APPENDIX E Groundwater Information

Transcription

1 APPENDIX E Groundwater Information

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3 Kleinfelder Midcoast Groundwater Study April 2009 Summary and Errata Prepared by Planning & Building Department San Mateo County California vpwin\policy\covers.vp 4/02/09 rp

4 Midcoast Groundwater Study Summary Summary of Kleinfelder Midcoast Groundwater Study Introduction... 1 Setting and Methodology... 1 General Conclusions... 4 Specific Conclusions... 5 El Granada Subbasin... 5 Arroyo de en Medio and Frenchman s Subbasins... 6 Airport Subbasin... 7 Moss Beach Subbasin... 8 Montara Terrace Subbasin Montara Creek Subbasin Martini Creek Subbasin Recommendations Supplemental Recommendation Attachments Attachment A: Map of Midcoast Area Groundwater Basins...A.1 Attachment B: Errata...B.1 Attachment C: Map of Vacant Lots...C.1

5 SUMMARY OF KLEINFELDER MIDCOAST GROUNDWATER STUDY INTRODUCTION Due to the use of individual wells to support existing and proposed development in the Midcoast, and potentially limited groundwater sources, San Mateo County commissioned the Midcoast Groundwater Study to assist in long-term groundwater basin and watershed planning. The initial purposes of the study were to evaluate Midcoast groundwater conditions and assess the suitability and long-term sustainability of Midcoast groundwater supplies. This was to include an analysis of the potential impacts of groundwater withdrawals on sensitive areas such as riparian and wetland habitats, and an estimation of safe yield. However, as the study progressed, it was determined that safe yield and groundwater/habitat relationships could not be accurately assessed due to the limited availability of well data, concerns regarding the accuracy of the data, and information gaps regarding surface water flows. As a result of these limitations, Kleinfelder developed generalized water balance models based on aquifer characteristics, average pumping rates, and rainfall amounts over a 50-year period ( ). These models estimate the basins inputs and outputs, and how variations in annual rainfall affect groundwater levels and storage. The Seal Cove area is not addressed by the report because well monitoring data was not available. A map of the groundwater basins and subareas is provided as Attachment A. While the study provides valuable information regarding Midcoast groundwater basins, additional data and analyses are needed to determine the specific amounts of water that can be extracted without causing adverse long-term impacts. The report submitted to the County by Kleinfelder contains errors that are corrected by the errata sheet and replacement pages included in this summary as Attachment B. SETTING AND METHODOLOGY The Midcoast can be generally divided into the following two groundwater storing geologic formations: Coastal marine terrace or stream valley alluvial deposits where groundwater is stored in loose, unconsolidated, coarse-grained sand. 1

6 Upland granitic bedrock where groundwater is stored in weathered rock openings and in rock fractures. The generalized water storage potential of Midcoast geologic formations is shown below: Good Source of Groundwater Marine Terrace Deposits Younger Alluvial Fans Stream Channel Deposits Poor or Limited Source of Groundwater Monterey Formation Colluvium Granitic Bedrock (Montara Mountain) Purisima Formation Although recharged by percolated rainwater, there is generally limited storage capacity in weathered or fractured granite; most groundwater flows to downstream aquifers. The Midcoast study area is divided into eight groundwater subbasins, which are further divided into the following 21 subareas. The subareas consist of similar geologic units within a subbasin, e.g., terraces, uplands, and stream valleys through which groundwater flows. Frenchman s Subbasin 1 Frenchman s Terrace Subarea 2 Frenchman s Upland Subarea 3 Frenchman s Stream Valley Subarea Arroyo de en Medio Subbasin 4 Miramar Terrace Subarea 5 Arroyo de en Medio Upland Subarea 6 Arroyo de en Medio Stream Valley Subarea El Granada Subbasin 7 El Granada Terrace Subarea 8 El Granada Upland Subarea Airport Subbasin 9 Airport Terrace Subarea 10 Dennison Upland Subarea 11 Dennison Stream Valley Subarea 13 San Vicente Upland Subarea* 2

7 14 San Vicente Stream Valley Subarea* Moss Beach Subbasin 12 Lower Moss Beach Subarea 13 San Vicente Upland Subarea* 14 San Vicente Stream Valley Subarea* 15 Dean Creek Subarea 19 Upper Moss Beach Subarea 20 Lighthouse Subarea Montara Creek Subbasin 16 Portola Subarea 17 Montara Creek Upland Subarea 18 Lower Montara Creek Subarea 21 Wagner Valley Subarea 22 Montara Terrace Subbasin 23 Martini Upland Subbasin *The San Vicente Upland and Stream Valley Subareas contribute to both the Airport and Moss Beach Subbasins. Of the 1,087 Midcoast well records in the County s database, only 539 were deemed usable for the study due to the lack of information on actual well locations. Of the remaining 539 well records only about half of them had useful data for evaluation such as depth to groundwater, groundwater production, specific capacity estimates or well log data. Twenty-six existing private Midcoast wells were monitored for one year to measure the depth to groundwater, estimate groundwater surface elevations and observe groundwater behavior. Pumping tests were conducted in four private wells to estimate transmissivity and hydraulic conductivity values of the aquifers. Transmissivity is the rate water flows within the aquifer under a unit hydraulic gradient. Transmissivity is dependent on the aquifer s hydraulic conductivity. Hydraulic conductivity is the ease of internal water flow through a porous medium. Two of the four test wells had high yields and would be adequate for municipal or irrigation purposes. These wells are located in the Airport Terrace subarea of the Airport Subbasin, and the Lower Moss Beach subarea of the Moss Beach Subbasin, respectively. The other two test wells showed low aquifer yield and very low transmissivity. These wells are located in the Dean Creek subarea of the Moss Beach Subbasin and the El Granada Subbasin at the edge of the El Granada Upland subareas, respectively. 3

