Nitrate occurrence and attenuation in the major aquifers of England and Wales

摘要

The current occurrence of nitrate in the major aquifers of England and Wales is presented and the evidence for denitrification is critically reviewed. Denitrification is the principal nitrate attenuation process in the subsurface and its potential for mitigating the widespread nitrate inputs is considered. The study focuses on the three most important major aquifers: the Cretaceous Chalk, Permo-Triassic Sandstone and Jurassic Limestone. Elevated ground-water nitrate concentrations are shown to be widespread and continue to rise, with leaching from soils predicted to remain significant. Some 60% of groundwater bodies in England may fail to meet the Water Framework Directive requirement of ‘good status’ by 2015. Acquiring reliable evidence of denitrification is non-trivial and typically comprises integrated assessment of electron donor conditions, nitrogen oxide products, stable isotope ratios, nitrogen/argon ratios, microbiology and comparative velocity estimates. The available field studies confirm that denitrification in unconfined aquifers is relatively limited. Detailed unsaturated zone studies of both the Chalk and Sherwood Sandstone have demonstrated only minor decreases in nitrate concentrations, estimated at just 1–2% of the nitrate load within infiltrating water. Such decreases are unlikely to significantly influence regional groundwater quality. Within the saturated zones of the Chalk, Sherwood Sandstone and Jurassic Limestone aquifers, denitrification was significant only once these aquifers became confined and dissolved oxygen depleted. However, evidence for denitrification is typically weak at the regional aquifer scale and low nitrate concentrations may sometimes be simply ascribed to dilution or non-arrival of plumes. Denitrification within the Chalk or Jurassic Limestone matrix, although geochemically possible, may not occur, as bacteria are potentially excluded by the narrow pore throats. Although it is concluded that denitrification is unlikely to lead to very significant mitigation of the nitrate problems manifest in these aquifers, further research is still warranted to better understand its role. Relatively little denitrification research has been conducted in the major aquifers over the last decade. Organic carbon controls, in-fracture v. in-matrix denitrification and recent trends in nitrate and denitrification at historically monitored sites should all be further investigated.