rfb_13199_for_info_only_10-groundwater_resources_investigation_report.pdf

Groundwater Resources Investigation Report Cole Grade Road Bridge Over Keys Creek Project

Valley Center San Diego County, California

Oracle No. 1003046

County of San Diego – DPW Materials Lab 5252 Kearny Villa Way, San Diego, California 92123

June 21, 2023 | Project No. 108611016

Geotechnical | Environmental | Construction Inspection & Testing | Forensic Engineering & Expert Witness

Geophysics | Engineering Geology | Laboratory Testing | Industrial Hygiene | Occupational Safety | Air Quality | GIS

Groundwater Resources Investigation Report Cole Grade Road Bridge Over Keys Creek Project Valley Center San Diego County, California Oracle No. 1003046

Prepared for:

County of San Diego – DPW Materials Lab 5252 Kearny Villa Way | San Diego, California 92123

June 21, 2023 | Project No. 108611016

Courtney Brooks Principal Hydrogeologist

Stephan A. Beck, PG, CHG Manager, Environmental Sciences Division

CJB/SB/gah/mp

5710 Ruffin Road | San Diego, California 92123 | p. 858.576.1000 | www.ninyoandmoore.com http://www.ninyoandmoore.com/

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 i

CONTENTS

GLOSSARY OF TERMS, ACRONYMS, AND ABBREVIATIONS I

EXECUTIVE SUMMARY I

1 INTRODUCTION 1

1.1 Purpose of the Project 1

1.2 Project Location and Description 1

1.3 Applicable Groundwater Regulations 2

1.3.1 California Environmental Quality Act 2

1.3.2 San Diego County Groundwater Ordinance (2013) 2

2 EXISTING CONDITIONS 3

2.1 Topographic Setting 3

2.2 Climate 4

2.3 Land Use 4

2.4 Water Demand 5

2.5 Geology and Soils 5

2.6 Hydrogeologic Units 5

2.7 Hydrologic Inventory and Groundwater Levels 6

2.8 Water Quality 8

3 WATER QUANTITY IMPACT ANALYSIS 8

3.1 50 Percent Reduction of Groundwater in Storage 8

3.2 Groundwater Overdraft Conditions 9

3.3 Well Testing 9

3.3.1 Guidelines for the Determination of Significance 9

3.3.1.1 Well Interference in Fractured Rock 9

3.3.1.2 Well Interference in Alluvial or Sedimentary Basins 10

3.3.1.3 Low Well Yield/Residual Drawdown 11

3.3.1.4 Groundwater-Dependent Habitat 11

3.3.2 Well Test Methodology 12

3.3.2.1 Well Test Description 12

3.3.2.2 Well Test Analysis 13

3.3.3 Potentially Significance of Impacts Prior to Mitigation 15

3.3.4 Mitigation Measures and Design Considerations 17

3.3.5 Conclusions 17

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 ii

4 SUMMARY OF PROJECT IMPACTS AND MITIGATION 17

4.1 Well Interference 17

4.2 Groundwater-Dependent Habitat 18

4.3 Mitigation Measures 18

5 LIMITATIONS 18

6 REFERENCES 20

7 LIST OF PREPARERS 21

TABLES

1 – Watershed Characteristics 2 – Average Annual Precipitation Data, 2016-2021 3 – Soil Type and Select Soil Properties 4 – Water Well Inventory 5 – Groundwater Water Quality Objectives

FIGURES

1 – Regional Location Map 2 – Watershed Boundary 3 – Site Plan 4 – Precipitation Map 5 – Current General Plan Land Use 6 – Geological Map 7 – Soils Map 8 – Well Inventory Map 9 – Well Interference Analysis 10 – Potential Groundwater Dependent Vegetation

APPENDICES

A – Boring Logs and Log of Test Borings B – Well Completion Reports C – Vegetation Map D – Valley Center Municipal Water District Service Area Map

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GLOSSARY OF TERMS, ACRONYMS, AND ABBREVIATIONS

ACB Articulated concrete block B Boron bgs Below ground surface CEQA California Environmental Quality Act Cl Chlorides County County of San Diego DEHQ County of San Diego Department of Environmental Health and Quality DPW Department of Public Works DWR California Department of Water Resources F Fluoride ft/d Foot per day Fe Iron GPM Gallons per minute HA Hydrologic Area HSA Hydrologic Subarea MBAS Methyl blue-activated substances Mn Manganese MSL Mean sea level NL Not listed NO3 Nitrate NOI Notice of Intent NPDES National Pollutant Discharge Elimination System NTUs Nephelometric turbidity units %Na Percent sodium Project Cole Grade Road Bridge Over Keys Creek Project PVC Polyvinyl Chloride RWQCB Regional Water Quality Control Board sf Square feet SO4 sulfate TDS Total dissolved solids TMDLs Total Maximum daily loads Turb Turbidity VaB Visalia sandy loam, 2 to 5 percent slopes VLCC1 Valley Center Weather Station

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 i

EXECUTIVE SUMMARY

Ninyo & Moore was retained by the County of San Diego (County), Department of Public

Works (DPW) to perform a Groundwater Resources Investigation for the Cole Grade Road

Bridge Over Keys Creek Project (Project). The Groundwater Resources Investigation was prepared to address California Environmental Quality Act (CEQA) requirements in accordance with the County’s Approved Guidelines for Determining Significance and Report Format and

Content Requirements, Groundwater Resources (County of San Diego, 2007) and County of

San Diego Planning & Development Services, Groundwater Scoping, Keys Creek Bridge, Groundwater Investigation Requirements Memorandum (County of San Diego, 2021).

