Salinity Problems in the Lower Colorado River Area

Document: State of California, Colorado River Board of California, "Salinity Problems in the Lower Colorado River Area," September, 1962, page 2424 COLORADO RIVER BOARD the wells tap only the bottom layer of the aquifer, all excess water applied in irrigation will have to percolate through almost the entire 100-foof depth in order to be picked up by the pumps and carried out of the valley. Thus in the eyes of some drainage experts, before the salinity of the Wellton-Mohawk drainage water could be decreased to normal levels by this operation, the entire aquifer would have to be flushed and the entire volume of water freshened. This would require many years, depending upon the volume and total salt load of the ground water in storage. Diversion to the Wellton-Mohawk Unit form the Colorado River for calendar year 1961 was almost 400,000 acre-feet. During the first 12 months of operation of the Wellton-Mohawk drain (March 1961through February 1962) about 175,000 acre-feet were returned to the river without much fluctuation in the quality of the drain water. Most of the individual smaples and monthly averages showed between 8 and 9 tons of total dissolved solids per acre-foot, or about 6,000 to 6,500 ppm. Data available to dare do not appear to indicate any downward trend in mineral content of the main drain dischargc. Chemical quality of the water from individual drainage wells in the Wellton-Mohawk covers a wide range. Some are as high as 17,000 ppm, although most are in the range of 4,800 to 7,200 ppm. (6.5 to 9.8 tons per acre-foot.) Outflow from the Wellton-Mohawk drainage channel during the five-month winter period of 1961-62 was only 19 percent of the total flow at the boundary but the drain contributed 54 percent of the total salt load. Outflow from the drainage channel averages approximately 300 cfs or more than 40 percent of the minimum winter delivery schedule of about 700 cfs at Morelos Dam. allowing for the pumpage at San Luis and the waste returns from United States projects in the limitrophe section below Morelos. In October and November of 1961, the actual flow at the northerly boundary above Morelos averaged about 950 cfs, so that during those months the Wellton-Mohawk drainage discharge constitured nearly a third of the flow of the river. The following table shows a comparison for each of the five months, October 1961 through February 1962, of the flow and salt load of the Wellton-Mohawk drainage channel with the flow and salt load in the river at the northerly boundary. Wellton-Mohawk Drain Nortberly boundary Month Flow (acre-feet) Salt (tons) Flow (acre-feet) Salt (tons) October 1961 18,630 158,700 59,270 199,700 November 1961 15,480 126,500 57,230 183,700 December 1961 10,690 85,900 81,720 230,400 January 1962 19,450 155,400 160,820 381,100 February 1962 16,260 134,500 67,540 226,300 In the preceding summer months the flow of the river had been of sufficient volume to dilute materially the highly saline discharges of the Wellton-Mohawk drainage channel. From March through September the discharge of the drain averaged only 7.5 percent of the total flow at the boundary, and the salt load about a third of the total. It was not until the fall and winter months, when the quantity of river water available for dilution was 60-75 percent less than in the summer, that the quality at the boundary deteriorated rapidly. The City of Yuma, as previously mentioned, reacted to the impact of the highly saline discharges more than a year carlier than did the Mexican irrigators.Epson Perfection 4870 Photo, 400 dpi, 8 bit, 2,563,615 byte