Catchment-Scale Quantification of Hyporheic Denitrification Using an Isotopic and Solute Flux Approach

Abstract

A dual-isotope and solute flux mass-balance was used to elucidate the processes that lead to attenuation of nitrogen contamination in an agriculturally impacted river. The River Wensum drains a lowland catchment with an area of 570 km² in East Anglia, eastern England. Analysis of nitrate concentration, δ¹⁵N(NO₃) and δ¹⁸O(NO₃) of samples from the River Wensum collected from upstream locations to the catchment outlet through all seasons and flow conditions showed a consistent pattern of increasing isotope values with decreasing nitrate concentrations downstream. δ¹⁵N(NO₃) and δ¹⁸O(NO₃) of catchment surface water and groundwater samples revealed a dominant influence from microbially cycled and nitrified source-nitrogen, which results in high nitrate concentrations in Chalk groundwater and upstream in the River Wensum. Denitrification of Chalk groundwater-baseflow in the hyporheic zone results in the downstream trend observed in the river. Hyporheic denitrification is estimated to remove 931 kg/day of nitrate-nitrogen by the catchment outlet, representing 31% of the potential riverine nitrate load. The use of dual-isotope and solute flux modeling at the catchment scale is a novel application to quantify denitrification within the river valley, demonstrating the importance of hyporheic zone processes in attenuating the impacts of anthropogenic contamination of hydrologic systems.

A stable isotope approach and its application for identifying nitrate source and transformation process in water

“…As discussed above, isotopic fractionation can complicate their use for purposes of NO 3 − source identification because the original isotopic fingerprint of the source materials is altered. However, on the other hand, the nature and degree of isotope fractionation can also provide a powerful tool (Wexler et al 2011) for tracing N transformation processes.…”

Microbial and hydrological influences on nitrate isotopic composition in an agricultural lowland catchment

“…The Chalk is exposed in the river valley and in the western catchment with nitrate concentrations often above 1000 lM (Taylor, 1984;Feast et al, 1998;Wexler et al, 2011). In the interfluves the Chalk is confined under the clay-rich Lowestoft Till with nitrate concentrations below the limit of detection (Feast et al, 1998;Wexler et al, 2011).…”

“…Dissolved nitrogen export is also subject to hydrological influences such as flow condition and baseflow index which expresses the proportion of mean river flow derived from stored groundwater as opposed to rainfall runoff. Denitrification within the hyporheic zone of the River Wensum has been shown to reduce the nitrate-nitrogen load exported from the Wensum catchment via the river by 31% (Wexler et al, 2011). Natural abundance stable isotopes of nitrogen and oxygen in nitrate are a powerful tool to track nitrogen cycling processes.…”

Stable isotopic evidence for high microbial nitrate throughput in a High Arctic glacial catchment

“…Further, these studies reported no isotopic effect during oxygen incorporations or any further isotopic exchange. Since then this concept has been widely used to understand and quantify microbial nitrate in the aquatic and sedimentary environment (Anisfeld et al, 2007;Ansari et al, 2012;Kendall, 1998;Tye and Heaton, 2007;Wexler et al, 2011 (10) (Buchwald and Casciotti, 2010;Kool et al, 2011;Snider et al, 2010;Spoelstra et al, 2007). This has certainly overcast the confidence in the traditional use of equation (10) (10) …”

“…Among the 30 transported pollutants, nitrate has been a major focus for polar researchers due to its well known role in ozone chemistry (Crutzen et al, 1992;Solomon et al, 1996;Waibel et al, 1999) and acid rain events (KERR, 1979;SCHINDLER, 1988;Wright et al, 1988). However, the role of transported nitrate in the Arctic glacial ecosystem with changing glacial coverage remains understudied.…”