Specifically, the purpose of this Groundwater Resources Investigation is to evaluate the potential effects of temporary, localized dewatering of groundwater during the construction phase of the Project, document the groundwater resources at the site and vicinity, evaluate potential impacts to groundwater resources resulting from the Project, and to recommended measures to mitigate potentially significant impacts.

The Project is located in the unincorporated community of Valley Center in the County of

San Diego on Cole Grade Road where it crosses Keys Creek between Cool Valley Ranch Road and Cool Valley Road (site; Figures 1 and 2). The Project will provide a new bridge crossing of

Cole Grade Road over Keys Creek intended to meet current roadway and bridge standards and accommodate the planned road widening (Figure 3). As originally designed, the Project included deep foundations to support the bridge structure, which required temporary dewatering to construct the bridge foundation. However, due to concerns about potential temporary impacts to groundwater resources and groundwater-dependent habitat and input from local property owners, the bridge foundation was re-designed to eliminate the need for dewatering during foundation construction.

In order to address potential scouring of the Keys Creek channel, the design includes the installation of articulated concrete block (ACB) on the east and west sides of the bridge structure, a portion of which will require temporary dewatering to prepare subgrade to accommodate placement of the ACB in the channel (Figure 3). The ACB installation is anticipated to require temporary dewatering in two stages (Stage 1 and Stage 2, Figure 3), with an estimated duration of 1.5 months for each stage and an estimated 6 months without construction dewatering between stages. The cumulative volume of dewatering effluent that may be generated during both stages of dewatering (3 months) may be on the order of

1.6 million gallons.

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For the purpose of this Groundwater Resources Investigation, the study area is a ½-mile radius around the new bridge structure (Project area). A summary of the findings and results of the

Groundwater Resources Investigation is provided below.

• Although the Project proposes temporarily, localized construction dewatering, the County requested an analysis of potential impacts to offsite production wells. Parcels within ½-mile radius of the site were evaluated based on the following water source criteria (Figure 9):

o A- Served by groundwater only o B - Served by groundwater and the local water district (Valley Center Water District) o C - Served by local water district only o D - Undeveloped / no water use

• The adjoining parcels are either served by the Valley Center Water District and groundwater or were undeveloped / no water use. Two parcels are serviced by groundwater only, and are located greater than 500 feet west of the site (Appendix D).

• The results of the well testing indicate that drawdown is not predicted to exceed the County’s most conservative well interference threshold of significance of a decrease in water level of 5 feet or more in offsite wells (County of San Diego, 2007) (Figure 8).

• Potential groundwater-dependent habitats, as mapped by a County biologist (Figure 10), consist of Southern Coast Live Oak Riparian Forests and are generally present along Keys Creek and tributaries within ¼-mile from the site. Project dewatering may result in a temporary drawdown of groundwater 3 feet or greater than historical low groundwater levels, which is the County’s threshold to evaluate potential impact to groundwater-dependent habitat (County of San Diego, 2007). It should be noted that readily-available data for historical low groundwater levels were not found / provided, therefore, as a conservative measure, groundwater levels from previous fieldwork (Ninyo & Moore, 2015; 2022b) were used.

• As a mitigation measure to offset potential temporary, localized impacts to groundwater-dependent habitat, Ninyo & Moore recommends that the extracted groundwater be beneficially used for irrigation of groundwater-dependent habitat (Figure 10). Irrigation of groundwater-dependent habitat should be overseen by an irrigation and landscaping specialist to limit the potential for overwatering and erosion. To accommodate the potential for construction dewatering occurring during a period of significant precipitation when irrigation of groundwater-dependent habitat may not be warranted, Ninyo & Moore recommends that the County obtain a Regional Water Quality Control Board National Pollutant Discharge Elimination System permit, so that if necessary, groundwater generated by temporary construction dewatering can be returned to the watershed through discharge into Keys Creek downstream from the Project.

• Based on the findings of this Groundwater Resources Investigation, the temporary construction dewatering anticipated to be associated with the proposed Project would have a less than significant impact to groundwater-dependent habitat, as defined by the County CEQA guidelines (County of San Diego, 2007), with the incorporation of the recommended mitigation measure (see bullet above).

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 1

1 INTRODUCTION

Ninyo & Moore was retained by the County of San Diego (County), Department of Public

Works (DPW) to prepare a Groundwater Resources Investigation Report for the Cole Grade

Road Bridge Over Keys Creek Project (Project).

1.1 Purpose of the Project

This Groundwater Resources Investigation Report was prepared to address California

Environmental Quality Act (CEQA) requirements in accordance with the County’s Approved

Guidelines for Determining Significance and Report Format and Content Requirements (County of San Diego, 2007), and County of San Diego Planning & Development Services, Groundwater

Scoping, Keys Creek Bridge, Groundwater Investigation Requirements Memorandum (County of San Diego, 2021). Specifically, this Groundwater Resources Investigation Report evaluates potential effects of anticipated temporary dewatering during the construction phase of the proposed Project, discusses groundwater resources at the site and vicinity, evaluates potential groundwater resources impacts resulting from the proposed Project, and recommends measures to mitigate potentially significant, temporary localized impacts to groundwater-dependent habitat.

