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

Abstract

Abstract. The rate and mechanism of nitrate removal along and between groundwater flow paths were investigated using a series of well nests screened in an unconfined sand and gravel aquifer. Intensive agricultural activity in this area has resulted in nitrate concentrations in groundwater often exceeding drinking water standards. Both the extent and rate of denitrification varied depending on the groundwater flow path. While little or no denitrification occurred in much of the upland portions of the aquifer, a gradual redox gradient is observed as aerobic upland groundwater moves deeper in the aquifer. In contrast, a sharp shallow redox gradient is observed adjacent to a third-order stream as aerobic groundwater enters reduced sediments. An essentially complete loss of nitrate concurrent with increases in excess N 2 provide evidence that denitrification occurs as groundwater enters this zone. Electron and mass balance calculations suggest that iron sulfide (e.g., pyrite) oxidation is the primary source of electrons for denitrification. Denitrification rate estimates were based on mass balance calculations using nitrate and excess N 2 coupled with groundwater travel times. Travel times were determined using a groundwater flow model and were constrained by chlorofluorocarbon-based age dates. Denitrification rates were found to vary considerably between the two areas where denitrification occurs. Denitrification rates in the deep, upland portions of the aquifer were found to range from <0.01 to 0.14 mM of N per year; rates at the redoxcline along the shallow flow path range from 1.0 to 2.7 mM of N per year. Potential denitrification rates in groundwater adjacent to the stream may be much faster, with rates up to 140 mM per year based on an in situ experiment conducted in this zone.

Aquifer-scale controls on the distribution of nitrate and ammonium in ground water near La Pine, Oregon, USA

“…For example, the history and fate of NO 3 contamination have been carefully evaluated in multidisciplinary local transect studies in areas with agricultural sources (e.g. Böhlke and Denver, 1995;Tesoriero et al, 2000;Böhlke et al, 2002;Puckett et al, 2002) and septic tank sources (e.g. Robertson and Cherry, 1992;Wilhelm et al, 1996;Aravena and Robertson, 1998).…”

Nitrate fate and transport through current and former depressional wetlands in an agricultural landscape, Choptank Watershed, Maryland, United States

“…Reducing conditions and abundant organic matter typical of wetlands promote removal of NO 3 through denitrification and may thus improve groundwater quality (Howarth et al 1996;Whigham and Jordan 2003). Denitrification occurs where groundwater containing NO 3 passes through any sufficiently reducing environment (Puckett 2004) and has been observed in riparian and hyporheic sediments (Kennedy et al 2009), and along upgradient groundwater flow paths (Tesoriero et al 2000;Spruill et al 2005;Böhlke et al 2007;Mehnert et al 2007;Denver et al 2010), as well as in wetland sediments (Puckett 2004;Whigham and Jordan 2003).…”

“…Reducing conditions and abundant organic matter typical of wetlands promote removal of NO 3 through denitrification and may thus improve groundwater quality (Howarth et al 1996;Whigham and Jordan 2003). Denitrification occurs where groundwater containing NO 3 passes through any sufficiently reducing environment (Puckett 2004) and has been observed in riparian and hyporheic sediments (Kennedy et al 2009), and along upgradient groundwater flow paths (Tesoriero et al 2000;Spruill et al 2005;Böhlke et al 2007;Mehnert et al 2007;Denver et al 2010), as well as in wetland sediments (Puckett 2004;Whigham and Jordan 2003).The effectiveness of wetlands for mitigating agricultural NO 3 depends primarily on local hydrology and the movement of water through the landscape, which controls how much NO 3 actually passes through the zone of wetland influence (Puckett 2004;Tesoriero et al 2009). Shallow groundwater flow systems around wetlands and other surface water bodies are commonly transient and complex and reflect influences of local geology, variable precipitation and recharge, and groundwater flow systems at local to regional scales (Winter 1983(Winter , 1999 is also delivered to streams along flow paths at depths beneath the influence of riparian forests or wetlands (Böhlke and Denver 1995;Hill 1996;Puckett 2004;Ator et al 2005a).…”

A hydro-economic modeling framework for optimal management of groundwater nitrate pollution from agriculture

“…The nitrate content is generally low because it is taken up in synthesis, leached by water percolating through the soil, or subjected to denitrification activity below the aerobic top layer of the soil. However, synthesis and denitrification rarely remove all nitrates added to the soil from fertilizers and nitrified wastewater effluents (Tesoriero et al, 2000). Accordingly, nitrates leached from soil are a major groundwater quality problem.…”

“…Nitrates can move with groundwater with minimal transformation and can migrate long distances from input areas if there are highly permeable subsurface materials that contain dissolved oxygen. This process can be affected by a decline in the redox potential of groundwater that can lead to a denitrification process (Tesoriero et al, 2000). Groundwater fate and transport models are essential for assessing the impact of protection alternative measures that protect groundwater quality and reduce contamination.…”