Nitrate as a tracer of groundwater flow in a fractured multilayered aquifer

Abstract

Knowledge of the processes that control nitrate migration and its geochemical evolution in the subsurface are fundamental for the regional management of polluted aquifers. In this paper, the spatial distribution and transient variations of nitrate concentrations, associated with manure fertilization, are used to depict hydrogeological dynamics within the sedimentary aquifer system of la Plana de Vic in the Osona region of Spain. Flow systems are identified from geological, hydraulic head, hydrochemical and isotopic data, and by considering nitrate itself as a tracer that indicates how flow paths are modified by human pressures. In this area, nitrates move through fractured aquitards in flows induced by groundwater pumping. Moreover, the lack of casing in the boreholes permits a mixing of groundwater from distinct layers inside the wells, which negates any benefits from the low-nitrate groundwater found in the deepest aquifer layers. Therefore, impacts on groundwater quality are related to intensive manure fertilization as well as to inadequate well construction and exploitation strategies.

Citation Menció, A., Mas-Pla, J., Otero, N. & Soler, A. (2011) Nitrate as a tracer of groundwater flow in a fractured multilayered aquifer. Hydrol. Sci. J. 56(1), 108–122.     

Groundwater availability in the Khulais Plain,western Saudi Arabia

Abstract

The Khulais plain lies within a typical arid area in western Saudi Arabia. Groundwater occurs within two aquifers in the area: the alluvium of the wadi system, and the sandy layers of the Cretaceous-Tertiary sedimentary succession. Detailed field investigations and laboratory analysis helped in determining the aquifer properties for each of the water-bearing units. Groundwater movement has been thoroughly studied, and distribution maps prepared to explain the variations in transmissivity, permeability, porosity and specific yield. An attempt has been made to estimate volumes of groundwater flow towards the plain. This study presents a first attempt towards determining groundwater availability in the Cretaceous-Tertiary succession of this part of the world.     

Hydrodynamic behaviour of karstic aquifers in Boroujerd,western Iran

Abstract

Considering the geological conditions of the southwest of Boroujerd and northwest of Doroud, Iran, karst development is analysed with respect to the hydrodynamic behaviour of the main draining springs of the units and the karstic aquifers are classified as either those developed in Cretaceous limestone or those developed in Oligomiocene limestone. For this purpose, the yields of the main karstic springs of the region—Absardeh and Zoorabad (Cretaceous karstic limestone aquifer), Kalamsooz and Azizabad (Oligomiocene karstic limestone aquifer)—were measured and analysed. Analysis of the recession curve is used for hydrodynamical analysis and to construct the conceptual model for estimation of karst development in the selected aquifers. Based on the results, the dynamic storage capacity of the saturated zone in Cretaceous limestone is evaluated as low to medium and that in Oligomiocene limestone as medium to high. The dynamic storage capacity of the unsaturated zone in Cretaceous limestone is evaluated as high and that in Oligomiocene limestone as low to medium. Moreover, the contribution of quickflow in karstic aquifers developed in the Cretaceous limestone drained by the Absardeh and Zoorabad springs is 23.5 and 82.2%, respectively, and that for the Kalamsooz and Azizabad springs (Oligomiocene limestone) is 5.7 and 22.5%, respectively. Flow in the Cretaceous limestone aquifer drained by the Zoorabad Spring is of concentrated type and the main flow occurs in the well-developed karstic conduits. The main flow in the Oligomiocene limestone aquifer, drained by the Kalamsooz Spring, occurs in a network of joints and fractures and the contribution of concentrated flow is very low. The transmissivity of the saturated zone in the karstic aquifer drained by the Zoorabad and Absardeh springs is medium to high and that for the Kalamsooz and Azizabad springs is found to be low.     