1.2 Project Location and Description

The Project is located in the unincorporated community of Valley Center in the County of

San Diego on Cole Grade Road where it crosses Keys Creek between Cool Valley Ranch Road and Cool Valley Road (Figures 1 and 2). Cole Grade Road is an approximately 7.9-mile, two-lane road that extends from State Route 76 south to its terminus at the intersection with

Valley Center Road. The current crossing at Keys Creek comprises a double 8-foot by 8-foot reinforced concrete box culvert. The existing drainage within Keys Creek generally flows to the west towards Keys Canyon and then into the San Luis Rey River.

The Project will provide a new bridge crossing of Cole Grade Road over Keys Creek intended to meet current roadway and bridge standards and accommodate planned road widening

(Figure 3). As originally designed, the Project included deep foundations to support the bridge structure, which required temporary dewatering to construct the bridge foundation. However, due to concerns of potential temporary, localized impacts to groundwater resources and groundwater-dependent habitat and input from local property owners, the bridge foundation was re-designed and the need for dewatering during foundation construction was eliminated.

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 2

In order to address potential scouring of the Keys Creek channel, the design includes the installation of articulated concrete block (ACB) on the east and west sides of the bridge structure, which will require temporary, localized dewatering to prepare subgrade to accommodate placement of the ACB in the channel. The ACB installation is anticipated to require temporary dewatering in two stages (Stage 1 and Stage 2, Figure 3), estimated to be

1.5 months during each stage, with an estimated 6 months without construction dewatering between the two stages.

For the purpose of this Groundwater Resources Investigation, the study area is a ½-mile radius around the new bridge structure (Project area; Figure 2).

1.3 Applicable Groundwater Regulations

Applicable groundwater regulations are discussed in the following sections.

1.3.1 California Environmental Quality Act

Under CEQA, lead agencies are required to consider potential impacts to groundwater and water quality when considering discretionary actions. Section 10 of Appendix G

(Hydrology and Water Quality) of the State CEQA Guidelines lists the following two questions related to groundwater resources:

1) Would the project violate any water quality standards or waste discharge requirements or otherwise substantially degrade surface or groundwater quality?

2) Substantially decrease groundwater supplies or interfere substantially with groundwater recharge such that the project may impede sustainable management of the basin?

The County’s Guidelines for Determining Significance and Report Format and Content

Requirements for Groundwater Resources provides guidance for evaluating potentially adverse environmental effects a project may have on groundwater resources and are based on CEQA guidelines (County of San Diego, 2007). The potential effects of the

Project are evaluated in Section 3.

1.3.2 San Diego County Groundwater Ordinance (2013)

The County currently manages anticipated future groundwater demand through the

County Groundwater Ordinance. This Ordinance identifies specific measures to mitigate potential groundwater impacts of projects requiring specified discretionary permits.

According to Section 67.722(B) of the County Groundwater Ordinance, for discretionary

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 3 permit applications (non-single-family subdivision projects), the following findings must be made:

1) For a water intensive use, that groundwater resources are adequate to meet the groundwater demands both for the project and the groundwater basin if the basin were developed to the maximum density and intensity permitted by the General Plan;

or

2) For all other projects, that groundwater resources are adequate to meet the groundwater demands of the project.

Because the Project does not have a listed discretionary permit within Section 67.711 of the County Groundwater Ordinance, the above criteria are not applicable and the Project is not subject to the County Groundwater Ordinance.

2 EXISTING CONDITIONS

The following subsections include details on describing the physical, geologic, and hydrogeologic characteristics of the Project and Project’s sub-watershed, and include:

topography, climate, land use, water demand, geology and soils, hydrogeologic units, hydrologic inventory, groundwater levels, and water quality.

2.1 Topographic Setting

The ground elevation in the vicinity of proposed improvements varies from approximately

1420 feet above mean sea level (AMSL) within portions of the Keys Creek channel to 1434 feet above AMSL within the existing travel lanes of Cole Grade Road at the Key Creek crossing. The east and west sides of the existing embankments are sloped with estimated inclinations on the order of 1.5:1 (horizontal to vertical).

The Project area is located in the Keys Creek Sub-Watershed, within the Middle San Luis Rey

River and San Luis Rey-Escondido Watersheds. A map depicting the sub-watershed is provided as Figure 2. Keys Creek Sub-Watershed characteristics are provided in the table below.

Table 1 – Watershed Characteristics

Sub-Watershed Area (acres)

Topography (feet AMSL) Surface Drainage Pattern

Keys Creek 23,500 130 to 2,175 Topography generally slopes toward Keys Creek and elevation decreases from the east portion of the watershed to northwest.

References: USGS Watershed Boundary Dataset

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2.2 Climate

According to the County of San Diego, General Plan Update, Groundwater Study (County of

San Diego, 2010), 30-year average annual precipitation at the Project area ranged from 18 to 21 inches (Figure 4). Local weather stations were additionally researched for more current data. The annual precipitation data for the Valley Center (VLCC1) weather station from 2016-2021 are provided in the table below.

Table 2 – Average Annual Precipitation Data, 2016-2021

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AV G

2016 4.15 0.10 1.61 0.55 0.31 0 0 0 0.68 0.02 1.62 4.60 13.6 2017 6.98 6.21 0.21 0 1.32 0.03 0 0.05 0.03 0 0.02 0.02 14.9 2018 3.45 0.94 2.43 0.04 0.07 -- -- -- -- 1.58 1.74 1.71 12.0 2019 3.19 9.02 1.64 0.20 1.64 0.05 -- -- 0.28 0.01 6.72 3.82 26.6 2020 0.38 0.54 5.36 6.42 0.06 0.23 -- -- 0 0.08 1.07 1.72 15.9 2021 3.08 0.06 2.23 0.20 0.12 0.02 0.22 0.46 0.28 0.19 0.03 5.14 12.0

AVERAGE ANNUAL PRECIPITATION 15.6

Notes:

-- No Data See Figure 4 for approximate location of VLCC1 weather station.

