Section 3.8 Hydrology & Water Quality

Transcription

1 Section 3.8 Hydrology & Water Quality Introduction The hydrology and water quality information contained herein is based on the City-approved April 2015 Preliminary Hydrology Analysis Site Development Permit: SP12-07P SunPointe Vesting Tentative Tract Map No ; and on the Conceptual Water Quality Management Plan (WQMP) SunPointe Vesting Tentative Tract Map No , City of Laguna Niguel, California, prepared by Hunsaker & Associates Irvine, Inc., included in Appendices K and L, respectively Environmental Setting Surface Hydrology The project is within the San Juan Creek Watershed. The San Juan Creek Watershed covers square miles and includes portions of Dana Point, Laguna Hills, Laguna Niguel, Mission Viejo, Rancho Santa Margarita, and San Juan Capistrano. Its main tributary, San Juan Creek, originates in the Santa Ana Mountains, in the eastern portion of Orange County (Orange County Public Works n.d.). Receiving waters in the proposed project area include Oso Creek (lower), Trabuco Creek, San Juan Creek, and ultimately the Pacific Ocean. The water body nearest to the project site is Oso Creek, approximately 0.1 mile to the east. Oso Creek joins with Arroyo Trabuco Creek approximately 1.6 miles to the south of the project site. No creeks cross the site. Existing Topography and Drainage The project area is situated on an east-facing hillside with moderate relief. Surface elevations range from approximately 250 feet to 455 feet above mean sea level. The site is bounded to the south by a slope descending to Rancho Capistrano; to the north by an existing residential development; to the west by a natural slope that ascends to the prominent, generally north-south trending ridgeline near the boundary of the adjacent Colinas de Capistrano community; and to the east by the Mercedes Benz of Laguna Niguel facility. The site terrain is generally steep, with on-site paved roads ranging from 1% to 15% slopes. In general, the rough-graded pads slope at approximately 1% toward the project s paved streets. Graded slopes upstream of the site to the west average a gradient of approximately 45%, while those downstream to the south-southeast average approximately 30% gradient. The existing site condition consists of a single road, Avenida del Caballo, bisecting the project site from the northeast to the southwest and 35 rough-graded residential lots. The project site is approximately 90% pervious surface (17.57 acres) and 10% (1.95 acres) impervious surface. The site has seven drainage areas identified as Areas A, B1, C1, D1, E1, F1, and G1, all of which drain southerly, while the southern portion of Area A drains east and north, toward Avenida del Caballo. The project site directly discharges to Oso Creek Channel, which ultimately drains to Arroyo Trabuco Creek. Figure depicts the existing drainage boundaries

2 Soils Area A is the largest drainage area, with approximately 20.7 acres of off-site and on-site terrain. Runoff from this area is at a volume of approximately 76.2 cubic feet per second (cfs) and drains into an existing developed area, Colinas de Capistrano community, to the north. Portions of slopes in Area A, north of Avenida del Caballo (existing street), drain southeasterly before flowing into the street, which then carries flows to the northerly direction. Both the western and northern portions of the site in Area A and the abutting graded lots and streets south of Avenida del Caballo eventually drain northerly and are conveyed to a gutter that discharges into an existing off-site catch basin downstream in Avenida del Caballo. This flow is then conveyed northeasterly in an existing off-site 36-inch storm drain prior to discharging to Oso Creek Channel. Northerly portions of Areas A and G1, along the south side of the existing trail (Colinas Bluff Trail), sheet flow to the trail and would not be affected by the proposed project. Runoff from the southern slopes (B1, C1 D1, E1, and F1) of the project site are conveyed as sheet flow and concentrated flow in a southerly/southeasterly downstream direction to an existing concrete channel. Runoff from the same southern slopes discharges into a grass swale and then into the Oso Creek Channel. Hydrologic soil ratings are based on a scale of A through D, where Soil Type A is the most pervious and Soil Type D is the least pervious. Based on the soils map contained in the Hydrology Manual, the project site is comprised of soil Type D. Therefore, infiltration on site is not feasible. Flooding The project site is not within a 100-year flood hazard area designated by the Federal Emergency Management Agency (FEMA). It is located within the FEMA-designated flood Zone X, which indicates areas of minimal flood hazard, outside the Special Flood Hazard Area and higher than the elevation of the 0.2% annual chance (or 500-year) flood. The project site is not located within a dam failure zone. Sulfur Creek Dam is located approximately 1.5 miles to the west but is separated from the project site due to topography and residential development, and dam failure would not impact the project site. Groundwater Hydrology and Quality The project site is in the San Juan Valley Groundwater Basin, which underlies the San Juan Valley and several tributary valleys in southern Orange County. San Juan Creek drains the San Juan Valley, and several other creeks drain valleys tributary to San Juan Creek. Historical average annual precipitation ranges from 11 to 15 inches. The primary water-bearing unit within the San Juan Valley Groundwater Basin is Quaternary alluvium, which ranges from a heterogeneous mixture of sand, silt, and gravel in the eastern portion of the basin, to coarse sand near the center, to finegrained lagoonal sediments in the western portion of the basin. San Juan Valley Groundwater Basin total storage capacity has been estimated to be approximately 90,000 acre-feet. Basin management is conducted by the San Juan Basin Authority. The western part of the basin is impaired due to high total dissolved solids content and water coming from springs in Thermal Canyon (which are high in fluorine)