Estimation of the inland extending length of the freshwater–saltwater interface in coastal unconfined aquifers

ABSTRACT

The inland extending length of the freshwater–saltwater interface toe is useful in studies of seawater intrusion in coastal areas. The submarine fresh groundwater discharge in coastal zones is affected not only by hydraulic conductivity and hydraulic gradient of the aquifer, but also by the position of the interface. Two observation wells at different distances from the coast are required to calculate the fresh groundwater flow rate in coastal unconfined aquifers. By considering that the submarine groundwater discharge is equal to the groundwater flow rate, the length of the interface toe extending inland can be estimated when the groundwater flow is at a steady-flow state. Aquifers with horizontal and sloping confined beds and without/with unique surface vertical infiltration are considered. Examples used to illustrate the application of these methods indicate that the inland extending lengths of the interface toe in aquifers with vertical surface infiltration are much shorter than those in aquifers without vertical surface infiltration, and the length of the interface in aquifers with a horizontal confining lower bed are smaller than those in aquifers with a confining lower bed sloping towards the sea. The extent of the interface on the northwestern coast near the city of Beihai in southern Guangxi, China, on 18 January 2013 was estimated as 471–478 m.

Editor M.C. Acreman Associate editor not assigned     

Predicting Pesticide Attenuation in a Fractured Aquifer Using Lumped‐Parameter Models

Neighboring springs draining fractured‐rock aquifers can display large differences in water quality and flow regime, depending on local variations of the connectivity and the aperture size distribution of the fracture network. Consequently, because homogeneous equivalent parameters cannot be assumed a priori for the entire regional aquifer, the vulnerability to pollution of such springs has to be studied on a case by case basis. In this paper, a simple lumped‐parameter model usually applied to estimate the mean transit time of water (or tracer) is presented. The original exponential piston‐flow model was modified to take land‐use distribution into account and applied to predict the evolution of atrazine concentration in a series of springs draining a fractured sandstone aquifer in Luxembourg, where despite a nationwide ban in 2005, atrazine concentrations still had not begun to decrease in 2009. This persistence could be explained by exponentially distributed residence times in the aquifer, demonstrating that in some real world cases, models based on the groundwater residence time distribution can be a powerful tool for trend reversal assessments as recommended for instance by current European Union guidelines.     

Groundwater quality in some Voltaian and Birimian aquifers in northern Ghana—application of mulitvariate statistical methods and geographic information systems

Abstract

Groundwater is a priceless resource in the economies of the rural populations in northern Ghana. A combination of multivariate statistical and spatial analytical techniques was applied to groundwater data from the Voltaian and Birimian aquifers in parts of northern Ghana. The objective was to classify the groundwater quality control parameters and determine whether the aquifers deliver groundwater of acceptable quality for domestic and commercial irrigation purposes. It was found that groundwater quality is dominated by the weathering of accessory minerals that are predominant in the Obosum and Oti beds of the Middle Voltaian in the north, and incongruent silicate mineral weathering ranks second among the major causes of variation. The two processes account for over 70% of the total variance in the hydrochemistry and interpolation maps generated for these two major factors are discussed. The mineral weathering and dissolution processes are less apparent within the Birimian aquifers. Four spatial groundwater types were distinguished by differences in EC and pH, which are high among the water types within the Obosum and Oti beds, and generally low within the Birimian aquifers. Mineral stability diagrams suggest that montmorillonite is the most stable clay mineral phase in the system, a significant finding in terms of cation exchange processes. This, in turn, has affected the irrigation quality of groundwater from the aquifers in the area. Our findings suggest that the Birimian aquifers are more suitable for irrigation, due to generally low salinities and sodicities, and those associated with the Obosum and Oti beds of the Middle Voltaian are less suitable.

Editor D. Koutsoyiannis; Associate editor Y. Guttman

Citation Yidana, S.M., Banoeng-Yakubo, B., Aliou, A.-S., and Akabzaa, T., 2012. Groundwater quality in some Voltaian and Birimian aquifers in northern Ghana—application of mulitvariate statistical methods and geographic information systems. Hydrological Sciences Journal, 57 (6), 1168–1183.     

