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This study determines the spatial and temporal distribution of nitrate content within the water-supply wells in the Mancha Oriental System (MOS) for the period 1998-2003 and presents an example of the multiple nitrate pollution sources in the El Salobral-Los Llanos Domain (SLD, southwestern Albacete). The groundwater resources of the MOS are used to maintain approximately 800 km 2 of irrigated crops and are the sole water supply for a total population of 275,000 inhabitants. The average nitrate content varies from <0.5 mg l −1 (detection limit) to 125 mg l −1 . In some areas of the MOS statistical analyses show a growing tendency between 2001 and 2003. Nitrate content shows a heterogeneous spatial distribution but the highest levels can be associated with large areas of irrigated crops. However, there are also points in which nitrate has been detected in significant quantities which are not found to be spatially linked to this kind of crop. The presence of nitrate in these areas can be explained considering other sources of pollution, such us wastewater, or due to pollutant transport from contaminated areas through groundwater flow in a multiple-layered karstic aquifer. These results lead necessarily to reconsider the agricultural-derived nitrate as the sole source of pollution and to analyze the effectiveness of the current wastewater treatment practices from effluents in the MOS.

To determine the effects of anthropogenic pressures on the ecology of rivers in the Segura Watershed, we examined stream physicochemistry and the macroinvertebrate community in relation to land use, flow regulation by reservoirs, and the Tajo‐Segura Inter‐basin Transfer. The inter‐basin transfer is thought to be the most important stressor because it affects the flow regime as well as water quality and quantity in an arid, typically water‐stressed environment. The concentrations of major ions were strongly impacted by agricultural and urban land uses and the inter‐basin transfer. There was a significant increase in conductivity and the concentrations of SO42− and other major ions downstream of the Tajo‐Segura Transfer. The macroinvertebrate community was less diverse and composed of more pollution‐tolerant taxa in areas affected by flow regulation and the inter‐basin transfer. The rivers of study are at risk for salinization because of inputs from land use and the Tajo‐Segura Transfer. Therefore, major ion and nutrient concentrations should be monitored. Currently, there is no limit specified for SO42− concentration in relation to aquatic ecosystem health in Spain. Measures are needed to reduce the effects of contamination and flow regulation to protect the aquatic ecosystem within the Segura Watershed. Copyright © 2012 John Wiley & Sons, Ltd.