8 Water balance analyses were conducted for the Midcoast groundwater subbasins using the rainfall record for the 55-year period ( ). Water balance analysis relates the amount of water entering and leaving the groundwater storage system. The difference between the amount that enters and leaves the aquifer is the amount that is retained in storage. A terrace aquifer water balance model was developed for use in the El Granada, Arroyo de en Medio, and Moss Beach Subbasin terrace areas. Other water balance assessment methodologies were applied for the Airport and Montara Terrace Subbasins, the Dean Creek and Upper Moss Beach subareas of Moss Beach Subbasin, and the Portola subarea of the Montara Creek Subbasin. Groundwater evaluations were not conducted for the Montara Creek Upland (Montara Knob), Lower Montara Creek and Wagner Valley subareas of the Montara Creek Subbasin, and the Martini Creek Subbasin. The study did not assess these areas because of insufficient groundwater data. No large-scale future development is anticipated in these areas. GENERAL CONCLUSIONS Midcoast aquifers that have a considerable groundwater surplus in average rainfall years can have a deficit in dry and very dry years. The marine terrace subareas appear to be in long-term hydrologic balance under current pumping conditions, and should remain in long-term balance with a moderate increase in water extractions. This conclusion assumes rainfall patterns experienced during the 55-year period used for analyses are representative of long-term future conditions, and that periods when the water table falls below sea level will be of short duration. Although the marine terrace aquifers appear to be in long-term balance, current pumping rates have lowered the water table to near sea level during dry years, and potentially below sea level during very dry years, posing risks of saltwater intrusion. Increased pumping over long periods of time, especially during drier years, will increase the amount of time that the water table falls near or below sea level. This increases the risk of saltwater intrusion. Potential groundwater deficits may occur more frequently in the granitic aquifers underlying Montara Terrace Subbasin, Upper Moss Beach and Dean Creek subareas of the Moss Beach Subbasin, and the Portola subarea of the Montara Creek Subbasin. Although these areas appear to be in general long-term balance, individual wells may go dry during prolonged dry years and pumping increases could cause local detrimental impacts. The data is limited to quantify these impacts and their distribution. 4

9 SPECIFIC CONCLUSIONS El Granada Subbasin The El Granada Subbasin is made up of the El Granada Terrace and El Granada Upland subareas, as shown on the attached Midcoast Study Area map. Approximately 93 acre-feet of groundwater is pumped annually from the El Granada Subbasin. This is associated with the extraction of approximately 27 acre-feet per year by about 97 wells in the upland subarea, and about 66 acre-feet per year from approximately 237 wells in the terrace subarea. The water balance model developed by Kleinfelder indicates that the elevation of the El Granada terrace water table may have fluctuated about 45 feet and averaged about 15.5 feet above sea level. In a dry year ( ), groundwater elevations were modeled to be 8.4 feet above sea level. In a very dry year ( ), the model indicates that groundwater elevations dropped 0.7 feet below sea level. The model suggests that there were six water years over the 55-year period when the water table approached (less than 5 feet above) sea level, or dropped below sea level. These were , , and Such significant lowering of the water table accounts for 11% of the 55 years studied, and occurred after two or more consecutive dry years. The model estimates that storage volume for the El Granada Subbasin aquifer ranged between about 0 and 1,580 acre-feet during the 55-year period. The average storage volume within the marine terrace subarea was estimated to be about 560 acre-feet. Kleinfelder was not able to quantify the amount of storage in the upland area of the Subbasin based on current information. During the dry year, the model estimated aquifer storage at about 300 acre-feet of groundwater. During the very dry year, there may have been a 26 acre-foot deficit. This could have caused the water table to fall below sea level and limited seawater intrusion to occur. The study concludes that although water levels fluctuate significantly, groundwater at the El Granada Subbasin appears to be in general long-term balance. Although groundwater deficits occur, there appears to be no longterm depletion trend and the aquifer can recharge itself following an average to wet year. 5

10 Planning staff estimates that there are 590 vacant lots designated for residential development within the urban area of the Subbasin. If each of these lots were developed with a single-family residence that obtained water from a well and connected to the sanitary sewer system, pumping would increase from 93 to 259 acre-feet per year, and the probability of the groundwater falling to levels near or below sea level in the mid-part of the terrace would increase from 11% to about 24%. A prolonged drop in groundwater levels below sea level may have detrimental impacts due to saltwater intrusion. Arroyo de en Medio and Frenchman s Subbasins The Arroyo de en Medio and Frenchman s Subbasins are made up of the Miramar Terrace, Arroyo de en Medio Upland, Arroyo de en Medio Stream Valley, Frenchman s Terrace, Frenchman s Upland, and Frenchman s Stream Valley subareas. These subbasins are shown on the attached Midcoast Study Area map. Both subbasins drain to the Miramar marine terrace. The hydrogeologic conditions in the Arroyo de en Medio Subbasin are similar to the conditions in the El Granada Subbasin. Approximately 169 acre-feet of groundwater is pumped annually from the Arroyo de en Medio Subbasin, 167 acre-feet of which is extracted from more than 80 wells in the terrace area. Approximately six active wells in the upland area extract about 2 acre-feet per year. The model estimates that the elevation of groundwater in the terrace area averaged 21 feet above mean sea level between 1950 and In a dry year ( ), groundwater elevations may have lowered to 13 feet above sea level. During a very dry year ( ), groundwater elevations were modeled to drop to 2 feet below sea level. There were four water years when the water table approached or dropped below sea level ( ), ( ), and ( ). This represents a frequency of 7%. The model estimates that the amount of water stored in the terrace area averaged 502 acre-feet. In a dry year ( ), groundwater storage was reduced to 309 acre-feet. During a very dry year ( ), the model indicates that the amount of water leaving the aquifer exceeded the amount entering by 37 acre-feet. The Frenchman s Subbasin was not modeled in detail due to the limited information for that area, e.g., lack of existing wells. 6

