Inactive supply wells as conduits for flow and contaminant migration: conditions of occurrence and suggestions for management

Abstract

Water supply wells can act as conduits for vertical flow and contaminant migration between water-bearing strata under common hydrogeologic and well construction conditions. While recognized by some for decades, there is little published data on the magnitude of flows and extent of resulting water quality impacts. Consequently, the issue may not be acknowledged widely enough and the need for better management persists. This is especially true for unconsolidated alluvial groundwater basins that are hydrologically stressed by agricultural activities. Theoretical and practical considerations indicate that significant water volumes can migrate vertically through wells. The flow is often downward, with shallow groundwater, usually poorer in quality, migrating through conduit wells to degrade deeper water quality. Field data from locations in California, USA, are presented in combination with modeling results to illustrate both the prevalence of conditions conducive to intraborehole flow and the resulting impacts to water quality. Suggestions for management of planned wells include better enforcement of current regulations and more detailed consideration of hydrogeologic conditions during design and installation. A potentially greater management challenge is presented by the large number of existing wells. Monitoring for evidence of conduit flow and solute transport in areas of high well density is recommended to identify wells that pose greater risks to water quality. Conduit wells that are discovered may be addressed through approaches that include structural modification and changes in operations.

Using geographic distribution of well-screen depths and hydrogeologic conditions to identify areas of concern for contaminant migration through inactive supply wells

“…As discussed by Gailey (2017), wells act as conduits for contaminant migration under the following conditions ( Fig. 2): (1) differences in head and water quality exist between water-bearing strata over a vertical section, (2) stratification of high-and low-hydraulic Clay (study area) conductivity sediments impede vertical flow and solute transport and (3) the screened intervals of wells vertically span the stratigraphy allowing relatively rapid vertical flow between waterbearing strata to occur through the wells (short-circuiting flow).…”

“…The scaling is necessary because gradients associated with flow through well casings are typically much lower than vertical gradients in groundwater systems at some distance from the wells as a result of (1) decreased resistance to vertical flow within the casing relative to naturally layered groundwater systems and (2) head losses related to passage through well screens and (3) convergent (divergent) flow to (from) the well through the porous medium. Over estimation of flow rates occurs unless the gradient is reduced (Gailey 2017). c. Rates of nitrate and TDS transfer across the clay are calculated for each well as the product of flow through the well and concentration in the PLSS section.…”

“…Two numerical groundwater flow and solute transport models presented by Gailey (2017) are used to compare the relative magnitudes of groundwater volumetric flows, fluxes per unit area (Darcy velocities) and extent of water quality impact from solute mass transfer through (1) regional aquitards and (2) wells that act as conduits. The models simulate a hypothetical groundwater system comprising two aquifers and an intervening aquitard.…”

“…Contaminant migration through inactive supply wells can negatively affect groundwater quality (Landon et al 2009;Mayo 2010;Jurgens et al 2014;Zuurbier and Stuyfzand 2017; see Gailey 2017 for a detailed literature review). With the exception of Clark et al (2008), studies on wells that act as conduits for contaminant migration address single wells; however, the combined effects of groups of wells may cause greater impacts (Gailey 2017). Because the number of wells is often large and the geographic areas can be vast, approaches are needed to evaluate potential impacts and focus limited resources on investigations and corrective measures where most beneficial.…”

Domestic well vulnerability to drought duration and unsustainable groundwater management in California’s Central Valley

“…During the recent drought, it is likely that some households also deepened their wells. In California and nationwide, drilling deeper wells is a common practice to adapt to declining groundwater levels [9], but is a costly and unsustainable solution that may furthermore result in cross-contamination due to interconnection of confined aquifers by well construction [64]. Moreover, the financial burden of pump lowering or well deepening might disproportionately impact disadvantaged populations [30] unable to afford chasing after declining groundwater levels.…”