Investigating river–aquifer relations using water temperature in an anthropized environment (Motril-Salobreña aquifer)

Heat was applied as a tracer for determining river–aquifer relations in the Motril-Salobreña aquifer (S Spain). The aquifer has typically been recharged by River Guadalfeo infiltration, nevertheless from 2005 a dam was constructed changing the traditional dynamic river flow and recharge events. Temperature measurements in river, air, and a number of boreholes located at different distances away from the river were monitored over a 6 year period. Temperature is easy to monitor and especially useful in anthropized aquifers due to their natural and non-pollutant characteristics. The thermal responses of recharge events have been analyzed with different methodologies for the characterization of the recharge process. Changes in groundwater temperatures recorded between 15 and 40 m deep are detected during the periods of higher river discharge. Temperature logs envelopes reveal areas of infiltration dominance and areas where vertical upward flows are present. For the quantification of the recharge volume, a model was constructed with VS2DHI. The study of the temperature measurements suggests an upper zone of groundwater temperature variability related with the direct infiltration from the river. Also there is a deeper zone where temperature changes are related with the heat transport associated to the main groundwater flow in the aquifer. With the evaluation of the temperature data by different means, the conceptual three-dimensional flow model has been strongly improved establishing the basis for a future 3D modelling.