Multiple Aquifers Research Articles

Geochemical characteristics of water produced from CBM wells and implications for commingling CBM production: A case study of the Bide-Santang Basin, western Guizhou, China

Multi-layer commingling coalbed methane (CBM) development causes interlayer interference, which can severely impede the efficient pressure drop in CBM reservoirs and make it difficult to identify the produced water source. In this study, geochemical analyses (conventional ions, stable isotopes, and trace elements) were conducted on 38 water samples collected from 11 CBM wells, an adjacent river and the coal mine in the Bide-Santang Basin, western Guizhou, China. Variations in water quality caused by aquifer recharge, water-rock interactions, and the oxidation environment were revealed using principle component analysis and hierarchical cluster analysis. Li, Ga, Rb, Sr, and Ba were selected as characteristic trace elements, whose concentrations tend to increase with increasing producing layer depth. The water samples recorded an obvious D drift trend relative to the local meteoric water line (LMWL). Fracturing fluid from contaminated wells recorded elevated Cl− and Na+ concentrations and high δ18O and δD values. The geochemical parameters of d (the degree of D drift), δD, δ18O, Cl− + Na+ and characteristic trace element concentrations in produced water serve as indicators of the production and water source of CBM wells. Templates were created to discriminate between sources of produced water, including shallow groundwater, coal seam water, and fracturing fluid water. High-production wells are characterized by producing coal seam water, while low-production wells are characterized by producing shallow groundwater. The upper CBM system is susceptible to shallow groundwater recharge that involves multiple limestone aquifers, so it is recommended that the middle and lower CBM systems should be a priority for CBM development.

Groundwater management under uncertainty using a stochastic multi-cell model

The optimization of spatially complex groundwater management models over long time horizons requires the use of computationally efficient groundwater flow models. This paper presents a new stochastic multi-cell lumped-parameter aquifer model that explicitly considers uncertainty in groundwater recharge. To achieve this, the multi-cell model is combined with the constrained-state formulation method. In this method, the lower and upper bounds of groundwater heads are incorporated into the mass balance equation using indicator functions. This provides expressions for the means, variances and covariances of the groundwater heads, which can be included in the constraint set in an optimization model. This method was used to formulate two separate stochastic models: (i) groundwater flow in a two-cell aquifer model with normal and non-normal distributions of groundwater recharge; and (ii) groundwater management in a multiple cell aquifer in which the differences between groundwater abstractions and water demands are minimized. The comparison between the results obtained from the proposed modeling technique with those from Monte Carlo simulation demonstrates the capability of the proposed models to approximate the means, variances and covariances. Significantly, considering covariances between the heads of adjacent cells allows a more accurate estimate of the variances of the groundwater heads. Moreover, this modeling technique requires no discretization of state variables, thus offering an efficient alternative to computationally demanding methods.

Mapping vulnerability of multiple aquifers using multiple models and fuzzy logic to objectively derive model structures

Driven by contamination risks, mapping Vulnerability Indices (VI) of multiple aquifers (both unconfined and confined) is investigated by integrating the basic DRASTIC framework with multiple models overarched by Artificial Neural Networks (ANN). The DRASTIC framework is a proactive tool to assess VI values using the data from the hydrosphere, lithosphere and anthroposphere. However, a research case arises for the application of multiple models on the ground of poor determination coefficients between the VI values and non-point anthropogenic contaminants. The paper formulates SCFL models, which are derived from the multiple model philosophy of Supervised Committee (SC) machines and Fuzzy Logic (FL) and hence SCFL as their integration. The Fuzzy Logic-based (FL) models include: Sugeno Fuzzy Logic (SFL), Mamdani Fuzzy Logic (MFL), Larsen Fuzzy Logic (LFL) models. The basic DRASTIC framework uses prescribed rating and weighting values based on expert judgment but the four FL-based models (SFL, MFL, LFL and SCFL) derive their values as per internal strategy within these models. The paper reports that FL and multiple models improve considerably on the correlation between the modeled vulnerability indices and observed nitrate-N values and as such it provides evidence that the SCFL multiple models can be an alternative to the basic framework even for multiple aquifers. The study area with multiple aquifers is in Varzeqan plain, East Azerbaijan, northwest Iran.

