THE EVOLUTION OF SALINITY IN THE RIO GRANDE AND ITS IMPLICATIONS FOR WATER SUSTAINABILITY

Rivers that flow through arid and semi-arid environments, such as the Rio Grande, are becoming saltier, resulting in the degradation of water quality used for agriculture and limiting its domestic uses. It is becoming increasingly important to understand how the sources and abundances of salts in these river systems are responding to changes in climate and increasing demands on water usage. Using the upper and middle Rio Grande as an example, we link climate change to salt chemistry and sources by comparing the > 80-year historical record of salt chemistry measured in the Rio Grande at El Paso, Texas, with river discharge data in the upper and middle reaches of the Rio Grande and the Pacific Decadal Oscillation (PDO) climate index. We use qualitative and statistical comparisons among these datasets, as well as comparisons with historical and new chemical data from other locations in and along the Rio Grande (including waste water treatment plants and groundwater) to examine changes in salt chemistry and sources over time. The results show that the PDO climate index, which largely tracks snowpack in the Rockies, and the composition of dissolved salts in the Rio Grande are statistically cross-correlated (p<0.05). Calcium-SO₄-rich waters in the Upper Rio Grande, derived largely from snow-melt and runoff, become more Na-Cl-rich downstream. During years of drought and less snowpack in the Rockies, the Rio Grande evolves to a more Na-Cl-rich composition. Covariations in river discharge and chemistry occur both annually and as part of larger decadal scales. There is a trend, starting about 25 years ago, where the relative influence of Na-Cl-rich water has continually increased in the Rio Grande system. This shift may reflect a tipping point in the relative influence of anthropogenic on the overall surface water budget of the Rio Grande. Climate change is likely accelerating this transition through the reduced precipitation in the headwaters region, resulting in a decoupling of surface flow in the upper and lower sections of the river. The salt chemistry of the Rio Grande is likely to be more Na-Cl-rich in the future, reflecting localized natural (groundwater infiltration) and anthropogenic (waste and industrial effluents, irrigation returns) influences.