HYDROSTRATIGRAPHIC AND CHEMICAL CHARACTERIZATION OF GROUNDWATER AND SURFACE WATER INTERACTIONS IN CACHE VALLEY, UTAH

Five east-west and two north-south hydrostratigraphic cross sections were drawn from driller's logs of wells in the southern half of Cache Valley, Utah. These cross-sections demonstrate that groundwater flow to streams is restricted by a continuous low-permeability layer, nearly 100 feet thick. This layer correlates with the lake-bottom deposits of the Bonneville (30,000-13,000 years ago) and Little Valley (140,000-90,000 years ago) cycles of Lake Bonneville. The most productive aquifers in the valley, collectively termed the principal aquifer, are in the southeast corner. Sands and gravels of the principal aquifer were deposited as alluvial fans and deltas draining the Bear River Range.

Groundwater chemistry in the principal aquifer is of the calcium-magnesium bicarbonate type, with total dissolved solids (TDS) averaging about 300±100 mg/L. TDS and the relative proportions of sodium, potassium and chloride increase from recharge areas in the east to discharge areas in the west. Oxygen-18 and deuterium analyses were performed on precipitation samples at three locations on the east valley benches, four surface water samples from streams entering the valley, and fourteen groundwater samples from either wells or springs in the principal aquifer. Precipitation and surface water values generally plot along the Global Meteoric Water Line (GMWL), although the precipitation values plot significantly lower on the GWML than the surface water values. Twelve of the groundwater samples clustered near the surface water values, suggesting that water from streams, rather than infiltrating precipitaion, recharges the principal aquifer. Twelve of the groundwater samples were analyzed for tritium. The tritium values of eight samples suggest recharge after 1952. The four well samples with tritium values dating prior to 1952 were analyzed for carbon-14. Two of these wells are completed in the principal aquifer and two west of it. Correcting for partial carbon dilution, the age difference between the different areas is on the order of tens of thousands of years.