Maps showing geologic terrane, drainage basins, overburden, and low flow of streams in Fairfax County and vicinity, Virginia

Stream Water Age Reveals Hydrological and Human Drivers of Inland Freshwater Salinization

“…The distributions of groundwater storage depths plotted in Figure 2f include both active groundwater (which is hydrologically connected to the stream and directly contributes to stream flow) and passive groundwater (which is not directly connected to streamflow but can strongly influence solute transport). The inferred depths of active groundwater are very small (< 3 cm) consistent with the local hydrogeology, which consists of a thin veneer of soil and saprolite overlaying fractured bedrock [24,42,43], and its hydrologic classification as a surface flow dominated catchment (i.e., k gw = 0.11 day −1 [44]). The median age of subsurface flow discharged to the stream in the Flatlick Branch catchment is ≈ 5.2 years (see Figure C4) [38], which in our model corresponds to a passive groundwater depth of S gw,p = 1.4 m (Section C.4 and Figure C5).…”

“…To test this hypothesis we developed and implemented a generalizable modeling framework, based on transient transit time distribution (T-TTD) theory [18], to estimate the time-varying mean age of water in Flatlick Branch, a deicer-impacted stream draining a 10.9 km 2 urban catchment (32% impervious cover in 2016) located in the Mesozoic Lowland hydrogeomorphic (HGMR) province, also known as the Triassic Lowland province, of Northern Virginia (Supplementary Information Figure A1) [24][25][26]. Flatlick Branch is tributary to the Occoquan Reservoir, a critical source of drinking water for up to 1 million people in Northern Virginia.…”