Simulation of ground-water flow and the movement of saline water in the Hueco Bolson aquifer, El Paso, Texas, and adjacent areas

Abstract

The Hueco bolson aquifer is being pumped at increasing rates to supply water for El Paso, Texas, and Ciudad Juarez, Mexico. Water-use projections for 1984-2000 indicate that the upward trend in pumping rates probably will continue, which will put an increasing burden on the limited freshwater resources of the aquifer. Near El Paso, saline water in the Rio Grande alluvium overlies freshwater in bolson deposits. Withdrawal of ground water has created a large cone of depression in the water table that is centered approximately under the El Paso-Ciudad Juarez urban area. The maximum depth of this cone in January 1984 was about 140 feet below the pre-development (before 1903) water table. The principal concern of the water-management agencies involved with the water supply for El Paso and Ciudad Juarez, and of the U.S. Department of Defense at Fort Bliss to the north of these cities is that the saline water underlying and surrounding the freshwater in the bolson deposits will begin to move into the intensively pumped freshwater zone. Intrusion of large volumes of saline water would contaminate the freshwater and make it unusable for municipal and industrial supply. Changes in ground-water quality, indicated by an increase in dissolved-solids concentrations in well water, have occurred near the El Paso airport and near the Rio Grande. This study focused primarily on simulation of historical and present (1984) ground-water flow and salinity of water in the Hueco bolson aquifer, to help understand and project the movement of saline water in the aquifer to January 2000. The objective of this study was to understand the movement of water resulting from historical and projected pumping, using numerical simulation of the transport of saline water. Results of the numerical simulations indicate that historical withdrawals of ground water have caused only slight movement of saline water, except near the Rio Grande. The saline water probably has moved laterally from the river alluvium into the bolson deposits and towards the wells located north and south of the river. Some downward movement of saline water has occurred; this movement was caused by withdrawals from wells screened only in the freshwater zone beneath the river alluvium. Simulation results further indicate that upconing of saline water into well screens probably is not, and will not become, a substantial problem. This conclusion is based on the assumption that the aquifer is anisotropic, specifically that horizontal intrinsic permeability is much greater than the vertical permeability. Results of the projected withdrawal simulations from 1984-2000 indicate that the general historical trend of saline-water movement probably will continue. The saline water in the Rio Grande alluvium is the major source of saline-water intrusion into the freshwater zone throughout the historical period and into the future on the basis of simulation results. Some saline water probably will continue to move downward from the Rio Grande alluvium to the freshwater below. Injection of treated sewage effluent into some wells will create a small zone of freshwater containing slightly increased amounts of dissolved solids in the northern area of the Texas part of the Hueco bolson aquifer. Many factors, such as well interference, pumping schedules, and other factors not specifically represented in the regional simulation, can substantially affect dissolved-solids concentrations at individual wells.