Freshwater Recharge Research Articles

Application of multivariate statistical procedures to the hydrochemical study of a coastal aquifer: an example from Crete, Greece

AbstractA variety of multivariate statistical procedures were applied to three separate sets of quantitative analytical data from a coastal aquifer located in Malia, Crete (Greece), in order to identify the major hydrochemical processes affecting the groundwater quality and to investigate the evolution of groundwater composition in three different sampling periods. Two of them were carried out on October 2001 and September 2002 at the end of the dry season and the third on April 2002 at the end of the wet period. Two factors were found that explained major hydrochemical processes in the aquifer. These factors reveal the existence of an intensive intrusion of seawater and mechanisms of nitrate contamination of groundwater. Bivariate plots of the scores of the two main factors showed that the seawater intrusion and nitrate pollution processes are maintained through three surveys and that the process of nitrate pollution increases from the first to the second dry survey. Q‐mode factor analysis and discriminant analysis of the three sampling periods clearly showed a seasonal variation of the whole chemistry of groundwater samples. This seasonal variation can be attributed to the freshwater recharge and seawater intrusion that affect the groundwater quality of the Malia aquifer. The results of trend surface analysis are in agreement with those of factor analysis. Moreover, the fourth‐order trend surfaces of EC, Cl− and NO3− showed that the salinization process is more intensive during the first dry period and the spatial variation of NO3− maxima plumes are strongly affected by the flow regime of the Malia aquifer. Copyright © 2007 John Wiley & Sons, Ltd.

Airborne geophysical mapping of aquifer water quality and structural controls in the Lower Okavango Delta, Botswana

Groundwater exploration in the Lower Okavango Delta to the west of Maun in Northern Botswana has focused on the search for shallow, fresh-water aquifers in Kalahari sediments. The target aquifers comprised thick packages of medium grained sands characterised by medium porosity and permeability, allowing for annual flushing and freshwater recharge during seasonal floods. Elsewhere, a combination of poor drainage, evaporation and evapotranspiration results in the widespread distribution of saline groundwater. The northeasterly-striking Tamalakane and Kunyere fault bounding the southern edge of the Okavango graben are still active, and in the recent past movements along these features has controlled the distribution of drainage systems across the Lower Delta. As a consequence, target freshwater palaeochannels may now be distant from present river channels, but still undergo recharge during annual Delta floods. The area is covered by Kalahari beds of variable thickness which overly Karoo sediments above a Damaran basement. Airborne geophysical mapping conducted by the Botswana Government at a scale akin to that of mineral exploration has played a major role in the groundwater exploration programme, as well as adding significantly to a structural understanding of the area. Airborne magnetic/DTM surveys have directly mapped the southern Thamalakane and Kunyere Fault zones as magnetic linears along fault-truncated magnetic lithologies of the Damaran basement, and as DTM scarp features at surface. Magnetic modelling of fault and dyke anomalies over the 65 km wide Okavango Dyke Swarm has quantified the expected step-wise northerly increase in thickness of the Kalahari sediments (from 10 m to 400 m). Basin or graben-like structures within the inter-fault area point to secondary fault linkages between the two major bounding fault zones. Airborne and follow-up time-domain EM (TDEM) surveys were used as hydrogeological mapping tools to locate and delineate potential freshwater-sand aquifers on the basis of their relatively high formation resistivities. Such aquifers appear as sinuous zones of moderate conductivity in an otherwise high-conductivity background environment reflecting the widespread distribution of saline groundwater away from permanent river channels and, to a possible lesser extent, clay-rich units interbedded with the sands. Airborne survey data was used to set the surface lateral limits of potential freshwater aquifers, and follow-up ground EM sounding surveys to determine the maximum thickness of such systems from mapping of the fresh-saline water interface. Over the better-developed aquifers this interface is found at depths of 60 m to 80 m. Elsewhere it may be within 10m of surface. Findings are exemplified by reference to follow-up work and borehole results over the Upper Thamalakane Aquifer which is proximal to the permanent river channel, and for which an isopach model was developed from extensive EM sounding work.

