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Groundwater sampled from wells completed in Eocene rocks is generally enriched with
sodium and bicarbonate. Groundwater from Towsley Formation wells has mixed anionic
and cationic species with no apparent chemical trend. Groundwater from San Gabriel
Formation wells is enriched with calcium and magnesium, with no dominant anion. Specific

hydrochemical parameters and their occurrence are discussed below.

Total Dissolved Solids: Relatively high concentrations of total dissolved solids (TDS) and
bicarbonate were detected in groundwater from Eocene rocks (Table 6; Plate 8). During
the course of groundwater sampling at the project property, TDS concentrations have ranged
from approximately 223 to 4,360 mg/l in all wells completed in various formations. The
greatest concentration of TDS was detected in a sample collected from monitoring well

MW-21 (4,360 mg/l) during the second quarter water quality monitoring event (Table 6).
This well is screened in the Towsley Formation and located near the Elsmere area (Newhall
Oil Field), in the northern portion of the project property. Groundwater from well MW-21
was also the source of the highest detected concentrations of sulfate, calcium, magnesium,
and potassium. The TDS concentration detected in MW-21 exceeded the United States

Environmental Protection Agency (USEPA) drinking water secondary recommended
standard of 500 mg/l for TDS by almost 8 times. In addition, TDS concentrations in
samples collected from wells MW-l, MW-2, MW-13, MW-17, C-9, C-10, C-ll, C-15, and
spring SP-4 also exceeded this recommended standard (Table 6). Concentrations of total
dissolved solids may be related to the formation in which wells are completed, and/or may
be impacted from nearby oil field brines.

pH: Values of pH have ranged from 3.3 in spring SP-2 to 8.3 in well MW-13
(Meredith/Boli & Associates, Inc., 1992b). In addition to low pH found in spring SP-2, high
sulfur content and odors were detected at this location where Towsley Formation exposures
contain active tar seeps. Low pH in spring SP-2 is probably related to natural oxidation of

reduced sulfur content in this location. This value was outside the range of 6.5 to 8.5,
recommended as the secondary drinking water standard by the USEPA. In contrast, an
elevated pH of 8.3 was detected in groundwater from the San Gabriel Formation. This
slightly alkaline condition is probably due to natural weathering of igneous/metamorphic
minerals in which hydronium ion (H30+ or H") is consumed and hydroxyl ion (OH') is
produced.

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