Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

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Area above Pilarcitos Creek at Half Moon Bay Gauge A water balance was conducted for the area above the Pilarcitos Creek at Half Moon Bay gauge (Table 9, Column C). This balance includes the upper Pilarcitos Creek watershed and provides an estimate of the groundwater outflow to the Lower Pilarcitos Creek groundwater basin. Based on the isohyetal map (Figure 4), the average annual rainfall on the 17,069 acre watershed is 34.08 inches (about 2.84 feet). No surface or subsurface inflow occurs in the watershed. After treatment at the Nunes Water Treatment Plant (WTP), minor amounts of imported water (77 AFY) are brought into the watershed from Crystal Springs Reservoir (see Appendix 6). Therefore, the total inflow to the Pilarcitos Creek watershed is about 48,552 AFY or approximately 365 percent greater than the upper Pilarcitos Creek watershed. Because of the proximity to the coast (fog belt), the ET for the entire Pilarcitos Creek watershed was assumed to be approximately five percent less than the average ET of the upper Pilarcitos Creek (76.5 percent) and Purisima Creek (74.8 percent) watersheds or about 71.5 percent. Accordingly, ET is estimated to be 34,660 AFY. Average streamflow for the Pilarcitos Creek at Half Moon Bay gauge station is 11,543 AFY for the record between 1967 and 2001. Subsurface outflow cannot be readily measured and is estimated from the residual of the water balance. Based on records provided by the City of San Francisco (see Appendix 5), exported water from the watershed amounts to about 760 AFY. Other diversions include the Pilarcitos Creek wellfield operated by CCWD (155 AFY) and Pilarcitos Lake diversions (913 AFY) (see Appendix 6). Therefore, the difference between the inflow (48,552 AFY) and outflow (48,031 AFY) suggests that 521 AFY is groundwater outflow. The undetected subsurface outflow for the Pilarcitos Creek watershed can also be estimated using Darcy’s Law (254 to 508 AFY). In terms of the surface water runoff coefficient for the Pilarcitos Creek watershed, calculations show that 0.68 AF/acre of water drains from the watershed. Precipitation at the Half Moon Bay Station ranges between 13.03 inches in 1972 to 52.92 inches in 1983; the long-term average is 26.40 inches. Therefore, total inflow to the Pilarcitos Creek watershed could range between 23,985 (49.4 percent of average) in dry years to 97,347 (200.5 percent of average) in wet years. The total estimated 33
groundwater inflow to the Lower Pilarcitos Creek groundwater basin ranges between 257 AFY in dry years to 1,107 AFY in wet years. 34
Page 1 and 2: 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: ET can be expressed in terms of per
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 and 58: An example was provided involving p
Page 59 and 60: Water Quality Groundwater quality i
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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