3 K:\Irvine\GIS\Projects\City_of_Laguna_Niguel\00406_15\mapdoc\DEIR Figure Drainage Area Map

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5 Perched groundwater has been documented on the project site; however, the depth to groundwater has been highly variable depending on the timing and location of the investigation. Because the project site is located within an active landslide area with perched groundwater requiring remediation, routine dewatering operations are necessary to maintain slope stability. As a result, four horizontal drains were installed at the southeastern section of the project site to drain the perched groundwater in order to reduce the water table level, as shown on Figure Flow from three of the four horizontal drains has been monitored periodically since May 22, Since this time, flow in individual horizontal drains fluctuates with a peak flow of approximately 0.43 gallon per minute. The maximum daily flow for all drains peaked initially at approximately 600 gallons per day. Over time, the collective flow from all drains has decreased to approximately 220 gallons per day. The horizontal drains have been shut off since May Groundwater from a groundwater monitoring well was sampled on November 11, 2012, for the following types of analyses. Volatile organic compounds (VOC) using EPA Method 8260B Chloride, fluoride, nitrite (as NO 2), and nitrate (as NO 3) using EPA Method 300 Priority pollutants (metals) using EPA Method 6010B Specific conductance (EC) and total dissolved solids (TDS) using ASTM Methods No VOC or priority pollutant metals were detected over the laboratory reporting limits except for the following. Potassium at 79,000 micrograms per liter (µg/l) Magnesium at 110,000 µg/l Calcium at 180,000 µg/l The concentrations of potassium, magnesium, and calcium are considered typical of local groundwater in the Capistrano Formation and reflect the mineral content of the marine clay. As indicated by the specific conductance and TDS concentrations (11.1 micromhos per centimeter and 8,200 milligrams per liter [mg/l], respectively), groundwater from this formation is considered brackish. This water exceeds the secondary maximum contaminant level for drinking water in California for TDS, which is set at 1,000 mg/l. TDS comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water. In California, secondary maximum contaminant levels are nonenforceable standards based on aesthetics rather than a health hazard. An elevated TDS concentration is not a health hazard but may cause the water to appear cloudy or colored, or to taste or smell bad. On August 14, 2014, representative groundwater samples from the three active horizontal drains were collected and submitted for the following types of analyses. VOC using EPA Method 8260B Total petroleum hydrocarbons by EPA Method 8105C Chloride and fluoride using EPA Method 300 Priority pollutants (metals) using EPA Method 6010B 3.8-3

6 No VOC or petroleum hydrocarbons were detected in any of the samples submitted. For metals, only copper was detected above the laboratory reporting limit at concentrations ranging from 62 µg/l to 69 µg/l. The California public health goal for copper in drinking water is 300 µg/l. No fluoride was detected above the laboratory reporting limits. Chloride was detected in all three samples submitted, at concentrations ranging from 1,050 mg/l to 1,150 mg/l. These chloride concentrations reflect the clayey marine origin of the Capistrano Formation. Surface Water Quality The City of Laguna Niguel and the project site lie within the jurisdictional boundaries of the San Diego Regional Water Quality Control Board (San Diego Water Board). The Water Quality Control Plan (Basin Plan) for the San Diego Region designates beneficial uses for all water body segments in its jurisdiction, and then sets criteria necessary to protect these uses. Consequently, the water quality objectives developed for particular water segments are based on the designated use and vary depending on such use. The San Diego Water Board has set numeric and narrative water quality objectives for several substances and parameters in numerous surface waters in the region. For those waters that do not have specific beneficial uses or water quality objectives, the tributary rule applies to streams. 1 Table describes designated beneficial uses for the receiving water bodies of the project site. Table Designated Beneficial Uses for Surface Water Bodies in the Project Vicinity Water Body Jurisdiction Designated Beneficial Uses Oso Creek San Diego Water Board Agricultural Supply (AGR), Industrial Use (IND), Water contact recreation (REC1), Non-contact water recreation (REC2), Warm freshwater habitat (WARM), Cold freshwater habitat (COLD), Wildlife habitat (WILD) Arroyo Trabuco Creek Source: San Diego Water Board 2012 San Diego Water Board Agricultural Supply AGR, Industrial Use IND, Water contact recreation (REC1), Non-contact water recreation (REC2), Warm freshwater habitat (WARM), Cold freshwater habitat (COLD), Wildlife habitat (WILD) The State Water Resource Control Board (SWRCB) identifies waters failing to meet standards for specific pollutants, which are then state-listed in accordance with the federal CWA Section 303(d). Table describes identified impairments for the receiving water bodies of the project site, which are all under the jurisdiction of the San Diego Water Board. Table (d) Listed Impairments Uses for Surface Water Bodies in the Project Vicinity Water Body Oso Creek (Lower) Trabuco Creek 303(d) Listed Impairments Metals (Selenium), Toxicity Pesticides (Diazinon), Nutrients (Phosphorus, Total N), Toxicity Source: Appendix L; SWRCB 2012 Integrated Report 303(d) Listed waters. Accessed: December 13, The tributary rule refers to any streams not specifically listed in the plan that are deemed to have the same beneficial uses and water quality objectives of the listed stream, river, or lake to which they are a tributary