Controls on Methane Occurrences in Aquifers Overlying the Eagle Ford Shale Play, South Texas.

Assessing natural vs. anthropogenic sources of methane in drinking water aquifers is a critical issue in areas of shale oil and gas production. The objective of this study was to determine controls on methane occurrences in aquifers in the Eagle Ford Shale play footprint. A total of 110 water wells were tested for dissolved light alkanes, isotopes of methane, and major ions, mostly in the eastern section of the play. Multiple aquifers were sampled with approximately 47 samples from the Carrizo-Wilcox Aquifer (250-1200 m depth range) and Queen City-Sparta Aquifer (150-900 m depth range) and 63 samples from other shallow aquifers but mostly from the Catahoula Formation (depth <150 m). Besides three shallow wells with unambiguously microbial methane, only deeper wells show significant dissolved methane (22 samples >1 mg/L, 10 samples >10 mg/L). No dissolved methane samples exhibit thermogenic characteristics that would link them unequivocally to oil and gas sourced from the Eagle Ford Shale. In particular, the well water samples contain very little or no ethane and propane (C1/C2+C3 molar ratio >453), unlike what would be expected in an oil province, but they also display relatively heavier δ13 Cmethane (>-55‰) and δDmethane (>-180‰). Samples from the deeper Carrizo and Queen City aquifers are consistent with microbial methane sourced from syndepositional organic matter mixed with thermogenic methane input, most likely originating from deeper oil reservoirs and migrating through fault zones. Active oxidation of methane pushes δ13 Cmethane and δDmethane toward heavier values, whereas the thermogenic gas component is enriched with methane owing to a long migration path resulting in a higher C1/C2+C3 ratio than in the local reservoirs.

Groundwater Simulations and Uncertainty Analysis Using MODFLOW and Geostatistical Approach with Conditioning Multi-Aquifer Spatial Covariance

This study presents an approach for obtaining limited sets of realizations of hydraulic conductivity (K) of multiple aquifers using simulated annealing (SA) simulation and spatial correlations among aquifers to simulate realizations of hydraulic heads and quantify their uncertainty in the Pingtung Plain, Taiwan. The proposed approach used the SA algorithm to generate large sets of natural logarithm hydraulic conductivity (ln(K)) realizations in each aquifer based on spatial correlations among aquifers. Moreover, small sets of ln(K) realizations were obtained from large sets of realizations by ranking the differences among cross-variograms derived from the measured ln(K) and the simulated ln(K) realizations between the aquifer pair Aquifer 1 and Aquifer 2 (hereafter referred to as Aquifers 1–2) and the aquifer pair Aquifer 2 and Aquifer 3 (hereafter referred to as Aquifers 2–3), respectively. Additionally, the small sets of realizations of the hydraulic conductivities honored the horizontal spatial variability and distributions of the hydraulic conductivities among aquifers to model groundwater precisely. The uncertainty analysis of the 100 combinations of simulated realizations of hydraulic conductivity was successfully conducted with generalized likelihood uncertainty estimation (GLUE). The GLUE results indicated that the proposed approach could minimize simulation iterations and uncertainty, successfully achieve behavioral simulations when reduced between calibration and evaluation runs, and could be effectively applied to evaluate uncertainty in hydrogeological properties and groundwater modeling, particularly in those cases which lack three-dimensional data sets yet have high heterogeneity in vertical hydraulic conductivities.

Three-Dimensional Geological Model of Quaternary Sediments in Walworth County, Wisconsin, USA

