Glacial Aquifer Research Articles

Interaction of esker groundwater with headwater lakes and streams

Groundwater–surface water interactions were studied in a Finnish esker aquifer system where some lakes suffer from periodic water level decline and others from eutrophication. Natural tracers (Ca2+, SiO2 and PO43-) were used on the aquifer scale and seepage meter measurements and water level recordings on the single lake scale to better understand the complex interactions between groundwater, lakes and streams in the area. The natural tracers showed that lakes located in lower landscape positions and connected with streams were richer in tracer concentrations and that their water chemistry resembled that of the groundwater. On the other hand, closed basin lakes located at higher altitudes were nutrient-poor seepage lakes with some groundwater through flow. The data suggests that the subsurface acts as a phosphate source for the surface water bodies and the nutrient rich groundwater inseepage dictates the trophic status of the lakes in the area. The seepage meter measurements verified a strong interaction between groundwater and surface water and revealed spatial distribution of lake seepage and a temporal co-variation in lake seepage rates. A statistical analysis of seepage meter and water level observation time series were useful to show groundwater flow regimes controlling lake seepage. Using the natural tracer analysis and seepage meter measurements, we developed a novel conceptual model for the study site where the differences in lake trophic state and water table behavior were explained by assigning lakes to local and regional groundwater flow regimes. The obtained results provide new knowledge on using natural tracers in complex glacial aquifer systems and can be used in integrated groundwater–surface water management.

A fifty-year chronicle of tritium data for characterising the functioning of the Evian and Thonon (France) glacial aquifers

Using lumped models and a transfer function model, this paper deals with the interpretation of exceptionally long (up to 50 years (y)) and precise tritium chronicles characterising the rainfall, recharge (efficient rainfall) and outflow from various types of glacial aquifers from the French Alps (Evian-Thonon area). The efficient rainfall tritium chronicle was computed from tritium measurements performed for 11 years (1969-1979) in a lysimeter. The evapotranspiration induces a mean 15% drop of the annual tritium signal. The three superficial glacial aquifers (two fluvio-glacial kame terraces and a lateral till) provide similar results: a best fit with an exponential flow model (EM) (playing the major role) combined in parallel with a piston flow model (PFM), and a rather short mean transit time (T 5-7 y). The deepest mineral aquifer (Evian) can only be fitted with the in a series combination of a highly dispersive model (DM; T 68 y; DP = 0.5) and a piston flow model (T 2.5 y) or, better, by the in a series combination of an EM (T 8 y) modelling the subsurface aquifer and a DM (T 60 y; DP = 0.75) and the same piston flow model (T 2.5 y) modelling the deep mineral aquifer, this latest combination of models providing the following parameters: T 70 y and median transit time 45.5 y. It is also to be noted that a very small part of the recharge; about 1.3%, avoids both the EM and the DM, and directly enters the PFM (at the Northern limit of the Gavot Plateau). These models are very sensitive regarding the T (±1 y, 0.25 y for the PFM), less so with DP. These results will prompt hydrologists to (re)work historical data to determine if important hydrologic information is available. The interest and limits of such a modelling, also for other constituents than tritium, along with the future for tritium as a tracer are discussed and it also provides new insights on the structure and functioning of alpine paleo glacial hydrosystems.

An integrated approach for addressing uncertainty in the delineation of groundwater management areas

Uncertainty is a pervasive but often poorly understood factor in the delineation of wellhead protection areas (WHPAs), which can discourage water managers and practitioners from relying on model results. To make uncertainty more understandable and thereby remove a barrier to the acceptance of models in the WHPA context, we present a simple approach for dealing with uncertainty. The approach considers two spatial scales for representing uncertainty: local and global. At the local scale, uncertainties are assumed to be due to heterogeneities, and a capture zone is expressed in terms of a capture probability plume. At the global scale, uncertainties are expressed through scenario analysis, using a limited number of physically realistic scenarios. The two scales are integrated by using the precautionary principle to merge the individual capture probability plumes corresponding to the different scenarios. The approach applies to both wellhead protection and the mitigation of contaminated aquifers, or in general, to groundwater management areas. An example relates to the WHPA for a supply well located in a complex glacial aquifer system in southwestern Ontario, where we focus on uncertainty due to the spatial distributions of recharge. While different recharge scenarios calibrate equally well to the same data, they result in different capture probability plumes. Using the precautionary approach, the different plumes are merged into two types of maps delineating groundwater management areas for either wellhead protection or aquifer mitigation. The study shows that calibrations may be non-unique, and that finding a "best" model on the basis of the calibration fit may not be possible.

