Nitrate pollution in the Warta River (Poland) between 1958 and 2016: trends and causes.

The article presents analyses of long-term water quality data from the Warta River between 1958 and 2016. A clear increasing trend in nitrate concentrations was observed from 1958 to the early 1990s. This trend was mainly related to the increasing use of fertilizers in Poland in this period. Then, after the early 1990s, a slow decreasing trend related to improvements in water and sewage management and more rational fertilizer use was observed after political and economic changes in Poland. The influence of long-term hydrological droughts on nitrate concentrations was also investigated. Sharp increases in the nitrate concentration in surface water were related to the accumulation of contaminants in the soil and aeration zone during drought periods and the subsequent transport of these contaminants to groundwater and surface water via recharge infiltration after each drought period. The presented results highlight the importance of surface water-groundwater interactions and suggest that groundwater protection in an entire catchment area is essential for surface water quality protection.