A three-dimensional (3D) geologic model was developed for Quaternary deposits in southern Walworth County, WI using Petrel, a software package primarily designed for use in the energy industry. The purpose of this research was to better delineate and characterize the shallow glacial deposits, which include multiple shallow sand and gravel aquifers. The 3D model of Walworth County was constructed using datasets such as the U.S. Geological Survey 30 m digital elevation model (DEM) of land surface, published maps of the regional surficial geology and bedrock topography, and a database of water-well records. Using 3D visualization and interpretation tools, more than 1400 lithostratigraphic picks were efficiently interpreted amongst 725 well records. The final 3D geologic model consisted of six Quaternary lithostratigraphic units and a bedrock horizon as the model base. The Quaternary units include in stratigraphic order from youngest to oldest: the New Berlin Member of the Holy Hill Formation, the Tiskilwa Member of the Zenda Formation, a Sub-Tiskilwa Sand/Gravel unit, the Walworth Formation, a Sub-Walworth Sand/Gravel unit, and a Pre-Illinoisan unit. Compared to previous studies, the results of this study indicate a more detailed distribution, thickness, and interconnectivity between shallow sand and gravel aquifers and their connectivity to shallow bedrock aquifers. This study can also help understand uncertainty within previous local groundwater-flow modeling studies and improve future studies.

A Simultaneous Model for Simulating the Propagation of Hydraulic Head in Aquifers with Leakage Pathways

A three-dimensional simultaneous solution model was developed for analysis of variable transient flows, specifically leakage associated with arbitrary groundwater aquifers. The model simultaneously calculates leakage rates using the hydraulic gradients between coupled leakage points at two leaky aquifers and evaluates the propagation of hydraulic heads in multiple aquifers resulting from that leakage. Important considerations for leakage simulation are variant leakage rates with time and leakage starting time. Two types of leakage pathways are specified in terms of the generated time, including (1) pre-existing leakage pathways and (2) abruptly-induced leakage pathways at specific times. The governing equation with a leakage term is composed of three finite difference equations, and its form depends on whether leakage pathways pre-exist or are induced at specified times according to known or assumed aquifer histories. The developed numerical code was validated by comparison with the results of the TOUGH2/EOS1 program for a three-dimensional conceptual domain.

(美国)新墨西哥州萨克拉门托山脉土壤水动力学和树的水摄取

In the southwestern United States, precipitation in the high mountains is a primary source of groundwater recharge. Precipitation patterns, soil properties and vegetation largely control the rate and timing of groundwater recharge. The interactions between climate, soil and mountain vegetation thus have important implications for the groundwater supply. This study took place in the Sacramento Mountains, which is the recharge area for multiple regional aquifers in southern New Mexico. The stable isotopes of oxygen and hydrogen were used to determine whether infiltration of precipitation is homogeneously distributed in the soil or whether it is partitioned among soil-water ‘compartments’, from which trees extract water for transpiration as a function of the season. The results indicate that “immobile” or “slow” soil water, which is derived primarily from snowmelt, infiltrates soils in a relatively uniform fashion, filling small pores in the shallow soils. “Mobile” or “fast” soil water, which is mostly associated with summer thunderstorms, infiltrates very quickly through macropores and along preferential flow paths, evading evaporative loss. It was found that throughout the entire year, trees principally use immobile water derived from snowmelt mixed to differing degrees with seasonally available mobile-water sources. The replenishment of these different water pools in soils appears to depend on initial soil-water content, the manner in which the water was introduced to the soil (snowmelt versus intense thunderstorms), and the seasonal variability of the precipitation and evapotranspiration. These results have important implications for the effect of climate change on recharge mechanisms in the Sacramento Mountains.

Hydrogeochemical analysis of multiple aquifers in a coal mine based on nonlinear PCA and GIS