11 The Frenchman s Terrace subarea is contiguous with the Miramar Terrace subarea with no apparent groundwater divide separating the two areas. They share the similar hydrogeologic properties and have similar quantities of water in storage. The Frenchman s Upland subarea is larger and higher than the Arroyo de en Medio Upland subarea and should provide somewhat more water to the aquifer. Given the similarities of the Arroyo de en Medio and Frenchman s Subbasins, the general conclusions for the former may be applicable to the latter. The study concludes that the Arroyo de en Medio Subbasin is in general long-term balance. Hence, the Frenchman s Subbasin is probably also in general long-term balance. Planning staff estimates that there are 230 vacant lots designated for residential use within the urban area of the Miramar Terrace subarea. If each of these lots were developed with a single-family residence that obtained water from a well and connected to the sanitary sewer system, the amount of groundwater pumped from the Arroyo de en Medio Subbasin would increase from 169 to 235 acre-feet per year, and the probability of the groundwater falling to levels near or below sea level would increase from 7% to about 18%. A prolonged drop in groundwater levels below sea level may have significant detrimental impacts due to saltwater intrusion. Airport Subbasin The Airport Subbasin is made up of the Airport Terrace, Denniston Upland and Denniston Stream Valley subareas. The San Vicente Upland and San Vicente Stream Valley subareas also contribute to the Airport Subbasin. This subbasin is shown on the attached Midcoast Study Area map. Approximately 513 acre-feet of groundwater is pumped annually from the Airport Subbasin. These withdrawals consists of 169 acre-feet of average annual pumping by the Coastside Community Water District, 224 acre-feet of average annual pumping by the Montara Water and Sanitary District, about 96 acre-feet of extractions by approximately six agricultural wells, and approximately 24 acre-feet of withdrawals by about 87 domestic and other wells. The water table drops during dry years, but can quickly rebound during wet years. Based on prior studies, the 55-year precipitation record, monitoring data from two wells within the Airport subarea, and other factors, Kleinfelder estimates 7

12 that the average annual inflow to the basin of 2,780 acre-feet per year equals the average annual output. As a result, the report states that the Airport Subbasin appears to be in long-term hydrologic balance. The volume of Denniston Creek water that enters the Airport Terrace subarea is a significant recharge factor that is not well understood because longterm gaging data are not available. It is difficult to estimate the water balance in the Airport Terrace subarea without a better understanding of this recharge. A 1991 study by Earth Sciences Associates referenced by the report concluded that at least 45 to 87 additional acre-feet could be annually pumped from the Airport Subbasin without detrimental impacts. The Kleinfelder study does not indicate whether or not additional groundwater is available for pumping due to significant hydrological uncertainties in the area. Planning staff estimates that there are 61 vacant lots designated for residential uses in the urban area of the Airport Subbasin. If each of these lots were developed with single-family residence that obtains water from a well, pumping would increase by 17 acre-feet per year. The report is inconclusive regarding the sustainability of such increases. Moss Beach Subbasin The Moss Beach Subbasin is made up of the Lower Moss Beach, Dean Creek, Upper Moss Beach, and Lighthouse subareas. The San Vicente Upland and San Vicente Stream Valley subareas also contribute to the Moss Beach Subbasin. This subbasin is shown on the Midcoast Study Area attached map. Approximately 32 acre-feet of groundwater is pumped annually from the Moss Beach Subbasin. This is based on the estimation that 54 wells in the Lower Moss Beach subarea extract 15 acre-feet per year, about 6 acre-feet is pumped annually from 20 wells in the Upper Moss Beach subarea, and approximately 15 acre-feet of water is extracted each year from 55 wells in the Dean Creek subarea, 4 acre-feet of which is returned to the watershed via 31 permitted septic systems. No long-term data was available to assess groundwater trends in the Lower Moss Beach subarea. Using the 55 years of precipitation data and current pumping rates, the water balance model developed by Kleinfelder indicates that water levels in the Lower Moss Beach subarea vary from year to year but have not approached sea level, and that the subarea is currently in general balance. 8

13 The model estimates that average storage volume for the Lower Moss Beach subarea is about 719 acre-feet. Following a very dry year ( ), the model estimates that groundwater storage is about 332 acre-feet. The water balance model developed by Kleinfelder indicates that in an average rainfall year, 720 acre-feet of water flows into the Lower Moss Beach subarea, and 719 acre-feet flows out. In a very dry year, the model estimates that 215 acre-feet flows into the subarea, and 421 acre-feet flows out. Kleinfelder s assessment hypothesizes that 75% of the water that recharges the Lower Moss Beach aquifer comes from the San Vincente watershed. Because data are not available to validate the assumed inflow from this watershed, Kleinfelder recommends implementation of a long-term stream flow and gauging program. Planning staff estimates that there are 160 vacant lots within the urban area of the Lower Moss Beach subarea designated for residential use. If each of these lots were developed with a single-family residence that obtains its water from a well, extractions from the subarea will be about three times the amount of existing withdrawals. The Upper Moss Beach and Dean Creek subareas store groundwater in weathered or fractured granite. Although the long-term sustainability of granitic sources is low, the percolation recharge rate from rainfall is generally high. During extended droughts, water in these subareas may drop significantly. During the 55-year precipitation record, the volume of recharge in the Dean Creek subarea was six times the pumping demand. However, in 9 of the 55 years (16%), percolation recharge was less than the total pumping volume. Planning staff estimates that there are nine vacant parcels in the Dean Creek subarea designated for residential use. If each of these lots were developed with a single-family residence that obtains water from a well, the frequency of years in which pumping exceeds recharge would remain relatively the same. In the Upper Moss Beach subarea, recharge was less than pumping demand in 14 out of the 55 years (25%). Between 1953 and 1964, there were seven years when pumping exceeded recharge (64%). However, the average estimated recharge rate during this 11-year period was over double the pumping demand. Planning staff estimates that there are 83 vacant parcels in the urban area of the Upper Moss Beach subarea designated for residential use. If each of these lots were developed with a single-family residence that obtains water 9

