1-5 DELINEATING SOURCES AND MOBILIZATION MECHANISMS OF TRACE ELEMENTS DURING MANAGED AQUIFER RECHARGE

Session: Advances in Managed Aquifer Recharge

Presenting Author:

Authors:

Abstract:

In 2018, the Hampton Roads Sanitation District launched the Sustainable Water Initiative for Tomorrow (SWIFT) program, a managed aquifer recharge (MAR) operation involving the injection of highly treated wastewater effluent into the Potomac Aquifer System (PAS) in southeastern Virginia. The SWIFT program aims to maximize groundwater yields and prevent land subsidence and saline intrusion risks to the PAS. However, a risk associated with MAR includes the mobilization of naturally occurring trace elements, such as arsenic (As) into groundwater, due to differences in redox geochemistry between injected water and ambient groundwater. Previous monitoring near the pilot SWIFT recharge well detected elevated As in groundwater for periods of time that coincided with changes in operational conditions, including well rehabilitation and recharge start-up after extended shutdowns. 

In this study, we characterized sediments to identify As sources within the PAS and evaluate potential mechanisms of As mobilization associated with MAR. To achieve this, we collected 111 samples from archived well cuttings and one sediment core, representing both screened and non-screened intervals of the aquifer. We analyzed samples using microwave-assisted acid digestion followed by inductively coupled plasma mass spectrometry (ICPMS) analysis, in addition to X-ray fluorescence (XRF) and laser diffraction techniques to measure As, iron (Fe), sulfur (S), and grain size. Results show a positive correlation between As, Fe, and S and a negative correlation of As with grain size in the samples (p-value = <0.0001), suggesting sulfide minerals as a primary As source. We developed a predictive model for As using multiple linear regression.  The association of As, Fe, and S with smaller grain size suggests As-bearing sulfide minerals are concentrated within finer-grained layers. To explore this, we separated sulfides from select fine-grained sediments using heavy liquid (lithium metatungstate). Scanning electron microscopy – electron dispersive spectra (SEM-EDS) analyses confirmed the presence of As-bearing pyrite. We are in the process of conducting batch experiments to test pyrite oxidation as a potential As mobilization mechanism. Careful assessment of aquifer mineralogy is key to optimizing operational conditions that prevent As and other trace element releases to groundwater, thereby safeguarding water quality at MAR sites. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8298

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