7 3.8.3 Regulatory Setting Federal State Clean Water Act (33 USC 1251 et seq.) The Clean Water Act (CWA) is the primary federal law protecting the quality of the nation s surface waters, including lakes, rivers, and coastal wetlands. The CWA prohibits any discharge of pollutants into the nation s waters unless specifically authorized by a permit. The applicable sections of the CWA are further discussed below. Section 102 requires the planning agency of each state to prepare a Basin Plan to set forth regulatory requirements for protection of surface water quality, which include designated beneficial uses for surface water bodies, as well as specified water quality objectives to protect those uses. Analysis of the degree to which discharges of runoff from the project may or may not adversely affect project receiving water beneficial uses and attainment by the receiving water indicates the degree to which the project may affect water quality of existing surface waters. Beneficial uses and water quality objectives have been established by the Regional Water Boards for their respective jurisdictions. Section 303(d) requires each state to provide a list of impaired surface waters that do not meet or are expected not to meet state water quality standards as defined by that section. It also requires each state to develop total maximum daily loads (TMDL) of pollutants for impaired water bodies. The TMDL must account for the pollution sources causing the water to be listed. Impaired waters with potential to be affected by the project are described in the Water Quality section above. CWA Section 402 establishes the National Pollutant Discharge Elimination System (NPDES) permit program, which is the primary federal program that regulates point-source and non-point-source discharges to WoUS. NPDES permits are issued by SWRCB and the nine geographically separated Regional Water Boards in California. There are both general and individual NPDES permits. General NPDES permits cover industrial, construction, and municipal stormwater discharges, and some point-source discharges for specific activities. Individual NPDES permits cover point-source discharges from wastewater facilities. Responsibility for the protection of water quality in California resides with SWRCB and the nine RWQCBs. SWRCB establishes statewide policies and regulations for the implementation of water quality control programs mandated by federal and state water quality statutes and regulations. The San Diego RWQCB and SWRCB implement a number of federal and state laws regarding water quality, the most important of which are the State of California s Porter-Cologne Water Quality Control Act and the federal CWA. Porter-Cologne Water Quality Control Act (California Water Code Section et seq.) The Porter-Cologne Water Quality Control Act requires the regulation of all pollutant discharges, including wastes in project runoff that could affect the quality of the state s water. The act also provides for the development and periodic reviews of Basin Plans that designate beneficial uses of California s major rivers and groundwater basins and establish water quality objectives for those 3.8-5

8 waters. Beneficial uses and water quality objectives are specified for the project area in the Basin Plan for the San Diego Region and are regulated by the San Diego Water Board. National Pollutant Discharge Elimination System Permits Construction General Permit Pursuant to CWA Section 402(p) and as related to the goals of the Porter-Cologne Water Quality Control Act, SWRCB has issued a statewide NPDES General Permit for Stormwater Discharges Associated with Construction Activity (Order No DWQ, NPDES No. CAS000002, as amended by Order DWQ and DWQ) (Construction General Permit), adopted September 2, 2009 (SWRCB 2012). Every construction project that disturbs 1 or more acres of land surface or that is part of a common plan of development or sale that disturbs more than 1 acre of land surface would require coverage under this Construction General Permit. To obtain coverage under this Construction General Permit, the landowner or other applicable entity must file Permit Registration Documents prior to the commencement of construction activity, which include a Notice of Intent (NOI) and Stormwater Pollution Prevention Plan (SWPPP) prepared by a Qualified SWPPP Developer, and mail the appropriate permit fee to SWRCB. Construction activities subject to the Construction General Permit include clearing, grading, and disturbances to the ground, such as stockpiling or excavation, that result in soil disturbances of at least 1 acre of total land area. The SWPPP has two major objectives: (1) to help identify the sources of sediment and other pollutants that affect the quality of stormwater discharges; and (2) to describe and ensure the implementation of best management practices (BMPs) to reduce or eliminate sediment and other pollutants in stormwater and non-stormwater discharges. BMPs are intended to reduce impacts to the maximum extent practicable (MEP), which is a standard created by Congress to allow regulators the flexibility necessary to tailor programs to the site-specific nature of municipal stormwater discharges. The SWPPP is required to be implemented and monitored regularly by a Qualified SWPPP Practitioner. Reducing impacts to the MEP generally relies on BMPs that emphasize pollution prevention and source control, with additional structural controls as needed. The Construction General Permit requires that specific minimum BMPs are incorporated into the SWPPP, depending on the project s sediment risk to receiving waters based on the project s erosion potential and receiving water sensitivity to sediment. Municipal Separate Storm Sewer System CWA Section 402 mandates permits for municipal stormwater discharges, which are regulated under the NPDES General Permit for Municipal Separate Storm Sewer Systems (MS4 Permit). Phase I MS4 Permit regulations cover medium (serving between 100,000 and 250,000 people) and large (serving more than 250,000 people) municipalities. Phase II (Small MS4 Permit) regulations require that stormwater management plans/programs be developed by municipalities with populations smaller than 100,000, including non-traditional Small MS4s, which are facilities such as military bases, public campuses, and prison and hospital complexes. MS4 Permits require that cities and counties develop and implement programs and measures, including BMPs, control techniques, system design and engineering methods, and other measures as appropriate, to reduce the discharge of pollutants in stormwater to the maximum extent possible. As part of permit compliance, these permit holders have created stormwater management plans for their respective locations. These plans outline the requirements for municipal operations, industrial and commercial businesses, construction sites, and planning and land development. These 3.8-6