Hydrogeochemical characteristic analysis of multiple categorical aquifers, including the delineation of hydrogeochemical spatial distributions, can provide a basic understanding of the groundwater flow system necessary for the prevention and control of groundwater damage associated with coal mines. Nonlinear principal component analysis (PCA) was used to analyse the groundwater chemical characteristics of water samples from five aquifers in Gubei Coal Mine, Anhui Province, China. In particular, Ca2+, Mg2+, Na+ + K+, HCO3−, Cl−, SO42−, and pH were analysed. To analyse the spatial distribution of groundwater chemistry in this Permian fractured aquifer, interpolation of principal component 1 (PC1) scores obtained from the nonlinear PCA was performed using the inverse distance weighting (IDW) method. Additionally, concentrations of the hydrogeochemical variables were plotted against depth. The relationships between water samples, variables, and water samples with variables were simultaneously analysed by nonlinear PCA. The results show: (1) nonlinear PCA explains more variability than standard PCA in the first two principal components (PCs); (2) the Cenozoic top aquifer has low concentrations of Na+ + K+ and Cl−; the Cenozoic middle aquifer and Cenozoic bottom aquifer are characterised by high concentrations of Ca2+ and Mg2+ and low concentrations of HCO3−; the shallow groundwater of the Permian fractured aquifer is recharged by the Cenozoic bottom aquifer, while other parts have high concentrations of HCO3− and low concentrations of Ca2+ and Mg2+. The Taiyuan limestone aquifer has similar groundwater chemistry to that of the Cenozoic bottom aquifer, and thus it may be recharged by the Cenozoic bottom aquifer in outcropping Taiyuan Formation strata; (3) the wide south-western and north-western areas of the Permian fractured aquifer may be recharged by the Cenozoic bottom aquifer; and (4) concentrations of Ca2+ and Mg2+ in the Permian fractured aquifer are negatively correlated with sampling depth, while the concentrations of Na+ + K+ and HCO3− are positively correlated with sampling depth, which verifies that the shallow groundwater of this Permian fractured aquifer is recharged by the Cenozoic bottom aquifer. The findings demonstrate that combining nonlinear PCA with GIS can be a powerful approach for conducting spatial nonlinear analysis of hydrogeochemical characteristics.

Geogenic As and Mo groundwater contamination caused by an abundance of domestic supply wells

Lacking a connection to a municipal water supply, each household in the municipality of Lithia, approximately 30 km southeast of Tampa, Florida (USA), is responsible for its own supply of drinking water, causing a high-density of private domestic supply wells (DSW) in this area. There, a multilayered aquifer system exists, which can be subdivided into three distinct hydro stratigraphic units, which are, from the top down: the Surficial Aquifer System (SAS), the Intermediate Aquifer System (IAS), and the Upper Floridan Aquifer System (UFA). Despite the relatively small area, the geochemical and hydrogeological setting in Lithia is complex, consisting of: i) extensive cyclical pumping in a municipal well field to the west, ii) large seasonal changes in hydraulic head, ii) multiple aquifers with different hydraulic heads, and iv) a large density of domestic supply wells (DSW). Within the zone of highest As concentrations, there are approximately 100 wells in an area of 2.5 km × 1.5 km. Most of these wells have large open screened intervals, often open to all three aquifers, allowing the downward flow of oxygenated and upward flow of anoxic groundwater. A survey of groundwater quality found that As and Mo concentrations in the DSW were up to 371 μg/L and 4740 μg/L, respectively. To obtain information about the individual aquifers, 5 well clusters with 4 monitoring intervals (approximately 50 m, 65 m, 80 m and 95 m below surface) and 8 push core wells (approximately 9 m below surface) were installed and sampled. In those wells, As and Mo were only elevated in a permeable layer within the IAS at a depth of 50 m. Values were up to 195 μg/L for As and up to 5050 μg/L for Mo. Using the tritium-helium (3H–3He) method, the ages of those samples high in As and Mo were determined to be 40, 30 and 30 years, respectively, while all other samples had ages older than 50 years. This indicated that mixing between young and old groundwater could be responsible for the high As and Mo concentrations. A good negative correlation for the whole data set was also observed between the concentration 3H and δ18O values, which together with hydrogeological modeling confirmed that the increased permeability created by the high density of DSW resulted in flow paths that permitted the perpetual mixing of shallow and deep groundwater. The release of the As and Mo appeared to be a consequence of changes to the physicochemical conditions in the aquifer, either via the introduction of oxygen-rich fluids into the IAS or the mixing of different fluids in the IAS or the introduction of oxygen-depleted fluids into the IAS. While the mobilization of geogenic trace metals is often associated with pumping-induced hydraulic gradient changes, we found that a certain density of multi-aquifer wells can be sufficient to alter hydrologic flow paths and induce the mobilization of geogenic trace metals even in the absence of significant pumping. In Lithia, the DSW effectively increased the local scale permeability of the aquifer, causing the mixing of oxygen-rich surface and deeper anoxic groundwater across a confining unit. Because the alteration to the hydrologic flow paths was a consequence of changes to the physical structure of the aquifer system rather than due to pumping, the alteration is not easily reversible, thus significantly complicating site remediation. Our results provide a cautionary warning of the risks of undue private DSW development in rapidly growing communities.