Groundwater compartmentalisation: a water table height and geochemical analysis of the structural controls on the subdivision of a major aquifer, the Sherwood Sandstone, Merseyside, UK

Abstract. Compartmentalisation, the subdivision of an aquifer into discrete and relatively isolated units, may be of critical importance for the protection of groundwater although it has been largely ignored in the groundwater literature. The Lower Triassic Sherwood Sandstone, in north west of England, UK, may be a good example of an aquifer that has been compartmentalised by numerous high angle faults with displacements of up to 300 m. The study was initiated to assess the local groundwater flow, the extent of seawater invasion and the controls on recharge in the aquifer and to try to understand whether the aquifer is broken into discrete compartments. Maps and schematic cross-sections of groundwater heads for the years 1993, and 2002 were prepared to trace any structural controls on the groundwater heads across the area. Studying the contour maps and cross sections revealed that: 1) there are substantial differences in groundwater head across some of the NNW-SSE trending faults implying that groundwater flow is strongly limited by faults, 2) an anticline in the east of the area acts as a groundwater divide and 3) the groundwater head seems to follow the topography in some places, although steep changes in groundwater head occur across faults showing that they locally control the groundwater head. The aquifer was thus provisionally subdivided into several hydrogeological sub-basins based on groundwater head patterns and the occurrence of major structural features (faults and a fold). Using groundwater geochemistry data, contour maps of chloride and sulphate concentration largely support the structural sub-division of the area into hydrogeological sub-basins. Scrutiny of groundwater geochemical data, averaged for each sub-basin, confirmed the degree of compartmentalisation and the occurrence of sealed faults. The variation of the geochemical composition of the groundwater not only relates to the different, localised geochemical processes and seawater intrusion but also relates to compartmentalisation due to faulting. Faults have limited the degree of mixing between the groundwater types thus retaining the specific characteristics of each sub-basin. Highly localised seawater intrusion is mainly controlled by low permeability fault close to the Irish Sea and Mersey estuary. There is effectively no invasion of seawater beyond the faults that lie closest to the coastline. Freshwater recharge to the aquifer seems to be highly localised and mainly occurs by vertical percolation of rain and surface water rather than whole aquifer-scale groundwater flow. This study provides a detailed understanding of the groundwater flow processes in Liverpool as an example of methods can be applied to groundwater management elsewhere.

Hydrogeochemistry and its relation to groundwater level fluctuation in the Palar and Cheyyar river basins, southern India

An investigation was carried out to understand the role of water level fluctuation on major-ion chemistry of groundwater in the Palar and Cheyyar river basins, southern India. As groundwater is the only major source of water for agricultural and drinking purposes in this area, it is important to know the effect of geological formations and agricultural activities on groundwater chemistry. Groundwater samples were collected once a month from 43 wells (641 samples in total), from January 1998 to June 1999, and analysed for major ions. The results indicate that the major-ion chemistry of the groundwater varies with respect to space. Groundwater occurring near the River Palar has a high concentration of major ions except calcium, due to the absence of any recharge from the river, whereas lower concentrations of major ions were observed in the central part of the study area due to the recharge of fresh water from a number of surface reservoirs. The major-ion chemistry of the study region is controlled by both mineral dissolution and anthropogenic activities. The relative contributions of mineral dissolution and anthropogenic contamination are estimated by a stoichiometric approach, which suggests that mineral dissolution is the dominant process in both the formations. The relation between water level fluctuations and major-ion chemistry indicates that major-ion chemistry is also greatly influenced by the water level fluctuations in different geological formations. Thus, the major-ion chemistry of groundwater in this region is greatly influenced by mineral dissolution, anthropogenic activities and water level fluctuations in different geological formations. Copyright © 2006 John Wiley & Sons, Ltd.