9 requirements may include multiple measures to control pollutants in stormwater discharge. During implementation of specific projects under the program, project applicants are required to follow the guidance contained in the stormwater management plans as defined by the permit holder in that location. SWRCB is advancing Low-Impact Development (LID) in California as a means of complying with municipal stormwater permits. LID incorporates site design, including among other things the use of vegetated swales and retention basins and minimization of impermeable surfaces, to manage stormwater to maintain a site s predevelopment runoff rates and volumes. Regional South Orange County Municipal Stormwater NPDES Permit (MS4 Permit) On May 8, 2013, the San Diego RWQCB approved a regional MS4 Permit for San Diego, southern Orange, and southwestern Riverside counties (Order No. R ). San Diego RWQCB, adopted Order No. R , an Order Amending Order No. R , NPDES Permit and Waste Discharge Requirements For Discharges From The Municipal Separate Storm Sewer Systems (MS4s) Draining The Watersheds Within The San Diego Region at its February 11, 2015 Board Meeting. The region-wide NPDES Permit (commonly referred to as the Regional MS4 Permit) sets the framework for municipalities, such as the City of Laguna Niguel, to implement a collaborative watershed-based approach to restore and maintain the health of surface waters. The Regional MS4 Permit requires development of Water Quality Improvement Plans that will allow permittees to prioritize and address pollutants through an appropriate suite of BMPs in each watershed. To implement the requirements of the Regional MS4 Permit, the co-permittees developed a Drainage Area Master Plan (DAMP) that includes a Model New Development and Redevelopment Program. Per the requirements in the DAMP and the Regional MS4 Permit, the permittees are required to adopt an LIP to implement the DAMP and Regional MS4 Permit in their jurisdictions. Using the LIP as a guide, the permittees will approve WQMPs for new development and redevelopment projects within their jurisdictions as part of the development plan and entitlement approval process. WQMPs for new development and significant redevelopment projects that fall under specific priority project categories must include Site Design, Routine Structural and Nonstructural, and Treatment Control BMPs; include an Operations and Maintenance Plan; and address LID retention/biofiltration and hydromodification criteria. The priority project categories are those determined by the San Diego RWQCB to have the greatest potential to affect receiving waters with polluted runoff. A Priority Development Project is defined as a redevelopment project that creates and/or replaces 5,000 square feet or more of impervious surface (collectively over the entire project site on an existing site of 10,000 square feet or more of impervious surfaces). This includes commercial, industrial, residential, mixed-use, and public development projects on public or private land. The proposed project is considered a Priority Redevelopment Project subject to permittee conditioning and approval for the design and implementation of post-construction controls to mitigate stormwater pollution. A Project WQMP is a plan for minimizing the adverse effects of urbanization on site hydrology, runoff flow rates, and pollutant loads. Hunsaker & Associates prepared the proposed project s Conceptual Water Quality Management Plan in April 2015 (Appendix L)

