New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande increases in salinity as it flows from Colorado to Mexico. Previous research has focused on identifying and quantifying sources of the salinity, mainly through modeling of nonreactive solutes, which illustrated the major importance of deep geologic brine. The investigation of reactive solutes leads to further understanding of Rio Grande chemistry. Historically the Rio Grande contained higher solute concentrations, evident in comparisons between 1905-1907, 1934-1953, and 1980-2004 river datasets. All major solute concentrations from the three datasets increase from the headwaters to Ft. Quitman, Texas. In current data, magnesium, calcium and bicarbonate loads decrease below Elephant Butte Reservoir, which is likely caused by precipitation of carbonate minerals within the reservoir and possibly in irrigated fields below Elephant Butte. The calcium trend correlates well with the bicarbonate trend, indicating similar sources and sinks control these solutes. Irrigation return flow is one factor contributing to reactive solute behavior. Analysis of irrigation and ground water samples coupled with a water mass balance for a representative site (Lemitar, NM) in the middle Rio Grande suggests that mineral dissolution during irrigation releases solutes into the Rio Grande (200 kg/ha/yr of calcium, 1300 kg/ha/yr bicarbonate, 100 kg/ha/yr of magnesium, 80 kg/ha/yr of sodium and 240 kg/ha/yr of sulfate). A geochemical mass balance-modeling program (NETPATH) and soil analyses were used to identify subsurface soil reactions. The dissolution of calcite, dolomite and gypsum as well as cation exchange reactions account for the solute addition to the Rio Grande. The trends observed at the Lemitar site provide insight in understanding compositional differences between current and historic Rio Grande chemistry. The anion data illustrate chemical similarities between current and historic main channel river chemistry to groundwater and applied irrigation water from the Lemitar site respectively.