View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTWater Quality in the Forebay from NitratesThe main water quality concern in the Forebay is the presence of nitrate at varyinglocations and times, in concentrations that exceed drinking water standards. TDS and nitrateconcentrations in groundwater in the Forebay are shown in Figure 2-24 for spring and fall2001. This may, in part, be due to nutrients and other dissolved constituents in agriculturalirrigation return flows and from septic tanks in unsewered areas.Monitoring by UWCD indicates that nitrate levels fluctuate widely over time. Nitrate levelsin groundwater remain relatively low during relatively wet periods when ample supplies oflow-nitrate Santa Clara River water are available to recharge the Forebay. However, nitratelevels in groundwater rise when there is insufficient river water for recharge. Thedistribution of nitrate, both laterally and with depth, is being assessed by UWCD. The TDSconcentrations of groundwater in the Forebay are similar to TDS concentrations ingroundwater across the Oxnard Plain and Pleasant Valley areas, which vary within therange of approximately 500 to 1,500 mg/L.Water Quality from Gasoline Storage and Dispensing FacilitiesFormer and current gasoline storage and dispensing facilities have the potential to degradewater quality, particularly from the potential release of fuel hydrocarbons containingmethyl tertiary-butyl ether (MTBE). One particular area of MTBE has recently been detectedat the Oxnard Forebay near the UWCD El Rio spreading grounds and wellfield. MTBE wasdetected in groundwater of the UAS in 2000 at a gasoline station across Vineyard Avenuefrom the El Rio wellfield. UWCD modified operations on the Vineyard Avenue (west) sideof the wellfield to prevent MTBE from being pulled towards the wellfield. Thesemodifications included preferential spreading of diverted river water in the ponds closestto Vineyard Avenue (to maintain a positive gradient away from the wellfield) anddecreased pumping in wells closest to the MTBE site. Thus, the overall capacity of thewellfield has been temporarily reduced. Treatment to remediate the MTBE began in 2002,and MTBE subsequently has decreased in concentration. There have been no detectionsduring the past two quarters. There will be continuing monitoring until the site is closed.UWCD is planning to drill its own monitoring wells between the MTBE site and the El Riowellfield as a long-term sentinel to protect the wellfield. It is not known when fulloperations of the west side of the El Rio wellfield can resume.2.6.4 Land SubsidenceHistorical groundwater elevation declines from overdraft have induced land subsidencethat was first measured in 1939 and has resulted in as much as 2.7 feet of land subsidence inthe southern part of the Oxnard Plain. Historical subsidence at benchmarks used to monitorland surface elevations is shown in Figure 2-25, taken from the USGS groundwatermodeling study. The USGS model simulated a total of 3 feet of land subsidence in thesouthern part of the Oxnard Plain. Model simulations indicate that most of the landsubsidence occurred after the drought of the late 1920s and during the agriculturalexpansion of the 1950s and 1960s. The results also indicate that subsidence occurredprimarily in the UAS prior to 1959; but, in the LAS between 1959 to 1993, there was anincrease in pumpage from the LAS during this time.W112003002SCO LW1458.DOC/ 033390002 26