Distribution of Iron, Manganese, Zinc and Atrazine in Groundwater in Parts of Palar and Cheyyar River Basins, South India

A study was carried out in a part of Palar and Cheyyar river basin to evaluate the current status of iron, manganese, zinc and atrazine concentrations, their origin and distribution in groundwater. Groundwater samples were collected during post-monsoon (March 1998 and February 1999) and pre-monsoon (June 1999) periods from 41 sampling wells distributed throughout the study area. The groundwater samples were analyzed for trace metals using AAS and atrazine using HPLC. The concentration of the trace elements in groundwater is predominant during pre-monsoon period. Distribution pattern indicates that the concentration of these elements increases from west to northeast and towards Palar river. Lower concentrations in the central part may be due to recharge of fresh water from the lakes located here. During most of the months, as there is no flow in Palar river, the concentrations of trace elements in groundwater are high. Drinking water standards indicate that Mn and Zn cross the permissible limit recommended by EPA during the pre-monsoon period. A comparison of groundwater data with trace element chemistry of rock samples shows the abundance of trace elements both in the rock and water in the order of Fe > Mn > Zn and Fe > Zn > Mn. This indicates that iron in groundwater is derived from lithogenic origin. Further, Fe, Mn and Zn have good correlation in rock samples, while it is reverse in the case of water samples, indicating the non-lithogenic origin of Mn and Zn. Atrazine (a herbicide) was not detected in any of the groundwater samples in the study area, perhaps due to low-application rate and adsorption in the soil materials.

Use of relational databases to evaluate regional petroleum accumulation, groundwater flow, and CO2 sequestration in Kansas

Large-scale relational databases and geographic information system tools are used to integrate temperature, pressure, and water geochemistry data from numerous wells to better understand regional-scale geothermal and hydrogeological regimes of the lower Paleozoic aquifer systems in the mid-continent and to evaluate their potential for geologic CO2 sequestration. The lower Paleozoic (Cambrian to Mississippian) aquifer systems in Kansas, Missouri, and Oklahoma comprise one of the largest regional-scale saline aquifer systems in North America. Understanding hydrologic conditions and processes of these regional-scale aquifer systems provides insight to the evolution of the various sedimentary basins, migration of hydrocarbons out of the Anadarko and Arkoma basins, and the distribution of Arbuckle petroleum reservoirs across Kansas and provides a basis to evaluate CO2 sequestration potential. The Cambrian and Ordovician stratigraphic units form a saline aquifer that is in hydrologic continuity with the freshwater recharge from the Ozark plateau and along the Nemaha anticline. The hydrologic continuity with areas of freshwater recharge provides an explanation for the apparent underpressure in the Arbuckle Group.

Simulation of Hydrosalinity Behavior Under Skimming Wells

The movement of underlying saline water inresponse to freshwater abstraction infresh-saline aquifers is dependent uponaquifer properties, well design andoperational plans. The responses of thesefactors under various well configurationsand pumping modes are important forsustainable freshwater abstraction. In thepresent study, three shallow skimming wellswith different well configurations weremonitored under different pumping modes inthe Indus basin of Pakistan. The field datawere used to calibrate MODFLOW and MT3Dmodels for each well system. The resultsshowed that the selected models have thecapability to simulate both the continuousand intermittent pumping operations inskimming wells. The calibrated models wereused to evaluate the sensitivity of variousaquifers, well design, and operationalparameters. Hydrosalinity of the pumpedwater increased with the increase invertical hydraulic conductivity,longitudinal dispersivity, well abstractionrate, well penetration ratio, and pumpingduration; while increase in horizontalhydraulic conductivity, effective porosity,specific yield, freshwater recharge,transverse dispersivity, and number ofboreholes in multi-borehole skimming wellsdecreased the salinity of the pumpedwater.

Improving the worthiness of the Henry problem as a benchmark for density‐dependent groundwater flow models

This study considers the worthiness of the Henry saltwater intrusion problem as a test case to benchmark density‐dependent groundwater flow models. Previously published results from a coupled versus uncoupled analysis indicate that the flow patterns associated with the standard Henry problem are largely dictated by the boundary forcing and not necessarily a result of density‐dependent effects [Simpson and Clement, 2003]. Results in this study show that the worthiness of Henry's problem can be improved by decreasing the freshwater recharge. This modification maintains the usual benefits of the standard test case, while at the same time increasing the influence of the density‐dependent effects. The numerical results for the modified Henry problem are rigorously tested against a new set of semianalytical results. The semianalytical result for the modified problem offers an additional improvement over the standard Henry problem that has typically only been tested using intercode comparisons.

