Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

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ate to annual runoff during the five drought years results in an estimated stream recharge of 509 AFY. Drought Recharge. To summarize, drought conditions, like those of 1987 through 1991, would result in a decline in recharge to the groundwater basin. Estimated recharge amounts under drought conditions are as follows: Rainfall recharge Deep percolation Subsurface inflow Pipeline leakage Stream recharge Total 158 AFY 118 AFY (not changed) 254 AFY 50 AFY (not changed) 509 AFY 1,089 AFY This is approximately half the recharge experienced during average rainfall years. This decrease would result in a decline in groundwater levels, available drawdown in wells, and well yields similar to summer conditions. Trends The Lower Pilarcitos Creek hydrogeologic analysis and water balance evaluation show that the Lower Pilarcitos Creek groundwater basin is characterized by substantial groundwater outflow to the ocean. Because of this large outflow, groundwater storage that accumulates during the winter wet season drains relatively quickly to the ocean during the summer. As a result, seasonal groundwater level changes are relatively large in comparison to long-term changes. The hydrograph (Figure 19) of Well 29F04, located near the proposed Lower Pilarcitos Creek wellfield, shows a gradual rise in groundwater levels through the 1960s and relatively stable levels until about 1985. The severe, prolonged drought of 1988 through 1993 is marked by a decline of groundwater levels. Groundwater levels since 1995 have varied widely; however, no net change is apparent between the mid-1980s and present. Changes in land use can affect the water balance of a groundwater basin. In Half Moon Bay, the most significant land use changes since the 1970’s have involved conversion of open space (mostly urban vacant land) to urban uses. This conversion results in loss of permeable land for rainfall recharge (unless mitigated). As described in the section on rainfall recharge, estimated rainfall recharge declined from 885 AFY with 45
1987 land use conditions to 822 AFY with 2002 land uses. This loss of recharge is offset somewhat by increased return flows of imported water used for landscaping irrigation. Conversion of irrigated cropland to urban uses has been limited in recent years. If such conversion occurs, the impact on the water balance would involve loss of rainfall recharge, which might be offset in part by decreased pumping for agricultural irrigation and return flows of water imported for landscaping. Previous Work Water balances were evaluated previously for the Lower Pilarcitos Creek groundwater basin by Geoconsultants (June 1987 and August 1991). The Geoconsultants’ water balances are similar to the water balance in this study in two major respects: • The study areas are similarly defined; while the Geoconsultants area includes 1,720 acres, the study area in this report includes 1,765 acres. • The total recharge or inflow values are comparable. Geoconsultants estimated total recharge at 2,719 AFY (1987) and 2,261 AFY (1991), while this report provides an estimate of 2,200 AFY. The water balances differ in overall methodology. Geoconsultants focused on evaluation of two major sources of recharge, rainfall recharge and runoff recharge. This study includes independent evaluation of all major inflow and outflow components and change in storage. Both studies include evaluation of rainfall recharge, taking into account rainfall, soil moisture holding capacity, ET, and land use. While the values and computations differ, the conceptual approach taken by Geoconsultants and this report is similar. In addition, the rainfall recharge results are similar for the Geoconsultants’ 1987 study (871 AFY) and this report (822 AFY). Geoconsultants’ 1991 study, addressing drought conditions, estimated rainfall recharge at 599 AFY. Geoconsultants estimated runoff recharge as 15 percent of runoff, resulting in estimates of 1,848 AFY (1987) and 1,662 AFY (1991). This study estimates stream recharge conservatively at 723 AFY. In considering perennial yield, Geoconsultants offers two approaches. One is to use one-third of the storage capacity in one year and the second is to use two-thirds of 46
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 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: echarge is one of the largest singl
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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