Piper, Garrett, and others (1953) demonstrated
that certain trace elements could be used to determine
the source of high-chloride water to wells in the Long
Beach and Santa Ana areas of southern California.
Since that time, the application of trace elements to the
study of the source and movement of seawater and
other brines in aquifers has become widespread (Jones
and others, 1999). In California, the approach was
refined for use in coastal aquifer systems having
multiple sources of chloride to distinguish mixtures of
native (fresh) water and seawater from mixtures of
native water and high-chloride water from surrounding
and underlying partly consolidated deposits (Izbicki,
1991). In this paper, bromide, iodide, barium, and
boron were used to determine the source of highchloride
water to wells and to evaluate the geochemical
evolution of water as it flows through unconsolidated
deposits underlying the East Bay Plain. The use of
several trace elements, each of which has slightly
different chemistry, allows increased constraint on the
interpretation of the source of high-chloride water and
of ground-water movement to wells.
Changes in selected trace-element
concentrations were evaluated as ratios relative to
changes in chloride concentrations. Chloride is highly
soluble and not readily sorbed on mineral surfaces or
organic material. In addition, with the exception of
evaporite salts, chloride does not occur at high
concentrations in most rock-forming minerals or
aquifer materials (Feth, 1981; Davis and others, 1998).
Ratios are especially sensitive to mixing, and the
addition of a small volume of water having different
chloride and trace-element compositions may produce
a large change in the trace element-to-chloride ratio of
water from a well. In this report, ratios are presented on
a millimole per millimole basis, rather than on a mass
per mass basis; therefore, ratios calculated from
different trace elements, each having different atomic
masses, are comparable.