Reference: IOWA State University, Iowa Environmental Mesonet

Evapotranspiration includes water lost to the atmosphere from the ground surface, evaporation from the capillary fringe of the groundwater table, and transpiration of groundwater by plants whose roots tap the capillary fringe of the groundwater table.

According to the Department of Water Resources, California Irrigation Management

Information System, the Project area is located within the South Coast Marine to Desert

Transition zone, with a mean annual reference evapotranspiration rate of 55.1 inches.

2.3 Land Use

The site primarily consists of the planned bridge crossing and portions of the Cole Grade

Road right-of-way and shoulder. The adjacent properties are zoned as rural residential, vacant or undeveloped land, and field crops. The surrounding community generally consists of semi-rural residential land uses, which are characterized by lower-density residential neighborhoods, recreation areas, agricultural operations, and related commercial uses that support rural communities. The current General Plan Land Uses within the Project area are shown on Figure 5.

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2.4 Water Demand

Based on an estimated temporary construction dewatering pumping rate of 12 gallons per minute

(gpm) (Ninyo & Moore, 2022b) and estimated duration of each of the two stages of dewatering to accommodate installation of the ACB (1.5 months each), the cumulative volume of dewatering effluent that may be generated during both stages of dewatering (3 months) may be on the order of 1.6 million gallons. After the ACB is installed, the Project will not impact future water demand.

2.5 Geology and Soils

The site is situated in the northern San Diego County section of the Peninsular Ranges

Geomorphic Province. This geomorphic province encompasses an area that extends approximately 900 miles from the Transverse Ranges and the Los Angeles Basin south to the southern tip of Baja California (Harden, 1998; Norris and Webb, 1990). The province varies in width from approximately 30 to 100 miles. In general, the province consists of rugged mountains underlain by Jurassic metavolcanic and metasedimentary rocks, and Cretaceous igneous rocks of the southern California batholith. The portion of the province in San Diego County that includes the Project area consists generally of Cretaceous-age Tonalite of Cole Grade (referred to herein as granitic rock) (Figure 6).

The soil at the site is classified as Visalia sandy loam, 2 to 5 percent slopes (United States

Department of Agriculture, 1973). The Visalia Series consists of moderately well drained, very deep sandy loams derived from granitic alluvium. Soil properties are provided in the table below.

The soil map for the Project area is provided as Figure 7.

Table 3 – Soil Type and Select Soil Properties

Soil Type Soil Unit Soil Moisture Holding Capacity (inches) Runoff Hydrologic Soil

Group Visalia Sandy Loam

(2 to 5%) VaB 5 to 11.5 Very Low A

Reference: United States Department of Agriculture, 1973

2.6 Hydrogeologic Units

The site has been mapped as being dominantly underlain by granitic rock. Surficial units at the site include fills and alluvium (Figure 6). Boring logs from the Groundwater Dewatering Study

(Ninyo & Moore, 2022b) and the log of test borings from the Geotechnical Design Report

(Ninyo & Moore, 2015) are provided in Appendix A. A general description of the subsurface soil and geologic conditions encountered at the Project is provided below.

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• Fill – Fill was encountered in borings from the ground surface to a maximum depth of 14 feet. As encountered, these fill materials generally consisted of moist, loose to medium dense silty sand, sandy silt, or well-graded sand.

• Alluvium – Alluvium was encountered beneath the fill to depths of 15 to 36 feet. As encountered, the alluvium consisted of very loose to medium dense clayey sand, loose to medium dense poorly-graded sand with silt, and very loose to medium dense silty sand.

• Granitic Rock – When encountered, the granitic rock generally consisted of grayish and yellowish brown, moist to wet, weathered, coarse-grained granitic rock.

With regard to groundwater, the site is located within the Bonsall Hydrologic Subarea (HSA) within the Lower San Luis Hydrologic Area (HA), and San Luis Hydrologic Unit. The Lower

San Luis HA has beneficial uses for municipal, agricultural, and industrial service supplies

(Regional Water Quality Control Board [RWQCB], 2021). Groundwater in the Project area is expected to mimic topography and flow towards Keys Creek or tributaries (Figure 2). Fluctuations in the groundwater levels may occur due to variations in precipitation, topography, subsurface stratification, local irrigation, pumping from agriculture/irrigation/domestic wells, and other factors, which may not have been evident at the time of our field evaluation and groundwater elevations may be encountered at depths or elevations other than presented in this report.

2.7 Hydrologic Inventory and Groundwater Levels

Groundwater wells were researched to obtain an inventory of water wells within the Project area.

Well completion reports for wells located within ¼-mile from the site were provided by the County of San Diego Department of Environmental Health and Quality (DEHQ). Additionally, wells up to

½-mile from the site were obtained from the California Department of Water Resources (DWR)

Online System for Well Completion Reports, DWR well completion report file repository, and

ArcGIS Map Service well finder. Well completion reports are provided in Appendix B. Well locations and mapped surface waters are depicted on Figure 8, and water well information within the Project area is summarized in the table below.

