IV. Ground Water
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).
Next - Section V. Cold Seeps
Geology Table of Contents