Unconsolidated deposits of sand, gravel, silt, and clay, which are Pliocene and younger and primarily of alluvial origin, compose the Eureka area aquifers (fig. 104). Near the coast, the alluvial deposits interfinger with estuarine sediments and locally are underlain by marine sediments. The thickness of the unconsolidated deposits ranges from only a few feet to as much as 1,000 feet (fig. 105). The unconsolidated deposits range from coarse to fine grained. The most permeable deposits are surficial alluvium and dune sands. Virtually all fresh ground water is withdrawn from these deposits, but deeper beds yield water in some places. The permeability of the unconsolidated sediments varies with location, however, and well yields vary accordingly. Consolidated and semicon-solidated rocks of minimal permeability form the boundaries of the aquifer system.
Distinct confining units are scarce in the unconsolidated deposits, but large total thicknesses of fine-grained sediments can impede vertical flow sufficiently to create an increase in hydraulic head with depth. Consequently, depending upon the permeability and depth of the water-yielding deposits at a particular location, ground water can be under either confined or unconfined conditions.
The primary fresh ground-water body in the Eureka area is in the Eel River Valley, where ground water under unconfined, or water-table, conditions is available nearly everywhere at depths of 30 feet or less. An exception is in the vicinity of Ferndale, where sediments are fine grained, have minimal permeability, and yield little water to wells except near the mouths of streams, where the sediments are coarse grained and fluvial. A perched water table is above clay beds that form a local confining unit in terrace deposits near the Eel River. Water in the deeper parts of the aquifer in the Eel River Valley, near Humboldt Bay in the Eureka Plain, and in the Mad River Valley, between Eureka and Arcata, is under confined or partially confined conditions.