Analysis and interpretation of natural variations in water table and groundwater recharge of coastal aquifer system in the coastal plain of Rio Grande do Sul, Brazil

This paper presents the results of recharge estimation and water table variation analyses of the Coastal Aquifer System, located in the Rio Grande do Sul Coastal Plain (Brazil). Water-level data from 11 wells were processed and analyzed, applying autocorrelation and cross-correlation methods to evaluate the aquifer memory effect and mean response time to rainfall events. The seasonality of the response time was evaluated through splitting the data series in windows of 90 days, overlapped by 45 days. Recharge rates were obtained with the water table fluctuation (WTF) daily applications (RISE and MRC), considering specific yields of 20%, 25% and 30%. Water-level data smoothing with moving averages and time series filters were necessary, to remove level rises not associated with precipitation events within the aquifer average response time, The autocorrelation and cross-correlation displayed a variability of responses through the wells, with median response times of level to precipitation from 0, 1 and 2 days (wells in aquifer layers of fine to medium sands) to 10 days (wells in aquifer layers of higher clay content). The recharge rates of 0.74 to 2.42 mm day(-1) and Recharge/Precipitation ratios from 19% to 46% were obtained (S-y = 25%, mean between RISE and MRC results). From these applications, the study presents the importance of level data processing and filtering to daily WTF applications and displays the differences in the behavior of the wells in the Coastal Aquifer System.