New Mexico Geological Society Annual Spring Meeting — Abstracts

A quasi-three-dimensional numerical model of groundwater flow in the Tertiary aquifers of the central San Juan Basin was developed. The modeled aquifer system included the Pictured Cliffs Sandstone, Kirtland-Fruitland Shale, Ojo Alamo Sandstone, Nacimiento Formation, and the San Jose Formation. In the majority of the study area, the hydrogeologic data necessary for groundwater modeling was sparse and would have been unrealistically difficult to obtain by traditional methods. However, the potential for future groundwater development makes a quantitative groundwater flow model desirable. To obtain the required data, a combination of carbon-14 groundwater ages, electrical resistivity and heat flow logs, and water levels in wens and springs was employed. Aquifer hydraulic conductivities were determined from the groundwater ages and hydrogeological characteristics. Preliminary estimates of aquitard hydraolic conductivities were made from heat flow data. Formation thicknesses were obtained from electrical resistivity logs. Transmissivities calculated from these data compared favorably with values determined by traditional aquifer pumping tests. The input data required at nodal points in the finite difference groundwater flow model was obtained by kriging and trend surface analysis of the observed hydrogeological parameter distributions. An additional benefit resulting from the use of kriging was the development of a quantitative index of confidence to be placed in model results. The flow model was calibrated by adjusting the hydraulic conductivity of the aquitard layers until agreement with observed water levels in the Ojo alamo Sandstone, Nacimiento Formation, and San Juan River were obtained. After conversion of constant head to constant flux boundary conditions, a transient simulation of a proposed well field pumping 24,000 acre-feet per year was performed. The results indicated that significant changes in regional water levels would occur, and that flow in the San Juan River would be slightly reduced in the area of Navajo Reservoir within five years of the inception of the groundwater pumping. The non-traditional approach empIoyed in this study provided a means to make a preliminary assessment of the effect of potential, major groundwater development in the San Juan Basin which would not have been possible to accomplish by customary methods.