High mountains and extensive watersheds occur nearly everywhere along the eastern MBNMS margin. The considerable rainfall in these watersheds is held in porous Tertiary marine strata that have been uplifted and dip seaward, forming fresh water aquifers.
Numerous aquifers create extensive ground water basins in the Monterey Bay region (Figure 7; Muir 1972, 1980). Many of these are contiguous with the offshore (Greene 1969). The most porous and highly permeable aquifers are located in Pleistocene and Pliocene sedimentary rocks. The 180- and 400-foot aquifers are found in the Pleistocene eolean Red Sands Formation. Probably the most extensive, best recharged aquifer is found in the Pliocene Purisima Formation. These aquifers are subjected to overdraft and are experiencing salt water intrusion (Johnson 1983).
Other ground water-bearing formations exist in perched alluvial deposits and in marine terraces. Deeper ground water sources can be found in the fractures and cracks of the granitic basement rocks and the fractured cherts of the overlying Miocene Monterey Formation, although quality is not as good as in the younger rocks. Some ground water is also available in the Franciscan Assemblage of rocks, but again quality is poor.
In the Santa Cruz Mountains, extensive recharge areas are present where fresh water aquifers are exposed (Muir 1972, 1974, 1977). Recharge of the basins is accomplished through the outcropping aquifers, though destructive land management is limiting the recharge ability. These aquifers feed springs in the coastal mountains that supply water to the creeks, river and streams. In winter months, water is commonly seen seeping from well-layered strata of aquifers exposed in the sea cliffs, especially in the Santa Cruz area and along the coastal bluffs of the Santa Lucia Mountains. Historically (and at times even now during extremely wet periods), fresh water also seeped out along the submarine canyon walls and on the seafloor, from faults that cut aquifers originating in the coastal mountains. For example, fresh water sapping appears to have occurred along headward parts of Monterey Canyon, where several shallow (180 and 400 ft.) aquifers crop out in the head scarps of large, well defined slumps (Greene 1970, 1977; Greene and Hicks 1990).
Further offshore, fresh water aquifers crop out on the seafloor of Monterey Bay and on the Monterey Canyon walls. These are the likely sites for intrusion of sea water into the aquifers (Figure 7). Heavy agricultural demand on coastal ground water sources has caused seawater to encroach progressively inland (Figure 7; Water Resource Agency 1994). The major part of this saltwater contamination is occurring in the Pajaro and Salinas Valleys (Bond and Bredehoeft 1987; Esmaili 1978; Johnson 1980; Muir 1972, 1974, 1977, 1980; State of California 1958, 1970). However, recent observations of bacterial mats and chemosynthetic communities along the northern wall of Monterey Canyon suggest that fresh water may still be flowing out of more northerly aquifers fed by rainfall in the Santa Cruz Mountains (Greene et al. 1993; Orange et al. 1995; Barry et al. 1993).
Section III. Geology and Tectonics