Comparison of Models for Delineating Wellhead Protection Areas in Confined to Semiconfined Aquifers in Alluvial Basins

Abstract

The 1986 Safe Drinking Water Act Amendments include provisions for state wellhead protection (WHP) programs that address wellhead protection areas (WHPAs). In many states, WHPAs are delineated based on time‐of‐travel (TOT) criteria. This study compares 250‐day and 15‐year TOT capture zones computed in a confined to semiconfined aquifer system in an alluvial basin using semianalytical and two‐ and three‐dimensional numerical ground‐water flow models, and evaluates the relative importance of several sources of uncertainty, such as aquifer hydraulic conductivity, aquitard leakance, vertical transit time, hydraulic gradients, transient pumping effects, well interference, and three‐dimensional aquifer geometries. A numerical model should be used to delineate 15‐year TOT capture zones for wells in confined to semiconfined aquifers in alluvial basins. A semianalytical program may be acceptable for computing the 250‐day TOT capture zones; however, such codes can be applied only under a very narrow range of conditions. Hydraulic conductivity plays a critical role in controlling the sizes and shapes of capture zones computed in confined to semiconfined aquifers. Small, circular capture zones are computed in low hydraulic conductivity areas. More complex geometries should be expected where hydraulic conductivities are higher and pumping wells are in close proximity to each other. Aquifers with horizontal hydraulic conductivities that are greater than 1,000 times the vertical hydraulic conductivity of the overlying aquitard are effectively fully confined, and larger capture zones would be computed for these aquifers than for semiconfined aquifers where significant leakage is induced by pumping. In addition, relatively large drawdowns are computed in low hydraulic conductivity areas, resulting in short vertical transit times. Vertical transit times are longer where aquifer hydraulic conductivities are higher.

Assessment of vulnerability and control measures to protect the Salbarua ecosystem from hypothetical spill sites

“…7, respectively. The capture zones in the northern area were comparatively large and more homogeneously rounded shaped comparative to those of the southern area because of the hydraulic conductivity values in this area and its lesser spatial variability as have been observed previously by Forster et al (1997) and Barry et al (2009). In contrast to the northern side of the aquifer, the shapes and sizes of the capture zones in the area of influence of potential sources of pollution originating at Askarza and Zerio reflect the flow pattern in the area.…”

Using Analytic Element Models to Delineate Drinking Water Source Protection Areas

“…Since variations in aquifer parameters can have a significant impact on the size and shape of a protection area (Bhatt 1993;Levy and Ludy 2000), the Ohio EPA needed a way to address parameter uncertainty. Numerous studies have evaluated different methods to account for uncertainty in capture zone delineation, especially in areas where site-specific data is lacking (Cole and Silliman 2000;Evers and Lerner 1998;Forster et al 1997;Jacobson et al 2002;Levy and Ludy 2000;van Leeuwen et. al.…”

“…In addition, local political subdivisions in Ohio have the authority to establish source water protection area overlay zones that can further restrict activities occurring within the protection area. With more and more land‐use restrictions being placed on protection areas, ensuring protection areas are scientifically defensible becomes crucial (Forster et al 1997; Frind et al 2002). This concern is echoed in a recent U.S. EPA evaluation of state source water assessment programs that indicated reports were not always “sufficiently detailed to back up decisions that face political resistance” (U.S.…”

Comparative Study of Methods for Delineating the Wellhead Protection Area in an Unconfined Coastal Aquifer

“…On the other hand, applying the three-dimensional numerical model gives rise to the possibility of integrating more hydrogeologic characteristics to suit the highly anisotropic and heterogeneous settings [11,12]. The commonly used model in delineating WHPA is MODFLOW [13,14]. Coupled with MODPATH, it provides numerical solution to tracking the pathlines of the particles moving within groundwater flow in a forward or backward manner [15]; thus, the area in which particles could move to can be delineated as the WHPA.…”