Multidisciplinary approach in a water salinity study of the southern San Pitch drainage, Sanpete County, Utah

Abstract:

Geologic mapping and geophysical techniques corroborate surface water surveys to identify regions in the subsurface that likely influence San Pitch River salinity in central Utah. Geologic mapping reveals that two members of the Arapien Shale are likely present in the subsurface beneath the areas where saline springs are found. Previous studies specified halite deposits in one member, and indicated the other member is known to contain halite in the general region. A total of 49 unique Transient Electromagnetic Method (TEM) stations were measured in the study area using a ground loop layout. Modeling of TEM data reveals one very shallow conductive body (1 to 10 ohm.m) between desilting basins and the San Pitch River that we interpret to be saline groundwater. A larger, deeper, and more continuous conductive body, observed in the northeast part of the study area, is interpreted as a geologic feature, most likely Arapien Shale. We measured or estimated discharge (flow) at 53 unique locations within streams and from seeps and springs, and estimated water quality (field parameters) at 172 different sites within the flow regime, measuring some sites multiple times during different seasons. Our results show that a 1.6 mile reach of the San Pitch River between the Highway 89 bridge and the confluence with Twelvemile Creek is a major source of salt loading; salt load increases from mostly less than 50 g/s above the bridge to nearly 300 g/s above the confluence. An addition of 80 to 90 g/s salt load from Twelvemile Creek, which carries salt from a 10-acre saline marsh, combines to bring the overall salt load carried by the San Pitch River at a point of irrigation use 3 miles downstream to between approximately 400 g/s in the spring to approximately 650 g/s in autumn. Our combined geologic, geophysical, and hydrologic assessment indicates that the source of salinity in the San Pitch River and Twelvemile Creek is dissolution of salt from the Arapien Shale and its erosional remnants by groundwater and seepage from irrigation works. The data collected for this study provide information necessary to make targeted management decisions to reduce salinity and provide for a sustainable supply of acceptable/suitable quality of irrigation water to users.