Testing the suitability of geologic frameworks for extrapolating hydraulic properties across regional scales

The suitability of geologic frameworks for extrapolating hydraulic conductivity (K) to length scales commensurate with hydraulic data is difficult to assess. A novel method is presented for evaluating assumed relations between K and geologic interpretations for regional-scale groundwater modeling. The approach relies on simultaneous interpretation of multiple aquifer tests using alternative geologic frameworks of variable complexity, where each framework is incorporated as prior information that assumes homogeneous K within each model unit. This approach is tested at Pahute Mesa within the Nevada National Security Site (USA), where observed drawdowns from eight aquifer tests in complex, highly faulted volcanic rocks provide the necessary hydraulic constraints. The investigated volume encompasses 40 mi3 (167 km3) where drawdowns traversed major fault structures and were detected more than 2 mi (3.2 km) from pumping wells. Complexity of the five frameworks assessed ranges from an undifferentiated mass of rock with a single unit to 14 distinct geologic units. Results show that only four geologic units can be justified as hydraulically unique for this location. The approach qualitatively evaluates the consistency of hydraulic property estimates within extents of investigation and effects of geologic frameworks on extrapolation. Distributions of transmissivity are similar within the investigated extents irrespective of the geologic framework. In contrast, the extrapolation of hydraulic properties beyond the volume investigated with interfering aquifer tests is strongly affected by the complexity of a given framework. Testing at Pahute Mesa illustrates how this method can be employed to determine the appropriate level of geologic complexity for large-scale groundwater modeling.

Effects of invasive fish and quality of water and sediment on macrophytes biomass, and their consequences for the waterbird community of a Mediterranean floodplain

Floodplains are among the most threatened ecosystems world-wide because of multiple stressors, i.e., invasive species, pollution and aquifer overexploitation; the Tablas de Daimiel National Park (Spain) is a clear example of these kinds of impact. This work aims to test whether invasive fish and/or the water and sediment quality are significant drivers of the decline of stonewort (Chara spp.) meadows in the Tablas de Daimiel, investigating how this could explain changes observed in the waterbird community. Bird surveys performed monthly between June 2010 and April 2014 have shown that herbivorous species like the red-crested pochard (Netta rufina) reached historical records between September 2010 and June 2011, but have decreased since then. Piscivorous waterbirds like the great cormorant (Phalacrocorax carbo) and herons increased in population after 2011, however. These changes may be due to the decline of Chara spp. meadows, connected to overexploitation by herbivores, or to changes in water and sediment quality. To test this hypothesis, we studied the growth of Chara spp. biomass in ten sites of the Tablas de Daimiel, where experimental exclosures were set up to exclude herbivory by birds, and bioturbation and herbivory by fish. Our results have shown that the absence of Chara spp. in the Tablas de Daimiel is mostly explained by presence of invasive fish (i.e. common carp). Moreover, the physicochemical characteristics of the water (lower values of conductivity and higher values of inorganic carbon and organic nitrogen), as well as of the sediment (lower values of inorganic and organic phosphorus), favour the increase of Chara spp., in the absence of the fish effect. These results led the National Park managers to begin the control of invasive fish as an urgent measure to assure the ecological conservation of this Mediterranean wetland.

Irrigation aquifer depletion: the nexus linchpin

Demand for agriculturally produced food and feed is virtually certain to rise as populations increase and economies of developing countries improve. Irrigation is currently supplying water for a disproportionately large amount of agriculturally sourced food and feed production needed to meet this growing demand. Irrigation supports 40 % of global food production, while it occupies only about 18 % of the agricultural landscape. Irrigation is groundwater sourced in critical food production regions of the world, and a variety of these aquifers are being drained; their rates of water recharge are well below extraction rates. Life expectancy of multiple irrigation aquifers, including selected aquifers in the USA, is decades or less if much improved water irrigation management practices are not implemented. Case examples from Kansas, Colorado, and California, home of arguably the most technologically advanced, economically stable, and well educated agricultural industry in the world, are used to address the question: “Are existing irrigation management approaches preferentially focused on sustaining existing economies or sustaining the longevity of the irrigation aquifers?” In general, irrigation aquifer water stress and need to conserve water is recognized by a vast majority of stakeholders. The need for stakeholders to maintain short-term financial integrity, which requires continued use of irrigation water, is creating stress in local water management policy development and delaying meaningful action. Pockets of forward thinking plans and even action are evolving, but these pockets must grow, and grow rapidly, to avert major negative social and financial consequences.

