Coupling a Discrete State Compartment Model and a Water-Rock Reaction Model With Application to a Large-scale Hydrologic System in Southern Nevada.

In this study, a hydrology based Discrete-State Compartment program (DSC), was coupled with a geochemical program, PHREEQC. This coupled program was then applied to a southern Nevada groundwater flow system. The DSC program is a mixing-cell model used to represent a groundwater flow system as a network of interconnected cells, through which water and dissolved materials are transported. PHREEQC is a computer program for simulating chemical reactions and transport processes in natural waters. Coupling the two programs provided the capability to simulate groundwater flow while considering water-rock chemical reactions simultaneously. Important water-quality controlling geochemical reactions including calcite dissolution and precipitation, silicate dissolution, clay mineral precipitation, and cation exchange were incorporated. The DSC model was calibrated with δD. The results of two model simulations are presented: the first is a simple mixing model with no water-mineral reactions, and the second is a mixing model with water-mineral and aqueous complexation reactions. Results of the coupled modeling demonstrate that incorporating reactive geochemistry is effective and useful in modeling a flow system. The simple mixing simulation produced a poor representation of the observed water chemistry while the coupled flow-geochemical reaction simulation more closely matched observed water chemistry along most flowpaths. Areas in the flow system where the modeled water chemistry did not match observed chemistry highlighted the need for more hydrologic information, more chemistry data, or additional water-rock reactions for these areas. This project demonstrated the efficacy of modeling large-scale hydrologic regions with sparse data by applying a compartmentalized approach with geochemical information simultaneously.