Karst hydrogeological controls and anthropic effects in an urban lake.

Abstract

Reduction in volume of surface water bodies can sometimes be associated with hydraulic connections to groundwater aquifers. This is often the case in karstic aquifers that are used for water supply. This paper analyzed possible karst hydrogeological controls and anthropic effects in the depletion of a 1 km2 lake in the municipality of Sete Lagoas, Minas Gerais, Brazil. Several techniques were used for the analyses: long-term pumping test coupled with stable isotopic samplings ( 18O and 2 H) to analyze for hydraulic connections and estimation of groundwater-lake water mixture proportion; geophysical surveys (electrical resistivity image, and vertical electrical sounding) for mapping subsurface discontinuities; potentiometric maps (static and dynamic levels) and urban-industrial expansion of well pumping rates; climatological water budget from 1961 to 2019 for climate variation analyzes; and a 3D-karst hydrogeological modeling. Results show climatic variation combined with urban-industrial expansion increases the demand for water in the region and directly impacts local potentiometric surfaces, and hence the lake water level. Geophysical surveys indicate that lake is karstically structured and identified preferential flowpaths for water infiltration and mixing. The pumping test shows a drawdown stabilization, from a transient-flow regime to a steady-state, indicating a hydraulic connection with a water source that isotopically is more evaporated (common in surface water), which contributes 13% to the mixture.