Detecting the flow relationships between deep and shallow aquifers in an exploited groundwater system,using long‐term monitoring data and quantitative hydrogeology: the Acque Albule basin case (Rome,Italy)

Five years of hydrogeological monitoring and field activities performed in the complex hydrogeological system of the Acque Albule basin (AAB) were conducted to define the hydrogeological setting, the relationship between deep and shallow aquifers and a conceptual groundwater flow model of this exploited area using conventional quantitative techniques. The basin, which is located close to Rome (Italy) on the west side of the Apennine chain and just north of the Colli Albani volcano, subsided after development of a north–south fault system (about 115 000 y bp). The AAB experiences intense hydrothermal activity, which has produced a large travertine deposit (80‐m thick). The travertine deposit constitutes a fractured aquifer that is the final destination of more than 5 m³ s^‐1 of water and is strongly dewatered by quarry activities. The complex hydrogeology of this basin was investigated, revealing two main hydraulically connected aquifers, one thermalised and partly confined into the limestone bedrock and one unconfined in the travertine. The two aquifers are separated by a non‐continuous clayey aquiclude. The hydrogeological survey and geological characterisation contributed to the development of the groundwater flow conceptual model. Analysis and comparison of the monitored levels highlighted the pattern of flow between the deep and shallow parts of the flow system. Copyright © 2012 John Wiley & Sons, Ltd.     

Using Domestic Well Records to Determine Fractured Bedrock Watersheds and Recharge Rates

This study presents an approach for delineating groundwater basins and estimating rates of recharge to fractured crystalline bedrock. It entailed the use of completion report data (boring logs) from 2500 domestic wells in bedrock from the Coventry Quadrangle, which is located in northeastern Connecticut and characterized by metamorphic gneiss and schist. Completion report data were digitized and imported into ArcGIS^® for data analysis. The data were processed to delineate groundwater drainage basins for the fractured rock based on flow conditions and to estimate groundwater recharge to the bedrock. Results indicate that drainage basins derived from surface topography, in general, may not correspond with bedrock drainage basins due to scale. Estimates of recharge to the bedrock for the study area indicate that only a small fraction of the precipitation or the amount of water that enters the overburden recharges the rock. The approach presented here can be a useful method for water resource‐related assessments that involve fractured rock aquifers.     

Assessment of the groundwater favorability of fractured aquifers from the southeastern Brazil crystalline basement

ABSTRACT

Groundwater favorability maps can aid groundwater exploitation in fractured aquifers, such as those of the Bação and Northern Bonfim complexes (Quadrilátero Ferrífero) southeastern Brazil, bringing alternatives for water supply in regions where water availability is at risk. These maps were obtained by means of the analytic hierarchy process (AHP), using six information levels: the height above the nearest drainage (HAND) model, the declivity map, three lineament maps (morphostructural, radiometric and magnetometric), and the lithological map. The E–W strike of the lineaments was emphasized for the integration, because it is the most frequent and subparallel to the present strike of the maximum principal stress σ₁. The favorability maps were validated by varying the input parameters and comparing the maps with 82 specific capacity values obtained from well tests. Although more data should be necessary to confirm this method, the results are promising and can be tested in other crystalline basement areas.     

Assessing hydrogeologic controls on dynamic groundwater storage using long‐term instrumental records of water table levels

This study analyzes a long‐term regional compilation of water table response to climate variability based on 124 long‐term groundwater wells distributed across New England, USA, screened in a variety of geologic materials. The New England region of the USA is located in a humid‐temperature climate underlain by low‐storage‐fractured metamorphic and crystalline bedrock dissected by north–south trending valleys filled with glacial and post‐glacial valley fill sediments. Uplands are covered by thin glacial till that comprises more than 60% of the total area. Annual and multi‐annual responses of the water table to climate variability are assessed to understand how local hydraulic properties and hydrogeologic setting (located in recharge/discharge region) of the aquifer influence the hydrologic sensitivity of the aquifer system to climate variability. This study documents that upland aquifer systems dominated by thin deposits of surface till comprise ~70% of the active and dynamic storage of the region. Total aquifer storage changes of +5 to ?7 km³ occur over the region during the study interval. The storage response is dominated by thin and low permeability surficial till aquifer that fills and drains on a multi‐annual basis and serves as the main mechanism to deliver water to valley fill aquifers and underlying bedrock aquifers. Whereas the till aquifer system is traditionally neglected as an important storage reservoir, this study highlights the importance of a process‐based understanding of how different landscape hydrogeologic units contribute to the overall hydrologic response of a region.     

