Relationship between precipitation events and groundwater recharge of the Serra Geral Aquifer System in an experimental watershed located in the municipality of Caxias do Sul, RS: Brazil

This paper aims to evaluate recharge methods and the relationship between precipitation pattern and variation of groundwater levels of the Serra Geral Aquifer System in the region of Caxias do Sul, Rio Grande do Sul. The study was carried out in an experimental basin that contains 4 deep wells inserted in the fractured aquifer, monitored from October 2020 to May 2022, and 2 piezometers installed in the phreatic aquifer, monitored from October 2021 to July 2022. As a result, it was found that the response of the fractured aquifer to precipitation events is on average 27 days. Also, there is a greater response of the aquifer to precipitation events with a volume greater than or equal to 50 mm/day. Recharge estimates through the water balance method represented about 24% and 18% of the total precipitation for the years 2020 and 2021, respectively. Whilst with the Water Table Fluctuation method, the estimated recharge of the fractured aquifer ranged from 3.6 to 6.8% of the total precipitation. In contrast, for the granular aquifer, recharge values ranged from 19.4 to 35.6% of the total precipitation. These results demonstrate that the recharge of the fractured aquifer is lower, which should be taken into account in recharge studies and reserve estimates. Therefore, when using recharge methods that do not take into account this difference between fractured and phreatic aquifers, it is possible that recharge estimates are overestimated and, consequently, will not reflect the reality of fractured aquifer recharge.