Airborne Electromagnetic Surveys for Groundwater Characterization

Airborne electromagnetic (AEM) surveys provide densely sampled data over large areas (typically several hundred sq. km) that cannot be covered effectively using ground-based methods. AEM data are inverted to estimate the three-dimensional distribution of electrical resistivity structures from shallow depths to several hundred meters. These models convey unparalleled details that are used to make inferences about hydrogeologic properties and processes at the watershed and local scale. This information is being used in groundwater models that are critical to water management decisions, to better understand geologic frameworks, and to improve climate change models. The U.S. Geological Survey (USGS) has been engaged in the application of AEM to many watershed and local scale groundwater projects within United States. We present the results of several frequency- and time-domain AEM surveys acquired by the USGS that have been used for mapping alluvial valleys, buried glacial aquifers, fault-bounded basins, and understanding permafrost distributions.

A simple method to assess unsaturated zone time lag in the travel time from ground surface to receptor

In contaminant travel from ground surface to groundwater receptors, the time taken in travelling through the unsaturated zone is known as the unsaturated zone time lag. Depending on the situation, this time lag may or may not be significant within the context of the overall problem. A method is presented for assessing the importance of the unsaturated zone in the travel time from source to receptor in terms of estimates of both the absolute and the relative advective times. A choice of different techniques for both unsaturated and saturated travel time estimation is provided. This method may be useful for practitioners to decide whether to incorporate unsaturated processes in conceptual and numerical models and can also be used to roughly estimate the total travel time between points near ground surface and a groundwater receptor. This method was applied to a field site located in a glacial aquifer system in Ontario, Canada. Advective travel times were estimated using techniques with different levels of sophistication. The application of the proposed method indicates that the time lag in the unsaturated zone is significant at this field site and should be taken into account. For this case, sophisticated and simplified techniques lead to similar assessments when the same knowledge of the hydraulic conductivity field is assumed. When there is significant uncertainty regarding the hydraulic conductivity, simplified calculations did not lead to a conclusive decision.

Irrigation Effects in the Northern Lake States: Wisconsin Central Sands Revisited

Irrigated agriculture has expanded greatly in the water-rich U.S. northern lake states during the past half century. Source water there is usually obtained from glacial aquifers strongly connected to surface waters, so irrigation has a potential to locally decrease base flows in streams and water levels in aquifers, lakes, and wetlands. During the nascent phase of the irrigation expansion, water availability was explored in works of some fame in the Wisconsin central sands by Weeks et al. (1965) on the Little Plover River and Weeks and Stangland (1971) on "headwater area" streams and lakes. Four decades later, and after irrigation has grown to a dominant landscape presence, we revisited irrigation effects on central sands hydrology. Irrigation effects have been substantial, on average decreasing base flows by a third or more in many stream headwaters and diminishing water levels by more than a meter in places. This explains why some surface waters have become flow and stage impaired, sometimes to the point of drying, with attendant losses of aquatic ecosystems. Irrigation exerts its effects by increasing evapotranspiration by an estimated 45 to 142 mm/year compared with pre-irrigated land cover. We conclude that irrigation water availability in the northern lake states and other regions with strong groundwater-surface water connections is tied to concerns for surface water health, requiring a focus on managing the upper few meters of aquifers on which surface waters depend rather than the depletability of an aquifer.