10 Local South Orange County Hydromodification Management Plan The Orange County Stormwater Program developed a Hydromodification Management Plan (HMP) in response to MS4 Permit requirements from the San Diego RWQCB. The permit requires the incorporation of low impact development and hydromodification requirements in new development and significant redevelopment projects. Priority Development Projects are required to implement hydrologic control measures and on-site management controls so that post-project runoff flow rates and durations do not exceed pre-development, i.e. naturally occurring conditions, flow rates, and durations where they would result in an increased potential for erosion or significant impacts to beneficial uses. Priority Development Projects may be exempt from HMP criteria based on specific channel conditions, including those that discharge runoff into conveyance channels whose bed and bank are concrete lined all the way from the point of discharge to ocean waters, enclosed bays, estuaries, or water storage reservoirs and lakes. General Waste Discharge Requirements for Discharges from Groundwater Extraction and Similar Discharges to Surface Waters On March 12, 2008, the San Diego RWQCB issued the General WDRs for Discharges from Groundwater Extraction and Similar Discharges to Surface Waters within the San Diego Region Except for San Diego Bay (Order No. R , Permit No. CAG919002) (Groundwater Discharge Permit). This permit regulates discharges of treated and untreated groundwater from construction to surface waters. It specifies the discharge prohibitions, receiving water limitations, monitoring and reporting program requirements, and general compliance determination criteria for groundwater dewatering during construction activities and drilling, construction, and purging of wells. Dischargers are required to collect and analyze representative groundwater samples for all constituents listed in the Groundwater Discharge Permit. Based on the results, dischargers would be required to provide treatment for any toxic compounds detected above the applicable screening levels. To obtain coverage under the Groundwater Discharge Permit, each permittee must submit an NOI to begin the application process. City of Laguna Niguel Local Implementation Plan/Jurisdictional Runoff Management Program (LIP/JRMP) The Local Implementation Plan/Jurisdictional Runoff Management Program (LIP/JRMP) is the City of Laguna Niguel s specific document that details how the stormwater programs are implemented within their local jurisdiction. The LIP/JRMP describes the programs and activities that the City is implementing to meet MS4 Permit requirements, with the goal of making meaningful improvement in water quality Impact Analysis Methodology A hydrology study was prepared in conformance with procedure and criteria outlined in the Orange County Hydrology Manual dated 1986, using Advance Engineering Software s (AES s) Version 20.0 (Appendix K). The project was previously approved under Tract Map No. 5241, with the Project receiving conceptual WQMP approval by the City on September 14, Revisions have been made to the approved WQMP to reflect the current proposed project. The WQMP is prepared pursuant to 3.8-8

11 the requirements of the County of Orange MS4 Permit, the City of Laguna Niguel Municipal Code and the Local Implementation Plan for jurisdiction runoff Management Program (Appendix L). Thresholds of Significance Criteria for determining the significance of impacts related to hydrology and water quality are based on criteria contained in Appendix G of the State CEQA Guidelines. The proposed project could have a significant impact on the environment if it would result in any of the following. HYD-1 HYD-2 HYD-3 HYD-4 HYD-5 HYD-6 HYD-7 HYD-8 HYD-9 Violate any water quality standards or waste discharge requirements. Substantially deplete groundwater supplies or interfere substantially with groundwater recharge, resulting in a net deficit in aquifer volume or a lowering of the local groundwater table level (e.g., the production rate of pre-existing nearby wells would drop to a level that would not support existing land uses or planned uses for which permits have been granted). Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner that would result in substantial erosion or siltation on site or off site. Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, or substantially increase the rate or amount of surface runoff in a manner that would result in flooding on site or off site. Create or contribute runoff water that would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff. Otherwise substantially degrade water quality. Place housing within a 100-year flood hazard area, as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map. Place within a 100-year flood hazard area structures that would impede or redirect flood flows. Expose people or structures to a significant risk of loss, injury, or death involving flooding, including flooding as a result of the failure of a levee or dam. HYD-10 Contribute to inundation by seiche, tsunami, or mudflow. Impacts and Mitigation Impact HYD-1. The Project Would Potentially Violate Any Water Quality Standards or Waste Discharge Requirements The proposed project would not violate any water quality standards or waste discharge requirements. A discussion of construction and operation as they relate to water quality standards and discharge requirements is presented below. The proposed project could result in an increase in surface water pollutants such as sediment, oil and grease, and miscellaneous wastes during construction activities. Construction activities associated with the proposed project would include demolition of existing improvements, grading, 3.8-9