Table 4 – Water Well Inventory

Well Completion

Number

Well Completion

Date Well Location

Well Depth (feet)

Screened Interval

(feet)

Production Characteristics

Depth to Groundwater (feet)

Well Use(s)

Flow Rate

(GPM)

First Water

Static Level

4574 03/05/1977 14049 Hilldale Rd 127 NL Domestic 10 28 15

12266 02/13/2019 14158 Cool Valley Rd 510

80-120, 320-340, 460-500

Private, Agriculture 30 10 15

59028 10/18/1979 14052 Hilldale Rd 300 20 - 280 Domestic 7.5 40 14

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Table 4 – Water Well Inventory

Well Completion

Number

Well Completion

Date Well Location

Well Depth (feet)

Screened Interval

(feet)

Production Characteristics

Depth to Groundwater (feet)

Well Use(s)

Flow Rate

(GPM)

First Water

Static Level

64873 01/25/1973 14060 Cool Valley Rd 80 20 - 40 Irrigation 25 30 15

64874 01/26/1973 14117 Hilldale Rd 125 NL Irrigation 15 40 15

127141 11/04/1978 14158 Cool Valley Rd 57 20 - 57 Irrigation 50 25 15

158298 06/11/1984 30547 Terrace View Ln 150 NL Irrigation 40 60 25

293814 10/14/2015 14219 Cool Valley Rd 810 NL Irrigation 30 700 --

328056 12/15/1989 30635 Shady Creek Ln 100 60 - 100 Domestic 25 10 15

391994 04/17/1992 31035 Cole Grade Rd 311 48 - 110 Domestic 28 71 60

456836 04/10/1995 14445 Cool Valley Rd 420 28 - 420 Domestic 30 180 85

575976 05/23/1994 0 White Star Ln 300 NL Domestic 16 -- 15 575978 05/23/1994 0 White Star Ln 301 48 - 68 Domestic 22 26 17

575985 09/30/1994 14444 Cool Valley Ranch Rd 803 31 - 110 Domestic 60 8 8

575986 10/17/1994 14444 Cool Valley Ranch Rd 903 33 – 73, 93 - 113 Domestic 27 93 30

575991 07/26/1995 30946 Rebecca Ln 427 NL Domestic 4.5 50 25

584025 08/16/1994 1444 Cool Valley Ranch Rd 833 NL Irrigation 80 76 13

738655 08/28/2001 14210 Cool Valley Rd 860 NL Irrigation 3 388 --

738659 08/21/2001 14210 Cool Valley Rd 1,010 NL Irrigation 25 40 --

771066 09/20/2000 14375 White Star Ln 301 52 – 112 Domestic 35 75 28

905553 03/18/2005 14451 Cool Valley Ranch Rd 860 NL Irrigation 25 66 --

Notes:

GPM = gallons per minute NL = not listed

Reference: DWR Well Completion Reports

During the aquifer well testing conducted in February 2022 (Ninyo & Moore, 2022b), the depth to groundwater, as measured from the test and observation wells, was 12 to 13 feet below ground surface (bgs), which generally corresponded to the depth of surface water observed within Keys Creek, approximately 40 feet south of the test well. During the 2014 and 2015 geotechnical evaluation (Ninyo & Moore, 2015), the observed depth to groundwater during drilling was approximately 14 feet bgs and the surface water within Keys

Creek was reportedly stagnant (Ninyo & Moore, 2015).

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2.8 Water Quality

According to the County of San Diego, General Plan Update, Groundwater Study, the site is located within a mapped area depicted as having the potential for nitrate impacts to groundwater based on a limited data set from water systems regulated by the DEHQ and State of California. The site is located outside of the mapped areas for radionuclides (County of

San Diego, 2010).

Total maximum daily loads (TMDLs) are the amount of a pollutant that can be discharged into a waterbody and still maintain its water quality standards / objectives. TMDLs were not found for

Keys Creek nor the Bonsall HSA. The following table provides the groundwater objectives within the Bonsall HSA.

Table 5 – Groundwater Water Quality Objectives

HSA TDS Cl SO4 %Na NO3 Fe Mn MBAS B Odor Turb

(NTU)

Color (Units) F

Bonsall 1,500 500 500 60 45 0.85 0.15 0.5 0.75 None 5 15 1 Notes:

Values reported in milligrams per liter or as noted.

TDS – total dissolved solids Cl – chlorides SO4 – sulfate %Na – percent sodium Fe – iron NO3 – nitrate NTU - nephelometric turbidity units

Mn – manganese MBAS – methyl blue-activated substances B – boron Turb – turbidity F – fluoride HSA – hydrologic subarea

Reference: Water Quality Control Plan for the San Diego Basin (RWQCB, 2021)

3 WATER QUANTITY IMPACT ANALYSIS

The County’s Guidelines for Determining Significance and Report Format and Content

Requirements, Groundwater Resources, provide guidance for evaluating potential adverse environmental effects a project may have on groundwater resources, based on CEQA guidelines (County of San Diego, 2007). Potential water quantity impacts, as defined by the

County’s Guidelines for Determining Significance (County of San Diego, 2007), are discussed in the following sections.

3.1 50 Percent Reduction of Groundwater in Storage The following condition, if met, would be considered significant impact to groundwater resources:

For proposed project in fractured rock and sedimentary basins, groundwater impacts will be considered significant if a soil moisture balance, or equivalent analysis, conducted using a minimum of 30 years of precipitation data, including drought periods, concludes

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 9 that at any time groundwater in storage is reduced to a level of 50 percent or less as a result of groundwater extraction.

