled only 85.4 m away. Some upward migration of low-density injected fluids has occurred, but at no site were the injected liquids detected in deep monitor wells, such as occurred at injection-well sites along the coasts of southeastern, west-central, and east-central Florida. The primary confinement of the injected liquids (i.e., deepest effective confining beds) consists of unfractured beds of low-permeability dolomite within the Oldsmar Formation, whose locations are also laterally and vertically variable. The origin and controls of the distribution of fractures in the Oldsmar Formation are poorly understood. Received, December 1997 Revised, June 1998, August 1998 Accepted, August 1998     

Multi-scale characterization of a complex karst and alluvial aquifer system in southern Germany using a combination of different tracer methods

Water suppliers face major challenges such as climate change and population growth. To prepare for the future, detailed knowledge of water resources is needed. In southern Germany, the state water supplier Zweckverband Landeswasserversorgung provides 3 million people with drinking water obtained from a complex karst and alluvial aquifer system and the river Danube. In this study, a combination of different tracing techniques was used with the goal of a multi-scale characterization of the aquifer system and to gain additional knowledge about groundwater flow toward the extraction wells in the Danube Valley. For the small-scale characterization, selected groundwater monitoring wells were examined using single-borehole dilution tests. With these tests, a wide range of flow behavior could be documented, including fast outflow within just a few hours in wells with good connection to the aquifer, but also durations of many weeks in low-permeability formations. Vertical flow, caused by multiple flow horizons or uprising groundwater, was detected in 40% of the tested wells. A regional multi-tracer test with three injections was used to investigate the aquifer on a large scale. For the highly karstified connection between a swallow hole and a spring group, high flow velocities of around 80 m/h could be documented. Exceptionally delayed arrivals, 250 and 307 days after the injection, respectively showing maximum velocities of 0.44 and 0.39 m/h, were observed in an area where low-permeability sediments overlay the karst conduits. With the chosen methods, a distinct heterogeneity caused by the geological setting could be documented on both scales.

     

A field trial for in-situ bioremediation of 1,2-DCA

Historic spillages of chlorinated hydrocarbons at a vinyl chloride plant in the Rotterdam Botlek area in The Netherlands have lead to contamination of the underlying aquifer. The principal contaminant is 1,2-dichloroethane (1,2-DCA). The contamination is temporarily contained by a pump-and-treat system. A field trial was carried out to investigate the feasibility of treating the dissolved phase of 1,2-DCA via reductive dechlorination by injection of an aqueous solution of methanol, ammonium chloride and sodium chloride into the confined aquifer using an array of eight boreholes. Biodegradation of 1,2-DCA was localised. This was attributed to limited mixing of the carbon substrate within the test zone. In addition, clogging of recharge wells complicated groundwater circulation.     

The effectiveness of artificial recharge in combating seawater intrusion in Salalah coastal aquifer, Oman

The Salalah central sewage treatment plant has been designed to treat 20,000 m³/day at the first stage and two further stages to double the initial capacity. The plant currently (2005) treats more than 15,000 m³/day effluents to a tertiary level, and after chlorination phase, the effluents are recharged into tube wells in a line parallel to the coast. The process aims to help stabilize the seawater interface and a part to be recovered from hand-dug wells/boreholes further inland and downstream. A three-dimensional flow and solute advection transport model was developed to assess the effectiveness of the proposed recharge scheme and to track the solute transport with respect to the design system. The advection transport model predicted that in 2020 the maximum pathlines of the injection fluids would reach the abstraction wells that are located 600 m, southward of the injection bores in about 1-year travel time in the case of the no-management interference and more than that southward under management interference. The developed flow predicted the wedge of the saline intrusion in 2019 is tracked up to 2.7 and 3.4 km from the shoreline with the injection and without the injection, respectively under constant underflow. The injection scheme is effective in pushing back the saline zone front by 700 m. This study argues that the treated wastewater would help to increase the water levels at the vicinity of the injection line and to reduce the influence of saline inflows from the coast. The reclaimed sewage recharge scheme is examined in the case of the Salalah coastal aquifer using groundwater simulation, which can also be applied to other regions with similar conditions.     