12 and installation of subdrains, horizontal drains, and groundwater wells; followed by installation of driveways and all infrastructure and home construction, all of which could result in increased polluted stormwater runoff. Water quality would be temporarily affected if disturbed sediments were discharged via existing stormwater collection systems. Increased turbidity resulting from construction-related sediment discharge can introduce compounds toxic to aquatic organisms, increase water temperature, and stimulate the growth of algae. Furthermore, concrete used for structures, footings, and other paving materials could be potential sources of water quality pollution if any of these materials were spilled or deposited on unprotected surfaces. In general, the delivery, handling, and storage of construction materials and wastes, as well as use of construction equipment, could also introduce the risk of stormwater contamination if not properly handled and contained. Staging areas or building sites can be sources of pollution because of the use and storage of equipment and materials during construction. Impacts associated with metals in stormwater include toxicity to aquatic organisms, such as bioaccumulation. Vegetation removal associated with site preparation work can result in erosion and surface water contamination from runoff. Construction impacts on water quality are potentially significant and could lead to exceedance of water quality objectives or criteria specified in the San Diego RWQCB s Basin Plan. Construction of the proposed project would disturb more than 1 acre and, therefore, would be required to prepare and implement a SWPPP, in accordance with the Construction General Permit. The SWPPP would list BMPs that would be implemented to protect stormwater runoff and would include monitoring of BMP effectiveness. At a minimum, BMPs would include practices to minimize the contact of construction materials, equipment, and maintenance supplies (e.g., fuels, lubricants, paints, solvents, adhesives) with stormwater. The SWPPP would specify properly designed centralized storage areas that keep these materials out of the rain. If grading must be conducted during the rainy season, the primary BMPs selected would focus on erosion control (i.e., keeping soil particles from detaching) and sediment control (i.e., keeping sediment on the site after its been detached). However, compliance with the Construction General Permit would require implementation of the BMPs included in the SWPPP to address these types of concerns and provide preventive measures that reduce the risk of having potential significant water quality impacts. The erosion and sediment control BMPs identified in the SWPPP would be regularly monitored and re-enforced to reduce impacts to less-than-significant levels. The proposed project includes construction of up to 71 small-lot, single-family residences on a acre site. This change in land use and associated increase in population, along with increases in vehicle use, would result in the potential for discharge of residential associated pollutants. Primary pollutants of concern include nutrients, pathogens, pesticides, metals, and toxicity. Further, leaks of fuel or lubricants, tire wear, and fallout from exhaust contribute petroleum hydrocarbons, heavy metals, and sediment to the pollutant load in runoff that could be transported to the receiving waters. However, to address the project s San Diego RWQCB South Orange County MS4 Permit and hydromodification requirements, the proposed project includes the use of biofiltration BMPs to capture and treat runoff along with vaults for temporary detention of stormwater prior to discharging off site. Figure depicts the location of the proposed treatment facilities. Runoff from residential units would be conveyed as sheet flow, gutter flow, and area drain flow to the project s proposed main storm drain line. Runoff from the project driveways and parking areas would be conveyed as gutter flow to catch basins for treatment prior to discharging into the project s main storm drain system. Runoff from the project s main storm drain line would be

13 K:\Irvine\GIS\Projects\City_of_Laguna_Niguel\00406_15\mapdoc\DEIR LEGEND DRAWN BY: TIH PREPARED BY: PREPARED FOR: DATE: 08/27/14 REVISED: 01/15/15 W.O. #: HONEYFLOWER IRVINE, CA TELEPHONE: (949) Figure WQMP Site Plan

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15 conveyed to the proposed detention vaults with proprietary biofiltration prior to discharging off site. Biofiltration BMPs are treatment methods designed to address the project s potential stormwater pollutants of concern. One type of biofiltration BMP consists of proprietary planters to which runoff is directed that use woody and herbaceous plants to mimic pollutant removal mechanisms that operate in forested ecosystems. Studies have demonstrated that, if maintained, biofiltration systems exhibit high removal rates (greater than 90%) for pollutants such as nutrients, metals, bacteria, and total suspended solids. Another biofiltration BMP is a proprietary vault with biofiltration whereby flows pass through a variety of filtration media and is temporarily stored in a vault prior to discharge off site. One commonly used facility is called a Modular Wetland. These features slow runoff velocity, which would reduce erosion potential, but would not infiltrate the water due to geotechnical stability constraints. The water is slowly released off site consistent with predevelopment conditions. Underground vaults are designed to minimize vector impacts that typically occur with standing water. The system is designed with interior slopes so the flow of water is constantly draining, preventing standing water. Compliance with MS4 Permit requirements, the installation and maintenance of biofiltration BMPs, and compliance with hydromodification stormwater management requirements would ensure that the project would have a less-than-significant impact on water quality standards during project operation. The San Diego RWQCB regulates discharge of groundwater into the storm drain system. The San Diego RWQCB has established waste discharge requirements for the release of groundwater. If the Applicant chooses to release groundwater into the storm drain system, it would need to obtain a discharge permit and comply with the waste discharge requirements and any applicable pretreatment requirements. The Applicant may also discharge groundwater flow into the sewer system operated by MNWD. The Applicant has met with MNWD and is pursuing discharge authorization. However, a final option is for the Applicant to design a system to reuse the water for slope irrigation, which would not require waste discharge permit. With implementation of Mitigation Measure MM HYD-1, the potential impact associated with violating a waste discharge requirement from the discharge of groundwater would be reduced to less-than-significant levels. Mitigation Measures MM HYD-1: Prior to the issuance of building permits, the Applicant will provide the City with either a groundwater discharge permit issued by the Regional Water Quality Control Board authorizing discharge of groundwater into the storm drain system, authorization from the Moulton Niguel Water District to discharge groundwater into the sewer system, or an acceptable design of a system to reuse the water for slope irrigation. Residual Impacts Implementation of Mitigation Measure MM HYD-1 would reduce impacts to less-than-significant levels

