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EXECUTIVE SUMMARY El Toro Groundwater Study

Monterey County

Background

El Toro Planning Area is a watershed-based planning area in Monterey County south of Salinas along the western margin of the Salinas Basin (Figure ES-1). Surface water in the El Toro watershed drains approximately 19,000 acres to Toro Creek, which flows northeastward into the Salinas River. Most of the development of the El Toro Planning Area is along Hwy 68, which connects Monterey and Salinas. The El Toro Planning Area includes five designated planning subareas based on local topographic drainage divides: Calera Creek, Watson Creek, Corral de Tierra, San Benancio Gulch, and El Toro Creek. However, water supply for the El Toro Planning Area is derived entirely from groundwater and major portions of the El Toro Planning Area subareas are hydrogeologically contiguous as are the aquifer systems beneath the northwest portion of the El Toro Planning Area and the adjacent Laguna Seca portion of the Seaside Basin along Hwy 68.

Decline of groundwater levels in some El Toro Planning Area wells during the 1980s, and the findings of a 1991 technical report (Staal, Gardner & Dunne, 1991), resulted in the County imposing a B-8 zoning overlay in November 1992 to portions of the El Toro Planning Area due to potential water supply limitations. The B-8 zoning limits development to single-family dwellings on existing lots of record since 1991. Figure ES-2 shows the El Toro Planning Area, the subareas, the extent of the B-8 zoning, water systems, and the locations of wells for which information was compiled and reviewed for this study.

Subsequent technical analysis by Fugro (1996), which included more detailed assessment of water demand and recharge, generally supported the findings reported by Staal, Gardner & Dunne (1991) including an estimated water supply surplus in all the subareas of the El Toro Planning Area with the exception of Calera Creek. Both the 1991 and 1996 reports, however, cautioned that demand approaches supply in most of the planning subareas and that build-out demand would exceed some estimates of supply. The Fugro report (1996) also noted that the accuracy of estimates of recharge could not be confirmed without better groundwater level data. An expanded water level monitoring program was recommended to verify the groundwater level trends prior to additional hydrogeologic investigation and analysis of water resources in the El Toro Planning Area. Groundwater level data from 1960 to 2007 were compiled and analyzed for 45 wells in the El Toro Planning Area and vicinity for this study and provide a key component of the basis for the findings.

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The primary objective of this study was to evaluate groundwater resource capacity of the El Toro Planning Area and recommend maintaining or revising the B-8 zoning overlay. Additional objectives and tasks completed for this study are consistent with recommendations of the Fugro report (1996) and included compiling water well and hydrostratigraphic information for the El Toro Planning Area; conducting aquifer testing; collecting and analyzing water samples from wells; developing a conceptual hydrogeologic model of the El Toro Planning Area; and evaluating hydrogeologic connectivity between existing subareas.

Groundwater Production and Sources of Information Compiled on Wells

The majority of groundwater production in the El Toro Planning Area is from the Santa Margarita marine sandstone and overlying Plio-Pleistocene continental deposits, which are also referred to as the Aromas and Paso Robles Formations. Many wells are screened in both the Santa Margarita sandstone and Plio-Pleistocene continental deposits units. In this report these units are referred to collectively as the El Toro Primary Aquifer System.

Water supply in the El Toro Planning Area includes two commercial well fields and associated distribution systems, hundreds of individual domestic wells, and at least 150 small community water systems (mutual water companies). The two commercial public water systems in the northern portion of the El Toro Planning Area, Toro Water System and Ambler Water System, are regulated by the State of California Department of Health Services (DHS). Information on these water systems was provided by DHS, the Monterey County Water Resources Agency (MCWRA) and California American Water (Cal Am). Files at the Monterey County Health Department’s Environmental Health Division (MCEHD) were a primary source of information for small community water systems. The MCWRA currently monitors water levels at 22 wells and historically monitored as many as 38 wells in the El Toro Planning Area.

Groundwater Quality and Aquifer Properties

Groundwater quality in the El Toro Planning Area is generally poor. Based on compilation of groundwater chemistry data from MCEHD and DHS files and analyses of samples collected from 25 wells for this study, arsenic concentrations exceed the primary maximum contaminant level (MCL) of 10 µg/l in 33% (27 of 82) of wells with available data. Also, iron, manganese, chloride, sulfate and/or total dissolved solids (TDS) exceed secondary MCLs in 78% (64 of 82) of wells with available data. Figure ES-3 shows a widespread distribution of wells in the El Toro Planning Area with water quality data that does not meet regulatory drinking water standards.

