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Pah Tempe Springs Salinity Investigation, Phase II

Project Chief: Steven Gerner, USGS Salt Lake City, Utah
Cooperator: Colorado River Basin Salinity Control Forum
Period of Project: June 2007 to December 2010

PROBLEM

The Virgin River, in Timpoweap Canyon near Hurricane, Utah, contains a substantial salt load that has been discharged from nearby Pah Tempe Springs.

Pah Tempe Springs (or Dixie Hot Springs or La Verkin Springs) are located along the Virgin River where the river cuts through Timpoweap Canyon in Washington County, Utah. These springs discharge approximately 100,000 tons of dissolved solids (salt) annually to the Virgin River. The salts discharged from Pah Tempe Springs are transported downstream by the Virgin River and contribute to the salinity of the Colorado River. During 1972-81 and 1983-84 the Bureau of Reclamation (BOR) conducted extensive studies of the salt discharge from Pah Tempe Springs, the impact of these salts on the Virgin River, and the feasibility of removing these salts from the Virgin River system. The BOR studies recommended that further study and planning be suspended until technological advances or other factors warrant reconsideration.

Phase I of a current U.S. Geological Survey study of dissolved solids in the Virgin River determined the average annual salt-load distribution in the river downstream of Virgin, Utah for the period 1992-2006. The BOR, in their earlier studies, had determined from data collected prior to 1980 that removing most of the dissolved solids discharged from Pah Tempe Springs would result in an initial reduction of 52,000 tons/yr in the dissolved-solids load in the Virgin River at Littlefield, Arizona. The current USGS study used similar computation methods applied to data from 1992–2006 and found the estimated dissolved-solids load reduction to be 72,000 tons/yr. The largest factor contributing to this increased estimated salt-load reduction is an apparent change in the amount of water and salt being lost through riverbed seepage in the reach between St. George, Utah and the narrows just upstream of Littlefield, Arizona. Prior to 1980, losses of water and salts in this reach were assumed to average 60 cfs and 126,000 tons per year, respectively. Preliminary results from Phase 1 of the current USGS study show that on average about 30 cfs and 40,000 tons of salt per year are being lost in this reach. It has generally been assumed that the salt and water lost returns to the river in outflow from Littlefield Springs; however, there are several important but unanswered questions related to sources and travel times of the water discharged from the springs. Additional studies are needed to confirm the results of the Phase I investigation and to improve the conceptual model of dissolved-solids transport in the Virgin River Basin.

OBJECTIVES

The results of Phase I of the current USGS study of dissolved solids in the Virgin River have shown substantial changes in the transport of dissolved solids in the river since the earlier BOR studies were concluded. Given these changes and questions about the hydrology of the Virgin River basin that have persisted since the early BOR studies, the following tasks are being proposed for Phase II of the current USGS study:

1.) Determine the source of water for Littlefield Springs and the approximate age of water discharged from the springs through analysis of stable and radioisotopes and noble gases in water samples collected from the Virgin River, Littlefield Springs, and the ground water in the Littlefield area. Additionally, these data might be used to develop a mixing model that would better determine the contributions of the various components to the Littlefield Springs discharge.

2.) Determine the current discharge and dissolved-solids concentration in water from Pah Tempe Springs and the seasonal variation in discharge by conducting tracer-dilution streamflow measurements and collecting water samples to be analyzed for major constituents. The tracer-dilution streamflow measurement may also provide the general location and relative discharge of individual or groups of springs and seeps in the riverbed.

3.) Acquire additional data for calibrating Virgin River dissolved-solids load models, particularly in the lower Virgin River Basin, by periodic specific-conductance measurements and collection of water samples to be analyzed for major ions and dissolved-solids concentrations at all sites where a USGS streamflow gage is located.

4.) Determine if replacing water discharged to the Virgin River from Pah Tempe Springs with more dilute water from Quail Creek Reservoir will affect the dissolution of salts in soil and rock if that water is diverted downstream for irrigation. Analytical results from additional water samples collected from diversions and drains in the agricultural areas should be applied as input to a geochemical model such as PHREEQC (a USGS computer program for simulating chemical reactions and transport processes).

5.) Determine the amount and variation of seepage occurring in the Virgin River reach between Bloomington, Utah, and the USGS streamflow gage above the Narrows in the Virgin River Gorge by conducting seasonal seepage studies.

RELEVANCE AND BENEFITS

The over-arching goal of the Colorado River Basin Salinity Control Program (Program) and its participating Federal agencies – the Bureau of Reclamation, the Bureau of Land Management, and the Natural Resources Conservation Service - is the cost-effective reduction of salinity in the River. This study will provide the Program with information needed to determine if it should proceed with the development and consideration of possible remediation scenarios for dissolved solids discharging from Pah Tempe Springs.

APPROACH

The study objectives will be accomplished through additional data collection related to each task and subsequent analysis of those data. The study chief will collaborate with USGS scientists who are experts in geochemical monitoring, age dating of water, or tracer methods of discharge determination. This collaboration is included in study design, data analysis, and presentation of results.

PRODUCTS

The results of the Pah Tempe Phase I investigation have been presented to the BOR in an administrative report. The results of the Pah Tempe Springs Phase II investigation will be presented in a USGS Scientific Investigations Report.