Saturated Zone Denitrification:  Potential for Natural Attenuation of Nitrate Contamination in Shallow Groundwater Under Dairy Operations

Abstract

We present results from field studies at two central California dairies that demonstrate the prevalence of saturated-zone denitrification in shallow groundwater with 3H/ 3He apparent ages of < 35 years. Concentrated animal feeding operations are suspected to be major contributors of nitrate to groundwater, but saturated zone denitrification could mitigate their impact to groundwater quality. Denitrification is identified and quantified using N and O stable isotope compositions of nitrate coupled with measurements of excess N2 and residual NO3(-) concentrations. Nitrate in dairy groundwater from this study has delta15N values (4.3-61 per thousand), and delta18O values (-4.5-24.5 per thousand) that plot with delta18O/delta15N slopes of 0.47-0.66, consistent with denitrification. Noble gas mass spectrometry is used to quantify recharge temperature and excess air content. Dissolved N2 is found at concentrations well above those expected for equilibrium with air or incorporation of excess air, consistent with reduction of nitrate to N2. Fractionation factors for nitrogen and oxygen isotopes in nitrate appear to be highly variable at a dairy site where denitrification is found in a laterally extensive anoxic zone 5 m below the water table, and at a second dairy site where denitrification occurs near the water table and is strongly influenced by localized lagoon seepage.

Legacy Nutrient Dynamics at the Watershed Scale: Principles, Modeling, and Implications

“…Given the presumed importance of denitrification at the watershed scale (30%-50% of NANI, Chen et al, 2014aChen et al, , 2015a, it is critical to constrain denitrification losses. A promising approach for constraining denitrification fluxes in the use of groundwater N 2 :Ar dissolved gas ratios to provide a direct measure of denitrification within a given groundwater flowpath (Bourbonnais et al, 2015;Hinshaw and Dahlgren, 2016;Singleton et al, 2007). The high spatial and temporal variability in natural and anthropogenic processes, nutrient inputs, transport, transformations, and outputs at the watershed scale greatly increases the difficulty for characterizing where and when nutrients accumulate in.…”

Controls on Nitrous Oxide Production in, and Fluxes from a Coastal Aquifer in Long Island, NY, USA

“…Anthropogenic NO 3 − comprises the bulk of DIN inputs to the coastal aquifer system [46], therefore NO 3 − concentrations are dictated more by inputs to the upper Glacial aquifer than processes in the coastal zone. Given the overall low concentrations of N 2 denitrification and N 2 O, when compared to other groundwater investigations in temperate sediments [11,[47][48][49] it is evident that much of the groundwater NO 3 − is not denitrified during transit through the coastal aquifer. Previous investigations in coastal aquifers do not consider DOC as a driver of N 2 O reduction of an influence on N 2 O flux to surface water.…”

“…Temporal variations in coastal aquifer heads can drive the saltwater-freshwater interface landward during winter months and seaward during the summer months (46), with additional freshwater recharge to catchment areas of subterranean estuaries due to recharge as snowmelt (49) or precipitation (50) increases water table height (48). Consequently, variations in freshwater discharge, with its elevated NO 3 concentrations, alter the overall composition of SGD (48) …”

An isotope study of the sources of nitrate in Malta’s groundwater

“…Figure 4. This is a feature generally indicative of the effects of partial denitrification (Kendall et al, 2007;Singleton et al, 2007).  15 N and …”