Evaluating drivers of hydrology, water quality, and benthic macroinvertebrates in streams of Fairfax County, Virginia, 2007–18

Abstract

contributed greatly to this study. Their program coordination, field expertise, and scientific insights have been critical to the success of this effort and will continue to be relied upon in future study years. Staff at the Fairfax County Environmental Services Laboratory performed the nutrient analyses for most samples collected; their work is sincerely appreciated. Many U.S. Geological Survey (USGS) staff have contributed to the design, operation, and interpretation of this study since its inception in 2007. These combined efforts have produced years of consistent data collection and foundational knowledge that greatly benefited this report. Technical reviews by Doug Chambers of the USGS and Jonathan Duncan of the Pennsylvania State University strengthened this report and are greatly appreciated. The authors also thank Kaycee Faunce of the USGS for helping identify and communicate the findings of this research.

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

“…The latter is very similar to the specific conductance measured previously in groundwater samples collected from the Mesozoic Lowland HGMR in Virginia (median 540 µS cm −1 , violin plot in Figure 3a, see also Supplementary Information Section H and Tables H3-H5), consistent with the idea that the end-member on the far right of Figure 3a is groundwater. The moderately high specific conductance of groundwater in the Mesozoic Lowland HGMR reflects the relative solubility of carbonate material in the region's Triassic deposits of sandstone and shale [25,26,42]. Not all of the hourly stream salinity measurements separate cleanly into the three age-dependent end-members noted above (e.g., in a few cases high stream salinity is associated with very old water, Figure 3a).…”

“…These processes drive the salinity of water toward some geology-specific value. Possible geochemical reactions include cation exchange in soils [5,17,50]) and, at our site, dissolution of carbonate materials in Triassic sandstone and shale deposits that tend to increase the specific conductance of groundwater and springs in the Mesozoic Lowland HGMR [25,26,42,43]. The older water is when it exits the catchment and enters the stream, the closer its salinity approximates the geology-specific value (represented in the figure by dark brown balls exiting the control volume along the longest flow paths).…”

“…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.…”