Heat flow and ground water flow in the Great Plains of the United States

Regional groundwater flow in deep aquifers adds advective components to the surface heat flow over extensive areas within the Great Plains province. The regional groundwater flow is driven by topographically controlled piezometric surfaces for confined aquifers that recharge either at high elevations on the western edge of the province or from subcrop contacts. The aquifers discharge at lower elevations to the east. The assymetrical geometry for the Denver and Kennedy Basins is such that the surface areas of aquifer recharge are small compared to the areas of discharge. Consequently, positive advective heat flow occurs over most of the province. The advective component of heat flow in the Denver Basin is on the order of 15 mW m⁻² along a zone about 50 km wide that parallels the structure contours of the Dakota aquifer on the eastern margin of the Basin. The advective component of heat flow in the Kennedy Basin is on the order of 20 mW m⁻² and occurs over an extensive area that coincides with the discharge areas of the Madison (Mississippian) and Dakota (Cretaceous) aquifers. Groundwater flow in Paleozoic and Mesozoic aquifers in the Williston Basin causes thermal anomalies that are seen in geothermal gradient data and in oil well temperature data. The pervasive nature of advective heat flow components in the Great Plains tends to mask the heat flow structure of the crust, and only heat flow data from holes drilled into the crystalline basement can be used for tectonic heat flow studies.     

Characterization of the aquifers of the Bangui urban area,Central African Republic,as an alternative drinking water supply resource

Abstract

This paper presents the results of a survey carried out in 2010 aimed at evaluating the type and quality of the groundwater resources of the Bangui region of the Central African Republic. This work is the first step towards the development of groundwater resources in the Central African Republic in order to find alternatives to direct pumping from the Ubangi River and provide the population of the suburbs with a safer drinking water supply from deep boreholes. By combining both geological and hydrogeochemical approaches, it appears that the geology of Bangui is favourable to the development of a secure and sustainable water supply from groundwater provided that the conditions of exploitation would be constrained by the local authorities. The deep Precambrian carbonate aquifers, known as the Bimbo and Fatima formations, are identified as target resources in view of the relatively good quality of their water from the chemical point of view, and the semi-confined structure of the aquifers that prevents the mixing with shallow aquifers that are already strongly affected by domestic and industrial pollution. The main difficulty in terms of exploitation is to appreciate the depth of the resource and the more or less fractured/palaeo-karstified type of the porosity.

Editor Z.W. Kundzewicz

Citation Djebebe-Ndjiguim, C.L., Huneau, F., Denis, A., Foto, E., Moloto-a-Kenguemba, G., Celle-Jeanton, H., Garel, E., Jaunat, J., Mabingui, J., and Le Coustumer, P., 2013. Characterization of the aquifers of the Bangui urban area, Central African Republic, as an alternative drinking water supply resource. Hydrological Sciences Journal, 58 (8), 1760–1778.     

A methodology for regionalizing 3‐D effective porosity at watershed scale in crystalline aquifers

An innovative approach for regionalizing the 3‐D effective porosity field is presented and applied to two large, overexploited, and deeply weathered crystalline aquifers located in southern India. The method derives from earlier work on regionalizing a 2‐D effective porosity field in that part of an aquifer where the water table fluctuates, which is now extended over the entire aquifer using a 3‐D approach. A method based on geological and geophysical surveys has also been developed for mapping the weathering profile layers (saprolite and fractured layers). The method for regionalizing 3‐D effective porosity combines water table fluctuation and groundwater budget techniques at various cell sizes with the use of satellite‐based data (for groundwater abstraction), the structure of the weathering profile, and geostatistical techniques. The approach is presented in detail for the Kudaliar watershed (983 km²) and tested on the 730 km² Anantapur watershed. At watershed scale, the effective porosity of the aquifer ranges from 0.5% to 2% in Kudaliar and between 0.3% and 1% in Anantapur, which agrees with earlier works. Results show that (a) depending on the geology and on the structure of the weathering profile, the vertical distribution of effective porosity can be very different and that the fractured layers in crystalline aquifers are not necessarily characterized by a rapid decrease in effective porosity and (b) that the lateral variations in effective porosity can be larger than the vertical ones. These variations suggest that within a same weathering profile, the density of open fractures and/or degree of weathering in the fractured zone may significantly vary from a place to another. The proposed method provides information on the spatial distribution of effective porosity that is of prime interest in terms of flux and contaminant transport in crystalline aquifers. Implications for mapping groundwater storage and scarcity are also discussed, which should help in improving groundwater resource management strategies.     