Based on the areal extent of the Keys Creek Sub-Watershed (23,500 acres), temporary, localized nature of the dewatering that may be associated with the Project, and that the extracted groundwater will be used for irrigation of groundwater-dependent vegetation or returned to the watershed/aquifer via discharge to Keys Creek through a Regional Water Quality

Control Board (RWQCB) National Pollutant Discharge Elimination System (NPDES) permit, this condition would not be met for the subject Project.

3.2 Groundwater Overdraft Conditions

The following condition, if met, would be considered significant impact to groundwater resources:

For fractured rock basins that have been demonstrated to be in an overdraft condition, any groundwater use will be considered a significant impact.

The site is not located in a fractured rock basin designated to be in an overdraft condition

(County of San Diego, 2007). Therefore, this condition is not applicable.

3.3 Well Testing

If any of the conditions in the following sections are met, they would be considered a significant impact to groundwater resources.

3.3.1 Guidelines for the Determination of Significance

This section discusses the potential, temporary impacts on local groundwater resources in relation to the State (RWQCB, 2021) and County (County of San Diego, 2007) CEQA

Guidelines.

3.3.1.1 Well Interference in Fractured Rock

As an initial screening tool, offsite well interference will be considered a significant impact, if after a five-year projection of drawdown, the results indicate a decrease in water level of 20 feet or more in the offsite wells. If site-specific data indicates water bearing fractures exist which substantiate an interval of more than 400 feet between the static water level in each offsite well and the deepest major water bearing fracture in the well(s), a decrease in saturated thickness of 5 percent or more in the offsite wells would be considered a significant impact.

Ninyo & Moore | Cole Grade Road Bridge Project Over Keys Creek, San Diego County, California | 108611016 | June 21, 2023 10

Based on the temporary nature of the dewatering associated with the Project and that the planned dewatering will occur in an alluvial aquifer, this condition is not applicable.

3.3.1.2 Well Interference in Alluvial or Sedimentary Basins

As an initial screening tool, offsite well interference will be considered a significant impact, if after a five-year projection of drawdown, the results indicate a decrease in water level of 5 feet or more in the offsite wells. If site-specific data indicates alluvium or sedimentary rocks exist which substantiate a saturated thickness greater than 100 feet in offsite wells, a decrease in saturated thickness of 5 percent or more in the offsite wells would be considered a significant impact.

The County requested an analysis of potential, temporary impacts to offsite production well be performed. Initially, parcels within ½-mile radius of the site were evaluated based on the following water source criteria provided by the County:

• A - Served by groundwater only

• B - Served by groundwater and the local water district (Valley Center Water District;

service area map provided in Appendix D)

• C - Served by local water district only

• D - Undeveloped / no water use

As depicted on Figure 9, the adjoining parcels are either served by the Valley Center

Water District and groundwater, served by Valley Center Water District only or undeveloped / no water use. Two parcels (assessor parcel numbers [APNs] 133-201-23 and -24) are serviced by groundwater only, and are located greater than 500 feet west of the site. The two parcels are greater than 150 feet from the nearest water lines located along Cole Grade Road and Hilldale Road.

As depicted on Figure 8 and Table 4, the wells located within 1,000 feet of the site were drilled to depths ranging from 125 to 1,010 feet bgs, and were predominately completed in the confined / semi-confined, decomposed granite / fractured granitic aquifer (granitic aquifer), not the alluvium.

Approximately 825 feet northwest of the site, near a stream bed, two wells (Figure 8, Well Completion Logs 4574 and 64874) were drilled to depths of approximately 125 feet bgs. During drilling, alluvium and decomposed granite were encountered to a depth of approximately 30 feet, and sanitary seals were placed at the alluvium and decomposed granite contact (depths of 28 and 30 feet bgs in each well, respectively).

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Approximately 1,000 feet west-southwest of the site, an irrigation well (Well Completion

Log 127141) was installed to a depth of 57 feet bgs adjacent to a stream bed. Alluvium was encountered to a depth of 15 feet bgs followed by decomposed granite from 15 to

35 feet bgs, and hard rock (granite) from 35 to 57 feet. The well was constructed with a

20-foot sanitary seal.

Review of available subsurface geologic and well completion data indicates that the production in the nearest offsite wells is from the granitic aquifer, not alluvium. However, since there are no site-specific data that conclusively demonstrate hydrologic isolation between the alluvial and underlying fractured granitic aquifers, the County assumes the alluvium, where dewatering would occur, is connected to the fractured granitic rock system for purposes of analysis.

The potential 5-foot aquifer drawdown elevation at the end of each stage of dewatering

(1.5 months) was calculated, plotted, and contoured (Figure 8). ModFlow estimated drawdown contours are based on the measured hydraulic conductivity value, factoring in the influence of water flowing in the stream. The 5-foot contour does not extend to the nearest offsite private production well; therefore, offsite well interference in an alluvial or sedimentary basin is not considered a significant impact (Ninyo & Moore, 2022b).

Site-specific data and well completion logs from production wells in the vicinity of the

Project indicate alluvium or sedimentary rocks with a saturated thickness greater than

100 feet do not exist in offsite wells. Therefore, the County well interference threshold of significance of a decrease in saturated thickness of 5% or more in the offsite wells in an alluvial or sedimentary basin does not apply to the Project.

3.3.1.3 Low Well Yield/Residual Drawdown

The Low Well Yield and Residual Drawdown thresholds for determining significance are associated with residential well tests and, therefore, not applicable to the subject Project.