Hydraulic and hydro-salinity behavior of skimming wells under different pumping regimes

Skimming wells are meant to extract top fresh water layer in the fresh-saline aquifer. Their development in the Indus basin occurred through private sector in a technological vacuum. As a result, these wells have some technical, environmental and social constraints, which hinder the sustainability of these wells. As an initial step to improve the well technology, the hydraulic and hydro-salinity responses of the fresh-saline aquifer under different pumping regimes need to be monitored. The present paper reports the hydraulic and hydro-salinity behavior of the Indus basin aquifer in Pakistan under field conditions at farmers’ wells. Two sites, having 6- and 16-strainer wells were monitored during July 2000–December 2001. The 6-strainer well was operated for 4 h with single-, 4- and 6-strainer arrangements and the spatial behaviors of specific drawdown were observed under these arrangements. The 16-strainer well was monitored continuously for the above period. The well discharge, pumped water quality and pumping duration was recorded of every pumping event under farmer’s practice to extract groundwater. The rainfall and temporal water table fluctuation was also recorded at this site. The impact of 24 years of well operation on groundwater quality was observed by comparing the hydro-salinity profiles of 1974 and 1998 under 3-strainer well. The results showed that the specific drawdown was higher for single-strainer and it decreased with the increase in number of strainers in skimming wells and hence reduced the chances of saline-upconing. Each strainer in multi-strainer well contributes equally in well discharge provided the horizontal distances among the strainers are equal. The pumped water quality in fresh-saline aquifer was a very sensitive function of fresh water recharge and pumping duration. It was observed that with the increase in daily operation from 2 to 12 h per day, the pumped quality deteriorated three-folds and there was also 30% reduction in well discharge due to high suction lift. It was observed that continuous operation of a 3-strainer well having discharge of 14 lps over the 24 years had raised the fresh-saline interface (iso-concentration line of 1.5 dS/m) to 9 m. Keeping in review the observations, it is recommended that the daily operation of 4–6 h keep the water quality within marginal limit (<1.5 dS/m) and the pumping operation is also cost-effective with only 15–20% reduction in well discharge for the study area.

Numerical Modeling of Saline Intrusion in Salar de Atacama

This paper presents the results of numerical simulations of groundwater circulation and solute transport at the Salar de Atacama through use of a numerical model to solve the two-dimensional problem of flow in an aquifer when considering the effects of variable density. The phenomena associated with solute transport are modeled by means of an advection-dispersion equation, and a linear relationship is assumed between fluid density and concentration of the dissolved solids. Simulations considered conditions of high groundwater evaporation, which depends on the depth of the phreatic surface. Results indicate that the discharge of groundwater occurs essentially in freshwater-saline water interface zones, where a number of lagunas begin. Different freshwater recharge scenarios were simulated, while it was verified that the effects of evaporation are important and minimize or buffer the variations in the phreatic surface and the discharges of groundwater that are the source of water supply for the lagunas.

Spatial patterns in soil water fluxes along a forest-marsh transect in the southeastern United States

Time series of water levels in piezometer nests along a forest-marsh transect near North Inlet, SC, show fluctuations that are attributable to recharge by precipitation and tidal flooding and to removal by evapotranspiration (ET) and seepage out of the soil. Volumes of water associated with these water level changes were estimated by correlating rain-induced water level increases with measured rainfalls. In the forest the ratio of water table rise to rainfall is about 10. This ratio increases with decreasing elevation to about 40 in the mid marsh where the antecedent soil moisture is generally higher. The relative influence of removal by ET and seepage and recharge by rain and tides varies systematically along the transect. In the mainland forest, loss of water by ET is somewhat less than infiltration, leading to a net recharge of fresh water which eventually discharges in the adjacent mid marsh. With decreasing elevation, the relative importance of recharge by rain decreases as recharge by tidal flooding increases. In the low marsh, however, these mechanisms of loss and recharge can not be discerned in the water level time series because the water table rarely, if ever, drops below the marsh surface.

