Modeling the Potential Impact of Seasonal and Inactive Multi-Aquifer Wells on Contaminant Movement to Public Water-Supply Wells1

Abstract

Wells screened across multiple aquifers can provide pathways for the movement of surprisingly large volumes of groundwater to confined aquifers used for public water supply (PWS). Using a simple numerical model, we examine the impact of several pumping scenarios on leakage from an unconfined aquifer to a confined aquifer and conclude that a single inactive multi-aquifer well can contribute nearly 10% of total PWS well flow over a wide range of pumping rates. This leakage can occur even when the multi-aquifer well is more than a kilometer from the PWS well. The contribution from multi-aquifer wells may be greater under conditions where seasonal pumping (e.g., irrigation) creates large, widespread downward hydraulic gradients between aquifers. Under those conditions, water can continue to leak down a multi-aquifer well from an unconfined aquifer to a confined aquifer even when those multi-aquifer wells are actively pumped. An important implication is that, if an unconfined aquifer is contaminated, multi-aquifer wells can increase the vulnerability of a confined-aquifer PWS well.

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

“…Changes in pumping stresses within the groundwater system also affected direction and magnitude of flow in conduit wells as well as the depth of impact by transport. Johnson et al (2011) reinforced the conclusions of others noting that (1) proximity to pumping wells as well as seasonality of pumping influenced the magnitude of vertical flows and (2) bypassing transport through both shallow waterbearing zones and lower hydraulic conductivity sediments reduced protection to deeper groundwater since less solute attenuation occurred.…”

“…Some include modeling and field data while others evaluate field data without modeling. The MODFLOW Multi-Node Well package (Halford and Hanson 2002;Konikow et al 2009) was used for many of the studies that model intraborehole flow (Hanson et al 2004; Konikow and Hornberger 2006;Zinn and Konikow 2007;Clark et al 2008;Johnson et al 2011;Yager and Heywood 2014) although some researchers represented flow through the inside of the well casing with an equivalent porous medium (Lacombe et al 1995;Mejia et al 2012). Some works consider a small spatial scale (the well and immediate vicinity), while others focus on larger areas (wellfields and regions).…”

Multi-aquifer susceptibility analyses for supporting groundwater management in urban areas

“…This means that the upper 13 aquifer can represent a secondary (i.e., indirect) source responsible for the contamination of the underlying deep aquifer. Together with polluted waters in the shallow aquifer, local heterogeneities in the aquitard and wells exploiting both aquifers (i.e., multi-aquifer wells) can have a relevant role in favouring the vertical migration of the contaminants, thus compromising the quality of the deep aquifer (Johnson et al, 2011;Filippini, 2017). Heterogeneities of the aquitard can include sectors with reduced thickness of fine sediments or zones where the layer is mainly comprised of a silty-sandy matrix (i.e., more permeable than a clay matrix), so that, locally, the vertical groundwater flow through the aquitard is favoured with respect to the horizontal groundwater flow.…”

“…One of the most common solutions to provide safe drinking water consists in pumping groundwater from a more protected deep aquifer. Nevertheless, the intensive exploitation of deep aquifers can contribute to increase their susceptibility by creating preferential flow paths from the contaminated shallow aquifer to the deepest ones (Johnson et al, 2011;Eberts et al, 2013).…”

Simulation of the Effects of Seasonally Varying Pumping on Intraborehole Flow and the Vulnerability of Public‐Supply Wells to Contamination

“…Zinn and Konikow () simulated groundwater flow and transport through long‐screened wells to demonstrate the potential effect of intraborehole flow on the groundwater age distribution within a simple conceptual aquifer system. Johnson et al () simulated the effect of intraborehole flow within inactive wells on the “zones of transport” to public‐supply wells and demonstrated the vulnerability of public‐supply wells in confined aquifers to shallow contaminant sources through intraborehole flow. Landon et al () utilized depth‐dependent sampling to document contaminant transport between multiple depths in aquifer systems through long‐screened public‐supply wells near York, Nebraska, and Modesto, California.…”