Surface drainage waters in humid regions underlain by silicate rocks range in pH from about 2 units above to several units below neutrality. Alkaline waters were observed in regions underlain by pyroclastic rocks and nepheline syenite, whereas acid surface waters were measured where the underlying rocks were gneisses, granites, and certain other igneous rocks. The pH of the surface water is interpreted as being influenced by the hydrolytic (alteration) products of the rocks - the more alkaline water being associated with the silicate rocks most susceptible to hydrolysis.

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