14 from a well, the frequency of years in which withdrawals will exceed recharge increases up to 67%. Under these conditions, the Upper Moss Beach subarea would be out of long-term balance. The study suggests that an un-quantified amount of additional groundwater from the aquifer may be available for pumping without significant saltwater intrusion. Before additional pumping is carried out, the report suggests that estimates on inputs and outputs should be refined and confirmed, particularly the volume of water that enters the subbasin from the San Vincente watershed. Montara Terrace Subbasin The Montara Terrace Subbasin is bounded by the Pacific Ocean to the west, Martini and Kanoff Creeks to the north, Wagner Valley to the east, and Montara Creek to the south, as shown on the attached Midcoast Study Area map. Approximately 50 acre-feet of groundwater is pumped annually from the Montara Terrace Subbasin. This is based on an estimated 184 wells supporting residential uses, and 11 septic systems that return some of this water to the groundwater basin. The Montara Terrace Subbasin consists of marine terrace deposits overlaying weathered granitic bedrock. In the upper areas of this subbasin, the marine terrace deposits are absent. The marine terrace deposits tend to thicken downslope, which may allow for a great water storage volume. Well monitoring during the water year showed generally stable water levels with a peak water-surface elevation occurring at the end of the rainy season. Rainfall-runoff modeling generated data indicate that if the current pumping rates of the existing wells in the Montara Terrace Subbasin were maintained during the 55-year period, for 14 of the years (25%), more water would be drawn from the aquifer system than enters it. Wells at higher elevations would likely be at more risk of increased drawdown and going dry. In a very dry year ( ), there may have been about a 50 acre-foot groundwater deficit, i.e., where outflow exceeded inflow. Planning staff estimates that there are 304 vacant parcels designated for residential use within the urban area of the Montara Terrace Subbasin. If each of these parcels were developed with a single-family residence that 10

15 obtained water from a well, the frequency of years where pumping exceeds recharge would increase to 38% if septic systems were used, and 53% if the residences connected to the sewer system. The resulting drawdown on the water table would depend on the proximity of individual wells, localized hydrogeologic characteristics, volume of water in storage from prior years, and numbers of consecutive dry years. Wells at higher elevations would likely be at more risk of increased drawdown and going dry. The study concludes that although there have been wide swings year to year between surplus and deficit in the Montara Subbasin, in general, the area appears to be in long-term balance. The study further concludes that overall limited additional water could be pumped; however, there would be significant risk of localized well interference, large well drawdowns in dry years and the risk of individual wells going dry in dry and very dry years. Montara Creek Subbasin The Montara Creek Subbasin is made up of the Portola, Montara Creek Upland (Montara Knob), Lower Montara Creek and Wagner Valley subareas. This subbasin is shown on the attached Midcoast Study Area map. Within the Montara Creek Subbasin, groundwater evaluations were conducted for the Portola subarea, but not for the Montara Creek Upland, Lower Montara Creek and Wagner Valley subareas. The study did not assess these areas because of insufficient groundwater data and unanticipated future large-scale development. Approximately 44 acre-feet of groundwater is pumped annually from the Portola subarea. This is based on an estimated 35 wells supporting residential uses, including two production wells operated by the Montara Water and Sanitary District, and 18 septic systems. The Portola subarea consists of weathered and fractured granitic rocks. Groundwater is primarily recharged by infiltration and percolation of rainwater falling on the area. Near the western edge of the area, groundwater from the Wagner Valley may also seep into the weathered granitic rocks to an unknown extent. Model generated data for the existing wells in the Portola subarea indicates that in 20 of the 55 years (36%) of precipitation records, more water would have been drawn from the groundwater system than enters it. Wells at higher elevations would likely to be at more risk of increased drawdown or of going dry. 11

16 In a very dry year ( ), there may have been about a 45 acre-foot groundwater deficit, i.e., where outflow exceeded inflow. In a dry year ( ), there may have been a 16 acre-foot groundwater deficit. Planning staff estimates that there are 51 vacant parcels designated for residential use within the urban area of the Montara Terrace Subbasin. If each of these parcels were developed with a single-family residence that obtained water from a well, the frequency of years where pumping exceeds recharge would increase to 45% if septic systems were use, and 51% if the residences connected to the sewer system. The study concludes that although there have been wide swings year to year between surplus and deficit in the Portola subarea, in general, the area appears to be in long-term balance. The study further concludes that additional pumping in the Portola subarea runs the significant risk of localized well interference, large well drawdowns in dry years and the risk of individual wells going dry in dry and very dry years. Martini Creek Subbasin The Martini Creek Subbasin, located north of Montara, is shown on the attached Midcoast Study Area map. Groundwater evaluations were also not conducted for the Martini Creek Subbasin because of the lack of sufficient groundwater data, and that future large-scale development there is not expected. RECOMMENDATIONS The study recommends that the following measures be considered by the County to improve the long-term groundwater use in the area: A long-term stream flow and gauging program should be implemented to better define the area s hydrology. The County s database of wells should be carefully evaluated and corrected where possible. A survey of well owners should be considered. This would help in correctly locating wells, recording well characteristics, and assessing actual pumping demands. 12

