Shallow
Groundwater Conveyance of Geologically Derived
Contaminants to Urban Creeks in Southern California
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Abstract
In California alone, there are currently
over 200 instances on
the EPA’s list of impaired water bodies with unknown sources
of excessive salinity or trace contaminants. This investigation focuses
on Orange County, CA, a region that has undergone extensive hydrological
modification, relies heavily on imported water for municipal supply,
and has come under regulatory scrutiny for elevated TDS, sulfate,
Cd, Ni, and Se. A survey of shallow groundwater weeps and springs,
discharging directly to urban creeks, reveals high concentrations
of TDS, sulfate, Cd, Ni, Zn, Cu, and Se that are often far in excess
of water quality standards. Isotopic (δ<sup>34</sup>S and δ<sup>18</sup>O) and geochemical evidence indicate that the source of sulfate
and TDS is weathering of sulfide minerals in the Capistrano Formation
marine mudstone and dissolution of secondary minerals formed during
past periods of sulfide oxidation, rather than anthropogenic inputs.
The relative availability of carbonate minerals along the flow path
appears to control pH, which then influences trace metal mobility
to surface waters. Stable isotopes of H<sub>2</sub>O indicate that
despite widespread use of imported water, meteoric recharge dominates
shallow groundwater inputs with municipal sources contributing only
13–29% of discharge. These findings highlight the importance
of understanding the hydrogeological setting to properly apportion
contaminant sources and conveyances