Aquifer properties were estimated for 60 well locations based on analysis of aquifer testing conducted for this study, analysis of pumping test data in MCEHD files, and calculation of transmissivity from specific capacity data in MCEHD files. Based on these data, 0.5 to 1 ft/day is a reasonable range for regional average bulk hydraulic conductivity

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of the El Toro Primary Aquifer System. Production rates reported to MCEHD are commonly based on short-term data that provide overly optimistic estimates of well yield that may not reflect sustainable pumping conditions.

Groundwater Production Potential

Large saturated thickness of the El Toro Primary Aquifer System is necessary to sustain significant pumping rates because bulk hydraulic conductivity values are low. Accordingly the majority of groundwater pumping is concentrated in the northwest portion of the El Toro Planning Area near Hwy 68 where the saturated thickness of the El Toro Primary Aquifer System is greatest. The saturated thickness of the El Toro Primary Aquifer System determines which portions of the El Toro Planning Area have good groundwater production potential (Figure ES-4).

In general, groundwater production potential is poor where the saturated thickness of the El Toro Primary Aquifer System is small—less than 100 to 200 feet. Groundwater production potential is also poor where the water table occurs in the Monterey Formation, such as the upland portions of the San Benancio and Watson Creek subarea. And, groundwater production potential is negligible in areas underlain by granitic or metamorphic basement rocks, such as the portion of the San Benancio Gulch subarea northeast of the Harper Fault and Calera Creek subarea south of the Chupines Fault. Four classifications of groundwater production potential (good, poor, possible, and neglible) are delineated on Figure ES-4.

The basal sand unit beneath the Monterey Shale is locally an important aquifer in the Upper Corral de Tierra Valley. Flowing artesian conditions reported in this area (Thorup, 1971; MCEHD files) likely result from pressurization at depth where the basal sand aquifer is truncated by faulting in Upper Corral de Tierra Valley. Potential for additional groundwater production is considered favorable in this area (Thorup, 1971), although the volume of groundwater in storage is minor in comparison to the much thicker El Toro Primary Aquifer System in lower Corral de Tierra valley and along Hwy 68.

Trends of Groundwater Levels

The MCWRA maintains a long-term record of water levels that facilitate assessment of groundwater elevations and trends of water levels in portions of the El Toro Planning Area. Trend lines plotted on graphs of water levels with time (hydrographs) were fitted to the entire data record to provide long-term average rates of change, and to data collected since 1999 to provide short-term average rates of change in recent years. Most of the long-term trend values are based on a sufficiently long period of record so that they reflect change in groundwater elevations that is independent of short-term fluctuations associated with rainfall variation. The trends of data collected since 1999 are more likely to reflect short term influence by rainfall because the period of record is only a few years.

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Approximately 80% of the long-term hydrographs exhibit a downward trend in groundwater level and the overall average long-term rate of change in groundwater level is -0.6 ft/yr. Recent short-term trends based on data collected since 1999 are downward for 90% of the hydrographs, and the average rate of change in groundwater since 1999 is -1.8 ft/yr. Figure ES-5 is a map with contoured values of water level change rate based on long-term trends. Similar decline in groundwater levels has been documented in the Laguna Seca area (Yates et al., 2002), which borders the El Toro Planning Area to the west along Hwy 68 and is hydrogeologically contiguous.

Water Budget

County Planning provided a refined inventory of existing and future development in the El Toro Planning Area for the hydrogeologic update reported by Fugro in 1996. The updated estimates of dwelling units and net water demand considered all individual parcels in 1995 and at build-out for each planning subarea. The estimate of additional number of dwelling units at build-out was based on assessment of individual parcels and included existing limitations on development. Every parcel that was unoccupied in 1995 was assumed to have one dwelling unit at build-out. Reasonable estimates of water duty factors and return flows for various land uses were assigned for annual water demand analysis. Estimated build-out demand was 2145 acre feet per year (AF/Y), a factor of 1.7 greater than the estimated demand for 1995 conditions (Fugro, 1996).