Arsenic Geochemistry and Hydrostratigraphy in Midwestern U.S. Glacial Deposits

Arsenic concentrations exceeding the U.S. EPA's 10 μg/L standard are common in glacial aquifers in the midwestern United States. Previous studies have indicated that arsenic occurs naturally in these aquifers in association with metal-(hydr)oxides and is released to groundwater under reducing conditions generated by microbial oxidation of organic matter. Despite this delineation of the arsenic source and mechanism of arsenic mobilization, identification of arsenic-impacted aquifers is hindered by the heterogeneous and discontinuous nature of glacial sediments. In much of the Midwest, the hydrostratigraphy of glacial deposits is not sufficiently characterized to predict where elevated arsenic concentrations are likely to occur. This case study from southeast Wisconsin presents a detailed characterization of local stratigraphy, hydrostratigraphy, and geochemistry of the Pleistocene glacial deposits and underlying Silurian dolomite. Analyses of a single core, water chemistry data, and well construction reports enabled identification of two aquifers separated by an organic-rich aquitard. The upper, unconfined aquifer provides potable water, whereas arsenic generally exceeds 10 μg/L in the deeper aquifer. Although coring and detailed hydrostratigraphic characterization are often considered impractical, our results demonstrate that a single core improved interpretation of the complex lithology and hydrostratigraphy. This detailed characterization of hydrostratigraphy facilitated development of well construction guidelines and lays the ground work for further studies of the complex interactions among aquifer sediments, hydrogeology, water chemistry, and microbiology that lead to elevated arsenic in groundwater.

Relative Vulnerability of Public Supply Wells to VOC Contamination in Hydrologically Distinct Regional Aquifers

A process‐based methodology was used to compare the vulnerability of public supply wells tapping seven study areas in four hydrologically distinct regional aquifers to volatile organic compound (VOC) contamination. This method considers (1) contributing areas and travel times of groundwater flowpaths converging at individual supply wells, (2) the oxic and/or anoxic conditions encountered along each flowpath, and (3) the combined effects of hydrodynamic dispersion and contaminant‐ and oxic/anoxic‐specific biodegradation. Contributing areas and travel times were assessed using particle tracks generated from calibrated regional groundwater flow models. These results were then used to estimate VOC concentrations relative to an unspecified initial concentration (C/C0) at individual public supply wells. The results show that the vulnerability of public supply wells to VOC contamination varies widely between different regional aquifers. Low‐recharge rates, long travel times, and the predominantly oxic conditions characteristic of Basin and Range aquifers in the western United States leads to lower vulnerability to VOCs, particularly to petroleum hydrocarbons such as benzene and toluene. On the other hand, high recharge rates and short residence times characteristic of the glacial aquifers of the eastern United States leads to greater vulnerability to VOCs. These differences lead to distinct patterns of C/C0 values estimated for public supply wells characteristic of each aquifer, information that can be used by resource managers to develop monitoring plans based on relative vulnerability, to locate new public supply wells, or to make land‐use management decisions.

Postaudit evaluation of conceptual model uncertainty for a glacial aquifer groundwater flow and contaminant transport model

Numerical groundwater flow and contaminant transport modeling incorporating three alternative conceptual models was conducted in 2005 to assess remedial actions and predict contaminant concentrations in an unconfined glacial aquifer located in Milford, Michigan, USA. Three alternative conceptual models were constructed and independently calibrated to evaluate uncertainty in the geometry of an aquitard underlying the aquifer and the extent to which infiltration from two manmade surface water bodies influenced the groundwater flow field. Contaminant transport for benzene, cis-DCE, and MTBE was modeled for a 5-year period that included a 2-year history match from July 2003 to May 2005 and predictions for a 3-year period ending in July 2008. A postaudit of model performance indicates that predictions for pumping wells, which integrated the transport signal across multiple model layers, were reliable but unable to differentiate between alternative conceptual model responses. In contrast, predictions for individual monitoring wells with limited screened intervals were less consistent, but held promise for evaluating alternative hydrogeologic models. Results of this study suggest that model conceptualization can have important practical implications for the delineation of contaminant transport pathways using monitoring wells, but may exert less influence on integrated predictions for pumping wells screened over multiple numerical model layers.

