Quantification of mixing proportions of surface water, groundwater, and precipitation in a regional basin using multiple techniques

Abstract

Increased precipitation in the 1970s led to base level shifts in the Rio Quinto basin, causing conflicts due to flooding. A regional study was conducted to analyze the water resource system, particularly the contributions of groundwater discharge and precipitation to surface water. Various techniques were employed, such as analyzing major ionic species and isotopes like δ18O and δ2H, along with methods like end-member mixing analysis (EMMA) and inverse geochemical modeling (PHREEQC). The analysis encompassed components explaining the geochemistry of groundwater across the entire basin. The results showed that surface and groundwater geochemistry results from mixing precipitation, local groundwater flows, and regional groundwater flows. EMMA and PHREEQC quantitatively showed that in the upper basin, surface waters receive 60% from precipitation and 40% from local groundwater flows on average. In the middle and lower basins, regional groundwater flows are minimal (<5%), with precipitation/local groundwater flows dominating (40% and 55%). Conversely, the upper basin exhibits variability, with precipitation contributing more to shallow wells (<20 m) and local groundwater flows to deeper wells (>20 m). In the middle and lower basins, regional groundwater flows increase (15%), local groundwater flows dominate (60%), and precipitation plays a lesser role (not exceeding 50%). Geochemical processes such as cation exchange, salt dissolution, and carbonate precipitation further explain the evolution of water chemistry in the system. These results help establish the origin, hydraulic connections, and contributions of various water sources, contributing to a comprehensive understanding of the water cycle in the study area.