Determination of traveltime‐related capture zones of wells in instances where there is a lack of adequate or reliable site‐specific values of hydraulic parameters and/or the heterogeneous character of the geologic materials may lead to designation and management of wellhead‐protection areas of dubious function. In these instances, Monte Carlo simulation can be used to determine traveltime‐related capture zones that account for the uncertainty in values of hydraulic and geologic parameters that cannot be accounted for by deterministically based flow models. Monte Carlo simulations using 100 randomly generated values of hydraulic conductivity and effective porosity were used to determine one‐year capture zones based on determination of percentile confidence regions from reverse‐tracked flowpaths emanating from a well completed in a leaky‐confined aquifer. The mean of the lognormal distribution used to generate the hydraulic‐conductivity values was taken to be 3.89 ft/d, the log of the average value from an aquifer test, and a standard deviation of 1.0 ft/d was used. The normal distribution used to generate the effective porosity values had a mean of 25 percent and a standard deviation of 3.5 percent. The values of hydraulic conductivity and effective porosity were ordered and then paired to induce correlation. Other relevant hydraulic and geologic parameters were fixed at average or observed values from the available data. Simulations were made using an analytical flow model in conjunction with a particle‐tracking program to obtain 100 sets of endpoint coordinates for 36 reverse‐tracked, one‐year flowpaths emanating from the well. Wellhead‐protection areas based on the 90th‐ and 75th‐percentile confidence regions of the distribution of 3,600 endpoints were determined by deleting the 10 and 25 most‐extreme (distant) endpoints, respectively, from the 100 sets of flowpath endpoints of the 36 flowpaths. Delineation of the wellhead‐protection areas was based on determination of the convex hull of the remaining endpoints. The distribution of remaining endpoints around the well also was used to determine optimum locations for construction of sentinel wells to detect contaminants moving toward the well along the most probable flowpaths.
Abstract. Combined use of the tritium/helium 3 (3H/3He) dating technique and particletracking analysis can improve flow-model calibration. As shown at two sites in the Great Miami buried-valley aquifer in southwestern Ohio, the combined use of 3H/3He age dating and particle tracking led to a lower mean absolute error between measured heads and simulated heads than in the original calibrated models and/or between simulated travel times and 3H/3He ages. Apparent groundwater ages were obtained for water samples
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.