Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

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LIST OF TABLES Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14 Table 15 Table 16 Table 17 Table 18 Table 19 Groundwater Basin Land Use Area Percent Impervious Area for Selected Land Use Types Summary of Streamflow Data near Half Moon Bay Summary of Test Boring/Well Construction Details for Lower Pilarcitos Creek Groundwater Investigation Summary of Hydraulic Data for Lower Pilarcitos Creek Groundwater Investigation Lower Pilarcitos Creek Test Wells – Operating Parameters (a) Winter, (b) Summer Summary of Hydraulic Data for Lower Pilarcitos Creek Groundwater Investigation at the Balboa Wellfield Summary of Gauge Station Areas and Average Discharges Pilarcitos Creek Regional Water Balance Groundwater Basin Land Use Areas for Water Balance Lower Pilarcitos Creek Groundwater Basin Rainfall and Recharge, Water Years 1987-2001 Land Use Types and Average Annual Recharge from Rainfall Summary of Water Balance, Lower Pilarcitos Creek Groundwater Basin, Water Years 1987-2001 Lower Pilarcitos Creek Groundwater Quality Groundwater Quality in AP Book 56 Areas Lower Pilarcitos Wellfield: Water Quality Characteristics Lower Pilarcitos Wellfield: Water Corrosivity/Scaling Characteristics of Groundwater Lower Pilarcitos Groundwater Study: Soil Characteristics and Corrosivity Lower Pilarcitos Wellfield: Well Pump Size and Capacity APPENDICES Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5 Appendix 6 Appendix 7 Appendix 8 Appendix 9 Rainfall Data at Half Moon Bay Airport Half Moon Bay Well Inventory Streamflow Data near Half Moon Bay Summary of Wells used for Cross Sections A-A’, B-B’, and C-C’ Water Diversions by City of San Francisco CCWD Summary of Water Usage Water Balance Spread Sheets Water Quality Photographs of Well Sites and Pipeline Routes iii
Executive Summary This report summarizes a study of the technical and economic feasibility of a proposed Lower Pilarcitos Creek groundwater project for Coastside County Water District (CCWD) in Half Moon Bay, California. The purpose of this study is threefold: • To evaluate the safe or perennial yield of the Lower Pilarcitos Creek groundwater basin, • To assess the potential for production from a proposed CCWD wellfield in the basin, and • To prepare a cost estimate to construct the infrastructure for groundwater production. In addition, this study responds to seven questions posed in the July 2002 request for proposal for the Lower Pilarcitos Creek Groundwater Basin Project, which are listed and addressed at the end of this executive summary. This report describes the hydrogeologic framework of the groundwater basin. In brief, marine terrace deposits form the main aquifer in the Lower Pilarcitos Creek groundwater basin. The predominantly-confined aquifer is about 30 feet thick in the vicinity of the proposed wellfield and is underlain by Purisima Formation. Relatively finegrained alluvial deposits overlie the aquifer. Groundwater flows from east to west, discharging into the Pacific Ocean. Along the shoreline, an interface exists between fresh groundwater and saltwater. The position of the fresh-salt water interface is dynamic and migrates depending upon the aquifer discharge. Reducing this discharge will move the interface inland. The total groundwater storage of the marine terrace aquifer averages 10,600 acre-feet (AF). Assuming that only one-third of the stored water can be used, then the usable storage is 3,533 AF. However, groundwater levels fluctuate on a seasonal basis by 10 to 20 feet, which is significant relative to an aquifer thickness of only about 30 feet near the proposed wellfield. The water balance equation for the groundwater basin addresses inflow, outflow, and change in storage. Change in storage is considered zero. While a small net error ES-1
Page 1 and 2: Coastside County Water District Hal
Page 3: LIST OF FIGURES Figure 1 Figure 2 F
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
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1987 land use conditions to 822 AFY
Page 57 and 58:
An example was provided involving p
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Water Quality Groundwater quality i
Page 61 and 62:
colloidal form. Accordingly, the al
Page 63 and 64:
Economic Feasibility This section a
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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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