IDENTIFICATION AND EVALUATION OF DIRECT NATURAL RECHARGE IN THE RIO VERDE FLUVIAL AQUIFER

The importance of natural recharge quantification to groundwater flow systems in fluvial aquifers, which is mandatory in semi-arid areas where groundwater constitutes the principal source of drinking water to rural and urban communities, must be recognized. Recharge estimation is fundamental since it is the basis for long-term sustainable water management. From all the factors included in groundwater resource evaluation, recharge (natural or induced) is the most difficult to estimate. By using a combination of geophysical, chemical, and isotopic techniques, it was possible to identify and evaluate natural recharge along ephemeral creeks in the Rio Verde fluvial aquifer. Unconsolidated permeable sediments derived from alluvial fans fed by rivers and streams, constitute the fluvial aquifer. The geophysical characterization of 2 sections (transversally to ephemeral streams) with field measurements along the year, resulted in temporal changes for the vertical and lateral resistivity values, indicating water infiltration along preferential pathways. The fieldwork included the seasonal collection of rain, surface, and groundwater samples for major, minor, trace, and environmental stable isotopes (δ18O, δ2H). Total rain was considered for the calculation of weighted average chloride concentrations in rainwater. With this value, the chloride mass balance approach was used for natural groundwater recharge evaluation. Chloride concentrations in groundwater indicated that natural recharge is within 5-15 % of total rain; an adjustment to this estimation is ongoing to consider natural run-off along the ephemeral creeks. The characteristics of natural recharge indicate that groundwater has low electrical conductivity (180-784 μmhos/cm), high dissolved oxygen (2.63-11.07 mg/l), pH in the range of 5.25-9.3, and is HCO3-Ca type. Information of additional rainwater samples allowed the adjustment of the slope and intercept for available the local isotopic water line. The new local meteoric water line equation allowed obtaining a better understanding of recharge processes; groundwater isotopic values show minimal evaporation effects, indicating a control for natural recharge by direct flow along preferential pathways.