MONITORING TEMPERATURE AS A TRACER AT THE GUARANI AQUIFER SYSTEM OUTCROP ZONE

The Guarani aquifer system is one of the most important transboundary aquifers in the world. Located in parts of four South American countries (Argentina, Brazil, Paraguay, and Uruguay), it covers an estimated area of 1.2 million km². Recharge and discharge of the aquifer is assumed to occur mainly in the outcrop areas of the Botucatu and Pirambóia Formations, where the Guarani aquifer system appears as an unconfined aquifer (OAS, 2009). Despite the importance of the aquifer, only a few studies have quantified its recharge and discharge. For more than 10 years, our team has monitored the groundwater level, stream flow rate, stream stage, and weather parameters within the Guarani aquifer recharge zone encompassed by the Ribeirão da Onça watershed. This 65.0 km2 watershed is dominated by agricultural activities, which include grassland, sugar cane, citrus, and eucalyptus farming. The Onça Creek drains the watershed and runs with an average water table at 1 m depth, and the base flow rate at the outflow is approximately 0.5 m3/s. Shallow groundwater flow directions generally mimic the slope of the land surface, with discharge mainly into the creek.

Temperature values have been registered in the monitoring wells and stream since 2011. The groundwater temperature is constant, with a mean value of 23.48 ± 0.13 °C. Stronger variation is observed for the stream temperature, which has a mean of 21.01 ± 2.18 °C, with a maximum and minimum of 25.19 and 14.93 °C, respectively. Based on these temperature data, our hypothesis is that the temperature could be used as tracer to evaluate stream–aquifer interaction, especially in the winter (June to August), when larger temperature differences between the aquifer and stream are observed. Preliminary results using a temperature probe with four thermistors confirmed our hypothesis along Onça Creek. In the next step, fiber-optic distributed temperature sensing was applied to measure synoptic high-resolution temperature distributions at both fine spatial and temporal scales and to identify stretches with gaining behavior. The collected data and research findings provide important background measurements and evaluations relevant to long-term monitoring needs within the Guarani aquifer system.