View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTReducing the Potential for Coastal Landward Flow in the LASAs previously described, the following indicates a moderate potential for landward flow inthe LAS, particularly during drier years and in the fall when water levels are seasonally low.• Simulated groundwater elevations at the end of the model calibration period in the thirdquarter 2000 (Figure 6-7) indicate that groundwater elevations were at about sea level inthe LAS along the northern Oxnard Plain coastline, remained at about sea levellandward across most of the northern Oxnard Plain ,and then increased to above sealevel in the Forebay area (Figure 6-7).• Simulated Base Case groundwater elevations for the third quarter 2020 (Figure 6-9)indicate that the groundwater elevations were about 10 feet below sea level in the LASalong the northern Oxnard Plain coastline, lower than they were for 2000 (at about sealevel). The water levels decreased to below sea level landward across most of thenorthern Oxnard Plain, lower than they were for 2000 (at about sea level inland andincreasing in the Forebay area).• The northern Oxnard Plain area hydrographs (Figure 6-16, page 2/3) indicate that theLAS groundwater levels are below sea level at the City Water Yard (no well) for mostyears of the 31-year simulation period and are occasionally below sea level at BlendingStation No. 3 (no well) for a few years during the 31-year simulation period.The two LAS wells at the City Water Yard (Nos. 20 and 21) would be used for groundwaterrecovery for each of the Phase 2 scenarios (2a, 2b, 2c, and 2c2) at a rate of 500 AFY for eachwell (total of 1,000 AFY from LAS). No other LAS wells would be used for recovery. Thewater level declines would be up to between 10 feet and 15 feet for each of the Phase 2scenarios, which would incrementally increase the moderate potential for landward flowthat exists at the coastline, particularly during drier years and in the fall when water levelsare seasonally low. In general, groundwater should be recovered form the UAS whenpossible to minimize this potential.Reducing the Interference with Operations at the Forebay Spreading GroundsExcessive groundwater elevation declines at the UWCD El Rio wellfield from recoveringGREAT Program recharged water at this location has the potential to interfere withpumping operations and other UWCD water delivery obligations by:• Exceeding the capacity of the UWCD extraction well network at the El Rio wellfield• Increasing pumping costs by making the extraction wells less efficient• Exacerbating water quality conditions (e.g., increasing nitrate concentrations andinducing MTBE migration toward the wellfield)The range of extractions to be potentially expected from recovering GREAT Programrecharge water at the UWCD El Rio wellfield was bracketed by Scenarios 2a (low of3,857 AFY) and 2b (high of 15,429 AFY). These scenarios resulted in moderate to large waterlevel declines in the UAS of up 20 feet for Scenario 2a and up to 30 feet for Scenario 2b in thearea of the El Rio wellfield. Because greater water level declines would have a higherpotential to interfere with pumping operations and other UWCD water delivery obligations,in general, groundwater recovery from the El Rio wellfield should limited where practicable(i.e., shift pumping to City Water Yard wells and/or City Blending Station No. 3 wells).W112003002SCO LW1458.DOC/ 033390002 100