Pleistocene recharge to midcontinent basins: effects on salinity structure and microbial gas generation

The hydrogeochemistry of saline-meteoric water interface zones in sedimentary basins is important in constraining the fluid migration history, chemical evolution of basinal brines, and physical stability of saline formation waters during episodes of freshwater recharge. This is especially germane for interior cratonic basins, such as the Michigan and Illinois basins. Although there are large differences in formation water salinity and hydrostratigraphy in these basins, both are relatively quiescent tectonically and have experienced repeated cycles of glaciation during the Pleistocene. Exploration for unconventional microbial gas deposits, which began in the upper Devonian-age Antrim Shale at the northern margin of the Michigan Basin, has recently extended into the age-equivalent New Albany Shale of the neighboring Illinois Basin, providing access to heretofore unavailable fluid samples. These reveal an extensive regional recharge system that has profoundly changed the salinity structure and induced significant biogeochemical modification of formation water elemental and isotope geochemistry.New-formation water and gas samples were obtained from Devonian-Mississippian strata in the Illinois Basin. These included exploration wells in the New Albany Shale, an organic-rich black shale of upper Devonian age, and formation waters from over- and underlying regional aquifer systems (Siluro-Devonian and Mississippian age). The hydrostratigraphic relations of major aquifers and aquitards along the eastern margin of the Illinois Basin critically influenced fluid migration into the New Albany Shale. The New Albany Shale formation water chemistry indicates significant invasion of meteoric water, with δD values as low as −46.05‰, into the shale. The carbon stable isotope system (δ13C values as high as 29.4‰), coupled with δ18O, δD, and alkalinity of formation waters (alkalinity ≤24.08 meq/kg), identifies the presence of microbial gas associated with meteoric recharge. Regional geochemical patterns identify the underlying Siluro-Devonian carbonate aquifer system as the major conduit for freshwater recharge into the fractured New Albany Shale reservoirs. Recharge from overlying Mississippian carbonates is only significant in the southernmost portion of the basin margin where carbonates directly overlie the New Albany Shale.Recharge of dilute waters (Cl− <1000 mM) into the Siluro-Devonian section has suppressed formation water salinity to depths as great as 1 km across the entire eastern Illinois Basin margin. Taken together with salinity and stable isotope patterns in age-equivalent Michigan Basin formation waters, they suggest a regional impact of recharge of δ18O- and δD-depleted fluids related to Pleistocene glaciation. Devonian black shales at both basin margins have been affected by recharge and produced significant volumes of microbial methane. This recharge is also manifested in different salinity gradients in the two basins because of their large differences in original formation water salinity. Given the relatively quiet tectonic history and subdued current topography in the midcontinent region, it is likely that repeated cycles of glacial meltwater invasion across this region have induced a strong disequilibrium pattern in fluid salinity and produced a unique class of unconventional shale-hosted gas deposits.

Assessment of groundwater quality and contamination from uranium-bearing black shale in Goesan-Boeun areas, Korea

This study was initiated to identify the impact of metals and uranium enriched soil and black shale in groundwater quality and contamination. From a Piper diagram, groundwater was classified into four types as (Ca+Mg)−HCO3 type, (Ca+Mg)−SO4 type, the mixed type of these two and Na−HCO3 type, reflecting the complicated nature of geology of the study area. Silicate weathering appeared to be the major water–rock interaction. In groundwater, metals including Cr, Pb, Cu and V, previously identified as being enriched in soils and black shale, were much lower in concentrations than Korean and US EPA drinking water guidelines. Instead, Fe and Mn caused major water-quality problems. In the artesian groundwater from an abandoned uranium mine, the uranium concentration was 21.3 µg L−1, slightly higher than EPA guidelines of 20 µg L−1. Heavy metals in groundwater appeared to be controlled mostly by sorptions on to Fe- and Mn-oxyhydroxides. They could be remobilised in groundwater with changes of pH and Eh conditions due to acid mine drainage from black shale or the recharge of fresh water. Uranium would be associated with carbonate and sulphate complexes in groundwater. Because of the remaining water-quality problems in the study area, we suggested containment of identified mine wastes, considering remedial measures for local problems with Fe and Mn, continuous monitoring of groundwater and developing groundwater from deep aquifers.