An attempt to understand the behavior of dissolved organic carbon in coastal aquifers of Pondicherry region, South India

Groundwaters of the coastal region are significant and serve as a fragile ecosystem in the sedimentary terrain. The Pondicherry region is characterized by different geological formations of distinct geological epochs Cretaceous, Tertiary and Quaternary. Ninety-three samples were collected from specific aquifers with respect to spatial distribution [Alluvium (31), Upper Cuddalore (13), Lower Cuddalore (13), Other Tertiary (7), Cretaceous (24) and Mixed—multiple completion aquifer (5)]. The collected samples were analyzed for major ions Ca2+, Mg2+, Na+, K+, HCO3−, Cl−, PO42−, SO42−, F and DOC. Stables isotopes of δ18O, δD and δ 13C were also analyzed along with heavy metals like Al, Ba, Fe, Sr, Cd and Zn. The δ18O ranges between −6.85 and −2.15 ‰, and δD ranges between −43.94 and −19.41 ‰. The distribution of these environmental isotopes in groundwater of this layered aquifer sequence was also attempted. A comparison of the isotopic data with the rainfall, Local Meteoric Water Line with a equation of δD = 7.398 * δ18O + 5.067, Indian Meteoric Water Line and Global Meteoric Water Line was performed. The δ13C values for groundwater ranges from −5.3 to −18.1 ‰. The result indicates that the groundwater in the study area is mainly meteoric in origin, and few samples show evidence of evaporation. The dissolved organic carbon (DOC) is a very important component in biogeochemical cycling of elements characterized by high susceptibility to leaching. The range of the DOC in the study area is 0–16 mg/L. Factor analysis was applied to classify the groundwater samples and to identify geochemical processes controlling groundwater geochemistry. The Alluvium aquifers show a complex hydrogeochemistry than the older aquifers. The major factor influencing the hydrogeochemistry of the region is rock–water interaction and anthropogenic processes. Hence, metal mobility of the groundwater shows the correlation of metal bonding and DOC. The relation between the DOC and heavy metal concentration in the aquifers was also identified.

Hydrogeological Model of an Urban City in a Coastal Area, Case study: Semarang, Indonesia

DOI:10.17014/ijog.3.1.17-27In Semarang City, groundwater has been exploited as a natural resource since 1841. The groundwater exploited in deep wells is concentrated in confined aquifers. The previous hydrogeological model was developed in one unit of aquifer and refined then by using several hydrostratigraphical units following a regional hydrogeological map without any further analysis. At present, there is a lack of precise hydrogeological model which integrates geological and hydrogeological data, in particular for multiple aquifers in Semarang. Thus, the aim of this paper is to develop a hydrogeological model for the multiple aquifers in Semarang using an integrated data approach. Groundwater samples in the confined aquifers have been analyzed to define the water type and its lateral distribution. Two hydrogeological cross sections were then created based on several borelog data to define a hydrostratigraphical unit (HSU). The HSU result indicates the hydrogeological model of Semarang consists of two aquifers, three aquitards, and one aquiclude. Aquifer 1 is unconfined, while Aquifer 2 is confined. Aquifer 2 is classified into three groups (2a, 2b, and 2c) based on analyses of major ion content and hydrostratigraphical cross sections.

