Vulnerability of Streams to Legacy Nitrate Sources

Abstract

The influence of hydrogeologic setting on the susceptibility of streams to legacy nitrate was examined at seven study sites having a wide range of base flow index (BFI) values. BFI is the ratio of base flow to total streamflow volume. The portion of annual stream nitrate loads from base flow was strongly correlated with BFI. Furthermore, dissolved oxygen concentrations in streambed pore water were significantly higher in high BFI watersheds than in low BFI watersheds suggesting that geochemical conditions favor nitrate transport through the bed when BFI is high. Results from a groundwater-surface water interaction study at a high BFI watershed indicate that decades old nitrate-laden water is discharging to this stream. These findings indicate that high nitrate levels in this stream may be sustained for decades to come regardless of current practices. It is hypothesized that a first approximation of stream vulnerability to legacy nutrients may be made by geospatial analysis of watersheds with high nitrogen inputs and a strong connection to groundwater (e.g., high BFI).

Quantifying watershed‐scale groundwater loading and in‐stream fate of nitrate using high‐frequency water quality data

“…At the intermediate and lowest BFI sites-the Potomac River and Difficult Run-groundwaterdischarged nitrate accounted for 69% (1.1 3 10 7 kg) and 58% (3.7 3 10 4 kg) of the total annual load discharged to the stream, respectively. The positive relationship between BFI and the fraction of total loads estimated to be groundwater-discharged loads is consistent with studies that have identified positive relationships between BFI and in-stream loads during base flow conditions [Tesoriero et al, 2009[Tesoriero et al, , 2013Spahr et al, 2010].…”

“…Therefore, end-member concentrations for groundwater discharge and runoff were determined using the winter BFI-nitrate relationships. Groundwater-discharged end-member concentrations (1.8 6 0.1-2.9 6 0.1 mg/L as N), which represent the nitrate concentration in the slowly varying subsurface flow paths contributing to the stream, were greater than runoff end-member concentrations (0.8 6 0.1-1.6 6 0.1 mg/L as N; [Sanford and Pope, 2013;Tesoriero et al, 2013]. Indeed, the age of groundwater in the Potomac River watershed has been estimated to range from 10 to 20 years [Michel, 1992;Phillips et al, 1999].…”

“…While data suggest that this assumption may be reasonable at these study sites, targeted collection of discrete nitrate samples during high flow, low BFI conditions among seasons would allow for definition of seasonally variable runoff end-member concentrations. Ratios of nitrate to chloride concentrations have been used to differentiate between hydrological (e.g., dilution) and biogeochemical (e.g., denitrification) processes influencing nitrate concentrations [Messer et al, 2012;Billy et al, 2013;Tesoriero et al, 2013]. Targeted sampling for major ions (including nitrate and chloride) during high BFI days among seasons would allow for assessments of the time-variable nature of groundwater-discharged endmember concentrations.…”

“…For example, our estimate that 58-73% of the annual nitrate loads delivered to the study streams is from groundwater suggests that mitigation actions aimed at intercepting nitrate in runoff may have a smaller impact on nitrate loads than anticipated at these streams. Coupling this approach for quantifying N loading to streams and in-stream retention/loss with methods that estimate N transport and removal along groundwater flow paths, such as recent work demonstrating that decades old nitrate is being discharged to streams in the Chesapeake Bay watershed and elsewhere [Sanford and Pope, 2013;Tesoriero et al, 2013], will provide a more holistic understanding of watershed-scale N processing and transport. …”

A lagged variable model for characterizing temporally dynamic export of legacy anthropogenic nitrogen from watersheds to rivers

“…2014). Nitrogen leaching from legacy sources has been recognized as a primary reason for these limited results, due to the long lag time (ranging from years to decades) elapsed between watershed N inputs and riverine export (Sebilo et al 2013;Sanford and Pope 2013;Tesoriero et al 2013). Exceedance of water quality targets has prompted regulatory agencies to reevaluate nutrient management standards, measures, and guidelines (Sharpley et al 2013).…”