17 Updated well information should be incorporated in the Distance-to-Wells spreadsheet and the spreadsheet should be used along with well site observation to evaluate minimum distances between proposed and existing wells, and to record instances of well interference. The County should select or construct strategic index monitoring wells in each subbasin to collect representative groundwater data on a long-term, periodic basis. In areas of marginal or limited groundwater production, the County may consider metering water use and water levels. Although fractured granite bedrock wells are generally unpredictable, the presence of interconnected fractures and joints that can provide reliable quantities of good quality water. Continued assessment of fractured granitic rock groundwater sources should be considered. Expanded distribution systems may be considered to even out groundwater supplies in the Midcoast area. In the event of extended lean rainfall years, alternative sources of water, including imported water, should be considered. Wells that are not in use should be considered for destruction in compliance with County and State guidelines. SUPPLEMENTAL RECOMMENDATION Since and have been dry years, the County should measure water levels in Midcoast wells to see how far they have gone down. This will provide hard data on what really happens during a dry year and provide valuable data for future calibration and validation of any modeling and further study. SAM:fc SAMT0348_WFM.DOC (4/20/09) 13

18 Attachment A Kleinfelder Midcoast Groundwater Study Study Area Map The groundwater subareas delineation are provided by Kleinfelder. Other data are derived from County GIS system. Map is produced by the Planning and Building Department Graphic Section. Meters ,250 2,500 3,750 Frenchmans Subbasin Frenchmans Terrace subarea Frenchmans Upland subarea Frenchmans Stream Valley subarea Arroyo de en Medio Subbasin Miramar subarea Arroyo de en Medio Upland subarea Arroyo de en Medio Stream Valley subarea El Granada Subbasin Graphic By: J. P. Walker El Granada subarea Date: 01/20/2006 El Granada Upland subarea Project No: File Name: A.1 Legend Airport Subbasin Airport subarea Denniston Upland subarea Denniston Stream Valley subarea Moss Beach Subbasin Lower Moss Beach subarea Dean Creek subarea Upper Moss Beach subarea Lighthouse subarea San Vicente Subareas San Vicente Upland subarea San Vicente Stream Valley subarea The San Vicente Upland and Stream Valley subareas contibute to both the Airport and Moss Beach Subbasins. Montara Creek Subbasin Portola subarea Montara Creek Upland subarea Lower Montara Creek subarea Wagner Valley subarea Plate Montara Terrace Subbasin Martini Upland Subbasin l:\planning\gis\gb\smc_groundwaterstudy-parcel.mxd ah

19 Midcoast Groundwater Study Summary Attachment B: Errata Revisions to Study 1. Third paragraph, page 1 - revise first sentence as follows: The San Mateo County Board of Supervisors has determined that because of the rapid growth use of individual wells within the Midcoast area of the County and the potential limited groundwater source in the area, a new comprehensive study of the hydrogeologic conditions of the area should be conducted. 2. Table 2, page 17 - correct data and revise footnote as follows: Hydrogeologic Units (Subareas) Number of Wells Table 2 Subarea Data Number of Septic Tanks Area (acres) Ocean Frontage (ft.) Potential New Residential Wells* 1 Frenchman s ,318 4 Miramar , El Granada , Airport , Lower Moss Beach , Upper Moss Beach Lighthouse Frenchman s 7 0 2, Arroyo de en Medio El Granada , Denniston 0 0 9, San Vicente 1 1 1, Dean Creek Portola Montara Knob Montara , Martini ,943 Stream Valleys 3 Frenchman s Arroyo de en Medio Denniston B.1

20 Hydrogeologic Units (Subareas) Number of Wells Table 2 Subarea Data Number of Septic Tanks Area (acres) Ocean Frontage (ft.) Potential New Residential Wells* 14 San Vicente Lower Montara Creek Wagner Valley Totals ,746 30,207 30,207 1,503 Numbers in left column refer to Hydrogeologic Units depicted on Plate 6. *Estimates of potential new residential wells within the Midcoast Urban Area were provided by Steve Monowitz (the San Mateo County Planning and Building Department,) and are based on an assumption that one single-family residence will be constructed and served by a well on each vacant lot in the R-1 and R-3 zoning districts as of 5/29/08 and 7/8/ First paragraph, page 29 revise second sentence as follows: For the Midcoast hydrogeologic study, hydrologic budgets were developed for subareas within the seven subbasins (Plate 3) and the urban limit lines shown on Plate Plate 2 - delete site reference. 5. Plate 7 replace Plate 7 contained in the report with the map of vacant parcels included as Attachment C. SAM:fc SAMT0348_WFM.DOC (4/20/09) B.2

21 RD Attachment C Midcoast Vacant Lots as of 2008 Legend Urban Rural Boundary EAST AVE KANOFF AVE 6TH ST CEDAR ST Montara LAS ALAMO ST FLORES RD Vacant Parcel (1562) Developed Parcels CALIFORNIA AVE CYPRESS AVE SUNSHINE VALLEY Moss Beach Meters CABRILLO HIGHWAY SAN RAMON AVE AIRPORT ST El Granada CALIFORNIA AVE AVENUE GRANADA BALBOA AVE Princeton CABRILLO AVE MAGELLAN AVE Miramar MIRADA RD l:\planning\gis\steve\midcoast_parcel gws ah The map is produced by Planning & Building Department Graphic Section based on 2008 Assessor's data..

