The nitrate time bomb : a numerical way to investigate nitrate storage and lag time in the unsaturated zone

摘要

Nitrate pollution in groundwater, which isudmainly from agricultural activities, remains an internationaludproblem. It threatens the environment, economicsudand human health. There is a rising trend inudnitrate concentrations in many UK groundwaterudbodies. Research has shown it can take decades forudleached nitrate from the soil to discharge intoudgroundwater and surface water due to the ‘store’ ofudnitrate and its potentially long travel time in theudunsaturated and saturated zones. However, this timeudlag is rarely considered in current water nitrateudmanagement and policy development. The aim of thisudstudy was to develop a catchment-scale integratedudnumerical method to investigate the nitrate lag time inudthe groundwater system, and the Eden Valley, UK,udwas selected as a case study area. The method involvesudthree models, namely the nitrate time bomb—audprocess-based model to simulate the nitrate transportudin the unsaturated zone (USZ), GISGroundwater—audGISGroundwater flow model, and N-FM—a model toudsimulate the nitrate transport in the saturated zone.udThis study answers the scientific questions of when theudnitrate currently in the groundwater was loaded intoudthe unsaturated zones and eventually reached theudwater table; is the rising groundwater nitrate concentrationudin the study area caused by historic nitrate load;udwhat caused the uneven distribution of groundwaterudnitrate concentration in the study area; and whether theudhistoric peak nitrate loading has reached the waterudtable in the area. The groundwater nitrate in the areaudwas mainly from the 1980s to 2000s, whilst theudgroundwater nitrate in most of the source protectionudzones leached into the system during 1940s–1970s;udthe large and spatially variable thickness of the USZ isudone of the major reasons for unevenly distributedudgroundwater nitrate concentrations in the study area;udthe peak nitrate loading around 1983 has affected mostudof the study area. For areas around the Bowscar,udBeacon Edge, Low Plains, Nord Vue, Dale Springs,udGamblesby, Bankwood Springs, and Cliburn, the peakudnitrate loading will arrive at the water table in the nextud34 years; statistical analysis shows that 8.7 % of theudPenrith Sandstone and 7.3 % of the St Bees Sandstoneudhave not been affected by peak nitrate. This researchudcan improve the scientific understanding of nitrateudprocesses in the groundwater system and support theudeffective management of groundwater nitrate pollutionudfor the study area. With a limited number ofudparameters, the method and models developed in thisudstudy are readily transferable to other areas.