Geochemical reaction path and mass balance modeling techniques were used to test the hypothesis that an eastwardly flowing plume of thermal water in the southern moat of the Long Valley caldera system reacts with hydrothermally altered intra-caldera tuffs and mixes with non-thermal groundwater. Our conceptual model is based on hypotheses in the literature and published geochemical and petrologic data. Mixing of thermal and non-thermal waters and reaction with wall rock were simulated using the reaction path code EQ3/6. Mass balance calculations were conducted to estimate the extent of water-rock interaction between the intra-caldera tuffs and fluids. A mixing ratio of 82% thermal and 18% non-thermal water reacting with altered tuff minerals closely matches Casa Diablo fluid compositions and minerals observed in petrographic studies. Results of this study show that the mineralogy and fluid chemistry observed in the shallow reservoir at Long Valley caldera are formed in an open system. Further, calcite precipitated in the system serves as a sink for high levels of CO(2) generated by the deeper magmatic system. Our study serves as an example that processes acting in a geothermal system can be effectively quantified using geochemical modeling and mass balance calculations. (C) 2011 Elsevier Ltd. All rights reserved.