Santa Margarita Basin

The boundaries of the Santa Margarita Basin are derived from geologic boundaries. The lateral boundaries of the basin generally follow the definable limits of the stacked Butano, Lompico and Santa Margarita aquifers. The lateral boundaries are further defined by two faults that impede groundwater flow: the Zayante Fault and the Ben Lomond Fault. The northwest corner of the proposed Santa Margarita Groundwater Basin is the intersection of the Zayante Fault and the Ben Lomond Fault. The basin boundary proceeds eastward from this intersection along the Zayante Fault to where the Butano sandstone meets the boundary of the existing Santa Cruz Purisima Formation Basin.

The boundary then follows the boundary of the existing Santa Cruz Purisima Formation Basin westward to a granitic outcrop. From there, the basin boundary proceeds southwestward along the spine of the granitic high. At the southern end of the Santa Margarita Groundwater Basin, the basin boundary follows the contact between the Lompico sandstone and the Locatelli Formation to its intersection of the Ben Lomond fault. The basin boundary follows the trace of the Ben Lomond fault north to the intersection of the Zayante Fault and the Ben Lomond Fault.

The San Lorenzo River is the most significant surface water feature in the Santa Margarita Groundwater Basin. The river runs in or near the western edge of the Basin; sub-parallel to the Ben Lomond fault. Important tributaries to the river include Lompico Creek, Zayante Creek, Bean Creek and Carbonera Creek. Loch Lomond is a man-made lake on Newell Creek: a tributary to the San Lorenzo River. The lake is approximately 2.5 miles long with a width of about 1,500 feet (McPherson and Harmon, 1998). The southern half of the lake lies within the proposed Santa Margarita Groundwater Basin. Loch Lomond is owned by the City of Santa Cruz, and water impounded in the lake provides a significant portion of the water used by the City of Santa Cruz.

The aquifers that comprise the Santa Margarita Groundwater Basin have been managed as a distinct groundwater resource since the early 1990s. The area of the Santa Margarita Groundwater Basin has been managed under an AB3030 Groundwater Management Plan since 1994 (Todd Engineers, 1994). As part of the Groundwater Management Plan implementation, annual reports have been produced that analyze and describe the status of the Santa Margarita Groundwater Basin. Starting in 2013, the annual reporting was changed to a biennial cycle because of the marginal year-to-year changes observed in the basin. The most recent report was produced in 2015, and reflected basinwide conditions during the 2014 water year (Kennedy Jenks Consultants, 2015a).

Recognizing that the stacked aquifers defined a regional groundwater resource, a groundwater model of the area was developed in 1997 (Todd Engineers, 1997). The groundwater model has been regularly updated and used for groundwater planning purposes. Significant model updates occurred in 2006 (EITC, 2006) and 2015 (Kennedy Jenks Consultants, 2015b).