Hydrologic framework of the Santa Clara Valley, California

The hydrologic framework of the Santa Clara Valley in northern California was redefined on the basis of new data and a new hydrologic model. The regional groundwater flow systems can be subdivided into upper-aquifer and lower-aquifer systems that form a convergent flow system within a basin bounded by mountains and hills on three sides and discharge to pumping wells and the southern San Francisco Bay. Faults also control the flow of groundwater within the Santa Clara Valley and subdivide the aquifer system into three subregions. After decades of development and groundwater depletion that resulted in substantial land subsidence, Santa Clara Valley Water District (SCVWD) and the local water purveyors have refilled the basin through conservation and importation of water for direct use and artificial recharge. The natural flow system has been altered by extensive development with flow paths toward major well fields. Climate has not only affected the cycles of sedimentation during the glacial periods over the past million years, but interannual to interdecadal climate cycles also have affected the supply and demand components of the natural and anthropogenic inflows and outflows of water in the valley. Streamflow has been affected by development of the aquifer system and regulated flow from reservoirs, as well as conjunctive use of groundwater and surface water. Interaquifer flow through water-supply wells screened across multiple aquifers is an important component to the flow of groundwater and recapture of artificial recharge in the Santa Clara Valley. Wellbore flow and depth-dependent chemical and isotopic data indicate that flow into wells from multiple aquifers, as well as capture of artificial recharge by pumping of water-supply wells, predominantly is occurring in the upper 500 ft (152 m) of the aquifer system. Artificial recharge represents about one-half of the inflow of water into the valley for the period 1970–1999. Most subsidence is occurring below 250 ft (76 m), and most pumpage occurs within the upper-aquifer system between 300 and 650 ft (between 91 and 198 m) below land surface. Overall, the natural quality of most groundwater in the Santa Clara Valley is good. Isotopic data indicate that artificial recharge is occurring throughout the shallower parts of the upper-aquifer system and that recent recharge (less than 50 yr old) occurs throughout most of the basin in the upper-aquifer system, but many of the wells in the center of the basin with deeper well screens do not contain tritium and recent recharge. Age dates indicate that the groundwater in the upper-aquifer system generally is less than 2000 yr old, and groundwater in the lower-aquifer system generally ranges from 16,700 to 39,900 yr old. Depth-dependent sampling indicates that wellbores are the main path for vertical flow between aquifer layers. Isotopic data indicate as much as 60% of water pumped from production wells originated as artificial recharge. Shallow aquifers not only contain more recent recharge but may be more susceptible to anthropogenic and natural contamination, as evidenced by trace occurrences of iron, nitrate, and volatile organic compounds (VOCs) in selected water-supply wells. Water-resource management issues are centered on sustaining a reliable and good-quality source of water to the residents and industries of the valley. While the basin has been refilled, increased demand owing to growth and droughts could result in renewed storage depletion and the related potential adverse effects of land subsidence and seawater intrusion. The new hydrologic model demonstrates the importance of the aquifer layering, faults, and stream channels in relation to groundwater flow and infiltration of recharge. This model provides a means to analyze water resource issues because it separates the supply and demand components of the inflows and outflows.

Evaluation and application of organic micro-pollutants (OMPs) as indicators in karst system characterization

This study presents chances and challenges associated with the application of organic micro-pollutants (OMPs) as indicators in karst system characterization. The methodology and options of possible indications were evaluated based on the interpretation of the spatial distribution of 54 compounds in groundwater in combination with a complex geological setting consisting of multiple aquifer horizons and tectonic faults. A high variety of OMPs are released mainly in an urban area leading to concentrations of several nanograms per liter up to micrograms per liter, which are detectable using a high-performance liquid chromatography with subsequent tandem mass spectrometry (HPLC-MS/MS) method. Since characteristic patterns of spatial distribution were repeatedly observed during a 2-year observation period, important criteria of the aforementioned indicator application are fulfilled. Triazoles, compounds with recent high emission rates, could be successfully applied for the identification of flow directions and the delineation of catchment areas. Concentrations and the number of OMPs are believed to be dependent on properties of covering rock layers. Therefore, OMPs can also be used as a validation tool for vulnerability mapping. Compounds, such as triazines, persistent in the system for more than two decades, demonstrate the interaction between different parts of the aquifer system and the hydraulic characteristics of a tectonic fault zone. Such indicator potentials complement those of artificial tracer tests. Point sources of OMPs and their impact on groundwater could be identified qualitatively. In combination with the interpretation of the geological setting, the distribution of OMPs provides essential information for the development of a conceptual hydrogeological model.

