Agricultural practices and geostatistical evaluation of nitrate pollution of groundwater in the Júcar River Basin District, Spain

Groundwater pollution by nitrates induced by intensive agriculture and overexploitation of water in the Jucar River Basin District is a major concern. This work is a contribution to understanding the spatial and temporal distribution of nitrates in groundwater and the possible relationships between the presence of nitrates and contextual variables, such as rainfall, seepage and lithology. A space-time table of groundwater measurements of nitrate levels between 1969 and 2012 (41 hydrological years) in 6381 monitoring locations is used. First, a statistical analysis was performed to find correlations between nitrate concentrations and the contextual variables of yearly rainfall and recharge amounts evaluated by the PATRICAL model. A geostatistical simulation approach was then used to build a space-time model of nitrate concentrations, including calculation of experimental variograms and fitting of a theoretical model. The results of the variograms were used to infer the maximum spatial correlation distance and the maximum time range of temporal correlation for the same location. Sequential Gaussian simulation was selected instead of kriging for the purpose of declustering of data locations. Finally, yearly maps of average nitrate concentrations were generated.The results show that nitrate concentrations have increased steadily from the early 1970s and have stabilized since 2005. Contamination by nitrates occurs predominantly in agricultural areas, such as the plateau of La Mancha and the coastal plain.