DETERMINING SALINITY SOURCES VIA HYDROLOGIC MEASUREMENTS, GEOLOGIC MAPPING, AND TEM ANALYSIS IN THE LOWER SAN PITCH RIVER DRAINAGE, SANPETE COUNTY, UTAH

The lower San Pitch River drainage system in Sanpete County, Utah, has highly variable water quality along different reaches fed by springs, seeps, and Twelvemile Creek. We determined the sources and extent of salinity in the lower river drainage and adjoining Gunnison Irrigation Company (GIC) canal system by assessing hydrogeologic conditions through surface water analysis, geologic mapping, and geophysical surveys.

Salinity, likely sourced from bedrock, is known to affect water quality in the region. We spent two seasons measuring water quality and quantity along different reaches of the San Pitch River and tributary Twelvemile Creek and nearby canals and springs. We estimated salt loading based on water-quality and stream-discharge measurements and identified sources of salinity by mapping gypsum/halite-bearing Jurassic Arapien Shale and applying Transient Electromagnetic Method (TEM) inversion models of the conductive subsurface. Water from 209 sites was measured for field parameters; 48 sites during fall 2014, 87 sites during spring 2015, and 74 sites were duplicated. Conductivity for all sites ranged from 359 to 77,500 μS/cm, the lowest from Twelvemile Creek and highest from a seep at the San Pitch River. About 67% of samples had TDS concentrations >1000 mg/L and ~20% had TDS >3000 mg/L (measured or estimated). We collected 83 flow measurements at 53 sites (some multiple times). Flow ranged from ~0.001 cfs at several seeps to 53.5 cfs for the river. Most salt dissolved in the lower San Pitch River and Twelvemile Creek is from interaction between shallow groundwater, canal seepage, or stream flow and the Arapien Shale (in shallow subsurface or stream bed).

We recommend transferring higher-salinity water from the San Pitch River below the confluence with Twelvemile Creek to the Highland Canal, which has adequate flow to dilute saline river water to usable quality. Below the confluence with Twelvemile Creek, the San Pitch River is a gaining stream and acquires significant flow from better quality seeps and springs (~2 to 2.5 cfs) before it reaches a lower point-of-use. Transferring water away from this reach would improve the water quality in the lower San Pitch River by reducing salinity and provide acceptable quality irrigation water for GIC and its water users.