Elevated As concentrations in groundwater of the Huhhot basin (HB), Inner Mongolia, China, and the western Bengal basin (WBB), India, have been known for decades. However, few studies have been performed to comprehend the processes controlling overall groundwater chemistry in the HB. In this study, the controls on solute chemistry in the HB have been interpreted and compared with the well-studied WBB, which has a very different climate, physiography, lithology, and aquifer characteristics than the HB. In general, there are marked differences in solute chemistry between HB and WBB groundwaters. Stable isotopic signatures indicate meteoric recharge in the HB in a colder climate, distant from the source of moisture, in comparison to the warm, humid WBB. The major-ion composition of the moderately reducing HB groundwater is dominated by a mixed-ion (Ca-Na-HCO 3 -Cl) hydrochemical facies with an evolutionary trend along the regional hydraulic gradient. Molar ratios and thermodynamic calculations show that HB groundwater has not been affected by cation exchange, but is dominated by weathering of feldspars (allitization) and equilibrium with gibbsite and anorthite. Mineral weathering and mobilization of As could occur as recharging water flows through fractured, argillaceous, metamorphic or volcanic rocks in the adjoining mountain-front areas, and deposits solutes near the center of the basin. In contrast, WBB groundwater is Ca-HCO 3 -dominated, indicative of calcite weathering, with some cation exchange and silicate weathering (monosiallitization).

Two extensive plumes (combined area >1000km2) have been delineated within the Ogallala aquifer in the Southern High Plains, TX, USA. Salinity varies within the plumes spatially and increases with depth; Cl ranges from 50 to >500mgl−1. Variable-density flow modeling using SUTRA has identified three broad regions of upward cross-formational flow from the underlying evaporite units. The upward discharge within the

Samples of deep groundwater, river water and rainwater were collected for d 18 O and d 2 H analyses from an area of $22,000 km 2 in the arsenic-affected districts of West Bengal, India, in order to examine groundwater recharge. A plot of d 18 O versus d 2 H of groundwater falls subparallel to the constructed local meteoric water line (d 2 H = 7.2 d 18 O + 7.7), suggesting a predominance of meteoric recharge with some evaporation. The stable isotopic signature of groundwater from the deeper part of the semiconfined main aquifer is similar to that of shallower groundwater, which suggests that deeper groundwater has been recharged in the present-day climatic regime, mostly from monsoonal rainfall. Groundwater in deeper isolated aquifers falls within the isotopic range observed for the main aquifer. A trend of isotopic depletion of groundwater that extends northward and westward from the Bay of Bengal (the monsoonal moisture source area) indicates a rain-out process following Rayleigh-type distillation. Some recharge may have taken place from the rivers Ganges and Bhagirathi-Hoogly, with minimal inflow from other rivers (Jalangi and Ichamati).

In the Coastal Plain of the southeastern USA, streams have commonly been artificially channelized for flood control and agricultural drainage. However, groundwater discharge along such streams has received relatively little attention. Using a combination of stream-and spring-flow measurements, spring temperature measurements, temperature profiling along the stream-bed, and geologic mapping, we delineated zones of diffuse and focused discharge along Little Bayou Creek, a channelized, first-order perennial stream in western Kentucky. Seasonal variability in groundwater discharge mimics hydraulic-head fluctuations in a nearby monitoring well and spring-discharge fluctuations elsewhere in the region, and is likely to reflect seasonal variability in recharge. Diffuse discharge occurs where the stream is incised into the semi-confined regional gravel aquifer, which is comprised of the Mounds Gravel. Focused discharge occurs upstream where the channel appears to have intersected preferential pathways within the confining unit. Seasonal fluctuations in discharge from individual springs are repressed where piping results in bank collapse. Thereby, focused discharge can contribute to the morphological evolution of the stream channel.

Recent studies in various settings across eastern North America have examined the movement of volatile organic compound (VOC) plumes from groundwater to streams, but few studies have addressed focused discharge of such plumes in unlithified sediments. From 1999 through 2002, we monitored concentrations of trichloroethene (TCE) and the non-volatile co-contaminant technetium-99 (99 Tc) along Little Bayou Creek, a first-order perennial stream in the Coastal Plain of western Kentucky. Spring flow contributed TCE and 99 Tc to the creek, and TCE concentrations tended to vary with 99 Tc in springs. Contaminant concentrations in stream water fluctuated seasonally, but not always synchronously with stream flow. However, contaminant influxes varied seasonally with stream flow and were dominated by a few springs. Concentrations of O 2 , NO À 3 , and SO 2À 4 , values of d 37 Cl DOCl in groundwater, and the lack of less-chlorinated ethenes in groundwater and stream water indicated that anaerobic biodegradation of TCE was unlikely. Losses of TCE along Little Bayou Creek resulted mainly from volatilization, in contrast to streams receiving diffuse contaminated discharge, where intrinsic bioremediation of VOCs appears to be prevalent.