Multiple Working Transport Hypotheses in a Heterogeneous Glacial Aquifer System

AbstractMultiple working hypotheses can be used to evaluate permissible alternative hydrogeological interpretations at sites with limited subsurface control. This approach was applied to test the viability of three conceptual aquifer system architecture models coupled with three hypothesized source locations for a 1,4‐dioxane plume in a heterogeneous glacial aquifer system in Washtenaw County, Michigan. The three alternative conceptual models characterized the site hydrogeology with increasingly complex distributions of hydrostratigraphic units: (A) an effective aquifer, (B) a layered confined aquifer, and (C) a discretely heterogeneous aquifer model. Each was incorporated into an independently calibrated numerical ground water flow (MODFLOW) model. Steady‐state and transient flow simulations of the alternative models were evaluated using both hydraulic flow field characteristics observed under natural conditions and the perturbed response after local remedial pumping activity began. Three plausible locations where 1,4‐dioxane could have entered the aquifer system were identified using historical information at the site: (1) manufacturing waste water disposal lagoons, (2) a 60 foot (18 m) deep kettle lake, and (3) a shallow impoundment on a local stream. Advective transport modeling (MODPATH) was used to assess the consistency of the hypothesized source locations with observed contaminant migration pathways inferred from the mapped location of the plume. Evaluation of the nine combinations of hydrogeologic conceptualizations and 1,4‐dioxane source locations led to elimination of four working hypotheses and discounting of two others, leading to reduced overall uncertainty and the development of new insights into the system behavior.

The Groundwater Resources of the Morainic Amphitheatre: A Case Study in Piedmont

Problem statement: The risk of groundwater pollution and preservation of groundwater quality of these resources are extremely important topics, particularly at the present time in which water is becoming a limitative factor for land development. In this context the main objective of this study was the evaluation of the pollution vulnerability of groundwater in the Ivrea Morainic Amphitheatre (IMA), situated in the North-West of Piemonte. Approach: The case study required the application of the combined approach SINTACS and GNDCI-CNR Basic Method. SINTACS, used only for the evaluation of vulnerability in the plain covered by alluvial materials, was not considered for the hilly area due to lack of sufficient data, necessary to assign values to the seven required SINTACS parameters. Instead the GNDCI-CNR Basic Method was used, based on a survey of hydrogeologic complexes. The hydrogeologic study had identified an unconfined aquifer in the plain area, situated in the fluvial permeable sediments. Moreover the study had identified a confining bed which separates the aquifer from a deep confined aquifer. Analysis of water table aquifer flow patterns and groundwater quality had led to the characterization of some relationships between different aquifers. Results: Vulnerability map showed an important difference between the water table aquifer and the glacial tills aquifers. The first one had a very high vulnerability, due especially to the small depth of middle-high permeable sediments in the water table. The second one, on the contrary, had a low degree of vulnerability. Conclusion: The vulnerability map must be a fundamental component of an overall groundwater protection scheme: Springs in moraines and the confined aquifer in the plain, which was not affected by groundwater contamination, had better characteristics for drinking water designations.

Simulated response of water quality in public supply wells to land use change

Understanding how changes in land use affect water quality of public supply wells (PSW) is important because of the strong influence of land use on water quality, the rapid pace at which changes in land use are occurring in some parts of the world, and the large contribution of groundwater to the global water supply. In this study, groundwater flow models incorporating particle tracking and reaction were used to analyze the response of water quality in PSW to land use change in four communities: Modesto, California (Central Valley aquifer); York, Nebraska (High Plains aquifer); Woodbury, Connecticut (Glacial aquifer); and Tampa, Florida (Floridan aquifer). The water quality response to measured and hypothetical land use change was dependent on age distributions of water captured by the wells and on the temporal and spatial variability of land use in the area contributing recharge to the wells. Age distributions of water captured by the PSW spanned about 20 years at Woodbury and >1,000 years at Modesto and York, and the amount of water <50 years old captured by the PSW ranged from 30% at York to 100% at Woodbury. Short‐circuit pathways in some PSW contributing areas, such as long irrigation well screens that crossed multiple geologic layers (York) and karst conduits (Tampa), affected age distributions by allowing relatively rapid movement of young water to those well screens. The spatial component of land use change was important because the complex distribution of particle travel times within the contributing areas strongly influenced contaminant arrival times and degradation reaction progress. Results from this study show that timescales for change in the quality of water from PSW could be on the order of years to centuries for land use changes that occur over days to decades, which could have implications for source water protection strategies that rely on land use change to achieve water quality objectives.

