AWWA WQTC60779

The Semitropic Water Storage District, located in the San Joaquin Valley near Wasco,California, has an existing groundwater-banking program, which under the first phase has beencontracted to supply 1,000,000 acre-feet of water to six partners in California. The Districtbanks water for its partners from the State Water Project (SWP)'s California Aqueduct duringwet years and returns water during dry years to the Aqueduct either by exchanging its waterentitlement or by pumping up to 300 cfs (194 mgd) of groundwater to the Aqueduct.Due to increasing water demands in California, the District is in the process ofdeveloping the second phase of the program, which is an additional 650,000 acre-feetgroundwater storage unit, called the Stored Water Recovery Unit (SWRU). Recharge to andrecovery from the new unit will be through a new wellfield with a delivery capacity of 150,000acre-feet per year in ten months, equal to a continuous discharge of 250 cfs (161 mgd), to theAqueduct. The proposed well field will ultimately consist of 65 wells in a roughly six squaremile area in the northwestern part of the District.A challenge with the new SWRU is that the arsenic concentration in the groundwater isapproximately 60 µg/L, much higher than the impending 10 µg/L maximum contaminant leveland the 2 µg/L As level in the Aqueduct water. The State of California to date has notestablished a policy or criteria regarding the mixing of raw water before it is treated for humanconsumption. The analysis described in this paper addresses a worst-case scenario in which thebanked groundwater is treated before it is co-mingled with the Aqueduct water and conveyed tofinal treatment. Therefore, the stored water may require treatment before it is discharged to theAqueduct, which will require implementing a treatment system to reduce arsenic concentrations in the water. Pending negotiations with the SWP, the maximum arsenic concentration allowablein the water discharged to the Aqueduct could be as low as 2 to 5 µg/L.A second challenge relates to operation of the treatment system. Since the water to betreated will be drawn from the ground only during periods of dry weather when it is needed tosupplement low summer water supply, the treatment facility is expected to operate for perhaps aslong as ten continuous months followed by two to five years of inactivity. The short use andprolonged idle periods represent some very serious operational issues for the planned treatmentsystem(s). The long downtimes can exacerbate corrosion on storage and pumping equipment,filter media may accrete, friable sorbant media will crumble, chemicals could degrade, andmicrobes will flourish.Wellfield development will be phased in as more subscribers contract with the Districtfor groundwater banking, but the District has not decided whether the treatment process willconsist of individual wellhead treatment systems, clustered wells discharging to one of severaltreatment plants, or one large plant that treats the water from all 65 wells. Operational andmaintenance (O&M) processes will have to be determined for all three of these options.Finally, the wellfield and treatment plant site is on either native undeveloped pasture landor active agricultural land. No sanitary sewers have been installed in the area, and the Districtdoes not to construct sanitary sewer service as part of the wellfield and arsenic treatmentsystem(s) construction project. Any treatment process would need to generate either no liquidresidual or have a residual stream that can be readily handled on-site. Nineteen technologies were evaluated using existing project and manufacturer literature.The technologies evaluated consisted of various ion exchange (IX) processes, adsorption ontodifferent media in a fixed or fluidized bed configuration, different co-precipitation processes, andphysical rejection using nanofiltration and/or reverse osmosis. A brief description o

Product Details

Edition:

Vol. - No.

Published:

11/15/2004

Number of Pages:

9

File Size:

1 file , 310 KB