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23 Midcoast Groundwater Study Phase III, San Mateo County, California Report prepared for: County of San Mateo Planning and Building Department Prepared by: Mark Woyshner Barry Hecht Jason Parke Travis Baggett Balance Hydrologics, Inc. June 2010

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25 TABLE OF CONTENTS 1. INTRODUCTION PURPOSE AND OBJECTIVES OF THE PHASE III STUDY COMMENCEMENT OF WORK ACKNOWLEDGMENTS MONITORING DATA RAINFALL STATIONS STREAM GAGING GROUNDWATER MONITORING Existing groundwater monitoring programs Search for inactive wells to monitor PREVIOUSLY REPORTED MONITORING DATA DISCUSSION OF DATA STATUS OF CURRENT DROUGHT DATA IMPLICATIONS BY SUBAREA El Granada Miramar and Frenchmans Creek Airport Moss Beach Montara and Ocean View Farms Portola and Wagner Valley Seal Cove CONCLUSIONS RECOMMENDATIONS SPECIFIC RECOMMENDATIONS BY SUBAREA (DISCUSSED IN SECTION 3.2) LIMITATIONS REFERENCES Final Report Text doc i

26 LIST OF TABLES Table 1. Table 2. Summary of well search for monitoring groundwater levels, Midcoast Groundwater Study Phase III, San Mateo County, California Annual rainfall and runoff records from previous drought to present, Midcoast Groundwater Study Phase III, San Mateo County, California Table 3. Summary of results from domestic water-well yield tests for home resale, 1991 through 2009, Midcoast Groundwater Study Phase III, San Mateo County, California Table 4. Table 5. Table 6. Comparison of Denniston Creek flow to previous drought, Midcoast Groundwater Study Phase III, San Mateo County, California Summary of current drought conditions, Midcoast Groundwater Study Phase III, San Mateo County, California Summary of methods and results by subarea, Midcoast Groundwater Study Phase III, San Mateo County, California LIST OF FIGURES Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Groundwater subareas, Midcoast Groundwater Study Phase III, San Mateo County, California Inactive wells monitored with dataloggers, Midcoast Groundwater Study Phase III, San Mateo County, California Inactive wells monitored by depth to water measurements only, Midcoast Groundwater Study Phase III, San Mateo County, California Inactive wells with cap glued on and not monitored, Midcoast Groundwater Study Phase III, San Mateo County, California Wells in use and not monitored, Midcoast Groundwater Study Phase III, San Mateo County, California Stream gaging stations, Midcoast Groundwater Study Phase III, San Mateo County, California Gaging stations with relatively high dry-season baseflow from late water year 2009 through early water year 2010, Midcoast San Mateo County, California Final Report Text doc ii

27 LIST OF FIGURES (CONTINUED) Figure 8. Figure 9. Gaging stations with relatively low dry-season baseflow from late water year 2009 through early water year 2010, Midcoast San Mateo County, California Gaging stations with little to no baseflow from late water year 2009 through early water year 2010, Midcoast San Mateo County, California Figure 10. Gaging stations with dry-season baseflow prior to the major storm of October 13, 2009, Midcoast San Mateo County, California Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Gaging stations with dry-season baseflow in November following the major storm of October 13, 2009, Midcoast San Mateo County, California Groundwater monitoring from dry-season baseflow into winter of water year 2010, Ocean View Farms, Montara and Portola areas, Midcoast San Mateo County, California Groundwater monitoring from dry-season baseflow into winter of water year 2010, Half Moon Bay Airport Aquifer, Moss Beach and Seal Cove areas, Midcoast San Mateo County, California Water levels measured in piezometers located at the north end of Pillar Point Marsh, San Mateo County, California Average percent of mean rainfall and runoff, water years 1997 to 2009, Midcoast Groundwater Study Phase III, San Mateo County, California Comparison of water levels measured in the Airport Aquifer, water years 1997 to 2010, San Mateo County, California Figure 17. Water levels measured in inactive well located near Arroyo de en Medio at 464 Third Avenue, Miramar, water years 1999 to present, San Mateo County, California Figure 18. Martini Creek flow above Old San Pedro Trail, water years 2004 to January 8, 2010, San Mateo County, California Figure 19. Figure 20. Figure 21. Montara Creek flow at Point Montara lighthouse old diversion dam, water years 2002 to present, San Mateo County, California Comparison of annual precipitation with the percentage of domestic wells tested that yielded greater than 2.5 gallons per minute, Midcoast Groundwater Study Phase III, San Mateo County, California Comparison of annual precipitation with specific capacity of domestic wells, Midcoast Groundwater Study Phase III, San Mateo County, California Final Report Text doc iii

28 LIST OF FIGURES (CONTINUED) Figure 22. Figure 23. Specific capacity of domestic wells tested by year, Midcoast Groundwater Study Phase III, San Mateo County, California Depth to water in domestic wells in use, Midcoast Groundwater Study Phase III, San Mateo County, California APPENDICES Appendix A. Long-term monthly rainfall records, Midcoast San Mateo County, California Appendix B. Baseflow gaging records, Midcoast San Mateo County, California Appendix C. Groundwater levels in MWSD and CCWD wells as far back as water year 2003, Midcoast San Mateo County, California Appendix D. Groundwater levels at LUST sites as far back as water year 1996, Midcoast San Mateo County, California Appendix E. Long-term groundwater levels in DWR monitoring wells as far back as water year 1953, Midcoast San Mateo County, California Final Report Text doc iv

