Identification and Modeling the Impact of Marine Shale Bedrock on Groundwater and Stream Salinity: Upper Colorado River Basin

Recent studies have shown that groundwater is a major contributor to stream salinity in the Upper Colorado River Basin. The primary salt sources are the marine shales and shale residuum that underlie the soils of much of the basin. A field site in the Price River Basin, a tributary to the Green and Colorado Rivers, was selected to study the physical and chemical factors that control the interactions between groundwater and these shales. Preliminary data were available at the site as a result of a Bureau of Reclamation study conducted by CH2M Hill. On the basis of the CH2M Hill study and the additional data collected during this study groundwater flow paths, salt transport and weathering processes were identified. Results show that the groundwater evolved from a calcium-bicarbonate water to a sodium-sulfate water with depth and distance along the flow paths. Geochemical equilibrium modeling and mass balance computations were performed using the USGS models PHREEQE and BALANCE. A preliminary saturated-unsaturated two-dimensional flow model (UNSAT) was implemented along the identified groundwater flow path. Once a satisfactory flow calibration was achieved, a solute transport model was then implemented to examine the relative importance of advective, dispersive and diffusive mixing processes along the flow profile. Preliminary management runs were made to study the effect of possible changes in land use practices. Results of these hypothetical cases suggest that water conservation methods (improved irrigation efficiency, canal lining and retiring irrigated land) will reduce return flow salt loads over the short run (about 50 years), when the transport of salts by displacement is most important. However, these salinity control alternatives are much less effective in the long range (> 50 years) because the diffuse salt loading from underlying marine shales is unaffected by groundwater flow rates in the alluvium. Although additional field data must be collected for verification the proposed model is a realistic first step towards a quantitative physically based approach to land use-salinity control issues.