New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande flows from Southern Colorado through New Mexico and West Texas down to the Gulf of Mexico. It serves as an important water supply for agricutural and municipal needs. In Rio Grande waters, total dissolved solids (TDS) increase from ~40 mg/L at the headwaters to 500-1500 mg/L at El Paso, Texas. The elevated TDS values in downstream water cause various problems such as reduction in crop productivity and deterioration of soil quality due to salt loading. A number of natural and anthropogenic factors may lead to increased salinity, so the exact sources and their relative contributions to the salt load remain unclear. U isotopes (e.g., ²³⁴U and ²³⁸U) fractionate naturally when released from rocks to waters during chemical weathering processes at Earth’s surface. It has been suggested that the degree of U isotope fractionation depends largely on local lithology and climate conditions, which affect chemical weathering and U release rates. U isotopes in natural waters thus have great potential to serve as natural tracers for chemical weathering processes, which in turn can help to determine the origins of dissolved solids (i.e., salts) and their history.

Here, we measured the U concentrations and isotope ratios for water samples collected along a ~ 1000 km stretch of Rio Grande (from the headwaters in Colorado to El Paso, Texas), as well as from streams and springs in the Jemez Mountains, a small drainage basin that recharges to Rio Grande in northern New Mexico. The comparison of these two case studies reveals different evolution histories for U in surface waters. In the Jemez Mountain region where human impacts are minimal, U isotope ratios in surface waters are largely controlled by rhyolite weathering, and both U concentrations (0.01-0.19 ppb) and (²³⁴U/²³⁸U) activity ratios (1.5-3.0) vary systematically with elevation (2600-2900 m). Here, solutes in streams largely represent mixing of two sources: young surface water (e.g., several months old) with low U concentrations and high (²³⁴U/²³⁸U) ratios and relatively old shallow groundwater that has higher U concentrations and lower (²³⁴U/²³⁸U) ratios. Similar ranges of U concentrations and (²³⁴U/²³⁸U) ratios are observed for the headwater regions of the Rio Grande. U concentrations in the Rio Grande increase significantly downstream (0.12 to 5.97 ppb) and correlate well with Ca²⁺, Mg²⁺, and HCO₃^- ions, revealing a control of carbonate dissolution/precipitation on river water chemistry. This is probaly due to a change of lithology. In addition, both (²³⁴U/²³⁸U) ratios (1.6-2.1) and U concentrations in the Rio Grande waters show strong seasonal patterns, reflecting the human impacts on river chemistry, such as the regulation of river flows by reservoirs and dams, agricultural irrigation return flows, and pumping of cold/geothermal aquifer waters.