Barstow is 96 mi northeast of Los Angeles in the Mojave Desert region of southern California, adjacent to the normally dry Mojave River. Groundwater in alluvial fill is the only reliable source of water for the main water purveyors (the city of Barstow and the U.S. Marine Corps Supply Center). The alluvial aquifer near Barstow has been subjected to contamination from percolation of industrial and municipal sewage for nearly 60 years. The contamination has forced the abandonment of several domestic wells because of taste, odor, and foaming, and it threatens the well field serving the U.S. Marine Corps Supply Center. An intensive investigation was made to determine (1) the nature of groundwater degradation; (2) the areal and vertical extent of the degradation; (3) the rate nd direction of movement of the degraded water; and (4) the effects of several proposed management practices designed to alleviate the problem.

A series of 53 observation wells was installed within the 10-sq-mi study area to supplement data from existing domestic and irrigation wells. Groundwater samples were analyzed for the usual chemical constituents plus arsenic, hexavalent and total chromium, dissolved organic carbon, detergents, ammonia, phosphates, and oil and grease. Concentration gradient for dissolved solids and several individual constituents were defined in 3 dimensions. The dispersive characteristics were investigated by use of a 2-well tracer-dilution test.

The chemical stratification found within the aquifer indicates that an old plume of degraded water (produced by percolation from sewage facilities near Barstow) occupies the lower part of the alluvial aquifer. Since 1910 this plume has moved down gradient about 4 mi. A more recent overlying plume of degraded water occurs near the downstream edge of the deeper plume. This recent plume is produced by effluent from sewage-treatment facilities installed in 1968. Detergent concentrations beneath this site reflect the current use of LAS-type detergents in contrast to the ABS types that are found in the deeper zones of degradation.

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Distribution of nitrogen and chloride has indicated that the gradual increase in dissolved-solids concentrations in the Marine Corps wells is caused by the use of treated sewage effluent on a local golf course. Areal and vertical mapping of the degraded water indicates that the water supply at the Marine base will be affected if no measures are taken to avoid contamination.

A digital water-quality model of the aquifer was built to aid in evaluating the effects of several alternative ground-water management practices. The model first was verified by comparison with existing hydrologic data and then used to calculate the water-level and water-quality conditions that might occur in the aquifer within the next 20 years. Model results indicated that by 1991 present sewage-percolation practices would result in dissolved-solids concentrations exceeding 900 mg/l in the Marine Corps well field.

Two of the alternative water-quality management practices that were evaluated with the model are:

(1) The oxidation ponds could be lined to prevent further percolation, and the effluent piped out of the study area. The model indicated that this would cause water-level declines in excess of 70% of the saturated thickness of the aquifer in the vicinity of the Marine Corps well field. This is probably unacceptable.

(2) The percolation of treated sewage effluent could be allowed to continue, and a line of barrier wells could be drilled between the sewage-treatment facility and the Marine Corps well field. The barrier wells would be pumped at a lesser rate than the rate of recharge from the oxidation ponds, to lessen the undesirable effects. The model indicated that by 1987 a constant rate of pumping from the barrier wells would effectively limit the movement of the degraded plume toward the well field. However, by 1991 the increasing quantities of percolation from the oxidation ponds would greatly exceed the barrier pumping rate and the highly concentrated parts of the degraded plume would approach the edge of the Marine Corps well field. Judicious choice of barrier pumping rates would probably e tend the effectiveness of the barrier beyond 1987.

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