Seasonal to Decadal Variability in Focused Groundwater and Contaminant Discharge along a Channelized Stream

Abstract

Focused groundwater discharge to streams is problematic at contaminated sites because high fluxes can limit natural attenuation in the hyporheic zone. However, information on location, spatial evolution, and temporal persistence of springs in unlithified sediments over multiyear time scales is limited. We examine discharge at point (~1‐m) to reach (~300‐m) scales along a stream that intercepts trichloroethene and technetium‐99 plumes from a Superfund site. During 2011 to 2012, we seasonally monitored stream and spring flow and contaminant concentrations, along with probing streambed temperatures on a grid in winter and summer, building on prior monitoring during 1999 to 2002. Baseflow measured by both gauging and dye dilution generally increased with distance downstream, and stream and spring discharge varied seasonally, from minima in October to January to maxima in February to June. Thermal anomalies identified by probing occupied approximately 3% to 6% of the reach and typically coincided with visible springs or seeps. Locations of anomalies were similar to those identified in summer 2002, although some orifices disappeared and others emerged. Vertical groundwater fluxes calculated from probing tended to be less than net fluxes calculated from stream discharge, perhaps in part because the assumption of one‐dimensional, steady‐state flow in calculating point fluxes was simplistic. Maximum contaminant concentrations and fluxes decreased between 1999 to 2001 and 2011 to 2012 as a result of partial capture by an upgradient pump‐and‐treat system. Our findings confirm that springs in unlithified sediments can remain stationary within a few meters over decadal time scales, and seasonal variability in discharge can be greater than decadal variability.

Polyfluoroalkyl substances requiring a renewed focus on groundwater‐surface water interactions

“…This method has been most often used in small streams and can quantify smaller groundwater flux values compared to comparative flow measurements with stream weirs; however, this technique cannot measure stream loss to the aquifer (i.e., losing reach conditions). An example dataset from a tracer test conducted on the stream in Vignette B is presented as an example in Figure 8 and another example of the practical application of this technique can be found in Tripathi et al (2021).…”

“…As most of these geochemical parameters are typically present at microgram to milligram per liter (μg/L; mg/L) concentrations, they can adversely influence the achievable PFAS removal efficiency. Moreover, constituent flux widely varies not just spatially, but also temporally (e.g., dry versus wet season) across the barriers (Keery et al 2007;Cheng et al 2020;Lemaire et al 2020;Tripathi et al 2021;Rosenberry et al 2021). Therefore, remedial design needs to account for uncertainties associated with site-specific contaminant mass flux and sorbent material performance under site-specific conditions.…”

“…Moreover, constituent flux widely varies not just spatially, but also temporally (e.g., dry versus wet season) across the barriers (Keery et al. 2007; Cheng et al 2020; Lemaire et al 2020; Tripathi et al 2021; Rosenberry et al 2021). Therefore, remedial design needs to account for uncertainties associated with site‐specific contaminant mass flux and sorbent material performance under site‐specific conditions.…”