Ground-water resources in the Lompoc area, Santa Barbara County, California

Abstract

Water supplies in the Lompoc plain are pumped from highly permeable gravel and sand, which make up the main water-bearing zone. The history of ground-water quality in the Lompoc area shows little change in the Lompoc upland and the Lompoc terrace, but an increase in the concentration of dissolved solids in the main water-bearing zone beneath the Lompoc plain. Most water pumped from this zone is used for irrigation and contains dissolved solids in concentrations ranging from less than 500 to more than 3,000 milligrams per litre. The average concentration is about 1,500 milligrams per litre. The total hardness is more than 1,000 milligrams per litre, and most of this water contains more than 750 milligrams per litre sulfate; the most abundant ions are calcium, sodium, sulfate, and bicarbonate. Changes in ground-water quality seem largely related to irrigation return, to inflow of native water of poor quality, to discharge of sewage effluent, and, in the shallow water-bearing zone, perhaps to the oxidation of sulfide minerals. Average annual recharge to ground water in the plain from the Santa Ynez River, local tributaries, underflow, and infiltration of rain is about 14,000 acre-ft (acre-feet) or 17 hm 3 (cubic hectometres). Discharge of ground water from the Lompoc plain is by pumping, evapotranspiration, streamflow, and underflow to the ocean. Pumpage during 1972 was about 23,000 acre-ft (28.4 hm 3 ), less than half of which probably returned to the main water-bearing zone. Ground water in storage is about 220,000 acre-ft (270 hm 3 ) in the Lompoc plain area, more than 100,000 acre-ft (120 hm 3 ) in the Lompoc terrace area, and more than 400,000 acre-ft (490 hm 3 ) in the Lompoc upland area. During the period 1941-72, the aggregate depletion of storage in the three areas was about 60,000 acre-ft (74 hm 3 ). Management alternatives to improve water quality in the rain water-bearing zone of the Lompoc area include increasing the quantity of water available for recharge by importing water and salvaging more runoff and treated sewage effluent, by coordinating future releases from Lake Cachuma, by providing additional storage space in the aquifer by selective pumping, by changing patterns of water use, by selective pumping and transport of highly mineralized ground water from the area, and by pumping ground water of favorable quality from older aquifers or adjacent basins. An adequate continuing water-quality monitoring program to evaluate changes would provide pertinent information for the prudent utilization of water resources.