Convergent Radial Tracing of Viral and Solute Transport in Gneiss Saprolite

Deeply weathered crystalline rock aquifer systems comprising unconsolidated saprolite and underlying fractured bedrock (saprock) underlie 40% of sub-Saharan Africa. The vulnerability of this aquifer system to contamination, particularly in rapidly urbanizing areas, remains poorly understood. In order to assess solute and viral transport in saprolite derived from Precambrian gneiss, forced-gradient tracer experiments using chloride and Escherichia coli phage ΦX174 were conducted in southeastern Uganda. The bacteriophage tracer was largely unrecovered; adsorption to the weathered crystalline rock matrix is inferred and enabled by the low pH (5.7) of site ground water and the bacteriophage's relatively high isoelectric point (pI = 6.6). Detection of the applied ΦX174 phage in the pumping well discharge at early times during the experiment traces showed, however, that average ground water flow velocities exceed that of the inert solute tracer, chloride. This latter finding is consistent with observations in other hydrogeological environments where statistically extreme sets of microscopic flow velocities are considered to transport low numbers of fecal pathogens and their proxies along a selected range of linked ground water pathways. Application of a radial advection-dispersion model with an exponentially decaying source term to the recovered chloride tracer estimates a dispersivity (α) of 0.8 ± 0.1 m over a distance of 4.15 m. Specific yield (S_y) is estimated to be 0.02 from volume balance calculations based on tracer experiments. As single-site observations, our estimates of saprolite S_y and α are tentative but provide a starting point for assessing the vulnerability of saprolite aquifers in sub-Saharan Africa to contamination and estimating quantitatively the impact of climate and abstraction on ground water storage.     

Alteration of the groundwater thermal regime caused by advection

ABSTRACT

Groundwater temperature at an arbitrary depth and at an arbitrary point is determined not only by heat transported by conduction but also by advection caused either by infiltration of rain, snowmelt or irrigated water, or by seepage from surface water bodies. Therefore, characteristic changes of groundwater temperature are observed in recharging and discharging areas within a groundwater flow system. The changes may be one-, two-, or three-dimensional, depending on individual situations. Since heat is a conservative quantity in the subsurface environment, groundwater temperature can be used as a tracer to reveal the regional structure of a groundwater flow system. A case study showing the importance of groundwater temperature in a regional groundwater survey is presented taking Nagaoka plain, Japan, as an example. The groundwater temperatures were measured in observation wells with diameters of 65 to 250 mm and depths of 20 m or more. Marked seasonal changes in temperature depth profiles showing advective effects in the horizontal direction from the Shinano River, and in the vertical direction from upper and lower aquifers, were observed. The temperature depth profiles were classified into six types. The distribution of these types does not contradict the regional structure of the groundwater flow system revealed by the potential distribution. As groundwater temperature is an easily measureable element in a hydrological survey, the method described in the present paper is appropriate for a field study in an uninstrumented groundwater basin.     

Infiltration-recharge through wadi beds in arid regions

Abstract

Groundwater recharge in arid regions is intermittent and usually occurs as a result of flood flow transmission losses in dry wadi channels. Hydrograph characteristics play a dominant role in determining the amount of channel abstraction in relation to the width of the wetted perimeter and the time of inundation, and the subsequent groundwater recharge. Large variations in the magnitude of channel losses result mainly from the diversity in inflow volumes. The magnitude of groundwater recharge in relation to bed transmission losses is dependent on flood volume and duration, soil moisture content and physical soil profile characteristics. Runoff volume and duration are the dominant factors influencing the cumulative infiltrated volume and recharge to shallow water tables. Taking into consideration the influence of various hydrological and channel characteristics, several regression equations are suggested to estimate the transmission losses from a wadi bed and the groundwater recharge.     