Precipitation is the principal source of water for the hydrologic budget of the El Toro Planning Area. Based on refined detailed modeling evaluation of recharge, which included down-scaling of recharge areas based on the proportion of land area underlain by poor water bearing geologic units, Fugro (1996) estimated that average annual recharge in the El Toro Planning Area is in the range of 2 to 3 inches, or 1902 to 2852 acre feet, which is approximately 10 to 20 percent of average rainfall.

The range of recharge estimated by Fugro (1996) brackets the estimated build-out demand for the El Toro Planning Area. For the lower recharge value they estimated an average water balance deficit of the approximately 250 AF/Y. For the higher recharge value they estimated a water balance surplus of approximately 700 AF/Y. The calculations reported by Fugro (1996) showed the largest water supply deficit at build-out in the Calera Canyon subarea and the largest surplus in the Watson Creek subarea.

Based on hydrogeologic analysis conducted for this study, however, limited saturated thickness of the El Toro Primary Aquifer System in the majority of the Watson Creek subarea indicates poor groundwater production potential and casts doubt on the existence of surplus groundwater supply in the Watson Creek subarea (Figure ES-4). Revision of the recharge modeling to account for limited saturated thickness of the El Toro Primary Aquifer System in the Watson Creek and the upper portions of San Benancio subareas

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was not part of the scope of this study, but would likely result in a range of recharge significantly lower than the estimated build-out demand.

Projected Depletion of Groundwater Storage

Downward trends for the majority of long-term hydrographs indicate that the rate of groundwater pumping from the El Toro Primary Aquifer System exceeds the rate of groundwater replenishment. Compilation of trend analyses for long-term hydrographs clearly shows groundwater overdraft conditions in the northern portion of the El Toro Planning area near Hwy 68, where the majority of pumping occurs (Figure ES-5).

Based on the geometry of the El Toro Primary Aquifer System (Figure ES-4) we have calculated and graphed average drop in groundwater level1 with time in response to a range of water balance deficits (Figure ES-6). For example, a constant deficit of 500 AF/Y for approximately 25 years results in a drop in groundwater level of 20 feet, which is similar to the average long-term rate of decline of 0.6 ft/yr estimated from the trend analyses of the long-term hydrographs. The calculations illustrated by Figure ES-6 suggest that the average recent rate of groundwater decline of 1.8 ft/yr in the El Toro Primary Aquifer System is consistent with a deficit in excess of 1000 AF/Y. Figure ES-6 also shows calculated groundwater level decline with time for an initial deficit of 500 AF/Y increasing 20 AF/Y and 50 AF/Y. This initial deficit and range of increasing pumping is consistent water balance calculations and records for the Ambler and Toro Water Systems, which both show pumping rates increasing at approximately 10 AF/Y (Figure ES-7). The last calculation shown on Figure ES-6 is groundwater level decline with time for an initial deficit of 500 AF/Y increasing 50 AF/Y for 20 years after which the rate increase is cut to 10 AF/Y. This calculation is intended to illustrate the potential benefit of long-term water supply conservation and recycling efforts.

Conclusions and Recommendations

Water level data compiled and reviewed for this study indicates that the primary aquifer system in the El Toro Planning Area is in overdraft. However, current and increasing rates of pumping can be sustained for decades in areas with large saturated thickness of the El Toro Primary Aquifer System because of the large volume of groundwater in storage. The most evident problem will be lowering of the water table below the screened intervals of existing wells completed in shallower portions of the aquifer system, which has already occurred in portions of the Corral de Tierra.

If long term declines in groundwater levels and reliance on groundwater storage are acceptable to the County, the B-8 zoning could be lifted in areas with large saturated thicknesses of the El Toro Primary Aquifer System where additional groundwater 1 These calculations assume uniform decline in groundwater level in the entire El Toro Primary Aquifer System; however, actual declines are expected to be greater near pumping wells and less away from pumping wells.

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production is feasible for several decades. However, if County Policy does not allow overdraft conditions and mining of groundwater, the B-8 zoning should be expanded to cover the entire extent of the El Toro Primary Aquifer System.

Because groundwater production diminishes with decreasing saturated thickness of the El Toro Primary Aquifer System, expansion of B-8 zoning is appropriate, and recommended for areas where the saturated thickness of the El Toro Primary Aquifer System is less than 100 to 200 feet, regardless of policy on overdraft. A notable exception, based on limited data, is the Upper Corral de Tierra Valley where the Basal Sand unit is a relatively productive aquifer. Significant additional groundwater production may be feasible in this area, but the quantity groundwater in storage is more limited than the much thicker El Toro Primary Aquifer System near Hwy 68.

