The Cascade Range and Modoc Plateau (CAMP) study unit covers approximately 15,000 square miles in northeastern California. The study unit was divided into six study areas on the basis of geologic features: Eastside Sacramento Valley, Honey Lake Valley, Cascade Range and Modoc Plateau Low-Use Basins, Quaternary Volcanic Areas, Shasta Valley and Mount Shasta Volcanic Area, and Tertiary Volcanic Areas (Shelton and others, 2013; Fram and Shelton, 2015).

The volcanic rocks of the CAMP study unit can be highly productive aquifers. Groundwater resides in porous zones between lava flows, fractures that formed during cooling of the lavas, lava tubes, and other features. These systems can be interconnected or isolated, resulting in variability in water levels, well yields, and water quality on local and regional scales. Sedimentary groundwater basins in the CAMP study unit primarily contain Quaternary-age alluvial fan, fluvial, and lake sediments (California Department of Water Resources, 2003). Aquifers composed of different materials can contain groundwater with different chemical compositions.

The primary aquifer system of the CAMP study unit is defined as those parts of the aquifer system tapped by wells and springs listed in the State of California’s database of public drinking-water supply sources. In the CAMP study unit, about 10 percent of these sources are springs. Of those that are wells, most are drilled to depths of 140 to 400 feet, consist of solid casing or a seal from the land surface to a depth of about 50 to 220 feet, and are open or have perforated casing below that depth. Water quality in the primary aquifer system can differ from that in the shallower and deeper parts of the aquifer system. Average annual precipitation ranges from 10 to 20 inches over much of the area to over 80 inches at high elevations in Lassen Volcanic National Park and on Mount Shasta. The study unit includes three major hydrologic regions: the Klamath River watershed, the Sacramento River watershed, and closed drainage basins of the North Lahonton area. About 80 percent of the land in the study unit is natural, and much of that land is in national forests. About 16 percent of the land is used for agriculture, and about 4 percent is urbanized.

Recharge to CAMP study unit aquifers is mainly from stream-channel infiltration, direct infiltration of precipitation and snow melt, flow through volcanic features, and in some areas, infiltration of irrigation water. Groundwater exits the aquifer system when it is pumped for irrigation or drinking-water supply, discharges into springs, streams and lakes, or evaporates from areas with a shallow depth to groundwater.