Hydrologic characteristics of the Koehn Lake area were investigated to determine the effects of external stresses on the system. Unconsolidated deposits are more than 900 feet thick in the central part of the basin. Cantil Valley fault, in the central part of the basin, acts as a barrier to the flow of ground water.
Average annual recharge to aquifers from percolation of surface water and from underflow, between 1958 and 1976, was 10,200 acre-feet. Average annual consumptive use, between 1960 and 1976, was 32,000 acre-feet, 21,800 acre-feet more than the recharge, implying that average annual withdrawal from storage is about 22,000 acre-feet. Storage depletion in 1976 was 50,000 acre-feet, considerably higher than the average. The cumulative storage depletion has caused a decline in water levels and a pumping depression about 5 miles southwest of Koehn Lake.
Ground water in storage in 1976 was about 4 million acre-feet, and ground water in storage above the 500-foot depth excluding the saline water under Koehn Lake was about 2 million acre-feet.
Water samples were collected from 24 wells for chemical analysis. Comparison of analyses made during this study with historical water-quality analyses indicated no significant change. A recent reversal in ground-water gradient southwest of Koehn Lake may allow the saline water below Koehn Lake to invade the fresh-water aquifer.