29 1. INTRODUCTION San Mateo County Midcoast extends from northern Half Moon Bay to Devils Slide along Highway 1. It encompasses the communities of Montara, Moss Beach, Seal Cove, Princeton, El Granada, and Miramar. Domestic water is supplied to the southern part of the region by Coastside County Water District (CCWD), providing services from Half Moon Bay to Princeton, and by Montara Water and Sanitary District (MWSD) on the north, servicing Montara and Moss Beach. Private wells are scattered through both service districts and outlying areas. With the exception of CCWD, which meets much of its demand through long-term contractual agreements with San Francisco Public Utilities Commission, the source of domestic water is from local surface water and groundwater resources. The carrying capacity of the local resources may be considered as the population that can be sustained using conservation measures during an extended drought without undue stress to valued natural habitats. The San Mateo County Board of Supervisors seeks to identify the groundwater yield that may be safely taken from the Midcoast aquifers. County staff has requested a multi-phased technical report that may be used at the basin/watershed planning level, including aquifer management alternatives that could lead to the development of a Groundwater Management Plan (GMP). A GMP could be used to guide and respond to effects of groundwater development on public health and natural resources, and may lead to expedited and smoother permitting and have tangible benefits and cost savings programs. Balance Hydrologics (Balance) prepared a comprehensive literature and data review as a Phase I of the Midcoast Groundwater Study (Woyshner and others, April 2002). Kleinfelder subsequently executed Phase II of the study (Clark and others, October 2008), which included depth-to-water measurements and pump tests in selected wells and a water balance assessment by subarea. The water balance model consisted of two principal components: 1) Monthly water balances were performed for the record of rainfall from 1958 to 2005 to estimate percolation and runoff from published soil properties and evapotranspiration averages; and 2) Annual totals of percolation and runoff were used to estimate subarea groundwater storage and groundwater level using aquifer properties. Water levels from monitoring wells were used to calibrate the model. The model approach was similar to the water balances assembled for the Draft Montara-Moss Beach Water Well EIR (Hecht and others, 1989) and the El Granada Groundwater Investigation Report (Laduzinsky, Hecht, and Woyshner, 1988) Final Report Text doc 1

30 The Phase II water balance model was applied to the following sub-basins: El Granada Sub-basin (Subareas #7 and #8); Arroyo de en Medio Sub-basin (Subareas #4, #5, and #6); results were extended to the Frenchmans Creek Sub-basin (Subareas #1, #2, and #3); and Moss Beach Sub-basin (Subareas #12, #13, #14, #15, #19 and #20). The water balance for the El Granada Sub-basin was the least complex and utilized the most data for calibration and verification, followed by Arroyo de en Medio. The Moss Beach water balance was more complex and subjectively calibrated and validated. Findings from water balance of the El Granada Sub-basin were generally consistent with findings from a water balance assessment in the El Granada Groundwater Investigation Report (Laduzinsky, Hecht, and Woyshner, 1988). Based on water balance results, the Phase II study concludes the following: a) the Midcoast aquifers have a considerable groundwater surplus in above average rainfall years but can have a deficit in dry and very dry years; b) the marine terrace subareas appear to be in long-term hydrologic balance and should remain in long-term balance with a moderate increase in water extractions; c) current pumping rates have lowered the water table to near sea level during dry years, and potentially below sea level during very dry years, posing risks of saltwater intrusion; and, d) increased pumping over long periods of time, especially during drier years, will increase the amount of time that the water table falls near or below sea level; and this increases the risk of saltwater intrusion. The Phase II study recommends the following programs: a) long-term stream gaging, b) longterm monitoring of subarea index wells, c) improve and expand the information in the County s well database, and d) enhance monitoring in subareas with marginal groundwater production. Stream gaging and groundwater monitoring is needed to calibrate and further refine the water balance models Final Report Text doc 2

31 In summary, in addition to groundwater recharge from direct rainfall, Miramar and Frenchmans terraces appear to receive significant recharge from Arroyo de en Medio, Frenchmans Creek, and ostensibly other minor drainages, continuing into the dry season and supporting local groundwater levels. Current groundwater storage in proximity to the creeks appears have not exceeded previous drought levels. Additional groundwater monitoring is needed at distance from the creeks. One inactive well volunteered by a homeowner in upper Miramar would serve for this purpose. Additional gaging of Arroyo de en Medio is needed to quantify persistence of baseflows, with results used to calibrate a water balance model. A subbasin water balance model and a groundwater flow model would assist groundwater management Airport The Phase II study concluded that the Airport Subarea is in long-term equilibrium but did not conduct a drought analysis. We found many inactive wells were available for monitoring in the Airport Terrace: We installed dataloggers in 6 inactive wells that were reasonably distributed across the subarea, and at a three-level piezometer nest at the north end of Pillar Point Wetland (Figure 2). Water level data were available from MWSD and CCWD production wells (Appendix C). Data were available from LUST site shallow monitoring wells since WY2003 (Appendix D). Data were available from a DWR monitoring well from 1953 to 1991(Appendix E). We also measured depth-to-water in 1 inactive well near the Pillar Ridge Manufactured Home Park (Figure 3) and 1 domestic well in use in Princeton (Figure 5). Monitoring data indicate that groundwater storage was not as depleted as during previous droughts and storm recharge appeared normal during dry-season 2009 relative to pre-drought conditions. The nested piezometers showed artesian groundwater at Pillar Point Marsh and wells west of the airport runway showed shallow groundwater (Figure 13), which is consistent with reports at the Big Wave site (Christopher A. Joseph & Associates, October 2009) Final Report Text doc 20

