View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTdistance form the coast, allowing rapid intrusion of seawater into the aquifers ifgroundwater elevations at the coast fall below sea level.• Cross Contamination. The introduction of poor quality water into the fresh watersupply has occurred via existing wellbore that were improperly constructed, improperlydestroyed, or have been corroded by poor quality water in the Semi-perched zone(discussed above).• Salt-Laden Marine Clays. The dewatering of marine clays, interbedded within the sandand gravel rich aquifers, yields high concentrations of chloride-enriched water. Thisdewatering is the result of decreased pressure in the aquifers, caused by regionalpumping stresses and lowering of water levels.• Lateral Movement of Brines from Tertiary Formations. The lateral movement of salinewater from older geologic formations has resulted from regional pumping.Areas of degraded water quality in the UAS and LAS from seawater intrusion anddewatering of sediments as interpreted by the USGS are shown in Figure 2-22. Forillustration purposes, chloride data are shown for spring and fall 2002 conditions for theOxnard Aquifer within the UAS and within the 410- to 580-foot zone within the LAS.Similar areas of degraded water quality occur in other aquifer zones within the UAS andLAS. Areas depict the extent of degraded water quality with chloride concentrations inexcess of 500 mg/L. There are two distinct lobes of saline water in both the UAS and LAS,one in the Port Hueneme area and the other in the Pt. Mugu Area. Figure 2-23 provides timeseries charts of chloride concentrations for select wells for the data shown in Figure 2-12. Itis noted that the interpretation of this extent of degraded water quality is limited by thenumber of monitoring wells.Upper Aquifer System. The two distinct lobes of saline water in the UAS located nearPort Hueneme and Pt. Mugu are interpreted to occur from seawater intrusion, while thesouthern lobe near Pt. Mugu is interpreted to also consist of a larger area of saline waterderived from sediments. Large decreases in chloride concentrations have been observed incertain wells in the vicinity of Port Hueneme, coincident with the recent increase ingroundwater elevations in this area to above sea level, which has reversed the seawaterintrusion. Groundwater levels in this area have been relatively high since 1995, and artesianconditions have not been uncommon. Chloride concentrations remain well above acceptablelevels in the vicinity of Mugu Lagoon, coincident with the groundwater elevations thatremain at and slightly below sea level in this area.Lower Aquifer System. The lobe of saline water in the LAS located near Port Hueneme isinterpreted to occur from seawater intrusion, while the southern lobe near Pt. Mugu isinterpreted to consist of saline water derived from sediments. The chloride concentrationsobserved in the vicinity of Port Hueneme appear to be decreasing. However, the extent andtrends in chloride concentrations are uncertain in the area north of Pt. Mugu due to thefaulting and folding of the LAS in this area. Water quality continues to deterioratethroughout the LAS at and inland of Pt. Mugu, coincident with the groundwater elevationsthat continue to be over 50 feet below sea level in the southern Oxnard Plain and PleasantValley areas. The intrusion of saline water is expected to broaden and intensify due to thepersistent presence of these severely depressed groundwater elevations and is one of theprimary water quality concerns on the Oxnard Plain.W112003002SCO LW1458.DOC/ 033390002 25