The saltpool benchmark problem – numerical simulation of saltwater upconing in a porous medium

Recently, a series of laboratory experiments was carried out in which a typical variable-density flow problem in a porous medium was investigated. A stable layering of saltwater below freshwater is affected by the recharge and discharge of freshwater on the top. The experiments were conducted with 1% and 10% initial salt-mass fraction contrasts. In this paper, we define a mathematical model problem, which is able to reproduce the experimental results within a reasonable accuracy. To this end, the sensitivity of the model with respect to model parameters is investigated. An inverse modelling leads to an appropriate choice of the model parameters and the definition of the mathematical benchmark problem. It will become clear that, in the 10% case, the transversal dispersion coefficient plays an important role. Detailed numerical investigations are carried out and reference solutions are obtained. For the high concentration case a very high spatial grid resolution using up to 16 million grid points is necessary. Error bounds are derived for the solutions without any a priori assumption on regularity and convergence.

Effect of strip recharge on sea water intrusion into aquifers

Parametric studies were carried out to analyse the effect on sea water intrusion of freshwater recharge through a finite width strip parallel to the coast. A vertically integrated one-dimensional finite element model was used for this purpose. The studies included the analysis of the effect of location, width, intensity and the period of recharge on sea water-freshwater interface motion. Relationships were established between the interface motion and the recharge parameters applicable to wide ranging practical cases. From the studies, the ideal location for recharge was identified to achieve the maximum repulsion of intrusion. The width of recharge also affects the interface motion and the widths greater than 2% of the initial intrusion length were effective in controlling the intrusion. The results indicated that the reduction of intrusion up to 30% could be achieved through strip recharge.

Thermal buffering of sedimentary basins by basement rocks: implications arising from numerical simulations

The Earth’s crust releases or absorbs heat energy in response to changes in the thermal regime. Numerical simulations of basin-scale heat transport which use the sediment–basement interface as a thermal boundary assume that near-surface temperature changes have no effect on basement rocks and vice versa (unbuffered model). We test this assumption by comparing numerical models of transient fluid flow and heat transport in the Arkoma foreland basin with and without thermal buffering by basement rocks. Thermal buffering by basement rocks reduces cooling (by fresh water recharge) of deep basin sediments near the fold–thrust belt and reduces warming (by upward fluid discharge) of basin margin sediments. The unbuffered model predicts a temporary warming of basin margin sediments which is largely an artefact of the model thermal boundary conditions. In all cases, the buffered model produces no significant thermal transient. Exaggerated temperature predictions also can occur in numerical simulations of uplift and erosion or heat refraction.

Ground-water quality deterioration in arid areas: a case study of the Zerqa river basin as influenced by Khirbet Es-Samra waste water (Jordan)

The effects of the Khirbet Es-Samra Waste Water Treatment Plant (KS) on the deterioration of ground-water quality at Seil Zerqa basin were investigated in this study. The plant is located in an arid area in the eastern part of Jordan. It was built in 1985 to treat the waste water of about 60% of the Jordan population. It is composed of two anaerobic and four facultative ponds which empty into four maturation ponds. The reservoir of these ponds is impermeable but, due to geologic and hydrogeologic features, leakage of waste water into the ground-water began to occur and the Upper Aquifer Complex (Amman-Wadi Sir) started to deteriorate. In addition, the water quality of Seil Zerqa and Wadi Dhuleil has deteriorated during the flow to the King Tallal dam, due to the increase of waste water and limited fresh water resources. Topography differences plays a large role in changing climatic elements, especially rainfall, due to orographic effects and temperature. The results of periodical chemical analysis show that the electrical conductivity increased from 700 μs cm−1at Hanna well in the KS upstream which is not affected, to more than 7000 μs cm−1at Muawad well in the KS downstream. The other cations and anions have also increased by many times. The electrical conductivity of the soil water increased from 5800 μs cm−1at 30 cm depth to more than 35,000 μs cm−1at 140 cm depth. The heavy metals and bacterial count of soil irrigated with waste water have also increased more than the normal dry soils of the arid and semi-arid soils. This continual seepage of waste water may change the ground-water flow direction, in addition to the increased recharge which is many times greater than it was before the plant was built. However, the recharge of fresh water is very limited in this area, which is characteristic of an arid environment. Moreover, Seil Zerqa and Wadi Dhuleil have become breeding sites for insects and developed a foul odour, due to the low levels of rainfall that may otherwise have diluted the treated waste water through the flow to the King Tallal dam.