3.3.1.4 Groundwater-Dependent Habitat

For projects that have groundwater-dependent habitat on or near the subject property, the following condition, if met, would be considered a significant impact to biological resources (as specified in the County Guidelines for Determining Significance, Biological

Resources, 2010a):

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The project would draw down the groundwater table to the detriment of groundwater dependent habitat, typically a drop of 3 feet or more from the historical low groundwater levels.

Potential groundwater-dependent habitats were mapped by a County biologist and are shown on Figure 10. Groundwater-dependent habitats, which consist of Southern Coast

Live Oak Riparian Forests, are generally present along Keys Creek and tributaries within

¼-mile from the site. Other vegetation mapped in the Project area include intensive agriculture, orchards and vineyards, coastal scrub, Coast Live Oak Woodland, undifferentiated dense woodland, and disturbed habitat (Appendix C).

The estimated drawdown of groundwater during Project dewatering was modeled using data collected from the Well Test (Section 3.3.2; Figure 10). Note that the drawdown depicted on Figure 10 is at equilibrium (i.e., when the pumping discharge volume essentially equals the volume of water recharging the soil being dewatered), which, if other parameters (e.g., rainfall, flow in Keys Creek) remain relatively constant, should not change significantly for each 1.5-month stage of dewatering. The estimated drawdown modeled should be considered a conservative estimate because historical low groundwater level in the alluvial aquifer, at or near the site are based on measurements collected in 2014-2015 and 2022 (Ninyo & Moore, 2015; 2022b).

3.3.2 Well Test Methodology

Well testing of the alluvial aquifer was performed in general accordance with the

County-approved Well Test Plan (Ninyo & Moore, 2022) and the results, including aquifer modeling, are summarized in the Groundwater Dewatering Study (Ninyo & Moore, 2022b), which is incorporated herein by reference. The test and observation wells locations are shown on Figure 3. The well testing was performed from January 31 to

February 3, 2022.

3.3.2.1 Well Test Description

Two 8-inch diameter borings and one 10-inch diameter boring were drilled to 30 feet bgs near the footprint of the proposed bridge structure (Figure 3), using a limited access drill rig equipped with hollow-stem augers, on December 7 and 8, 2021. The three borings were converted to groundwater wells (one 4-inch diameter test well and two 2-inch diameter observation wells), which were constructed of Schedule 40, 0.020-inch slotted

Polyvinyl Chloride (PVC) casing from approximately 30 to 10 feet bgs and Schedule 40

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PVC solid casing from 10 feet bgs to the surface. Groundwater was measured at 12 to

13 feet bgs during drilling.

On February 1, 2022, step testing was performed using average pumping rates of 0.2, 1.6, and 1.2 gallons-per-minute (gpm) to estimate a sustainable pumping rate for the constant rate test. Prior to performing step testing, static depth to groundwater was measured at 12.5 feet below top of casing in the test well, which was consistent with the previous measurements.

From February 2 to 3, 2022, an approximately 24-hour constant rate test was performed.

Due to pump issues, which resulted in an inconsistent pumping rate, the constant rate test was restarted after groundwater recovered to static (pre-testing) levels. The constant rate test was performed at a rate of approximately 1.6 gpm. Following completion of the constant rate test, the depth to groundwater in the test well recovered to its static level within 2.5 hours. According to the flow totalizer, approximately

2,679 gallons of groundwater were pumped and temporarily stored in the holding tank. In addition to the data collected from the transducers, manual groundwater level measurements were recorded from the test well during well pumping and recovery. Well test data are included in the Groundwater Dewatering Study report (Ninyo & Moore, 2022b).

Groundwater that was temporarily stored in the holding tank and four 55-gallon drums

(from well development), approximately 2,899 gallons, was used to irrigate oak trees on the east and west sides of Cole Grade Road on February 4, 2022. An irrigation and landscaping specialist contracted by the County oversaw irrigation activities.

3.3.2.2 Well Test Analysis

The output from the well testing, described above, was analyzed using the hydrogeologic platform AQTESOLV, which calculated estimated hydraulic conductivity values ranging from 1 foot per day (ft/d) to 1.3 ft/d. For this study, the higher, more conservative, hydraulic conductivity estimate was selected for use in the groundwater model to estimate the approximate flow rates that could be expected to achieve project dewatering objectives and be used in the preparation and submittal of the Notice of

Intent (NOI) for an NPDES construction dewatering permit. Other model input, such as base of ACB and target dewatering elevation, were derived from field measurement, review of the 100 percent plan submittal, and input from the County. Model results are provided in the Groundwater Dewatering Study report (Ninyo & Moore, 2022b).

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Static depth to groundwater before testing was measured at 12.5 feet below top of casing in the test well. The base of the alluvial aquifer is estimated to be 31 feet below grade, based on the stiff, interbedded clay encountered in the test well boring. Based on the subsurface lithology described in the geotechnical boring drilled to 45 feet bgs adjacent to the test well boring location (B-5, Ninyo & Moore, 2015), this clay is underlain by granitic bedrock, which is interpreted to act as an aquitard between the fractured granitic bedrock aquifer and the alluvial aquifer to be dewatered.

Based on the calculated hydraulic conductivity estimates from aquifer testing analyses

(1 ft/d to 1.3 ft/d) and saturated aquifer thickness in the test well of 18.5 feet (31 feet -

12.5 feet), the local transmissivity (essentially the ability of an aquifer to transmit groundwater throughout its entire saturated thickness, calculated as the product of hydraulic conductivity times aquifer thickness) is estimated to be 18.5 ft2/day to

24.05 ft2/day.

