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43 l\') ))WELLTON-MOHAWK, ARIZONA ~ ~ Floodplain areas of the Colorado River such as the Wellton- ~~ Mohawk, Palo Verde, and the Colorado Indian Reservation irrigated areas function differently than the upper basin salinity control units which are underlain by extensive shale deposits providing an unlimited supply of salt to be picked up by irrigation return flow, The Wellton-Mohawk irrigation and drainage District represents a hydro-geol-chemical system similar to parts of the central Valley of California, but on a small scale. Salinity and other toxic minerals threaten a flourishing agricultural industry in California, The problem is different, disposal of saline drainage effluent is needed to sustain irrigate agriculture in California. The objective of the Wellton-Mohawk salinity control project is to reduce salinity damages to downstream water users, particularly the people of the Republic of Mexico, The Wellton-Mohawk project in Arizona represents a typical aquifer along the Lower Colorado River flood plain. It operates as a closed basin with only token inflow and outflow in an area with less than four inches annual precipitation, Water is pumped into the area from the Colorado River and measured where it arrives in a lined canal, Water is measured and amounts entered into a computer for individual billing to each turnout by 34 ditch riders who are on 24 hour call, Farmers have been irrigating 65,000 acres of high value crops very efficiently with level basins for over 20 years, There is no surface runoff. Yields are excellent for over 30 crops. Deep percolating irrigation water mixes in an aquifer about 100 feet deep. About 109 wells pump the water into a lined drain ditch, which since 1978 has been dumped into the Gulf of California. Water quantity and quality has been intensively monitored into the area, onfarm, and out of the area since the project began in 1962. Prior to Colorado River water being delivered, farmers irrigated from wells, Water was being recirculated into the ground water aquifer and salinity concentration reached 6200 mg/L and irrigation was threatened, with the addition of 850 mg/L Colorado River water, 65,000 acres are irrigated, A high water table developed and 109 wells are used to pump drainage flow into a lined "drain ditch, Studies by USBR demonstrated and documented that the salinity of drain water from tile drains placed six feet below the ground surface would be the same as salinity of drain water from deep wells, Over 200 observation wells are monitored monthly for water chemistry and depth to water table, and changes in ground water storage is computed. The drain water was dumped back into the Colorado River raising the salinity concentration of water flowing to Mexico,