Development of analytical models to evaluate transport and attenuation in a multi-layer aquifer during managed aquifer recharge

Abstract:

The Sustainable Water Initiative for Tomorrow (SWIFT) project is a managed aquifer recharge (MAR) initiative of Hampton Roads Sanitation District (HRSD) to address environmental and water supply challenges facing the Tidewater area of Virginia. SWIFT involves supplying wastewater effluent treated to drinking water standards (i.e., SWIFT Water) to the Potomac Aquifer System (PAS) through multi-screen injection wells. In May 2018, HRSD began recharge at the SWIFT Research Center, a 1 million gallon per day (MGD) demonstration facility in Suffolk, VA. The Research Center includes a groundwater monitoring network to aid in aquifer characterization and water quality impact assessment. This network includes a depth-discrete monitoring well, as well as multiple conventional monitoring wells.

A combination of intrinsic and artificial tracers was used to quantify transport and attenuation in the multi-layer aquifer system. Because of the difference in quality between SWIFT Water and the native groundwater of the PAS, SWIFT Water acted as an intrinsic tracer for the initial recharge front. Conductivity, sulfate, 1,4-dioxane, and total organic carbon (TOC) were among groundwater constituents measured in SWIFT Water and at the monitoring well network. A bromide tracer test was conducted in 2020 to quantify subsurface transport properties and to further assess water quality changes in the PAS that may have resulted during recharge operations. The flow distribution to each screen in the recharge well was directly measured using an in-situ spinner flowmeter during MAR operations. Analytical models were employed to simulate transport of groundwater constituents of interest to nearby monitoring wells. Results indicated that the use of multiple tracers was effective in characterizing transport and attenuation in the multiple layers in the aquifer. Aquifer parameters derived from initial analytical models were validated by the use of multiple tracers as well as by later, more complex models. The use of SWIFT Water as an intrinsic tracer also allowed for a larger scale of characterization, with substantial injection volumes allowing the tracer reach to extend beyond the capacity of an artificial tracer.