Investigating sources and pathways of perfluoroalkyl acids (PFAAs) in aquifers in Tokyo using multiple tracers

We employed a multi-tracer approach to investigate sources and pathways of perfluoroalkyl acids (PFAAs) in urban groundwater, based on 53 groundwater samples taken from confined aquifers and unconfined aquifers in Tokyo. While the median concentrations of groundwater PFAAs were several ng/L, the maximum concentrations of perfluorooctane sulfonate (PFOS, 990ng/L), perfluorooctanoate (PFOA, 1800ng/L) and perfluorononanoate (PFNA, 620ng/L) in groundwater were several times higher than those of wastewater and street runoff reported in the literature. PFAAs were more frequently detected than sewage tracers (carbamazepine and crotamiton), presumably owing to the higher persistence of PFAAs, the multiple sources of PFAAs beyond sewage (e.g., surface runoff, point sources) and the formation of PFAAs from their precursors. Use of multiple methods of source apportionment including principal component analysis–multiple linear regression (PCA–MLR) and perfluoroalkyl carboxylic acid ratio analysis highlighted sewage and point sources as the primary sources of PFAAs in the most severely polluted groundwater samples, with street runoff being a minor source (44.6% sewage, 45.7% point sources and 9.7% street runoff, by PCA–MLR). Tritium analysis indicated that, while young groundwater (recharged during or after the 1970s, when PFAAs were already in commercial use) in shallow aquifers (<50m depth) was naturally highly vulnerable to PFAA pollution, PFAAs were also found in old groundwater (recharged before the 1950s, when PFAAs were not in use) in deep aquifers (50–500m depth). This study demonstrated the utility of multiple uses of tracers (pharmaceuticals and personal care products; PPCPs, tritium) and source apportionment methods in investigating sources and pathways of PFAAs in multiple aquifer systems.

Insights gained from geochemical studies in the Waterloo Moraine: Indications and implications for anthropogenic loading

The Waterloo Moraine is a historic and continuing groundwater resource for the growing Region of Waterloo in southern Ontario, Canada, providing hundreds of thousands of litres of fresh water daily to the regional water supply. The complex stratigraphy of the Moraine is characterized by a system of multiple aquifers separated by regionally discontinuous aquitards, complicating resource assessments. The Region’s extensive monitoring and production well network has been sampled repeatedly over the last four decades, providing insights into how long-term pumping and urban development have affected aquifer geochemistry. Various environmental tracers were used to identify groundwater residence time and mixing including 3H, 3H/3He, CFCs, SF6, 14C, and 4He. Studies on the Waterloo Moraine provided important contributions to the application of these environmental tracers by validating unsaturated zone transport theory and documenting the effects of localized atmospheric CFC contamination. These studies have also provided important information about groundwater flow and mixing in the Moraine. Both historical and recent hydrogeological studies identified urban applications of road deicers (salt) and rural agricultural practices (nitrate loadings) as potential problems for future use of groundwater as a domestic water supply for the region, with Cl and NO3-N concentrations in some areas approaching 900 and 17 mg/L, respectively. The effects of these urban and rural contaminant issues are explored, and efforts to mitigate and reduce the impact, including the effects of best (beneficial) management practices, are discussed. The potential impacts of these non-point source contaminants on the public water supply should be considered when planning new residential developments.

Improved Particle Tracking Proxy for Assessing Plume Migration during Geologic Sequestration

Abstract During geologic carbon sequestration, it is of immense importance that the migration of the injected plume of CO 2 be monitored in order to ensure containment within the storage volume. We have previously described a framework for predicting future plume migration, taking into account routinely recorded well data (like injection rate or bottom-hole pressures). The cornerstone of our prediction process is the use of a particle-tracking proxy that provides the ability to assess rapidly the flow connectivity of multiple aquifer models without the need to use computationally expensive numerical simulators. In this paper, we extend the proxy to explicitly represent multiphase flow effects, buoyancy effects and the effect of fluid viscosity and compressibility. The resultant proxy is thus robust for representing plume migration corresponding to flow and transport of CO 2 in aquifers exhibiting complex heterogeneity and for different physical flow and transport mechanisms.