Evaluation of salinity and selenium loading to lower Ashley Creek, Uintah County, Utah

Project Number: 97169HH
Cooperating Agency: Bureau of Reclamation
Project Chief: David Naftz, USGS Salt Lake City, Utah
Project Period: 2002-2005

Problem: Ashley Creek is the principal source of water for Vernal, Utah, and adjacent areas in Ashley Valley. Most residents in Ashley Valley are serviced by a municipal sewer system that discharges wastewater to five adjoining lagoons located on hills just east of Ashley Valley. These lagoons are not completely sealed; consequently, some recharge to the underlying ground-water system occurs. The sewage lagoons are built directly on soils derived from the Mancos Shale and on the Mancos Shale itself, a formation containing as much as 2 percent soluble salts by weight as well as the trace element selenium.

The selenium load in Ashley Creek that can be directly attributed to seepage from the sewage lagoons may be a high as 1 kilogram per day and potentially adversely affecting bird egg hatchability and aquatic biota. For several reasons, the Ashley Valley Waste Lagoons are currently being replaced by a wastewater-treatment facility (WWTF). The sewage lagoons will no longer be used once the treatment facility is operational, and seepage from them will cease. To quantify the effects of sewage lagoon closure on Ashley Creek salinity and selenium loads, it will be necessary to implement a comprehensive investigative program.

Objectives: The objective of this study is to quantify change in salinity and selenium loading in the upper Green River Basin. Salinity loads in Ashley Creek are affected by many factors and vary both seasonally and annually. Closure of the sewage lagoons will cause the current hydrologic system to evolve to a new state of equilibrium. This change provides an opportunity to study processes that affect both salinity and selenium loading in an area representative of much of the Colorado Plateau.

Relevance and Benefits: The proposed investigation will provide information on two Federal priority issues identified in Strategic Directions for the Water Resources Division: drinking water quality, and the suitability of aquatic habitat for biota. Salinity is the most important water-quality issue in the Colorado River Basin. The results of this investigation will directly determine the impact of the Ashley Creek Salinity Control Project and will help determine the effectiveness of these programs throughout the Colorado River Basin.

Approach: The approach includes quantifying discharge to the study reach using continuous gages for surface-water inflow and conducting seepage studies of the reach to define diffuse ground-water inflow. Water quality is monitored for Total Dissolved Solids (TDS) and dissolved selenium concentrations. Consistent losses of salt and selenium loads were observed after the new treatment plant became operational. During the second seepage study, boron and strontium isotopes were collected to assist in the identification of sewage and non-sewage salinity sources along the study reach.

Products: A poster on preliminary study results was prepared and presented at the USGS Western Region Research meeting in March 2004. The study was completed in 2005 and a journal article describing the results of this study is currently in progress.

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