The Grants Pass Batholith of Cretaceous age has intruded the
Triassic Applegate Group and metasedimentary rocks of the Jurassic
Galice Formation in the Murphy area of Josephine County, southwest
Oregon. The Applegate Group, mafic volcanic and sedimentary rock,
has been partially (nonequilibrium) metamorphosed under greenschist
facies conditions. The batholith, a hornblende-biotite granodiorite,
has produced a contact aureole (hornblende-hornfels facies)
in the country rocks.
Electrical conductance (EC), pH, temperature, and HC0₃ were
measured on 43 samples of well water and Applegate River water in
the field. Analyses for Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, Cl⁻, F⁻, S0₄⁻⁻, total Fe,
and SiO₂ (aq) were performed in the laboratory. Two water types
based on EC are apparent. A low EC type (< 500 μmho/cm @ 25°C) is
characteristic of HC0₃⁻ waters; high EC is associated with Cl⁻ waters.
Grus from the weathered pluton forms satisfactory aquifers with low
EC. Applegate Group rocks form a clay-rich aquifer containing both
high and low EC water. Wells located within the contact aureole in
the Galice Formation yield high EC waters.
Low EC waters are associated with areas of high recharge equiva
lent to the Local zone, the shallowest zone of Toth's (1963) model. Water quality enigmatically is characterized also by a molar Mg/Ca
ratio of unity. High EC waters are associated with the emergence
of deeper water along hydrogeologic boundaries.
Except for localized areas producing nonpotable water, aquifers
can generally be described as suitable for domestic purposes. The
ability of ground water resources to sustain development at recent
rates is, however, severely limited.