Design of effective and efficient pump‐and‐treat systems requires capture zones of recovery wells to closely circumscribe the contaminant plume. Overestimation of capture zones incurs the undesired expense of treating clean water. Underestimation of capture zones enables contaminants to escape downgradient. Recovery wells in unconfined aquifers commonly penetrate only part of the aquifer either because of its large saturated thickness or the shallow vertical extent of contamination. The capture‐zone geometry of a partially penetrating pumping well can differ greatly from that of a fully penetrating one because of vertical flow components near the well. The differences in geometry are far greater if the medium is anisotropic (Kh≠ Kv). To estimate the capture‐zone geometry of a partially penetrating pumping well, a steady‐state, finite‐difference model was constructed to simulate flow to a well in a regional flow system. The model was used to simulate differences in the velocity field created by changes in (1) the depth of well penetration, (2) the magnitude of the regional hydraulic gradient, and (3) the degree of anisotropy. Following each simulation particle tracking was performed to determine the maximum width, depth, and distance to the stagnation point of the capture zone. Graphs were developed between capture‐zone width, relative capture‐zone depth, distance to the stagnation point, versus the ratio of Q/q (pumping rate/specific discharge). The graphs enable estimates to be made of these geometric parameters for a variety of pumping rates, regional hydraulic gradients, hydraulic conductivities, anisotropy ratios, and degrees of partial penetration. The results show that for isotropic conditions, particularly for small ratios of Q/q and wells that penetrate less than 40 percent of the aquifer, the shape of a capture zone can deviate significantly from that of a fully penetrating well. For anisotropic conditions, these differences are more pronounced and apply to a wider range of Q/q ratios and well penetration depths. In sequences of sediment, anisotropy is produced by textural, architectural, and stratigraphic elements that occur at different scales. Capture zones should be estimated using Kh:Kv ratios determined from pumping tests. This assures that the measured degree of anisotropy is commensurate with the scale of elements encountered by the stress created by a pumping well. Tabulated Kh:Kv ratios from analysis of pumping tests in unconfined aquifers suggest there are few isotropic sand and gravel aquifers. Recognition of this characteristic and consideration of the effects of partial penetration and regional hydraulic gradient on the geometry of capture zones may lead to the design of more efficient and effective pump‐and‐treat systems.

Abstract. Portions of many regional-scale aquifers in midcontinent sedimentary basins exhibit large salinity gradients that significantly impact the velocity field and solute distribution through time. A two-dimensional, numerical transport model was constructed to examine the role of salinity-derived variable-density flow on changes in the velocity field and solute distributions in a near-surface, regionally extensive aquifer as brine is displaced by infiltrating meteoric water. The Silurian-Devonian carbonate aquifer in the western portion of the Appalachian Basin was used as a framework to insure that realistic flow velocities and salinities were used in the assessment. The variable-density effects on brine displacement are observed by examining the differences in the velocity fields and solute distributions produced by uniform-density and variable-density simulations. The effects include the change from an intraformational displacement pattern to a cross-formational displacement pattern with the development of flow reversals and partitioning of regional flow cells into smaller flow cells. Variable-density effects also are manifest in the solute distributions by slowing the displacement of brine and influencing the magnitude of the salinity gradient. A sensitivity analysis used to examine the influence of flow and transport parameters on the transient development and migration of salinity gradients shows that increasing cross-formational leakage into the regional aquifer causes flow velocities to decrease, which magnifies the influence of the variable-density behavior by slowing the displacement of brine. The sensitivity analysis also shows that increasing the value of dispersivity causes an increase in the variable-density effects. However, the effects of variable-density flow are relatively insensitive to changes in values of horizontal and vertical anisotropy assigned to the aquifer or to the presence of an overlying transmissive layer. IntroductionVariable-density groundwater flow is observed in many different hydrogeologic settings and at various scales. In this study we examine a hydrogeologic setting where natural salinity gradients are large and produce significant salinity-derived variable-density flow effects. In particular, we examine the role that salinity-derived variable-density flow plays in the flushing of brine by infiltrating meteoric water in the near-surface portion of a regionally extensive aquifer. In this regional-scale setting, density-driven flow is coupled with hydrologically driven flow creating a positive feedback system that is computationally intensive to simulate using numerical models [Voss, 1984].This hydrogeologic setting, the scale of the flow system, and our numerical approach are different from the many previous studies of the saltwater/freshwater interface in coastal aquifers [Henry, 1964 Scope of Work and ApproachEven though near-surface salinity gradients occur in many midcontinent sedimentary basins [Gupta and Bair, 1997], little work has been done to characterize the...

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Regional depressurization due to groundwater withdrawal from a large confined midcontinent aquifer may cause significant changes in the regional flow field and solute distributions through time. This study addresses the effects of depressurization in the Silurian-Devonian carbonate aquifer in the western flank of the Appalachian Basin in Ohio, where salinity-derived variable-density flow effects are significant. This complex hydrogeologic environment is examined using an interpretative, transient, numerical flow and transport model to examine the hydrodynamics involved in the salinization process, to simulate changes in the velocity field and solute distributions over the past 100-year period, and to forecast the effects of three different depressurizing scenarios over the next 100-year period. The results indicate that large-scale depressurization can lead to significant changes in the regional flow patterns, resulting in changes in solute distributions and salinization of a portion of the aquifer. Depressurization creates vertical flow gradients within the carbonate aquifer that transport brine from underlying and overlying shale units into the updip freshwater portion of the aquifer. Model results show the development of flow reversals in the downdip portions of the aquifer that facilitate transport of brine from the deeper portions of the carbonate aquifer. These processes result in significant water quality degradation in the updip portion of the aquifer, which is extensively used for municipal, industrial, and domestic water supplies. Our results indicate the need to monitor water levels, pumping rates, and water quality so that future management decisions regarding the sustainability of the resource are based on complete and accurate field data. IntroductionRegional depressurizing and dewatering of water supply aquifers can create dramatic effects by reorienting groundwater flow patterns over broad areas. A prime example is the withdrawal of water from the Cambrian and Ordovician aquifers in the Chicago, Illinois, region, where extensive reorientation of flow patterns has been observed [Sasman et al., 1977]. These regional hydrodynamic changes are commonly a result of long-term mining of groundwater resources and impact the sustainability of these important resources. With respect to groundwater development, sustainability refers to the balance between withdrawals by pumping and replenishment from recharge and leakage, whereby water levels are not lowered or water quality degraded to the detriment of future users. This paper examines the hydrodynamic processes causing salinization of freshwater supplies in an extensive midcontinent aquifer caused by prolonged groundwater abstraction. Spatial variations in fluid density are common and can be significant in regional-scale aquifers in midcontinent sedimentary basins [Garven and Freeze, 1984a, 1984b; Hanor, 1987; Bethke, The salinityderived variable-density nature of these aquifers can lead to complex hydrodynamic behavior in response to depressurization. Sali...

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