An Integrated Isotopic/Physical Approach to a Numerical Model of Groundwater Flow in the San Juan Basin

Natural resource developments in the San Juan Basin of New Mexico have the potential to affect the availability of groundwater supplies. To quantify possible impacts on the basin’s Tertiary aquifers, a quasi-three-dimensional numerical model was developed. Due to the limited amount of present groundwater usage, the detailed hydrogeologic information often was not available. The modeling effort utilized a combination of groundwater dating by carbon-14, geostatistics, and geophysical methods to determine appropriate hydrogeological parameters for the groundwater flow model. After calibration, transient analysis of a potential well field site was performed. This simulation indicated that regional water level changes will result from the proposed pumpage.

Groundwater dating was employed to determine the hydraulic conductivity of basin aquifers. Several conceptual models were examined to analyze the effects of dispersion and diffusion on groundwater ages. For the particular aquifers studied, the results indicate that the magnitude of these processes is approximately equivalent to the uncertainty inherent in the measurement of carbon-14 activity. A probabilistic conceptual model was developed to explain the reduction in the effective solute-transport velocity as predicted by the derivation of a stochastic solute transport equation.

The geostatistical technique of kriging was used to estimate hydraulic head values from available data for use in the groundwater flow model. Electric logs were employed to obtain point observations of formation thicknesses. These values were analyzed by multiple regression techniques to obtain appropriate model inputs. Thermal logs were used to estimate the hydraulic conductivity of basin aquitards.