Additional suggestions and recommendations for improved management, utilization, and preservation of water resources in the El Toro Planning Area are summarized below:

• Eliminate the designated planning subareas for water resource management that are based on watershed topographic boundaries because they are not relevant to the groundwater aquifers, which are the sole source water supply in the El Toro Planning Area.

• Establish a formal collaborative groundwater management program for the Laguna Seca and El Toro Planning Areas because they are hydrogeologically contiguous.

• Install dedicated groundwater monitoring wells in the El Toro Planning Area (several already exist in Laguna Seca). Also, utilize additional inactive wells as monitoring wells.

• Evaluate feasibility of a wastewater reclamation program for golf course and possibly also domestic property irrigation in the Corral de Tierra area to reduce the rate that water is exported with sewage to the Salinas Valley. Possibilities include treating sewage effluent and developing separate greywater recycling and distribution systems.

• Evaluate feasibility of retaining surface water runoff and enhancing aquifer recharge.

• Evaluate feasibility of surface water impoundment in upper the Calera Canyon area for supplementary water supply.

• Evaluate feasibility of additional groundwater production and storage in Upper Corral de Tierra Valley with distribution to lower portions of Watson Creek and Calera Canyon.

• Conduct a geotechnical assessment to evaluate potential risk of ground subsidence near Hwy 68 with continued decline of groundwater levels.

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Calera Creek

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Study Well Locations, Water System Boundaries and B8 Zoning

El Toro Groundwater StudyMonterey County, California

Figure

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June 2007

3,000 0 3,0001,500 Feet

ToroWater System

ToroWater System

Ambler ParkWater System

NOTES:MCWRA = Monterey County Water Resources AgencyMPWD = Monterey Penninsula Water Management DistrictWater systems represented by color-shaded groups of parcels

This figure was originally produced in color. Reproduction in black and white may result in loss of information.

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Calera Creek

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Groundwater Chemistry Showing Locations with MCL Exceedences

El Toro Groundwater StudyMonterey County, California

Figure

ES-3P:\GIS\ElToro\project\20070524mtg\GWChemv2.mxd

June 2007

3,000 0 3,0001,500 Feet

Legend#* Historical Groundwater Sample

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Laguna Seca Area

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Chupines Fault

Calera Creek

Watson Creek

Corral De Tierra

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Primary Aquifer System Saturated Thickness and Groundwater Production Potential

El Toro Groundwater StudyMonterey County, CA

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June 2007

4,000 0 4,0002,000 Feet

NOTES:Saturated thickness based on January 2001 water levels, Anderson & Nichols isopach maps (1981) and USGS digitalelevation model. Geology adapted from Clark et al. (2000).

This figure was originally produced in color. Reproduction in black and white may result in loss of information.

Legend

Estimated Saturated Thickness (feet)

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Estimated Annual Rate of Change of Groundwater Elevation

Based on Long-Term TrendsEl Toro Groundwater StudyMonterey County, California

Figure

ES-5P:\GIS\ElToro_Monterey\project\20070524mtg\wlChangeRatev2.mxd

June 2007

3,000 0 3,0001,500 Feet

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NOTES:Long term rate of change of groundwater elevation basedon trend analysis of available water level data for period from1960 to 2006.ft/yr = feet per year

This figure was originally produced in color. Reproductionin black and white may result in loss of information.

Change of Groundwater Elevation (ft/yr)

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Calculated Drop in Water Level with Time for a Range of Overdraft ScenariosEl Toro Groundwater Study

Monterey County, CA

Figure

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June 2007

NOTES:- Drainable Porosity = 0.1- Based on recent data, production rate from Ambler Park and Toro Water Systems are each increasing at 10 AF/Y- Calculation assumes uniform drop in groundwater level in entire QTc-Tsm aquifer system, but

actual drop is expected to be greater near pumping wells and less away from pumping wells.

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Toro Water Service & Ambler Park Annual Pumping

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AF/y

Alco Toro Wells Cal Am Ambler Wells Linear Best-Fit Trend

Each increasing ~10 afy

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Toro and Ambler Annual Pumping and Trends

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~10 AF/Y