An extensive saline plume (>250 km 2) within the regionally important uncon®ned aquifer in the Neogene Ogallala Formation overlies the Panhandle oil and gas ®eld in the Southern High Plains, Texas, USA. Relative to upgradient Ogallala water, the plume waters have d 18 O (À6.7 to À8.8-) and dD (À42 to À88-) values that tend to be depleted and have higher Cl (>150 mg/l) and SO 4 (>75 mg/l) concentrations. Various end-member-mixing models suggest that the plume composition re¯ects 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 in¯uenced by the Amarillo uplift, pinch-out of the laterally adjoining con®ned 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.

Information on groundwater chemistry in the central Ganges basin can provide insights into recharge, provenance, and fate of solutes in arsenic (As)-affected areas upstream of the more intensively studied Bengal basin. The geological and geomorphological units of the region are much more discernible than the Bengal basin aquifers. Moreover, the region is less affected by groundwater abstraction, which complicates interpretation of As distributions in the Bengal basin. The study area extends from the northern edge of the Indian craton outcrops to the foothills of the Himalayas. Geologic units in the area can be broadly classified as pre-Cenozoic metamorphics and volcanics (PC), older alluvial deposits of the Ganges and its tributaries (OA), younger or active alluvial deposits of the Ganges and its tributaries in the basin (YA), and sediments of the Himalayan foothills (piedmont, PD). Stable-isotopic analyses indicate groundwater in these units has been recharged by meteoric or surface water that has generally undergone some evaporation. The hydrochemical facies is generally a Ca-HCO 3 type. While most of the solutes in the YA groundwater are derived from carbonate dissolution, many of the PD, PC and OA groundwater samples are influenced by silicate weathering, suggesting that leaching of metamorphics and volcanics acts as a major source of solutes. Redox conditions are highly spatially variable (oxic to methanic, dominated by metal reduction), with no systematic depth variation within sampled aquifers. More than 75% of YA and PD groundwater samples have As P 0.01 mg/L, but As was detected in only one OA sample and no PC samples. Arsenic is probably mobilized by reductive dissolution of Fe-Mn (oxyhydr)oxides in the alluvium, with possibility of competitive anionic mobilization. Hence, relative to the Bengal basin, in addition to lower groundwater abstraction influence, groundwater chemistry in the study area reflects a greater variety of differences in the geological and geomorphological settings of the aquifers.

Irrigation, urbanization, and drought pose challenges for the sustainable use of ground water in the central Couloir sud rifain, a major agricultural region in north-central Morocco, which includes the cities of Fès and Meknès. The central Couloir is underlain by unconfined and confined carbonate aquifers that have suffered declines in hydraulic head and reductions in spring flow in recent decades. Previous studies have surveyed ground water flow and water quality in wells and springs but have not comprehensively addressed the chemistry of the regional aquifer system. Using graphical techniques and saturation index calculations, we infer that major ion chemistry is controlled (1) in the surficial aquifer by cation exchange, calcite dissolution, mixing with deep ground water, and possibly calcite precipitation and (2) in the confined aquifer and warm springs by calcite dissolution, dolomite dissolution, mixing with water that has dissolved gypsum and halite, and calcite precipitation. Analyses of 2 H and 18 O indicate that shallow ground water is affected by evaporation during recharge (either of infiltrating precipitation or return flow), whereas deep ground water is sustained by meteoric recharge with little evaporation. Mechanisms of recharge and hydrochemical evolution are broadly consistent with those delineated for similar regional aquifer systems elsewhere in Morocco and in southern Spain.

Because interactions between ground water and tributaries may influence contaminant loading to rivers, we delineated seepage along Little Bayou and Bayou Creeks in McCracken County, Kentucky, during a two-year period. From the Paducah Gaseous Diffusion Plant, on the divide between the creeks, trichloroethene and technetium-99 plumes extend several km toward the Ohio River. Gaining conditions occur where the creeks are incised into coarse sediments in the river's flood plain. Such conditions were marked by upward hydraulic gradients within the bed; maximum specific discharge (q) > 0.24 m d-1 ; relatively narrow ranges of stream, piezometer, and bed temperatures; relatively cool bed and bank temperatures in summer and early autumn; detections of trace solutes in stream water; and observations of springs, boils, and seeps. Evidence of losing or nonet discharge conditions included downward or lateral hydraulic gradients; minimal q values (indicative of stream-water flow through the bed); and relatively broad annual ranges of stream and piezometer temperatures. Mixing calculations using 6 18 0 and cisupport inferences about gaining and losing reaches. Seepage rates and directions changed during dry periods in summer and early autumn and following Ohio River flooding in spring. Discharge of uncontaminated ground water dilutes contaminants in Little Bayou Creek.

