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The Cristo lagoon, situated on Neogene deposits in the northern Spanish Meseta, occupies a shallow depression on a Pleistocene piedmont. The development of the lacustrine depression on the piedmont was favoured by the fault network, reinforced by substrateloss by weathering, probably during the late Quaternary. Even during the hot summer season, salinity is low, with concentrations of total dissolved solids (TDS) being around 150 mg L–1. Only when the lagoon is almost dry do TDS concentrations exceed 500 mg L–1, sometimes rising as high as 1700 mg L–1. Whenthe lake level is high, lake chemistry is dominated by Na+, Ca2+, HCO3– and Cl–. During drier stages, there is a relative increase in Ca2+, Mg2+, Cl–, and SO42–, trending toward a calcium chloride-sulphate brine. Values of pH are above 9 during late spring and summer, resulting primarily from evaporative degassing favoured by the shallow depth of water, and secondarily from photosynthesis by the abundant submerged macrophytes. The infilling deposits, less than 0.5 m thick, are dark brown, massive, sandy muds consisting of quartz and clays (illite, kaolinite, smectite), all of which are allogenic in origin. The main source of dissolved sulphate was the oxidation of sulphides during weathering of lower Palaeozoic rocks in the catchment area. The 13C-depleted nature of dissolved inorganic carbon indicates an origin mostly by respiration and oxidation of organic matter. Geomorphology and hydrogeochemistry indicate a flow-through lake dominated essentially by groundwater flows.