管涌与滤层的研究(Ⅱ): 滤层

对管涌和滤层两个相关联问题的研究和发展过程进行了综述和评论;同时也给出了过去的研究成果,并与之进行比较。在滤层研究方面,指出了由平均粒径一点控制到两点和级配曲线多点控制的发展趋势;研究表明:太沙基的滤层规格比较保守,应结合内外水动力学研究砂砾滤层和织物滤层的适用规格。而粘土的滤层设计,关键是看能否发生贯穿性裂缝,其设计思想悬殊;井管滤层的淤堵失效应研究抽洗复苏的良策。     

Impacts of at-site wastewater disposal systems on the groundwater aquifer in arid regions: case of Sfax City, Southern Tunisia

Groundwater in Sfax City (Tunisia) has been known since the beginning of the century for its deterioration in quality, as a result of wastewater recharge into the aquifer. An average value of 12 × 10⁶ m³ of untreated wastewater reaches the groundwater aquifer each year. This would result not only in a chemical and biological contamination of the groundwater, but also in an increase of the aquifer piezometric level. Quantitative impacts were evaluated by examining the groundwater piezometric level at 57 surface wells and piezometers. The survey showed that, during the last two decades, the groundwater level was ever increasing in the urban area with values reaching 7 m in part; and decreasing in Sidi Abid (agricultural area) with values exceeding −3 m. Groundwater samples for chemical and microbial analysis were collected from 41 wells spread throughout the study area. Results showed significantly elevated levels of sodium, chlorides, nitrates and coliform bacteria all over the urban area. High levels (NO₃: 56–254 mg/l; Na >1,500 mg/l; Coliforms >30/100 ml) can be related to more densely populated areas with a higher density of pit latrine and recharge wells. Alternatively results showed a very variable chemical composition of groundwater, e.g. electrical conductivity ranges from 4,040 to19,620 μs/cm and the dry residual varies between 1.4 and 14 g/l with concentrations increasing downstream. Furthermore a softening of groundwater in Set Ezzit (highly populated sector) was observed.     

Pollution characteristics of alluvial groundwater from springs and bore wells in semi-urban informal settlements of Douala,Cameroon, Western Africa

Alluvial groundwater from springs and bore wells, used as the major source of water for drinking and other domestic purposes in the semi-urban informal settlements of Douala, Cameroon, has been studied. Six representative springs, four bore wells and two hand dug wells, situated in the Phanerozoic basin were selected, from which a total of 72 water samples were analyzed for chemical characteristics and indicators of bacterial contamination. The results showed anthropogenic pollution, evident from high concentrations of organic (up to 94.3 mg NO₃/l nitrate) fecal coliform and fecal streptococcus detected in the springs and bore wells (with values of 2,311 and 1,500 cfu/100 ml, respectively). The pH ranged from 3.4–6.5, which is lower than the guidelines for drinking water. Groundwater samples from background upstream inland natural areas W1 and W2 had low electrical conductivity (54.2 and 74.8 μs/cm, respectively) and major ions, which increased downstream in the valleys, peaking in the more densely settled areas. An acceptable concentration of solutes was observed for the bore wells except for a single sample from B4. The bore-well sample B4 registered the highest microbial content (2,130 cfu/100 ml) and nitrate level(26 mg/l), which could be due to the bottom of this well lying just at or close to the zone of mixing between sewage and groundwater. The absence of a direct correlation between nitrate and fecal matter suggests multiple sources of contamination. The shallow alluvial aquifer consists of unconsolidated deposits of gravel, sand, silt and clay. The springs, therefore, receive direct recharge from the ground surface with limited contaminant attenuation, which leads to water quality deterioration, especially during the rainy season. This shows the urgent need to put basic service infrastructures in place. The local population should be sensitized to the importance of chlorinating and boiling drinking water to prevent health hazards.     

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.