Purpose and Scope: The purpose of this report is to describe the study methods and results of a characterization study of concentrations and loads of selenium in three tributaries to the Colorado River in the Grand Valley. The study area consists of three subbasins—Persigo Wash, Adobe Creek, and Lewis Wash—each representing transitional agricultural to urban, agricultural, and urban land-use types, respectively. These subbasins were selected because they represent different land-use types and because the tributaries that drain each subbasin contribute moderate to high concentrations and loads of selenium to the Colorado River.
Six sampling trips were conducted from December 2004 through March 2006 to collect information needed to characterize selenium sources within each tributary subbasin. Of the six trips, two were conducted on each tributary, one in early winter and one in early spring. Water samples were collected for analysis of dissolved selenium, total nitrogen, and total dissolved solids (hereinafter referred to as “salinity”). Concentrations of total nitrogen and salinity can covary with selenium concentration and load (Butler and others, 1996) and thus were deemed important constituents to analyze for this study.
Streamflow was measured by using the tracer-dilution and standard current-meter methods. Where practical to use, the tracer-dilution method (Zellweger and others, 1989; Kimball, 1996) was considered to be the most accurate way to measure streamflow given stream channel conditions (silts and clays) and the small streamflow rates typical of each tributary. The sample data were analyzed and high loading areas in each tributary subbasin are discussed and reported. An historical account of land-use change and salinity control in the study area also is provided.