The northern California volcanic-rock aquifers are located in the Modoc Plateau and the Cascade Mountains in volcanic terranes (fig. 137) that extend into Oregon. Unlike most of the other principal aquifers in Segment 1, these aquifers are not distinct, readily identifiable aquifers because they contain water in fractures, volcanic pipes, tuff beds, rubble zones, and interbedded sand layers, primarily in basalts of Miocene age or younger. Areas in which permeable zones are sufficiently large and interconnected to provide a good source of water to wells are usually found only through exploratory drilling because surficial fracturing might not reflect fracturing in the subsurface.
Well depths commonly are 75 to 200 feet, but some wells are reported to be as deep as 2,700 feet. Yields from wells completed in basalt commonly range from 100 to 1,000 gallons per minute and can be as large as 4,000 gallons per minute. Because of the unpredictable distribution of the permeable zones, however, exploration is speculative, and, in many cases, several dry holes are drilled for every productive well. Drilling near fault zones is usually a good strategy because the stress and shear forces generated in these zones can cause exceptional fracturing of the rock, and thus, the probability of large ground-water yields is high. Unconsolidated lake-bed deposits that overlie the volcanic rocks in this region have minimal permeability, contain highly mineralized water, and are not productive aquifers.
Population in this part of California is sparse, and ground-water use is minimal; recharge to the aquifers exceeds withdrawals. The potential for development of the ground-water resource could be much greater, particularly if exploration could be optimized by locating areas with a high degree of subsurface fracturing.