32 Groundwater was high in the Airport Aquifer when compared to the previous drought, 1987 to 1992 (Figure 16). Static (not pumped) groundwater levels in MWSD wells were higher than predrought levels (Appendix C). LUST site groundwater levels (in Princeton) were within a normal range (Appendix D). Previous investigations identified that baseflows in Denniston Creek provide significant recharge to the Airport Terrace through the dry season (LSCE & ESA, 1992, 1991, 1987). During dry-season 2009, baseflows were gaged in Denniston Creek at two stations (Figure 6). The upper station was located at the canyon mouth below the reservoir, and the lower station was located below Capistrano Road at Princeton. Similar to findings during the previous drought, we observed a net loss of flow in the creek (Figure 10), which can be attributed to groundwater recharge and evapotranspiration. 11 In addition, flows were compared to measurements taken in 1990, during the previous drought. Denniston Creek flowed continuously through dry-season 2009, with higher flows than were recorded during the scattered measurements made throughout the previous drought. The measurement with lowest flow was taken in June 1990 (Table 4); lower flows and drier conditions in general would have persisted through the dry season of This comparison of the 2009 flow data with 1990 measurements suggests that the current drought is less severe than the previous drought. Baseflows in 2009, however, were significantly lower than during 2008 (Table 4). In summary, groundwater storage was not as depleted as during previous droughts and storm recharge appeared normal during dry-season 2009 relative to pre-drought conditions. Groundwater recharge from Denniston Creek through the Airport Terrace is significant during the dry season. The agricultural irrigation ponds at the northeast portion of the Airport Subarea, filled from diversion of flow in San Vicente Creek, also should provide recharge to that portion of the Airport Subarea. Groundwater levels at Pillar Point Marsh support normal marsh conditions and conditions potentially enabling sea-water intrusion were not observed. Additional analysis should include developing dry-season groundwater contour maps to compare with those reported during the 1987 to 1992 drought (LSCE & ESA, 1992, 1991, 1987). 11 Along San Mateo County Midcoast, reference evapotranspiration during September and October averages 2.5 to 3.3 inches per month (Snider, 1999), or about an average of 0.1 inches per day. With roughly 1 mile of stream riparian corridor at an average width of 130 feet between the two gages, the estimated evapotranspiration is roughly 30 gallons per minute. The difference in average flow as seen in Figure 10 is roughly 50 gallons per minute. Therefore, groundwater recharge is estimated at 20 gallons per minute, or roughly 3 acre-feet per month Final Report Text doc 21

33 Wells are available for continued monitoring and reported subsurface information are available for the sub-basin. A water balance model, drought analysis, and a groundwater flow model would assist groundwater management. Gaging Denniston Creek would greatly assist calibration of the models. In addition, the Airport Terrace is an ideal location for regional reference evapotranspiration (ETo) monitoring. California Irrigation Management Information System (CIMIS) only estimates ETo for the Midcoast and measured ETo would assist with calibration of all water balance models on the Midcoast Moss Beach The Phase II study concluded that a) Lower Moss Beach is in long-term equilibrium and not overdrafted during dry and critically dry years with current groundwater draws; b) additional groundwater may be available for pumping without inducing salt-water intrusion; c) Upper Moss Beach and Dean Creek Subareas are in long-term equilibrium, overdrafted during dry years and droughts, and recharges readily during wet years; and at buildout, Upper Moss Beach would not be in long-term equilibrium. Two inactive wells were offered by well owners in the Lower Moss Beach Subarea for monitoring during the Phase III study (Figure 2). One well was near San Vicente Creek and one well was near the Seal Cove subarea. In the Upper Moss Beach Subarea, no inactive wells were volunteered but we measured depth-to-water in a well in use when the residence was out of town for a few days (Figure 5). In lower Sunshine Valley (Dean Creek), depth-to-water was measured in an inactive well near wetlands (Figure 3). Data from LUST site shallow monitoring wells were available for Lower Moss Beach since WY2005 and for Upper Moss Beach since WY2007 (Appendix D). San Vicente Creek was gaged at Fitzgerald Marine Reserve (Figure 6). San Vicente Creek through its Lower Moss Beach reach was dry during the dry season and developed some flow following the October 13th storm (Figure 9). Dean Creek was dry with few isolated pools near its mouth during the dry season. Groundwater levels near San Vicente Creek were shallow and were generally unchanged and groundwater levels near the wetland in lower Sunshine Valley appeared normal (Figure 13). 12 A recent study of low flows on lower Pilarcitos Creek (Parke and Hecht, 2010) also identifies lack of local evapotranspiration data as a major limitation to assessing streamflow losses affecting steelhead habitat on that stream. ETo data collected for the Airport Subarea would benefit the entre Midcoast Final Report Text doc 22

34 Montara Pacific Ocean 20 Moss Beach 12 Marti ni Creek Kanoff Creek Daffodil Canyon Montara Creek Dean Creek &% &% &%&% 21 &% 16 &%&% San Vicente Creek «1 &% &% &% 9 &% &% &% &% El Granada Denniston Creek Deer Creek Arroyo de en Medio Groundwater Sub-areas Frenchmans Terrace Frenchmans Uplands Frenchmans Stream Valley Miramar Terrace Arroyo de en Medio Uplands Arroyo de en Medio Stream Valley El Granada Terrace El Granada Uplands Airport Terrace Denniston Uplands Denniston Stream Valley Lower Moss Beach San Vicente Uplands San Vicente Stream Valley Dean Creek / Sunshine Valley Portola Montara Knob Lower Montara Creek Upper Moss Beach Lighthouse Wagner Valley Montara Terrace Martini Uplands Seal Cove Mavericks Groundwater sub-area boundaries adapted from the Phase II study. Legend Creek Watershed group Active production well &% CCWD well &% MWSD well Ü Half Moon Bay 4 1 Frenchmans Creek / 0 2,000 4,000 Feet Figure C1: Active production wells for Montara Water and Sanitary District and Coastside County Water District, Midcoast Groundwater Study Phase III, San Mateo County, California figures.mxd 2010 Balance Hydrologics, Inc.