QUANTIFYING CYCLING OF CA, MG, AND NA IN DRINKING WATER RESERVOIRS NEAR ROANOKE, VIRGINIA

Freshwater reservoirs are essential sources of public drinking water, but their water quality is influenced by the presence of geogenic metals such as magnesium (Mg), calcium (Ca), and sodium (Na). These metals, derived from geological weathering and hydrological inputs, can play significant roles in water hardness and salinity. However, the mechanisms driving their inputs and removal remain understudied in drinking water reservoir systems. Addressing this gap is vital for optimizing water treatment processes.

To investigate these mechanisms, we measured Mg, Ca, and Na concentrations in the water column and suspended sediment in two seasonally stratified drinking water reservoirs in southwestern Virginia: Falling Creek Reservoir (FCR) and Beaverdam Reservoir (BVR). FCR is equipped with a hypolimnetic oxygenation system (HOx), which can be activated to maintain oxic conditions in the hypolimnion, whereas BVR serves as a reference reservoir. Using data collected from 2018 to 2023, we examined seasonal trends of metal concentrations in water and suspended sediment and analyzed the role of oxygenation in metal removal.

Our results reveal distinct stratification patterns for Mg, Ca, and Na during the summer months with higher concentrations of Mg and Ca observed in the hypolimnion compared to the epilimnion. Sodium exhibited a more uniform distribution throughout the water column but showed occasional spikes. Across both reservoirs, we measured higher cumulative metal fluxes in FCR compared to BVR, particularly in the hypolimnion. The presence of the HOx system in FCR likely influences geochemical processes involving Ca and Mg under oxic conditions. This study demonstrates the critical role of oxygenation systems in managing geogenic metal dynamics, contributing to improved strategies for water quality management in drinking water reservoirs. The findings provide actionable insights for optimizing reservoir operations and ensuring safe drinking water supplies.