Water withdrawn from the Kenai Group and the strata in the Capps coal field locally contains objectionable concentrations of iron and hydrogen sulfide, both probably derived from the coal beds. Locally, methane gas that has been reported in water from some wells near Homer is also probably derived from the coal units. Carbonate rocks comprise known or potential aquifers in some parts of Alaska. The springs are thought to discharge from a network of solution cavities and conduits that have developed by partial dissolution of the carbonate rocks. Carbonate rocks on Admiralty Island in southeast Alaska locally yield large quantities of ground water, probably from a system of caves and solution conduits.
Metamorphic rocks yield water in substantial quantities only where they have been fractured.
Fractured slate and metagreywacke in the upland areas near Anchorage supply water to numerous domestic wells. Fractured slate and metamorphosed volcanic rocks on Kodiak Island generally yield less than 15 gallons per minute to wells, but locally yield as much as 100 gallons per minute. Little is known about the water-yielding potential of the widespread volcanic rocks in Alaska, but they are permeable at least locally, where hot springs issue from them. The permeability in these rocks may be a combination of fractures produced when the rocks cooled and weathered and vesicular layers that developed on the tops of individual basalt flows. Basaltic rocks in the Pribilof Islands yield sufficient water to supply small communities.