Multiple Geochemical and Isotopic Approaches for Assessing Ground Water NO₃⁻ Elimination in a Riparian Zone

Abstract

During the last 40 years, nitrate (NO3−) has become one of the most common ground water contaminants. Stream riparian zones are considered important ecological ecotones that decrease the NO3− load of ground water discharging into streams. This study uses NO3−/Cl− ratios, natural abundances of15N and18O in NO3−, and an in situ15NO3− tracer experiment to evaluate NO3− reducing processes occurring in ground water within a narrow grassed buffer strip bordering a stream in an agricultural watershed. The NO3−/Cl− ratios (1.9‐0.0 [M/M]) indicate that both NO3− consuming processes and mixing of two ground water flow regimes with different NO3− loads contribute to the drop observed in ground water NO3− concentrations within the riparian zone. δ15N and δ18O of the ground water NO3− within the riparian zone (δ15N = 5.1 to 48.8 %c; δ18O = 1.1 to 17.8 %c) were enriched compared to the ground water below the adjacent cultivated field (δ15N = 3.8 to 10.1 %0; δ18O = 0.5 to 6.2 %c). A significant linear relationship (r2=0.97) between ground water δ15N and δ18O in NO3− was found, which is consistent with NO3− consumption by microbial denitrification. The estimated enrichment factors for15N are a factor of 1.5 higher than for18O. The in situ15NO3− tracer experiment conclusively confirmed that denitrification is occurring within the ground water of the riparian zone and demonstrates that denitrification rates can be directly measured in situ.

Subsurface Mobilization of Phosphorus in an Agricultural Riparian Zone in Response to Flooding from an Upstream Reservoir

“…Hydraulic gradients are low at the site compared to what has been observed in other riparian wetlands in the region (e.g., Macrae, 2003;Mengis et al, 1999;Stainton, 2000). Hydraulic conductivities, which are generally characteristic of a sand-silt substrate (Freeze and Cherry, 1979) are comparable in magnitude to what has been observed elsewhere in the region (e.g., Macrae, 2003;Mengis et al, 1999;Stainton, 2000;Warren et al, 2001), but more variable than what has typically been observed within individual studies, demonstrating the fact that a large degree of heterogeneity exists at this site. Surface and shallow soils (50 cm) are more conductive than deeper soils, facilitating a greater groundwater flux in this layer.…”

Reduced nitrate concentrations in shallow ground water under a non-fertilised grass buffer strip

“…Additionally, process understanding can be attained by e.g., determination of Cl/NO 3 ratios differences in natural abundance of 15 N (denoted as d 15 N) of NO 3 -N in groundwater. An increase in the Cl/NO 3 ratio indicates that nitrate removal processes occur, e.g., denitrification (Altman and Parizek 1995;Mengis et al 1999). Crop uptake also results in increasing Cl/NO 3 ratios because relative NO 3 uptake rates exceed Cl uptake rates (Marschner 2002).…”

“…The efficiency was mainly related to the width of the buffer strip. Typically, these studies were conducted on sloping fields with shallow groundwater levels (< 3 m) and/or with a shallow, nearly impermeable soil layer (e.g., Hefting and De Klein 1998;De Klein and Hefting 1998;Dhondt et al 2002;Lowrance et al 2000;Haycock and Burt 1993;Haycock and Pinay 1993;Novak et al 2002;Mengis et al 1999). Such conditions (i.e.…”

“…Since measuring actual loads of nutrients towards open water systems is complicated, we followed the approach commonly used, namely measuring nitrate concentrations in shallow groundwater (Dhondt et al 2002;Altman and Parizek 1995;Lowrance et al 2000;Sabater et al 2003;Borin and Bigon 2002;Haycock and Burt 1993;Haycock and Pinay 1993;Kuusemets et al 2001;Novak et al 2002;Mengis et al 1999). We also considered changes in Cl/NO 3 ratios and in d 15 N in groundwater as indicators for nitrate removal.…”

Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths

“…As such, "extra" reduction capacity must be present in the sediments for appreciable amounts of nitrate reduction to occur. Organic carbon is the most common electron donor, and its role in the reduction of nitrate has been studied in both field [e.g., Trudell The ability of stream riparian zones to remove nitrate has been promoted as a way to reduce the effects of pollution from nonpoint sources [Osbourne and Kovacic, 1993;Mengis et al, 1999]. Stream riparian zones are defined as those areas which have direct interaction between terrestrial and aquatic ecosystems with boundaries of the riparian zone extending outward to the extent of flooding [Gregory et al, 1991].…”