AWWA MTC57592

In February 2000, the U.S. Court of Appeals affirmed a lower court ruling that the US Bureau of Reclamation must provide drainage service to the irrigators in the San Luis Unit which comprises an area of more than 700,000 acres. Reclamation recently completed a yearlong effort to evaluate various alternatives for the treatment and disposal of about 150,000 acre-ft/year of drainwater, that is 134 million gallons per day if spread out evenly throughout the year. The final preferred alternative consists of the following components: drainwater collection and reuse; biological treatment; desalination; and, disposal to evaporation ponds. The next phase of the project is a feasibility study which consists of field investigations and preparation of detailed designs and cost estimates. The feasibility study will be used to obtain congressional authorization and funding to implement drainage service and comply with the court order. Water quality analyses and projections of future water quality indicate there will be substantial challenges for desalting the drainwater. The total dissolved solids concentration is expected to range between 5,000 and 20,000 mg/L across the Valley over the 50-year planning period. Recovery will be limited by hardness which exceeds 2,000 mg/L as CaCO3 at some locations. The drainwater contains boron (>20 mg/L) which is not well rejected by reverse osmosis membranes below pH 10. The product water would be reused for irrigation of commercial agricultural crops, many of which are adversely impacted by boron concentrations as low as 5 mg/L. Valley soils contribute selenium to the drainwater which is well rejected by the membranes. The reverse osmosis concentrate, therefore, will be difficult to dispose of because it will contain elevated levels of selenium that already have proven toxic to wildlife within existing impoundments of concentrated drainwater. The field investigation for desalination involves pilot testing of reverse osmosis and nanofiltration membranes at two or more locations in the Valley. Reclamation is partnering with the California Department of Water Resources, local farmers, water districts, and membrane manufacturers for the pilot tests. A pilot system has been deployed and is currently evaluating pretreatment requirements. The next six months will provide information needed to address the challenges, determine the constraints for successful operation, and perform a detailed design and cost estimate for a full-scale desalination plant. This portion of the study will evaluate RO pretreatment requirements, performance, and chemical precipitation or absorption of selenium from the concentrate. Includes 3 references, tables, figures.