EVALUATION OF CHEMICAL METHODS TO DISCERN WATER SOURCE AND MIXING IN KARST

Understanding and modeling groundwater/surface water mixing, flow paths and resulting geochemical interactions in karst terrain is difficult due to variable hydraulics and hydrochemical conditions. Both chemical and isotopic tracers have been previously used to constrain the variables and processes characterizing these complex systems. The Lake Logan Martin Dam site on the Coosa River, St. Clair/Talladega counties, Alabama is underlain by the Knox Group and exhibits karstic features, including near-vertical solution channels, extreme anisotropy and heterogeneity. The dam has been subject to leakage problems since its construction in the sixties and a program of monitoring and remedial grouting was implemented to decrease seepage and protect against subsidence. Numerous dye tracer studies conducted at the site have determined some preferential flow paths and recent isotopic studies have identified areas of continued leakage.

This study examines the use of rare earth elements (REE) to model water source and ground/surface water interactions in karsted carbonate terrain. Ground and surface water samples have been collected seasonally (spring, summer and fall) from the Lake Logan Martin Dam site. The collected samples (11 per season) are representative of surface, well and weir locations at the dam. The samples were chemically analyzed by ICP-OES, ion chromatography and ICP-MS for major, minor, and trace constituents in addition to REE. Measured field data included pH, temperature and specific conductance. The spring season data shows a range of pH and temperature values indicative of water source. A Piper plot indicates the water samples are generally Ca²⁺+ Mg²⁺, HCO₃^- in composition. Major element chemistry does not adequately discriminate between the water types. Plots of the shale normalized REE data show variations in patterns as well as concentrations between surface, weir and ground waters. The REE data show that the weirs, constructed to monitor leakage, are a mixture of ground and reservoir waters. Temporal trends will also be discussed.