Subsurface flow pathway and hydrochemistry of a geographically isolated depression wetland in the Serra Geral Diabase Formation, São Paulo State, Brazil

Abstract Connectivity among water bodies is crucial for hydrological and ecological processes and fundamental for water management. Hydrochemistry and potentiometric studies on an isolated wetland allow to assess landscape influence, flow pathways, and subsurface connection. This study focuses on an isolated wetland over diabase on a topographic depression, with no visible river connection, and aims to determine the influence of rainfall and landscape on water composition and investigate subsurface flow toward the Santa Gertrudes stream. Water levels were monitored, and samples were collected from four wells during wet and dry seasons. Physical and chemical water parameters were measured, and ion concentrations determined. Pearson’s correlations highlighted codependence between physical and chemical parameters and ions and Piper’s diagram classified water composition. Six potentiometric maps were generated using interpolated hydraulic loads from monitoring campaigns. The findings show fluctuations in water level (0.11 to 0.93 m during wet periods and 1.25 to 1.79 m during dry periods), indicating rainwater directly affects water availability. Water composition was categorized as Sodium Bicarbonate and electrical conductivity values (greater than 100 µS/cm) may indicate that upstream wells are retaining salts from the surrounding crops. Strong positive correlation between sodium, bicarbonate and carbonate, and sulfate with electrical conductivity suggests simultaneous release of ions due to rock and soil mineral alteration. Potentiometric data reinforce the hypothesis of a subsurface connection between the studied area and the Santa Gertrudes stream. Future studies could benefit from expanding the monitoring network to include more GIWs and applying additional methods, such as isotope tracers.