Ground-water geochemistry of the Albuquerque-Belen Basin, Central New Mexico

Anderholm, Scott

The Albuquerque-Belen Basin, located in central New Mexico, is dependent on ground and surface water for irrigation and municipal use. The geochemistry of ground water in the basin was studied as part of the Southwest Alluvial Basins Regional Aquifer-Systems Analysis. The purpose of this study was to define the areal distribution of different water qualities, use the distribution to help define the ground-water flow system, and identify processes resulting in differences in ground-water quality in the Albuquerque-Belen Basin. The Albuquerque-Belen Basin contains as much as 18,000 feet of basin-fill sediments of the Santa Fe Group, which form the principal aquifer in the basin. The majority of ground-water inflow to the principal aquifer occurs as infiltration of surface water through river channels, infiltration of surface inflow from adjacent areas, infiltration of excess irrigation water, groundwater inflow from adj acent bedrock units, and ground-water inflow from the upgradient Santo Domingo Basin. In general, ground water flows from the margins of the basin toward the basin center and then southward to the adj acent Socorro Basin. The maj ori ty of ground-water outflow is evapotranspiration, ground-water pumpage, and ground-water outflow to the Socorro Basin. The chemistry of inflow water to the aquifer has the largest effect on the distribution of different water qualities in the Albuquerque-Belen Basin. In the southeastern area of the basin, inflow is derived from Paleozoic and Mesozoic rocks that contain gypsum. Specific conductance ranges from about 1,000 to 1,200 microsiemens per centimeter at 25 °Celsius in this area of the basin, and calcium and sulfate generally are the dominant ions. On the eastern side of the basin, inflow is derived from Precambrian and Paleozoic rocks. Ground water in this area of the basin has a specific conductance usually less than 400 microsiemens per centimeter, and calcium and bicarbonate are the dominant ions. Along the southwestern margin of the basin, ground water enters the basin from adj acent Paleozoic rocks and from the infiltration of surface water from adjacent areas. These two waters mix in the aquifer. The inflow from adjacent bedrock units has a specific conductance generally greater than 20,000 microsiemens per centimeter. This water contains large concentrations of sodium and chloride. The mixing of this water and the infiltration of surface water from adjacent areas, which generally has a small specific conductance, resul t in ground water with a large range of specific conductance. Sodium and sulfate are the dominant ions in ground-water inflow from Cretaceous rocks along the western margin of the basin. In the northern area of the Albuquerque-Belen Basin, ground-water inflow from the Jemez geothermal reservoir mixes with local recharge water and ground water in the aquifer. Large concentrations of silica and chloride generally are indicators of ground water from the Jemez geothermal reservoir. In a large area west of Albuquerque, sodium is the dominant cation in ground water. In this area of the basin, the exchange of calcium and magnesium for sodium probably is a dominant process affecting ground-water quali ty. This is the same area of the basin that is underlain by relatively fine grained sediments as indicated by drillers' and geophysical logs. Ground water in the Rio Grande valley is affected by the infiltration of excess irrigation water. Excess irrigation water generally has a larger specific conductance than other ground water in the Rio Grande valley, so the mixing of these waters results in shallow ground water with a generally larger specific conductance than the deeper ground water.