16 Impact HYD-2. The Project Would Not Substantially Deplete Groundwater Supplies or Interfere Substantially with Groundwater Recharge, Resulting in a Net Deficit in Aquifer Volume or a Lowering of the Local Groundwater Table Level (e.g., the Production Rate of Pre-Existing Nearby Wells Would Drop to a Level That Would Not Support Existing Land Uses or Planned Uses for Which Permits Have Been Granted) Implementation of the proposed project would not substantially deplete groundwater supplies or interfere substantially with groundwater recharge, resulting in a net deficit in aquifer volume or a lowering of the local groundwater table level. Pervious surface area pre-project is acres, or 90%, and post-project pervious surface area would be 12.3 acres, or 63%. However, based on information found in the Conceptual WQMP, on-site soils consist primarily of Alo Clay, which is classified as a Hydrologic Soil Group D soil, characterized as having very slow infiltration rates when thoroughly wet. Therefore, in the current condition, the project site does not substantially contribute to groundwater recharge due to poor infiltration rates. No substantial change would occur in the post developed condition. Furthermore, monitoring two on-site wells and three horizontal drains, including intermittent routine opening of the drain valves to allow perched groundwater to drain plus an extensive subdrain system to collect shallower water typified from storm water or irrigation, are an important component of the proposed project to attain geologic stability factors for safe residential development. Therefore, not only are the project site s soil conditions not conducive to groundwater infiltration, but the presence of groundwater could destabilize the site and, therefore, must be removed. However, based on the collection of the well data, the groundwater present under the project site is not part of an underground aquifer that supplies drinking water and is considered perched groundwater. Therefore, removal via the horizontal drains of perched brackish water from the project site would not cause a significant impact on groundwater supplies. As such, the proposed project would not substantially deplete groundwater supplies or interfere substantially with groundwater recharge such that there would be a net deficit in aquifer volume or a lowering in the groundwater table. Impacts would be less than significant, and no mitigation measures are required. Mitigation Measures Mitigation measures are not required. Residual Impacts Impacts would be less than significant. Impact HYD-3. The Project Would Not Substantially Alter the Existing Drainage Pattern of the Site or Area, Including Through the Alteration of the Course of a Stream or River, in a Manner That Would Result in Substantial Erosion or Siltation on Site or off Site Implementation of the project would alter the existing drainage pattern of the site; however, this alteration would not include the alteration of the course of a stream or river in a manner that would result in substantial erosion or siltation on or off site. As described in the existing setting, the site has seven drainage areas, all of which drain southerly (while the south drainage portion of Area A drains east and north) toward Avenida del Caballo. The northern portion of the site, the graded lots,

17 and streets south of Avenida del Caballo drain toward Avenida del Caballo, where they are then conveyed northeasterly. Runoff is then conveyed in an existing storm drain in Avenida del Caballo prior to discharging to Oso Creek. Runoff from the southern slopes of the project site (Areas B1 to F1) is conveyed as sheet flow and concentrated flow in a southerly/southeasterly downstream direction to an existing concrete channel. Runoff from the same southern slopes discharges southerly to Oso Creek. After implementation of the proposed project, runoff from residential units would be conveyed as sheet flow, gutter flow, and area drain flow to the project s proposed catch basin prior to discharging to the main storm drain in Avenida de Caballo. Runoff from the project driveways and parking areas would be conveyed as gutter flow to catch basins for treatment prior to discharging into the project s main storm drain system. Runoff from the project s main storm drain line would be conveyed to the proposed vaults with proprietary biofiltration for detention prior to discharging off site. Runoff would then be conveyed downstream and discharged as in pre-project conditions (via a 36-inch storm drain line and Oso Creek). The proposed biofiltration BMPs are intended to satisfy the project s requirements for Low Impact Development, which involves engineering design considerations to minimize impacts on the environment that encourage water conservation and stormwater treatment. These facilities would capture water runoff and provide a first flush to remove the pollutants before it enters the storm drain system. Water quality flows (non-stormwater flows and the Design Capture Volume) from the project s on-site drainage areas would be conveyed to biofiltration BMPs for treatment. Examples of these non-stormwater flows are sprinkler runoff or car washing run-off (nonstormwater) and the first 0.85 inch of a storm event (Design Capture Volume). Existing conditions would not change significantly, as drainage flows would continue toward Avenida del Caballo as planned. The proposed storm drains are all connected to the underground vault system to temporarily detain water prior to exiting the site on Avenida del Caballo. The proposed design for erosion control and filtering through biofiltration BMPs would reduce sedimentation and other pollutants within the stormwater drainage system. These project features, incorporated into the design as modular wetlands and an underground detention vault, would prevent substantial erosion and siltation on or off site. Therefore, although the proposed project would result in alteration of the existing drainage, it would not result in substantial erosion or siltation on or off site, and impacts would be less than significant and would not require mitigation measures. Mitigation Measures Mitigation measures are not required. Residual Impacts Impacts would be less than significant

