Land drainage

Land drainage systems (whether traditional tiles or modern perforated plastic pipe drains) accelerate the discharge of temporary or permanent near-surface groundwater to streams. This practice thus affects the convective flux of groundwater N2O. Reay et al (2003, 2004a, 2004b) conducted a study on N2O emission from a drainage system serving crop fields and discharging into a ditch. They demonstrated that N2O discharged by 14 field drain outfalls was rapidly emitted to the atmosphere during transport in the open ditch, which was enhanced by turbulence in the stream flow. An EF (CEF2) of 0.002 resulted from the N2O to NO3- mass ratio at the drain outfalls. The total range of CEF2 for artificial drainage is similar to CEF2 that was reported for groundwater (Table 8.1). However, this range is based on only a few studies and it is thus not clear if artificial land drainage leads to elevated convective N2O fluxes to streams in comparison with natural drainage.

Until now there have been no direct comparisons between N2O measurement at groundwater monitoring wells and corresponding field drainage outlets. Moreover, the effect of accelerated groundwater discharge via field drains on convective N2O fluxes to streams has not yet been studied. By comparing excess N2 and N2O in denitrifying aquifers, Weymann et al (2008) found typical patterns showing that, with increasing excess N2, i.e. ongoing denitrification progress during passage through the aquifer, N2O levels initially increased to a certain level, but then gradually decreased and finally disappeared after NO3- was completely consumed by denitrification. Highest N2O levels were generally observed at intermediate reaction stages, when 2060 per cent of the NO3- was denitrified. It was concluded that low N2O fluxes via artificial drainage might be due to short groundwater residence times in aquifers. However, one might expect that field drainage fluxes can be high under conditions when denitrification at the groundwater surface is rapid, causing substantial reaction progress even during the short residence time of groundwater passing through drainage systems. But this needs to be confirmed.

So far, there are no estimates of CEF1 for artificial drainage systems. Because significant denitrification is a frequent phenomenon in near-surface groundwater (Well et al, 2005b), it can be expected that in many cases some of the leached NO3~ is denitrified before groundwater is discharged via this path. Consequently, CEFi must be lower than CEF2 (see Equations 8.3 and 8.4 and section above). But due to the relatively short groundwater residence time, this discrepancy is probably lower compared to groundwater without artificial drainage. It would also be interesting to determine CEF3 for drainage in order to check its impact on the overall N2O emission of the total aquatic pathway including rivers and the ocean. Estimating CEF1 and CEF3 using excess N2 (see above) will not be possible by direct measurement at drainage outlets, since the water equilibrates with the atmosphere during passage through the drains. Thus excess N2 in the water would be lost before sampling. To measure excess N2 it would thus be necessary to measure it in groundwater monitoring wells installed within the groundwater body of the drained area, which has not been done up to now.

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