Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

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asin (9,500 AFY) was allocated among municipal and agricultural pumpers (Cities of Arroyo Grande, et al., 2002). This agreement, based on a previous groundwater basin assessment, specifies an allocation of 200 AFY to subsurface outflow to the ocean, similar to the 235 AFY indicated by the above example. 49
Water Quality Groundwater quality is a concern in the Lower Pilarcitos Creek groundwater basin, because many wells yield water with excessive iron and manganese, and locally high chloride concentrations. Accordingly, groundwater quality data are reviewed for CCWD test wells and other wells investigated by CCWD for their 1991 drought relief program. In addition, quality data are reviewed from selected private wells as recorded in selected San Mateo County Department of Health Services records. Potential sources of anthropogenic (human-caused) contamination are identified in the study area. CCWD Test Wells and Drought Relief Wells Figure 20 shows the locations of the CCWD Lower Pilarcitos Creek test wells and other wells investigated by CCWD for the drought relief program. The latter wells include the Nerhan, Holiday Inn, Corporate Yard, Amesport 1 and 2, Nurserymens 1 and 3, and Stone Pine Center 1, 4, and 6 wells. Table 14 summarizes the water quality for the CCWD test wells and drought relief wells. Appendix 8 contains water quality data for all the wells and an explanation of the major and minor cations and anions and trace elements in water. Diagnostic Analysis. Figure 21 shows three selected water quality diagnostic diagrams that were applied to water quality data from test wells and drought relief wells. The water differentiation diagram (Figure 21a) compares gypsum and sodium chloride contents and is used to indicate the source of salts in an aquifer. The diagram indicates that the CCWD test wells are characterized by low sodium chloride relative to the other wells. The diagram also suggests that the slightly elevated salt contents in the other wells probably were derived from salt spray incorporated into the aquifer during its formation. The elevated chlorides are not from saltwater intrusion. The calcium-sulfate diagram (Figure 21b) shows how much gypsum is in the water. Gypsum is a somewhat soluble evaporite mineral that can accumulate in an aquifer generally during its formation and therefore, it also indicates the influence of salt spray, which can be a significant source of salts. Accumulation of gypsum also is indicative of relatively low permeability sediments and a relative lack of flow (flushing) through the aquifer. The calcium sulfate diagram indicates that the test well water has only moderate amounts of gypsum, indicating a fairly permeable aquifer flushed with recharge, 50
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Coastside County Water District Hal
Page 3 and 4:
LIST OF FIGURES Figure 1 Figure 2 F
Page 5 and 6:
Executive Summary This report summa
Page 7 and 8: criteria to a monthly evaluation of
Page 9 and 10: top of well screens. Pumping at hig
Page 11 and 12: 1997 (Nelson, 1997 and 1998). This
Page 13 and 14: Figure 4 illustrates the areal dist
Page 15 and 16: • Urban: urban, urban residential
Page 17 and 18: should be noted that Geoconsultants
Page 19 and 20: slopes. In addition, the Pilarcitos
Page 21 and 22: The bedrock has been affected by fo
Page 23 and 24: Cross section A-A’ (Figure 16) wa
Page 25 and 26: Lower Pilarcitos Creek Test Well An
Page 27 and 28: well construction. The drilling flu
Page 29 and 30: drawdown is defined as the distance
Page 31 and 32: Because drilling depths are shallow
Page 33 and 34: Marine Terrace Hydraulic Properties
Page 35 and 36: 1993. Review of these maps indicate
Page 37 and 38: (1.000 g/cm 3 ), allowing the fresh
Page 39 and 40: Watershed Water Balance Two water b
Page 41 and 42: ET can be expressed in terms of per
Page 43 and 44: groundwater inflow to the Lower Pil
Page 45 and 46: This is a simplified accounting of
Page 47 and 48: 3,000 homes exist within the basin.
Page 49 and 50: (Geoconsultants, June 1987 and Augu
Page 51 and 52: To estimate consumption of this wat
Page 53 and 54: echarge is one of the largest singl
Page 55 and 56: 1987 land use conditions to 822 AFY
Page 57: An example was provided involving p
Page 61 and 62: colloidal form. Accordingly, the al
Page 63 and 64: Economic Feasibility This section a
Page 65 and 66: Overall, groundwater is characteriz
Page 67 and 68: The overall capacity of the five we
Page 69 and 70: Maintenance costs are estimated to
Page 71 and 72: Conclusions This study indicates th
Page 73 and 74: Earth Sciences Associates, February
Page 75: Todd, David K., 1980, Groundwater H
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