Recent work has demonstrated the utility of reactive barriers for transformation or immobilization of contaminants in ground water. However, reaction-induced changes in hydraulic conductivity may compromise the reactive barrier by diverting flow or focusing breakthrough of contaminants. For an acidic, ferric solution flowing through calcareous sand in column experiments, we investigated how hydraulic-conductivity reduction, reaction-front propagation, and fingering depend upon the reactive solid volume, reactive surface area, and pore-water velocity. Hydraulicconductivity reductions are greater with larger initial pore-water velocity, calcite surface area, or Ž. calcite volume. Reductions in hydraulic conductivity up to four orders of magnitude result Ž. primarily from CO g exsolution rather than from ferric oxyhydroxide precipitation and may be 2 at least partly reversed as bubbles migrate. Although fingering is both driven and repressed by pore-scale hydraulic changes, the velocity of the reaction front, width of the primary reaction zone, and maximum length of the largest finger appear to be insensitive to macroscopic changes in hydraulic conductivity at a constant flow rate. Reaction-front velocity increases as the ratio of initial calcite volume to pore-water velocity decreases, whereas zonal width and maximum finger length appear to increase as the Damkohler number decreases for a given calcite grain-sizë distribution. These results offer guidelines for improving the efficiency of reactive barriers when the reaction rate equals or exceeds the rate of mass transfer.

In arid and semi-arid environments, artificial recharge or reuse of wastewater may be desirable for water conservation, but NO y contamination of underlying aquifers can result. On the 3 Ž. semi-arid Southern High Plains USA , industrial wastewater, sewage, and feedlot runoff have been retained in dozens of playas, depressions that focus recharge to the regionally important High Ž. Plains Ogallala aquifer. Analyses of ground water, playa-basin core extracts, and soil gas in an 860-km 2 area of Texas suggest that reduction during recharge limits NO y loading to ground 3 water. Tritium and Cl y concentrations in ground water corroborate prior findings of focused 15 Ž. recharge through playas and ditches. Typical d N values in ground water) 12.5‰ and correlations between d 15 N and ln C y suggest denitrification, but O concentrations G 3.24 NO-N 2 3 mg l y1 indicate that NO y reduction in ground water is unlikely. The presence of denitrifying and 3 NO y-respiring bacteria in cores, typical soil-gas d 15 N values-0‰, and decreases in NO y-3 3 NrCl y and SO 2y rCl y ratios with depth in cores suggest that reduction occurs in the upper 4 vadose zone beneath playas. Reduction may occur beneath flooded playas or within anaerobic microsites beneath dry playas. However, NO y-N concentrations in ground water can still exceed 3 drinking-water standards, as observed in the vicinity of one playa that received wastewater. Therefore, continued groundwater monitoring in the vicinity of other such basins is warranted.

Samples of deep groundwater, river water and rainwater were collected for d 18 O and d 2 H analyses from an area of $22,000 km 2 in the arsenic-affected districts of West Bengal, India, in order to examine groundwater recharge. A plot of d 18 O versus d 2 H of groundwater falls subparallel to the constructed local meteoric water line (d 2 H = 7.2 d 18 O + 7.7), suggesting a predominance of meteoric recharge with some evaporation. The stable isotopic signature of groundwater from the deeper part of the semiconfined main aquifer is similar to that of shallower groundwater, which suggests that deeper groundwater has been recharged in the present-day climatic regime, mostly from monsoonal rainfall. Groundwater in deeper isolated aquifers falls within the isotopic range observed for the main aquifer. A trend of isotopic depletion of groundwater that extends northward and westward from the Bay of Bengal (the monsoonal moisture source area) indicates a rain-out process following Rayleigh-type distillation. Some recharge may have taken place from the rivers Ganges and Bhagirathi-Hoogly, with minimal inflow from other rivers (Jalangi and Ichamati).

Recent studies in various settings across eastern North America have examined the movement of volatile organic compound (VOC) plumes from groundwater to streams, but few studies have addressed focused discharge of such plumes in unlithified sediments. From 1999 through 2002, we monitored concentrations of trichloroethene (TCE) and the non-volatile co-contaminant technetium-99 (99 Tc) along Little Bayou Creek, a first-order perennial stream in the Coastal Plain of western Kentucky. Spring flow contributed TCE and 99 Tc to the creek, and TCE concentrations tended to vary with 99 Tc in springs. Contaminant concentrations in stream water fluctuated seasonally, but not always synchronously with stream flow. However, contaminant influxes varied seasonally with stream flow and were dominated by a few springs. Concentrations of O 2 , NO À 3 , and SO 2À 4 , values of d 37 Cl DOCl in groundwater, and the lack of less-chlorinated ethenes in groundwater and stream water indicated that anaerobic biodegradation of TCE was unlikely. Losses of TCE along Little Bayou Creek resulted mainly from volatilization, in contrast to streams receiving diffuse contaminated discharge, where intrinsic bioremediation of VOCs appears to be prevalent.