Decadal Delays in Groundwater Recovery from Nitrate Contamination Caused by Low O₂ Reduction Rates

Abstract

Nitrate (NO −3 ) is one of the main pollutants in agriculturally impacted groundwater systems. The availability and reactivity of electron donors control the prevalent redox conditions in aquifers and past nitrate contamination of groundwater can be ameliorated if denitrification occurs. Using aqueous geochemistry data and the stable isotope composition of dissolved nitrate ( 15 N and 18 O), we found that nitrate concentrations above the World Health Organization drinking water guideline were caused predominantly by manure and to a lesser extent by synthetic fertilizer applications and that denitrification was not a significant nitrate removal process in an aquifer in southern Germany underlying agricultural land with intensive hog farming. We also applied environmental isotopes ( 2 H and 18 O, 3 H/ 3 He, and 14 C) linked with a lumped parameter approach to determine apparent mean transit times (MTT) of groundwater that ranged from <5 years to >100 years. Furthermore, we determined low reduction rates of dissolved oxygen (O 2 ) of 0.015 1/year for first-order kinetics. By extrapolating the O 2 reduction rates beyond the apparent MTT ranges of sampled groundwater, denitrification lag times (time prior to commencement of denitrification) of approximately 114 years were determined. This suggests that it will take many decades to significantly reduce nitrate concentrations in the porous aquifer via denitrification, even if future nitrate inputs were significantly reduced.

Monte Carlo Simulations as a Decision Support to Interpret δ15N Values of Nitrate in Groundwater

“…The study area is located near Hohenthann in Lower Bavaria, Germany, and it is affected by high nitrate concentrations in groundwater, reaching up to 120 mg/L (Wild et al 2018). As displayed in Figure 1a, approximately 65% to 80% of the study area is agriculturally used with intensive hog farming and the cultivation of crops, predominantly maize.…”

“…As displayed in Figure 1a, approximately 65% to 80% of the study area is agriculturally used with intensive hog farming and the cultivation of crops, predominantly maize. The subsurface is characterized by Quaternary and Tertiary clastic sediments dominated by sandy gravels, where a main aquifer is formed at around 45 m depth and deeper (Figure 1b), consisting of the Younger Upper Freshwater Molasse (UFMy), the Northern Gravel Series, and the Fluviatile Freshwater Layers of the Upper Freshwater Molasse (UFM) (Wild et al 2018). Several smaller perched aquifers are present at shallow depths (upper 45 m below ground level bgl), which have been developed locally above clay layers.…”

“…for a catchment area, it is difficult to characterize the selfpurification potential of the groundwater system. Under such situations, the microbial reduction of nitrate can be evaluated using stable isotope ratios of δ 15 N and δ 18 O of nitrate and may enable the identification of aquifers that are more susceptible to nitrate contamination (Aravena et al 1993; Kendall and McDonnell 1998;Böhlke et al 2002;Einsiedl and Mayer 2006;Wild et al 2018). In several field studies, it was demonstrated that during denitrification both δ 15 N and δ 18 O of nitrate increase, that is, enrich in heavier isotopes, while nitrate concentrations decrease (Kendall and McDonnell 1998).…”

“…In a previous study (Wild et al 2018), we analyzed stable isotopes of nitrate (yielding δ 15 N and δ 18 O) and concluded that nitrate contamination of groundwater near Hohenthann, Germany, may has originated from different nitrate sources comprising mineralization of soil organic nitrogen, as well as nitrification of ammonia and urea included in fertilizers. It was also stated by the authors that denitrification may have occurred in two of the wells screened in the perched, but not significantly in the main aquifer of the studied groundwater system.…”

Water Quality and Hydromorphological Variability in Greek Rivers: A Nationwide Assessment with Implications for Management

“…Thus, in catchments with polluted groundwater that act as a source of nitrogen, diffuse pollution mitigation actions will not have the desirable outcome. Reducing the nitrogen levels in these rivers will be extremely difficult considering that residence time of groundwater may span several decades [53][54][55]. Thus, an integrated management that considers the status of both surface and groundwaters will likely have higher chances of success.…”