Source and transport controls on the movement of nitrate to public supply wells in selected principal aquifers of the United States

In 2003–2005, systematic studies in four contrasting hydrogeologic settings were undertaken to improve understanding of source and transport controls on nitrate movement to public supply wells (PSW) in principal aquifers of the United States. Chemical, isotopic, and age tracer data show that agricultural fertilizers and urban septic leachate were the primary sources of large nitrate concentrations in PSW capture zones at Modesto, California (Central Valley aquifer system) and York, Nebraska (High Plains aquifer). Urban septic leachate and fertilizer (possibly nonfarm) were the primary sources of large nitrate concentrations in PSW capture zones at Woodbury, Connecticut (glacial aquifer system), and Tampa, Florida (Floridan aquifer system), respectively. Nitrate fluxes to the water table were larger in agricultural settings than urban settings, indicating that it would be beneficial to reduce PSW capture zone areas in agricultural regions. Mixing calculations indicate that about 50 to 85% of the nitrate in water from the PSW could be from those modern anthropogenic sources, with the remainder coming from sources in old (>50 years) recharge or sources in young recharge in undisturbed settings such as forests. Excess N2 concentrations and age tracers showed that denitrification at Modesto occurred gradually (first‐order rate constant of 0.02/a) in a thick reaction zone following a ∼30‐year lag time after recharge. Denitrification generally was not an important nitrate sink at Woodbury. At York and Tampa, denitrification occurred rapidly (0.5 to 6/a) in thin reaction zones in fine‐grained sediments that separated the anoxic PSW producing zones from overlying oxic, high‐nitrate ground water. Particle tracking showed that a major pathway by which anthropogenic nitrate reached the York and Tampa PSW was by movement through long well screens crossing multiple hydrogeologic units (York) and by movement through karst features (Tampa), processes which reduced ground water residence times in the denitrifying zones. These results illustrate how PSW vulnerability to nitrate contamination depends on complex variations and interactions between contaminant sources, reaction rates, transit times, mixing, and perturbation of ground water flow in contrasting hydrogeologic settings.

Persistent Elevated Nitrate in a Riparian Zone Aquifer

Streamside vegetated buffer strips (riparian zones) are often assumed to be zones of ground water nitrate (NO3(-)) attenuation. At a site in southwestern Ontario (Zorra site), detailed monitoring revealed that elevated NO3(-) -N (4-93 mg L(-1)) persisted throughout a 100-m-wide riparian floodplain. Typical of riparian zones, the site has a soil zone of recent river alluvium that is organic carbon (OC) rich (36 +/- 16 g kg(-1)). This material is underlain by an older glacial outwash aquifer with a much lower OC content (2.3 +/- 2.5 g kg(-1). Examination of NO3(-), Cl(-), SO4(2-), and dissolved organic carbon (DOC) concentrations; N/Cl ratios; and NO3(-) isotopic composition (delta15N and delta18O) provides evidence of four distinct NO3(-) source zones within the riparian environment. Denitrification occurs but is incomplete and is restricted to a narrow interval located within ~0.5 m of the alluvium-aquifer contact and to one zone (poultry manure compost zone) where elevated DOC persists from the source. In older ground water close to the river discharge point, denitrification remains insufficient to substantially deplete NO3(-). Overall, denitrification related specifically to the riparian environment is limited at this site. The persistence of NO3(-) in the aquifer at this site is a consequence of its Pleistocene age and resulting low OC content, in contrast to recent fluvial sediments in modern agricultural terrain, which, even if permeable, usually have zones enriched in labile OC. Thus, sediment age and origin are additional factors that should be considered when assessing the potential for riparian zone denitrification.

