Assessing groundwater quality impacts of capturing high magnitude flows for managed aquifer recharge

Abstract:

Managed aquifer recharge (MAR) is increasingly used to alleviate disparities in water availability during wet and dry periods. While MAR can provide multiple benefits and help ensure freshwater security, artificial recharge can alter the native geochemical and hydrological conditions of an aquifer resulting in the release of toxic, geogenic contaminants and subsequent degradation of groundwater quality. Arsenic is a particular challenge at MAR sites owing to its ubiquity in subsurface sediments and toxicity at trace concentrations with a World Health Organization (WHO) guideline of 10 µg/L in drinking water. Currently, there is limited guidance on evaluation and management of the mobilization of geogenic contaminants during MAR. We conduct a regional analysis of surface water composition and geochemical compatibility of local aquifers across Texas, United States. In Texas, recent legislative changes have aimed to promote the use of unappropriated, high magnitude flow (HMF) events for storage in underlying, depleted aquifers. While recent studies have shown HMFs (i.e., streamflows in the 95^th percentile) have the potential to provide a large source of water for MAR, the viability of capturing HMFs for recharge depends on overcoming several limitations including infrastructure constraints, community and ecosystem impacts, and groundwater quality considerations including arsenic mobilization. Using publicly available datasets and geochemical modeling, we evaluate the hydrochemical composition of surface water and temporal variations with HMFs. We model the mineralogical environment of local aquifers and potential mechanisms of arsenic release if aquifers artificially recharge with excess surface water supplies. In particular, we focus on common geochemical triggers of arsenic mobilization including potential (1) shifts in redox conditions, (2) desorption via increasing pH to values > 8.5, and (3) competitive ligand exchange. This work provides an understanding of groundwater quality considerations for the increased adoption of MAR in Texas and, more broadly, a framework for evaluating geochemical impacts in the context of MAR site suitability.