Molybdate transport in a chemically complex aquifer: Field measurements compared with solute‐transport model predictions

A natural‐gradient tracer test was conducted in an unconfined sand and gravel aquifer on Cape Cod, Massachusetts. Molybdate was included in the injectate to study the effects of variable groundwater chemistry on its aqueous distribution and to evaluate the reliability of laboratory experiments for identifying and quantifying reactions that control the transport of reactive solutes in groundwater. Transport of molybdate in this aquifer was controlled by adsorption. The amount adsorbed varied with aqueous chemistry that changed with depth as freshwater recharge mixed with a plume of sewage‐contaminated groundwater. Molybdate adsorption was strongest near the water table where pH (5.7) and the concentration of the competing solutes phosphate (2.3 micromolar) and sulfate (86 micromolar) were low. Adsorption of molybdate decreased with depth as pH increased to 6.5, phosphate increased to 40 micromolar, and sulfate increased to 340 micromolar. A one‐site diffuse‐layer surface‐complexation model and a two‐site diffuse‐layer surface‐complexation model were used to simulate adsorption. Reactions and equilibrium constants for both models were determined in laboratory experiments and used in the reactive‐transport model PHAST to simulate the two‐dimensional transport of molybdate during the tracer test. No geochemical parameters were adjusted in the simulation to improve the fit between model and field data. Both models simulated the travel distance of the molybdate cloud to within 10% during the 2‐year tracer test; however, the two‐site diffuse‐layer model more accurately simulated the molybdate concentration distribution within the cloud.

Implications of Electrical Resistivity Data Regarding Ground-Water Lenses on San Salvador Island, Bahamas

Fresh ground-water lenses in the Bahamas may be mapped using electrical resistivity surveys utilizing the large resistivity contrast between fresh-water and salt-water saturated carbonate rock. Ideally, this contrast is a factor of 18 or more; however, porosity variations introduce even larger resistivity variations. The relevant resistivity-porosity relationship determined for surficial rock samples from San Salvador Island is R o = 1.04 R w /Φ 219 , where R o and R w are the resistivities of rock and pore water, and Φ is the porosity. These criteria were used to determine the depth of the fresh-water/salt-water interface of fresh-water lenses in the Sandy Point area of San Salvador Island, based on interactive inversion of vertical electrical sounding (VES) data collected during the past 15 years. The resulting computed resistivity layer models indicate a major fresh-water lens of maximum thickness of approximately 30 m under the peripheral hills of Sandy Point with little or no fresh water in the low-lying interior of the region. Pseudo-depth sections and high-resistivity layers at depth suggest confined fresh-water aquifers at depth beneath brackish near-surface ground water in parts of the interior, with fresh-water recharge from the peripheral hills. A resistivity profiling survey of the interior indicates possible conduits connecting several of the brackish blue holes.

Dating Quaternary groundwater flow events: a review of available methods and their application

Abstract During the Quaternary (the last 1.6 Ma), episodic groundwater flow occurred to depths of several hundred metres. In many regions, climatic fluctuations induced episodes of rapid and slow groundwater flow. The former, termed here ‘groundwater flow events’, were characterized by extensive fresh-water recharge, and flushing of pre-existing water. It is important to identify and characterize such events to: (i) calibrate predictive models for future flow, thereby underpinning Performance Assessments for underground waste repositories; (ii) establish what proportion of global fresh-water resources was recharged during early Holocene or Pleistocene flow; and (iii) help interpret the timing and nature of Pleistocene climatic changes. To date, hydrogeochemical, mineralogical and petrological data have been interpreted in terms of post-glacial Holocene groundwater flow, or pre-Quaternary palaeofluid flow. Typically, Pleistocene recharge before the last glacial maximum has been unresolved. The effect of any given Quaternary climate change on the flow of pre-existing groundwaters is rarely identified. This paper considers the methods available to date these flow events, and finds that mineralogical evidence has been under-used to interpret Quaternary groundwater flow. Used together with hydrogeochemical data, mineralogical observations offer the best prospect of improving our understanding of Quaternary groundwater flow events.