Main sources and processes affecting dissolved sulphates and nitrates in a small irrigated basin (Lerma Basin, Zaragoza, Spain): Isotopic characterization

• Hydrology was controlled by irrigation rather than precipitation inputs.
• Soil sulphate and Tertiary salts sufficed to explain sulphate concentrations.
• Nitrate accounted for 90% of nitrogen, and was mainly originated from fertilizers.
• Natural attenuation of nitrate pollution was inferred in surface diffuse flow.

Irrigated agriculture affects the quality of water bodies receiving irrigation return flows by both salinization and nitrate pollution, which are controlled by site-specific factors such as geology or agriculture management. In this work, coupled hydrogeochemistry and isotopic data are used to determine the factors controlling water salinization and nitrate pollution in a small irrigated basin in Northeast Spain. This basin is representative of a large irrigated surface in the Middle Ebro Valley, presenting perched aquifers developed over Quaternary glacis and half of its surface under pressurized irrigation. Identification of the controlling factors and the differences between both environmental problems (salinization and nitrate pollution) were established through chemical composition and stable isotope analyses (δD and δ18O-[H2O]; δ34S and δ18O-[SO42]; δ15N and δ18O-[NO3−]) of collected samples in groundwater, springs and surface water during the irrigated and the non-irrigated season. The isotopic composition of water indicated no significant evapoconcentration and a higher influence of irrigation water (rather than precipitation water) on the hydrology of the basin. Sulphate was used as a tracer for salinization. There was no positive correlation between nitrate and sulphate, indicating differences in the controlling factors for each compound. Sulphate content was significantly higher in surface water than in groundwater, and a mixture of soil and local gypsum sulphates explained the isotopic composition of most of the collected samples. One sampling location presented samples affected by fertilizers. Nitrate concentration was significantly lower in surface water than in groundwater, with synthetic fertilizers being the main source, especially the ammonia/urea components. The isotopic composition of surface water suggested a low degree of denitrification while circulating in a diffuse pathway over a low permeability substrate. All water quality information was incorporated into a conceptual model of the study site.