Occurrence of Nitrous Oxide in the Central High Plains Aquifer, 1999

Abstract

Nitrogen-enriched groundwater has been proposed as an important anthropogenic source of atmospheric nitrous oxide (N 2 O), yet few measurements of N 2 O in large aquifer systems have been made. Concentrations of N 2 O in water samples collected from the 124 000 km 2 central High Plains aquifer in 1999 ranged from < 1 to 940 nM, with a median concentration of 29 nM (n ) 123). Eighty percent of the N 2 O concentrations exceeded the aqueous concentration expected from equilibration with atmospheric N 2 O. Measurements of N 2 O, NO 3 -, and 3 H in unsaturatedzone sediments, recently recharged groundwater, and older groundwater indicate that concentrations of N 2 O in groundwater increased over time and will likely continue to increase in the future as N-enriched water recharges the aquifer. Large concentrations of O 2 and NO 3 -and small concentrations of NH 4 + and dissolved organic carbon in the aquifer indicate that N 2 O in the central High Plains aquifer was produced primarily by nitrification. Calculations indicate that the flux of N 2 O from the central High Plains aquifer to the atmosphere from well pumping and groundwater discharge to streams was not a significant source of atmospheric N 2 O.

Recovery of groundwater N2O at the soil surface and its contribution to total N2O emissions

“…If the correct concentration gradients would be incorporated in the flux calculation, the data of Ronen et al (1988) were within the range of the N 2 O fluxes reported by , but still higher than the N 2 O fluxes reported in this study according to the different N 2 O concentrations in the groundwater. Other previous studies tend to confirm our result of relatively low diffusive N 2 O fluxes from the groundwater to the atmosphere (McMahon et al 2000;Hiscock et al 2003). In the light of these studies and of our results, indirect N 2 O emissions via the diffusive pathway seem to be hardly significant.…”

Nutrient dynamics in relation to surface–subsurface hydrological exchange in a groundwater fed chalk stream

“…High concentrations of nitrous oxide have been reported from the groundwater of agricultural catchments and are thought to be a result of nitrification in the unsaturated zone Hiscock, 1997, 1998;McMahon et al, 2000). This is likely to be the case in the Lambourn catchment and the high background N 2 O concentration in the river bed may reflect similarly high N 2 O concentration in the underlying groundwater.…”

Groundwater N&lt;sub&gt;2&lt;/sub&gt;O emission factors of nitrate-contaminated aquifers as derived from denitrification progress and N&lt;sub&gt;2&lt;/sub&gt;O accumulation

“…soils, indirect emissions from ground and surface waters result from nitrogen leaching and runoff to adjacent systems (Well et al, 2005a;Nevison, 2000). The knowledge of these indirect emissions is limited because few studies have tried to relate subsurface N 2 O concentrations to N leaching from soils (Clough et al, 2005) and investigations of N 2 O in deeper aquifers are rare (Ronen et al, 1988;McMahon et al, 2000;Hiscock et al, 2002).…”

“…However, until now there have been no systematic investigations of N 2 O dynamics in these aquifers. N 2 O emissions from groundwater were thought to comprise a significant fraction of total agricultural N 2 O emissions (IPCC, 1997), but recent studies show in agreement that their significance is lower (McMahon et al, 2000;Hiscock et al, 2003;Höll et al, 2005;Reay et al, 2005;Well et al, 2005a;Sawamoto et al, 2005). Consequently, the nitrous oxide emission factor from aquifers and agricultural drainage water (EF5-g) was corrected downwards from 0.015 to 0.0025 by the Intergovernmental Panel on Climate Change (IPCC) in 2006, taking the data of Hiscock et al (2002Hiscock et al ( , 2003, Reay et al (2004Reay et al ( , 2005 and Sawamoto et al (2005) as a basis.…”