Orange County Basin - Metropolitan Water District of Southern ...

Chapter IV – Groundwater Basin Reports
Orange County Basins
Groundwater Contaminants
Key constituents of concern for the Orange County Basin include: total dissolved solids, nitrate,
volatile organic compounds (VOCs), perchlorate, colored water, and NDMA. The nitrate and
VOC contaminants are located in the shallow aquifers and none of these contaminants have
significantly impacted the groundwater basin’s operation. In many portions of the groundwater
basin, shallow water is prevented from migrating into deeper aquifers due to aquitards
(impervious formations). Additionally, OCWD has aggressively initiated programs to address
contaminants of concern before they become larger issues. A summary of these constituents is
provided in Table 10-6.
TDS concentrations range from 221 mg/L in Seal Beach area to more than 1,000 mg/L in
portions of the Irvine subbasin (OCWD, 2006). Increasing concentrations in forebay areas of the
Main basin are shown in Figure 10-11, which have led to management approaches such as using
low TDS groundwater replenishments and desalters. Areas of high TDS (>1,000 mg/L) in the
Irvine subbasin areas are shown in Figure 10-12.
Nitrate concentrations generally range from 4 to 7 mg/L in Forebay and 1 to 4 mg/L in the
Pressure area with an average of about 2.2 mg/L (OCWD, 2004; OCWD, 2006). Areas with
concentrations above 10 mg/L are located in inland areas. Nitrate concentrations that exceed the
MCL occur only in a small number of areas in the Orange County Basin (OCWD, 2004). Nitrate
plumes that are generally located in the shallow aquifers in the basin are shown in Figure 10-12.
Localized shallow aquifer areas, as shown in Figure 10-12, are also affected by high
concentrations of volatile organic compounds (VOCs), most notably trichloroethylene (TCE) and
tetrachloroethylene (PCE). OCWD has active remediation projects to remove these VOC and
nitrate contaminants before they reach the Principal aquifer system.
Perchlorate has primarily been detected in the Forebay area and in only one well in the
Orange County Basin at concentrations at or below the current State notification level of 6 µg/L.
As discussed above, the presence of colored water is significant in the Orange County Basin.
However, colored groundwater is limited to the Lower aquifer system primarily near the coast
and ranges from 25 color units to 230 color units. The area of colored groundwater is shown in
Figure 10-13. Most production wells along the coast pump from the Principal aquifer, which is
above the colored water.
Blending Needs
The local retail producers serve a blend of groundwater and MWD imported water supplies. The
blend percentage primarily depends upon the BPP. The average TDS of blended water in agency
systems ranges from 315 mg/L to 560 mg/L with an average of 450 mg/L. Nitrate
concentrations of blended water range from 0.1 to 3.5 mg/L with an average of 1.4 mg/L
(OCWD, 2006).
FINAL IV-10-18 September 2007