QUANTIFYING INDUCED INFILTRATION BY MUNICIPAL PRODUCTION WELLS USING STABLE ISOTOPES OF WATER

A common source of groundwater is alluvial aquifers adjacent to surface water bodies. Production wells can induce infiltration of surface water into the aquifer minimizing drawdown. However, inducing infiltration can lessen downstream surface-water flows and may introduce surface-water contaminants into the aquifer. At two sites in southwest Ohio, we investigated the use of stable isotopes 𝛿¹⁸O and 𝛿²H to quantify the amount of production-well water emanating from induced infiltration. One site was in Oxford, OH where three production wells are set along Four Mile Creek; the other was the much larger Bolton wellfield in Fairfield, Ohio comprising 13 production wells along the Great Miami River. Over the course of a year at both wellfields we sampled surface water, water from 3 productions wells and water from monitoring wells hypothesized to be both influenced and not influenced by induced infiltration. Because groundwater in this region is recharged mainly in the winter, we expected that it would have the lowest isotopic means and variances, and surface water would have the greatest. Production well water would be quantifiable by a mixture of the two. At Oxford Wellfield, surface water did have the highest isotopic means while groundwater not influenced by induced infiltration had the lowest. Simple mixing calculations using 𝛿¹⁸O and 𝛿²H were consistent and indicated that water from the largest production well had about 42% groundwater and 58% induced infiltration. The smallest production well and the one farthest from the creek contained about 84% groundwater and 16% induced infiltration. At the Bolton wellfield, the 2 monitoring wells least likely to be influenced by any induced infiltration had the lowest isotopic means as expected. However, river water did not have the highest isotopic means and differed substantially at the two sites from which it was sampled. For 2 production wells, data deviation from our conceptual model prohibited the use of a mixing calculation to estimate the degree of induced infiltration and possibly indicated that almost all production-well water came from induced infiltration. At a third production well, the 𝛿 ²H values indicated about half induced infiltration while the 𝛿 ¹⁸O values indicated 91% induced infiltration. Reasons for the discrepancies are being investigated.

Keywords: Induced infiltration, stable isotopes, 𝛿 ¹⁸O and 𝛿 ²H