Intense pumping in regional groundwater discharge areas: The problem of crop salinization in an inland aquifer (Cenozoic Duero basin aquifer, Spain)

Study region: Duero river basin, Castilla y León, Spain. Study focus: This study aims to identify the origin of crop salinization in the discharge zone of the Cenozoic Duero aquifer system. The combination of hydrogeological, geophysical, and geochemical techniques have been used to understand this problem. New hydrogeological insights from the region: Regional saline groundwaters flow through this aquifer system, discharging into the Duero River and mixing with local fresh groundwaters. Time domain electromagnetic profiles and vertical electric conductivity measurements indicate that salinity markedly increases below 150–200 m depth. Shallow groundwaters (local flows) are fresh, about 500 mg/l of total dissolved solids (TDS), and dominated by Ca2 + - HCO3- compositions. Groundwaters sampled in the boreholes located in the Duero river floodplain (100–150 m depth) are of Na+ - HCO3- - Cl- composition and have salinities about 1500 mg/L (TDS). Deeper groundwaters are more saline about 5000 mg/L (TDS) and have Na+ - Cl- compositions. Isotopic ratios (δ18O, δD) of regional groundwaters are lower than those of the average local precipitation and the Duero river, evidencing that groundwater recharge occurred at higher altitudes and/or lower temperatures. δ18O and δ34S from dissolved sulphate suggest mixed sources of dissolved sulphate with a major contribution from Miocene and Triassic gypsum. Intense groundwater pumpingproduces the uprise of the fresh-saline groundwater interphase. When irrigation boreholes intercept the saline groundwater salinization of cropsoccurs.