The test well specific capacity at the end of testing (the amount of groundwater available per foot of drawdown) is estimated to be 0.18 gpm/ft (1.6 gpm pumping rate / 8.86 ft of drawdown). Note that this value is unique to the test well and, during construction dewatering, the pumping wells’ actual specific capacity will vary depending on the wells’ design, performance, and efficiency, stream flow, weather conditions, shoring methods, and other factors.

As originally designed, the Project required temporary dewatering to construct the bridge foundation. However, due to concerns from local property owners about potential temporary, localized impacts to groundwater resources and groundwater-dependent habitat, the bridge foundation was re-designed to eliminate the need for dewatering during foundation construction.

In order to address potential scouring of the Keys Creek channel, the Project includes the installation of articulated concrete block (ACB) on the east and west sides of the bridge structure, a portion of which is anticipated to require temporary dewatering to prepare subgrade to accommodate placement of the ACB in the channel. ACB installation is anticipated to require temporary dewatering in two stages to maintain traffic flow on Cole Grade Road (Stage 1 and Stage 2, Figure 3), with an estimated duration of 1.5 months during each stage and an estimated 6 months without construction dewatering between the two stages. A target dewatering elevation of

1418 feet AMSL, approximately 2 feet below the base of the ACB, was provided by the

County for ACB installation.

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The groundwater model processing platform, Groundwater Vistas, was utilized to create groundwater simulations utilizing the ModFlow finite difference method (USGS, 2022).

Computerized finite difference groundwater models were set out on a grid with uniform

10 feet row and column spacing over an area 8,000 feet by 4,000 feet centered between the two points where Cole Grade Road crosses over Keys Creek. The model assumes that homogeneous and isotropic aquifer conditions exist horizontally across the grid and that Keys Creek will be flowing at the time of construction.

The model was run in transient mode with 1-day stress periods until steady-state groundwater conditions were achieved. The transient model predicts 2 wells / extraction points spaced approximately 100 feet apart with an average flow rate of approximately

6 gpm per well / extraction point could require approximately 3 to 5 days to reach the anticipated target dewatering elevation and equilibrium. Once equilibrium is reached, drawdown should not change significantly for the 1.5-month duration of dewatering, assuming that other parameters, such as precipitation and flow in Keys Creek, do not change significantly.

Based on an estimated combined dewatering pumping rate of 12 gpm for 2 wells

(Ninyo & Moore, 2022b) and estimated duration of each of the two stages of dewatering to accommodate installation of the ACB (1.5 months), the cumulative volume of dewatering effluent that may be generated during both stages of dewatering (3 months) may be on the order of 1.6 million gallons.

The actual dewatering rates and time it will take to reach the anticipated dewatering elevation and achieve a firm base to accommodate ACB installation will be influenced significantly by the dewatering system design, well construction/development/efficiency, extraction pump size and efficiency, associated discharge rates and duration, coffer dam

/ shoring effectiveness, actual saturated thickness of the aquifer and flow conditions at the time of construction, as well as variations in seasonal conditions (e.g., actual flow in

Keys Creek, precipitation, irrigation schedules, and soil moisture/saturation). Additionally, the construction means, methods, sequencing, and other conditions will be factors that influence actual groundwater dewatering rates and durations.

3.3.3 Potentially Significance of Impacts Prior to Mitigation Based on the temporarily, localized nature of the construction dewatering, and that the extracted groundwater will be used for irrigation of groundwater-dependent vegetation or returned to the watershed/aquifer via discharge to Keys Creek through an RWQCB

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NPDES permit, the Project would not result in groundwater overdraft conditions, and the low well yield and residual drawdown criteria do not apply because this is not a residential project/development, the Project satisfies the requirement listed in the County

Groundwater Ordinance for discretionary permit applications (non-single family subdivision projects) (County of San Diego, 2013), and adequate groundwater resources is not an issue. As requested in the County memorandum (County of San Diego, 2021), an analysis of potential impacts for well interference and groundwater-dependent habitat is provided below.

Well Interference Analysis: Based on review of provided well logs, production wells in the vicinity have sanitary seals from the surface to depths of 20 to 30 feet and therefore are likely producing groundwater from the underlying, fractured granitic aquifer. The

Project proposes to dewater within the overlying alluvial aquifer to meet the target dewatering elevation and therefore is not expected to significantly affect groundwater conditions in the granitic aquifer. However, since there are no site-specific data that conclusively demonstrate hydrologic isolation of the aquifers, as a conservative measure, it has been assumed that the alluvium where dewatering would occur is connected to the fractured rock system for purposes of analysis.

Therefore, the anticipated groundwater elevations at the end of each of the two, 1.5-month stages of dewatering were calculated and plotted to evaluate whether the temporary Project dewatering would result in drawdown in production wells in the site vicinity greater than 5 feet (i.e., impact offsite well production through well interference).

Based on the results of this evaluation, the effects of temporary construction dewatering would extend approximately 150 feet in the up-gradient and down-gradient directions and 200 feet in the cross-gradient directions from the extraction points and not impact production wells in the site vicinity (Figure 8).

Groundwater-Dependent Habitat: Groundwater-dependent habitats, which consist of

Southern Coast Live Oak Riparian Forests, are generally present along Keys Creek and tributaries within ¼-mile from the site (Figure 10). Readily-available data for historical low groundwater levels in the vicinity of the Project were not found or provided, therefore, as a conservative measure, the documented groundwater levels

(Ninyo & Moore, 2015, 2022b) were used to evaluate potential impacts to groundwater-dependent habitat.