18 Impact HYD-4. The Project Would Not Substantially Alter the Existing Drainage Pattern of the Site or Area, Including Through the Alteration of the Course of a Stream or River, or Substantially Increase the Rate or Amount of Surface Runoff in a Manner That Would Result in Flooding on Site or off Site Implementation of the proposed project would alter the existing drainage pattern of the site; however, this alteration would not include the alteration of the course of a stream or river, or increase the rate or amount of surface runoff in a manner that would result in flooding on or off site. The drainage area for the project site has seven drainage areas, as discussed above, totaling approximately 28 acres. The size of the drainage area exceeds the size of the project site because offsite areas drain onto the property. In the proposed development condition, the boundary of Drainage A area would increase slightly and the boundaries of the remaining drainage areas would slightly decrease. The critical storm event analyzed is the 100-year storm. The following table provides a comparison of drainage area and stormflow, referred to as Q 100 for the 100-year storm event, in both the existing and developed conditions. Table Change in Drainage Area (Acres) Existing Condition Proposed Condition Drainage Area Area (ac) Q100 (cfs) Area (ac) Q100 (cfs) Change in Area (ac) A B C D E F G Total Area Source: Appendix K. ac = acres; cfs = cubic feet per second; Q100 = 100-year storm even As shown in Table 3.8-1, during the 100-year event, all drainage areas except for Drainage Area A show a decrease in storm runoff. Drainage Area A is planned to increase in size by approximately 2.24 acres. To account for this increase in runoff, the proposed project design includes an underground detention vault with a storage capacity of 1.69 acre-feet of volume. Flows during storm events are captured in this storage vault and released over time. As a result, the peak storm flow during the 100-year event would be reduced from 83.8 cfs to 75.8 cfs, which is less than the existing condition of 76.2 cfs. Therefore, all drainage areas would show a reduction in storm flows. Furthermore, the proposed underground detention vault would capture smaller storm events, which complies with current hydromodification requirements consistent with the City s MS4 permit. In total, the vault would provide 0.68 acre-foot of detention, 0.52 acre-foot of hydromodification, and 0.75 acre-foot of water quality benefits. Therefore, the proposed project would not cause the rate or amount of surface runoff to result in flooding on or off site, and impacts would be less than significant and no mitigation measures are required

19 Mitigation Measures Mitigation measures are not required. Residual Impacts Impacts would be less than significant. Impact HYD-5. The Project Would Not Create or Contribute Runoff Water That Would Exceed the Capacity of Existing or Planned Stormwater Drainage Systems or Provide Substantial Additional Sources of Polluted Runoff As described in Impact HYD-4 above, implementation of the proposed project would not create or contribute runoff water that would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff. For each of the seven drainage areas, the runoff during the 100-year storm event would be less in the developed condition than in the existing condition through the capture of storm flows in an underground vault with a storage capacity of 1.69 acre-feet of volume. Furthermore, the planned biofiltration BMPs would treat stormwater before discharge and prevent additional sources of polluted runoff from entering the stormwater drainage system. Therefore, the proposed project would not create or contribute runoff water that would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff, and impacts would be less than significant and no mitigation measures are required. Mitigation Measures Mitigation measures are not required. Residual Impacts Impacts would be less than significant. Impact HYD-6. The Project Would Not Otherwise Substantially Degrade Water Quality Implementation of the proposed project would not otherwise substantially degrade water quality. As described in Impact HYD-1 through Impact HYD-5 above, the proposed project would result in less-than-significant short-term construction and long-term operational impacts on water quality. Construction impacts would be reduced through the implementation of BMPs during construction, and biofiltration BMPs would be incorporated as a project design feature. Therefore, implementation of the proposed project would be in compliance with existing stormwater regulations and would not otherwise substantially degrade water quality. Impacts would be less than significant, and no mitigation measures are required. Mitigation Measures Mitigation measures are not required. Residual Impacts Impacts would be less than significant

20 Impact HYD-7. The Project Would Not Place Housing within a 100-Year Flood Hazard Area, as Mapped on a Federal Flood Hazard Boundary or Flood Insurance Rate Map or Other Flood Hazard Delineation Map The project site is not within a FEMA-designated 100-year flood zone. Therefore, no impact would occur. Mitigation Measures Mitigation measures are not required. Residual Impacts No residual impacts would occur. Impact HYD-8. The Project Would Not Place within a 100-Year Flood Hazard Area Structures That Would Impede or Redirect Flood Flows The project site is not within a FEMA-designated 100-year flood zone, and the proposed project would be designed to minimize the potential for ponding or flooding on or off site. Therefore, no impact would occur. Mitigation Measures Mitigation measures are not required. Residual Impacts No residual impacts would occur. Impact HYD-9. The Project Would Not Expose People or Structures to a Significant Risk of Loss, Injury, or Death Involving Flooding, Including Flooding as a Result of the Failure of a Levee or Dam The project site is not within a FEMA-designated 100-year flood zone, and is not downstream of a large dam. Therefore, the possibility of exposing people or structures to a significant risk of loss, injury, or death involving flooding as a result of the failure of a levee or dam is very low. Consequently, no impact would occur. Mitigation Measures Mitigation measures are not required. Residual Impacts No residual impacts would occur. Impact HYD-10. The Project Would Not Contribute to Inundation by Seiche, Tsunami, or Mudflow The project site is not near a large inland water body, it is located approximately 5 miles from the Pacific Ocean, and it is generally considered too far away to be subject to a tsunami. Additionally, the site is not within the vicinity of a large reservoir and thus is not subject to seiche. Furthermore,