Microbiological Response to Well Pumping

To better constrain sampling strategies for observing biologically sensitive parameters in ground water, we vigorously pumped for 120 h a lightly pumped well completed in a confined glacial aquifer while observing how various physical and chemical parameters evolve in the water produced. The parameters commonly monitored when sampling a well stabilized within about an hour, after 5 wellbore volumes were produced; these parameters include temperature, pH, dissolved oxygen, oxidation-reduction potential (Eh), and electrical conductivity. The concentrations of ferrous iron, sulfide, and sulfate and various biological or biologically sensitive parameters, including the concentrations of dissolved hydrogen and methane, direct cell counts, and the microbial community profile, in contrast, required more than 8 h or 36 well volumes to stabilize. We interpret this result to mean that the zone of influence of the wellbore on biologic processes in the aquifer extends beyond the commonly recognized zone where physical properties are affected. A second period of adjustment of these biologically sensitive parameters began after about 50 h of pumping, following displacement of 230 wellbore volumes, and continued to the end of the experiment. During this period, the cell density and the composition of the microbial community suspended in the water samples changed. This finding indicates that the microbial community in and near the wellbore changed in response to pumping and the changes affected aspects of the composition of water produced from the well. The study demonstrates the importance of allowing adequate pumping time when sampling ground water for the analysis of biologically sensitive parameters.

Assessment of groundwater discharges into West Neck Bay, New York, via natural tracers

A field experiment to compare methods of assessing submarine groundwater discharge (SGD) was held on Shelter Island, NY, in May 2002. We evaluated the use of radon, radium isotopes, and methane to assess SGD rates and dynamics from a glacial aquifer in the coastal zone. Fluxes of radon across the sediment-water interface were calculated from changes in measured surface water inventories following evaluation and correction for tidal effects, atmospheric evasion, and mixing with offshore waters. These fluxes were then converted to SGD rates using the measured radon concentration in the groundwater. We used the short-lived radium isotopes to calculate a horizontal mixing coefficient to assess radon loss by mixing between nearshore and offshore waters. We also made an independent calculation of SGD using the Ra-derived mixing coefficient and the long-lived 226Ra concentration gradient in the bay. Seepage rates were calculated to range between 0 and 34 cm day −1 using the radon measurements and 15 cm day −1 as indicated by the radium isotopes. The radiotracer results were consistent and comparable to SGD rates measured directly with vented benthic chambers (seepage meters) deployed during this experiment. These meters indicated rates between 2 and 200 cm day −1 depending on their location. Both the calculated radon fluxes and rates measured directly by the automated seepage meters revealed a clear reproducible pattern of higher fluxes during low tides. Considering that the two techniques are completely independent, the agreement in the SGD dynamics is significant. Methane concentration in groundwater was very low (∼30 nM) and not suitable as SGD tracer at this study site.

The positive influence of quality assurance in 2 consecutive large EORTC trials involving radiotherapy for breast cancer

Using lumped models and a transfer function model, this paper deals with the interpretation of exceptionally long (up to 50 years (y)) and precise tritium chronicles characterising the rainfall, recharge (efficient rainfall) and outflow from various types of glacial aquifers from the French Alps (Evian–Thonon area). The efficient rainfall tritium chronicle was computed from tritium measurements performed for 11 years (1969–1979) in a lysimeter. The evapotranspiration induces a mean 15% drop of the annual tritium signal. The three superficial glacial aquifers (two fluvio-glacial kame terraces and a lateral till) provide similar results: a best fit with an exponential flow model (EM) (playing the major role) combined in parallel with a piston flow model (PFM), and a rather short mean transit time (T 5-7 y). The deepest mineral aquifer (Evian) can only be fitted with the in a series combination of a highly dispersive model (DM; T 68 y; DP = 0.5) and a piston flow model (T 2.5 y) or, better, by the in a series combination of an EM (T 8 y) modelling the subsurface aquifer and a DM (T 60 y; DP = 0.75) and the same piston flow model (T 2.5 y) modelling the deep mineral aquifer, this latest combination of models providing the following parameters: T 70 y and median transit time 45.5 y. It is also to be noted that a very small part of the recharge; about 1.3%, avoids both the EM and the DM, and directly enters the PFM (at the Northern limit of the Gavot Plateau). These models are very sensitive regarding the T (±1 y, 0.25 y for the PFM), less so with DP. These results will prompt hydrologists to (re)work historical data to determine if important hydrologic information is available. The interest and limits of such a modelling, also for other constituents than tritium, along with the future for tritium as a tracer are discussed and it also provides new insights on the structure and functioning of alpine paleo glacial hydrosystems.

