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Geochemical processes contributing to the contamination of soil and surface waters in the Rio Conchos basin, Mexico

¹ Geography, Geology and Planning, Missouri State University, Springfield, Missouri 65897, USA
² Department of Geosciences, University of Texas at El Paso, El Paso, Texas 79968, USA
³ Geography, Geology and Planning, Missouri State University, Springfield, Missouri 65897, USA

A geochemical characterization of the middle Rio Conchos basin is presented based on two contaminants, salts (Ca and Na) and toxic metalloids (As and Sb). Their content in surface water and sediment samples was determined, and their spatial distribution was mapped to show the relationship to each other, to the geology and hydrology, and to other potential factors affecting their distribution (i.e., prevailing winds). Correlation analyses between salts, toxic metalloids, and associated elements, and their spatial distribution aided in determining their sources, which included mines, rock outcrops, urban centers, irrigation waste water, and agricultural runoff. The salinity of the Rio Conchos reached a critical level after receiving waters of its contaminated tributary Rio Chuviscar and irrigation drain returns from the Irrigation District 005, but further downstream the water quality improved when it mixed with Carich water, significantly reducing its Na concentration. Based on its spatial distribution, the content of As in alluvial material was found to be associated with the presence of Ag-Pb mines and to a lesser degree to Oligocene ignimbrites. A correlation of As with Sb, Cu, and Bi suggests that natural sources are the dominant contribution of As within the area, although concentrations above permissible level for water were found in river water samples at a few places where sewage was also present, suggesting an additional (anthropogenic) important source of As. A characterization of natural sources affecting the chemistry of surface waters is a first step toward understanding the natural processes taking place and for documenting natural background levels that are needed to predict the response of the environment to various human activities.

Keywords: arsenic • Chihuahua • Chihuahuan Desert • Rio Conchos • salts • metalloid • water chemistry • water resource • water pollution