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. ~ 00 ~ the preplugging level of about 26,000 tons per year to about 7,000 tons per year. At the end of fiscal year 1985, monitoring of seeps and wells was terminated. Water levels in the observation wells had stabilized, and springs and seeps remained dry or filled with standing water indicating the well plugs remained intact. The estimated cost effectiveness of this reduction is $14 per ton. A planning report concluding the Meeker Dome Unit study was published July 1985. 15. Moapa Valley Unit (USDA) 8) The project covers a S,aDD-acre irrigated area on Muddy River upstream of Lake Mead. The project includes ir.stal1ation of 17 miles of underground piped delivery system, on-farm water management. and salinity control practices. By reducing overirrigation and excessive deep percolation, it is estimated average annual salt load reductions to the Colorado River system will be 19,200 tons. This unit has met the prerequisite for construction and is awaiting funding. SCS published its report on Moapa Valley in rebruary 1981. 16. Palo Verde Irrigation District (Reclamation and USDA) The Palo Verde Irrigation District (PVIDI is a privately developed district located in Riverside and Imperial Counties, California. Water for irrigation is diverted from the Colorado River at the Palo Verde Diversion Dam and is conveyed through 253 miles of main canals and laterals to serve approximately 90,000 acres of cultivated land. The irrigation return flows are collected in a 149-mile drainage system and returned to the Colorado River. PVID is downstream of Parker Dam, where the major Colorado River diversions are made to areas impacted by salinity. Control of PVID's return flow would not have the damage reduction impact of other units upstream of Parker Dam. A record of water and salt budget for PVID since 1951 shows that for most years the return flow carried about 10 percent more salt to the river than was dive~ted form the river. Because the drainage flow is so large (about 500 it Is), no alternative beneficial use for the water has been apparent. Consequently, investigations have focused on ways to minimize the increment of salt load that the drainage carries in excess of the salt load diverted with the irrigation water. The most recent land brought into production is in the southern end of Palo verde Valley, and drains there collect water with the highest salinity concentrations. This land has been under irrigation tor only 20 to 30 years, a relatively short time in comparison to the irrigation history of the valley which began about 1880. During the 1960's many drains were deepened 5 to 6 feet. This accounts tor a drop in the water table from an average of 5.5 feet in 1957 to 9.5 feet in 1983. The most recent investigation by Reclamation focused on the possible sources of the incremental increase of salt load, apparently coming primarily from the southern end of the valley. Several new observation wells were completed at various depth intervals. The different hydrostatic heads of the different intervals indicate no evidence of a rising saline ground water flow. Ground water table elevations indicate that saline water is not flowing horizontally into the valley. Electrical conductivity of extracts of VII-24