Temporal analysis of spring water data to assess nitrate inputs to groundwater in an agricultural area (Osona, NE Spain)

Abstract

Non-point agricultural pollution is a major concern in groundwater management. To investigate nitrate input to the subsurface through groundwater recharge, thirteen natural springs were sampled. Discharge, electrical conductivity (EC), nitrate concentration, pH value and water temperature were monitored every two weeks from January 2010 till February 2011 at selected springs in the Osona region (NE Spain). Two extensive hydrochemical analyses were also conducted at the beginning and at the end of the survey. Springs are classified in four groups describing their hydrological response, based on discharge, EC and nitrate data. Geostatistical analysis provides an additional insight into the discharge and nitrate temporal pattern. Even though discharge and EC can be related to specific hydrogeological behaviours, nitrate content shows uniform values in most of the springs with only a minor influence from external factors such as rainfall events, fertilisation regimes and geological traits. Such evenness of outflow might be attributed to a homogenisation of the subsurface processes that determine nitrate infiltration after decades of intensive fertilisation using pig manure. Accumulated nitrate mass load and variograms confirm this result. Assuming that spring data are representative of nitrate leaching towards the underlying aquifer, nitrate content of infiltrating recharge in shallow aquifers should therefore show a steady value over time with only small fluctuations due to natural processes. Nevertheless, temporal fluctuations in nitrate content in aquifers could be also attributed to flow regime alterations due to human groundwater withdrawal.