DELINEATING MECHANISMS OF TRACE ELEMENT MOBILIZATION DURING MANAGED AQUIFER RECHARGE

In 2018, the Hampton Roads Sanitation District launched the Sustainable Water Initiative For Tomorrow (SWIFT) program, a managed aquifer recharge (MAR) site involving the injection and storage of treated wastewater effluent into the Potomac Aquifer in southeastern Virginia. The SWIFT program aims to maximize groundwater yields and prevent land subsidence and saline intrusion risks to the Potomac Aquifer System (PAS). However, one potential risk of MAR is that the injection of water differing from the ambient groundwater can alter the native geochemistry, resulting in the mobilization of naturally occurring toxic trace elements from aquifer sediments into groundwater. To evaluate this risk for the SWIFT MAR project, we characterized selected PAS sediments to evaluate the role of grain size distribution, TOC, and depth on the concentrations of trace elements in aquifer materials. We also explored trace element associations to determine methods most appropriate for measuring low concentrations of arsenic in PAS sediments. Preliminary results from ICP-MS, XRF, grain size distribution, and combustion analysis reveal correlations between trace elements, grain size, and carbon/sulfur. Microwave-assisted digestion of sediment followed by ICPMS analysis shows positive correlations between iron and arsenic, manganese, sulfur, chromium, and lead (N=52, p-value = <0.0001). Carbon correlates with iron (N=52, p-value = <0.0001), arsenic (p-value = <0.0001), manganese (p-value = 0.0084), and sulfur (p-value = <0.0001). XRF can be utilized to determine trace elements in PAS sediments without the need for digestion; however, arsenic values fell below the detection limit. Thus, an established correlation between arsenic and iron from the sediment digestion with ICPMS analysis (N = 52, p-value = <0.0001) suggests that we may be able to use % iron as a proxy for determining arsenic in the field using XRF.

Our next step is to utilize batch experiments to test known hypotheses of trace element mobilization from PAS sediments to groundwater. In addition, we are planning large-scale column experiments to define an experimental approach for evaluating trace element mobilization potential using PAS sediment samples under different operational conditions. Results from our research can help determine the risks of trace element mobilization at MAR sites.