THE HYDROGEOLOGY OF THE BOTHWELL POCKET AREA, NORTHERN UTAH, WITH EMPHASIS ON POTENTIAL ENCROACHMENT OF POOR-QUALITY GROUND WATER FROM GREAT SALT LAKE

The Bothwell Pocket is an agricultural basin in northern Utah experiencing increased residential development. The principal source of water for the area is ground water from the basin-fill aquifer. The basin-fill material is composed of Quaternary to Tertiary, unconsolidated to semi-consolidated fluvial and lacustrine gravel, sand, silt, and clay deposits; these deposits have a maximum thickness exceeding 765 feet. The area has a history of poor ground-water quality, and water managers are concerned that large ground-water withdrawals have the led to encroachment of poorer quality ground water from Great Salt Lake northward into the Bothwell Pocket.

To determine if any decline in the potentiometric surface exists that would promote salt-water intrusion from Great Salt Lake, we measured water levels in 24 wells before and after the start of irrigation and constructed potentiometric-surface maps for each season. To determine if water quality in wells in the Bothwell Pocket area has been affected by subsurface encroachment of saline waters from Great Salt Lake, we analyzed concentrations of chloride and bromide in 30 water wells and springs, and compared these data to concentrations of the same species present in waters near Great Salt Lake.

During the irrigation season, pumping from wells in the center of the Bothwell Pocket creates a cone of depression extending across the basin, with a maximum measured drawdown of 40 feet; water levels in these wells recover when the irrigation season ends. Water levels in wells near irrigation canals south of this cone of depression increase up to 6 feet during the irrigation season due to canal seepage, and water levels in wells in the southern part of the study area have little change between seasons. No significant long-term change in non-irrigation water levels has occurred since water levels were measured during a 1971 study. Bromide and chloride concentrations, respectively, range from 0.09 to 5.61 mg/L and from 143 to 4100 mg/L. If an influence from saline water near Great Salt Lake existed, the data trend would show the weight ratio of bromide to chloride decreasing, coupled with increasing chloride concentration for wells nearer the more saline waters. Our data show no prevalent trend, and thus do not indicate encroachment of saline water.