Water quality and supply on Cortina Rancheria, Colusa County, California

Abstract

Cortina Rancheria covers an area of 1 square mile in Colusa County, California, near the western edge of the Sacramento Valley. Local sources of water for residents of the rancheria are of poor quality or limited availability. Domestic needs are presently met by water from a hand-dug well and from a drilled well with a potential yield of 15 gallons per minute. Water from both wells fails to meet California State drinking-water standards, primarily because of high concentrations of chloride and dissolved solids. High concentrations of sodium and boron pose additional problems for agricultural use of the water. The dissolved ions originate in Upper Cretaceous marine sediments of the Cortina Formation, which occurs at or near land surface throughout the rancheria. Small quantities of fresh ground water may occur locally in the Tehama Formation of Pliocene age, a thin layer of which overlies the Cortina Formation in the eastern part of the rancheria. Canyon Creek, the largest stream on the rancheria, has a drainage area of 3.1 square miles and flows only during winter and spring. Water from one of the rancheria's three springs meets drinking-water standards, but it flows at little more than a trickle during summer. The generally poor quality of ground and surface water on the rancheria is typical of areas along the west side of the Sacramento Valley, as is indicated by statistical analysis of regional data. Local and seasonal variations in water quality, however, offer possibilities for improving the water supply by storing winter runoff or by developing local sources of better quality ground water. Additional collection of hydrologic information could indicate more precisely the quantity and quality of surface and ground water on Cortina Rancheria. Principal features of a possible data-collection program would include monitoring of discharge and water quality in three springs and in Canyon Creek, electromagnetic terrain conductivity surveys, and monitoring of water levels and quality in two existing wells and several proposed test wells.