Conceptual models for carbonate aquifers. 1969

The very diverse types of ground‐water behavior in carbonate terrains can be classified by relating the flow type to a particular hydrogeologic environment each exhibiting a characteristic cave morphology. The ground water may move by diffuse flow, by retarded flow, or by free flow. Diffuse flow occurs in less soluble rocks such as extremely shaley limestones or crystalline dolomites. Integrated conduits are rare. Caves tend to be small, irregular, and often little more than solutionally widened joints. Retarded flows occur in artesian environments and in situations where unfavorable stratigraphy forces ground water to be confined to relatively thin beds. Network cave patterns are characteristic since hydrodynamic forces are damped by the external controls. Solution occurs along many available joints. Free flowing aquifers are those in which solution has developed a subsurface drainage system logically regarded as an underground extension of surface streams. These streams may have fully developed surface tributaries as well as recharge from sinkholes and general infiltration. Characteristic cave patterns are those of integrated conduit systems which are often truncated into linear, angulate, and branchwork caves. Free Flow aquifers may be further subdivided into Open aquifers lying beneath karst plains and Capped aquifers in which significant parts of the drainage net lie beneath an insoluble cap rock. Other geologic factors such as structure, detailed lithology, relief, and locations of major streams, control the details of cave morphology and orientation of the drainage network.     

A double-porosity model for a fractured aquifer with non-Darcian flow in fractures

Abstract

Non-Darcian flow in a finite fractured confined aquifer is studied. A stream bounds the aquifer at one side and an impervious stratum at the other. The aquifer consists of fractures capable of transmitting water rapidly, and porous blocks which mainly store water. Unsteady flow in the aquifer due to a sudden rise in the stream level is analysed by the double-porosity conceptual model. Governing equations for the flow in fractures and blocks are developed using the continuity equation. The fluid velocity in fractures is often too high for the linear Darcian flow so that the governing equation for fracture flow is modified by Forcheimer's equation, which incorporates a nonlinear term. Governing equations are coupled by an interaction term that controls the quasi-steady-state fracture—block interflow. Governing equations are solved numerically by the Crank-Nicolson implicit scheme. The numerical results are compared to the analytical results for the same problem which assumes Darcian flow in both fractures and blocks. Numerical and analytical solutions give the same results when the Reynolds number is less than 0.1. The effect of nonlinearity on the flow appears when the Reynolds number is greater than 0.1. The higher the rate of flow from the stream to the aquifer, the higher the degree of nonlinearity. The effect of aquifer parameters on the flow is also investigated. The proposed model and its numerical solution provide a useful application of nonlinear flow models to fractured aquifers. It is possible to extend the model to different types of aquifer, as well as boundary conditions at the stream side. Time-dependent flow rates in the analysis of recession hydrographs could also be evaluated by this model.     

Aquifer Vulnerability Assessment Based on Sequence Stratigraphic and 39Ar Transport Modeling

A large‐scale groundwater flow and transport model is developed for a deep‐seated (100 to 300 m below ground surface) sedimentary aquifer system. The model is based on a three‐dimensional (3D) hydrostratigraphic model, building on a sequence stratigraphic approach. The flow model is calibrated against observations of hydraulic head and stream discharge while the credibility of the transport model is evaluated against measurements of ³⁹Ar from deep wells using alternative parameterizations of dispersivity and effective porosity. The directly simulated 3D mean age distributions and vertical fluxes are used to visualize the two‐dimensional (2D)/3D age and flux distribution along transects and at the top plane of individual aquifers. The simulation results are used to assess the vulnerability of the aquifer system that generally has been assumed to be protected by thick overlaying clayey units and therefore proposed as future reservoirs for drinking water supply. The results indicate that on a regional scale these deep‐seated aquifers are not as protected from modern surface water contamination as expected because significant leakage to the deeper aquifers occurs. The complex distribution of local and intermediate groundwater flow systems controlled by the distribution of the river network as well as the topographical variation (Tóth 1963) provides the possibility for modern water to be found in even the deepest aquifers.     

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.