GEOCHEMICAL MODELING OF SPRINGS IN BIG BEND RANCH STATE PARK, TEXAS

As part of an inventory and assessment of springs in the Big Bend Ranch State Park (BBRSP), we have compiled 76 existing water chemistry analyses. The analyses have been used to characterize the different sub-watersheds in the park and to relate the water chemistry to the bedrock geology. The springs are mostly calcium-bicarbonate facies and have relatively low total dissolved solids (TDS; 230 to 660 mg/L). These general chemical characteristics are due to the relatively shallow flow paths and short residence times in the primarily silicic igneous rock aquifers that are isolated from deeper circulation due to the presence of aquitards in the volcanic stratigraphy. In order to better understand the chemical evolution of the water, we have attempted inverse modeling using PHREEQC to produce probable phase transfers of minerals along flow paths. Review of existing literature for bulk mineralogy of the rock units and precipitates present at spring exposures has led to a selection of mineral phases that produce unsatisfactory model results. Specifically, the abundances of K and Cl required the inclusion of unrealistic phases in the model. We are now in the process of analyzing aquifer rocks in varying states of alteration using X-ray diffraction and X-ray fluorescence to determine the alteration phases in order to improve the model results. Additionally, we are reanalyzing springs for silicon, aluminum, and iron, elements that are lacking from most of the compiled data. We are analyzing the geology of possible flow paths using ArcHydro and geologic cross sections in ArcGIS, and confirming these analyses with field observations. Finally, we are collecting new local precipitation data and building an evaporation component into the new models.