Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

Abstract

for using a Monte Carlo approach. When denitrification could be quantified, we reconstructed the initial NO3-N concentration and NO3-N/Cl -ratio at the point of entry to the groundwater system. The addition of NO3 -to the local groundwater system from temporary manure piles and pens equalled or exceeded NO3 -additions due to leaching from earthen manure storages at these sites. Nitrate attenuation at both sites is attributed to a spatially 20 variable combination of mixing and denitrification, but is dominated by denitrification. On-site denitrification reduced agriculturally derived NO3 -concentrations by at least half and, in some wells, completely. These results indicate that infiltration to groundwater systems in glacial sediments where NO3 -can be naturally attenuated is likely preferable to off-farm export via runoff or drainage networks. The application of isotopes of nitrate to constrain a mixing model based on concentrations of Cl -and NO3 -, which can be routinely monitored in 25 groundwater, provides a relatively simple method to assess the sources and fate of agriculturally derived NO3 -in these settings.

Groundwater nitrogen and chloride following closure of an earthen manure storage in southern Alberta

“…Nitrate‐N concentrations were elevated (>10 mg L −1 ) in wells less than 50 m downgradient of the EMS (D‐P10‐2, DMW13a), suggesting evidence of groundwater contamination (Bourke et al., 2019). Well D‐MW2 also had NO 3 –N concentrations that were slightly greater than the anthropogenic threshold of 3 mg L −1 (i.e., 3.63 mg L −1 ).…”

“…The use of NO 3 –N/Cl − ratios to identify sources of anthropogenic N and Cl − (i.e., the EMS) and its variation among these sources can show if anthropogenic NO 3 –N at different locations within an aquifer is derived from the same or different sources (Bourke et al., 2019). In our study, this ratio was determined to be best represented by D‐P10‐2, which was located directly downgradient of the EMS.…”

“…The CFO site, previously described by Bourke et al. (2019), was part of a larger study conducted from 2010 to 2015 investigating the effects of CFOs and manure management on groundwater quality in Alberta, Canada. The site, a combined 100‐head dairy and 2,000‐head beef feedlot, was located 25 km northeast of Lethbridge, Alberta, Canada, in an area with a high density of CFOs and nearly all cropland under irrigation (Bourke et al., 2019).…”

“…(2019), was part of a larger study conducted from 2010 to 2015 investigating the effects of CFOs and manure management on groundwater quality in Alberta, Canada. The site, a combined 100‐head dairy and 2,000‐head beef feedlot, was located 25 km northeast of Lethbridge, Alberta, Canada, in an area with a high density of CFOs and nearly all cropland under irrigation (Bourke et al., 2019). The CFO occupied an area of about 23 ha and included an EMS, barns, dairy cow pens, beef feedlot pens, a catch basin (for collection of runoff water from the feedlot pens), a dugout, feed storage areas, and a residence (Figure 1).…”

“…Considering the limited literature and research on EMS closures, this provided an opportunity to investigate the dynamics of manure components in shallow groundwater given that no specific EMS decommissioning procedures were followed. Previous work showed that this EMS had a significant effect on shallow groundwater quality in the immediate vicinity during operation and prior to closure (Bourke et al., 2019; Kohn, Iwanyshyn, & Olson, 2020; Lorenz et al., 2014). The objective of this study was to document changes in shallow groundwater quality by monitoring N and Cl − movement below and near an EMS following closure.…”