Arsenic in Glacial Aquifers: Sources and Geochemical Controls

A total of 176 wells in sand-and-gravel glacial aquifers in central Illinois were sampled for arsenic (As) and other chemical parameters. The results were combined with archived and published data from several hundred well samples to determine potential sources of As and the potential geochemical controls on its solubility and mobility. There was considerable spatial variability in the As concentrations. High concentrations were confined to areas smaller than 1 km in diameter. Arsenic and well depth were uncorrelated. Arsenic solubility appeared to be controlled by oxidation-reduction (redox) conditions, especially the presence of organic matter. Geochemical conditions in the aquifers are typically reducing, but only in the most reducing water does As accumulate in solution. In wells in which total organic carbon (TOC) was below 2 mg/L and sulfate (SO4(2-)) was present, As concentrations were low or below the detection limit (0.5 microg/L). Arsenic concentrations >10 microg/L were almost always found in wells where TOC was >2 mg/L and SO4(2-) was absent or at low concentrations, indicating post-SO4 (2-)reducing conditions. Iron (Fe) is common in the aquifer sediments, and Fe oxide reduction appears to be occurring throughout the aquifers. Arsenic is likely released from the solid phase as Fe oxide is reduced.

Matching Solute Breakthrough with Deterministic and Stochastic Aquifer Models

Two different deterministic and two alternative stochastic (i.e., geostatistical) approaches to modeling the distribution of hydraulic conductivity (K) in a nonuniform (sigma2ln(K)) = 0.29) glacial sand aquifer were used to explore the influence of conceptual model selection on simulations of three-dimensional tracer movement. The deterministic K models employed included a homogeneous effective K and a perfectly stratified 14 layer model. Stochastic K models were constructed using sequential Gaussian simulation and sequential i ndicator simulation conditioned to available K values estimated from measured grain size distributions. Standard simulation software packages MODFLOW, MT3DMS, and MODPATH were used to model three-dimensional ground water flow and transport in a field tracer test, where a pulse of bromide was injected through an array of three fully screened wells and extracted through a single fully screened well approximately 8 m away. Agreement between observed and simulated transport behavior was assessed through direct comparison of breakthrough curves (BTCs) and selected breakthrough metrics at the extraction well and at 26 individual multilevel sample ports distributed irregularly between the injection and extraction wells. Results indicate that conceptual models incorporating formation variability are better able to capture observed breakthrough behavior. Root mean square (RMS) error of the deterministic models bracketed the ensemble mean RMS error of stochastic models for simulated concentration vs. time series, but not for individual BTC characteristic metrics. The spatial variability models evaluated here may be better suited to simulating breakthrough behavior measured in wells screened over large intervals than at arbitrarily distributed observation points within a nonuniform aquifer domain.

Passive groundwater control system for a motorway

Considerable public discussion was generated during the planning stages of a rural motorway in Ireland regarding the possible effects of the road drainage system on an ecologically important fen, which was about 5 km distant. To overcome this potential problem, a passive groundwater level control system was devised to limit the drawdown by maintaining the water levels 1·75 m above the road surface. This groundwater level corresponded approximately to the seasonal low water levels recorded in the aquifer prior to the construction of the motorway. This is achieved by constructing the motorway within a sealed cross-section with a permeable layer beneath this section, called the underdrainage system, which is connected to drains that outflow at the higher level. This underdrainage system controls the water pressures and flows beneath the sealed section. This paper presents the concept of the passive groundwater control system in a non-homogeneous glacial aquifer, and analyses the effects of this control system on the groundwater around the motorway. The selection of the design criteria and site control methods for the material within the sealed section is also discussed. The results of site monitoring that show the successful performance of the system in operation are also given.