Unravelling recharge mechanisms along the Guarani aquifer system (SAG) outcrop in southern Brazil

Understanding recharge is an important step in quantifying the sustainability and vulnerability of the groundwater reserve and promoting the adequate implementation of management policies. In southern Brazil, the Guarani Aquifer System (SAG) is increasingly utilised as a source of water for both rural and urban populations and agricultural activities. Being 90% confined, the regional SAG outcrop zones are generally considered the recharge areas, yet local recharge mechanics have yet to be constrained. Water table time series data over a 2-year period from 21 monitoring wells in the southern outcrop zone were investigated to assess the local recharge of the SAG. Recharge estimates calculated using the water table fluctuation (WTF) method were compared to the time series data in terms of auto- and cross-correlation factors. In turn, alluding to the memory effect of the water table that is controlled by the flow pathways to the well and its response to local rainfall events, it is apparent that the local conditions strongly affect water percolating through the unsaturated zone, with evidence of both matrix and preferential flow pathways feeding the monitoring sites. Matrix flow is broadly occurring within the study area, and preferential flow has a more restricted occurrence yet is associated with the highest rate of recharge. Although future studies may quantify the actual recharge amount related to each mechanism, this work explicitly shows that preferential flow mechanisms should not be ignored, providing insight into aquifer sustainability and its vulnerability to modern contaminants.