4.8 HYDROLOGY AND WATER QUALITY

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1 4.8 HYDROLOGY AND WATER QUALITY INTRODUCTION The Hydrology and Water Quality chapter of the Draft EIR describes existing drainage and water quality setting for the project site, and evaluates potential impacts of the South of Woodward Avenue (SOWA) project (proposed project) with respect to flooding, surface water resources, and groundwater resources. Information for this chapter was primarily drawn from the Manteca General Plan, 1 the Manteca General Plan EIR, 2 the Manteca Municipal Services Review, 3 and the City s 2013 Storm Drain Master Plan EXISTING ENVIRONMENTAL SETTING The section below describes the existing hydrological features of the project site and the surrounding region, as well as the water quality of the existing resources in and around the project site. Regional Drainage The San Joaquin River is the major drainage feature in the area. The City of Manteca is located on the relatively higher ground between Lone Tree Creek to the north and the Stanislaus River to the south, with the San Joaquin River approximately four miles to the southwest and west. French Camp Slough is a tributary to the San Joaquin River and is the southwestern boundary for the City. The South San Joaquin Irrigation District (SSJID) owns a complex network of irrigation laterals and drains that run throughout the City s limits. The laterals and drains deliver irrigation water to various farming operations, and convey excess irrigation water and field runoff to downstream drains. Eventually, the water is conveyed to a large central drain called the French Camp Outlet Canal, which drains to the French Camp Slough and eventually the San Joaquin River. The backbone of the City s storm drains is a long-standing relationship with the SSJID, who permits the City to use the SSJID drains and laterals. The SSJID owns the drains and laterals and the City s Public Works Department operates and maintains the storm drainage system. The relationship was formalized in a 2006 agreement that allows the City to use the SSJID facilities through the year 2026, limited to the availability of SSJID capacity. Use of the SSJID laterals for City drainage is limited because capacity must be maintained for irrigation flows, and hydraulic grade lines are maintained higher for water deliveries. The City relies on the SSJID facilities to convey stormwater runoff to the San Joaquin River, and would continue to do so through buildout of the Manteca General Plan. A master plan of the City s storm drain system was adopted in 2006 in order to forecast needs of the system as established in the 2003 General Plan for Manteca. However, an updated master plan, the City s 2013 Storm Drain Master Plan, has been prepared and adopted

2 Meteorological events, such as intense precipitation, may adversely affect the natural drainage of the region. In addition, seasonal snowmelt from the Sierra Nevada mountain range to the east contributes to the volume of water in the local hydrologic system. Urbanization contributes to an increased volume in the hydrologic system by increasing impervious surfaces, which do not allow for infiltration of water into the soil, and, thereby, result in increased velocities and volumes of runoff. Local Drainage To avoid exceeding the capacity of SSJID facilities, drainage basins are located throughout the City to help provide storage and attenuation for storm drainage flows prior to being pumped into the SSJID s facilities. Some of the basins also delay releasing water for a longer period to further reduce the potential of downstream flooding. Most detention basins are joint-use facilities such as parks that provide recreation and other uses when not being used for stormwater detention. The pump stations are sized according to City design criteria and their operation controlled by water levels in downstream drains. A total of 10 water level monitoring stations currently exist throughout the City s storm drainage system, which are used to obtain real-time water level measurements at critical flow points in the system in order to prevent flooding. A Supervisory Control and Data Acquisition (SCADA) system is used by the City to remotely monitor and control the storm drainage pump stations and water level monitoring stations. Figure illustrates the City s existing storm drainage system, including the locations of existing detention basins (the areas indicated in turquoise), pump stations, and water level monitoring stations. The proposed project is located within Drainage Zone 36, in the South Drain storm drain subshed, as shown in Figure The South Drain subshed is bound by the City limits on the west and south, State Route (SR) 120 on the north and extends past SR 99 on the east. As shown in the figure, the laterals surrounding and within the project site include Laterals X, XAA, and D.D. The South Drain subshed is partially developed and is rapidly growing. Runoff from the developments in the South Drain area currently drains to Lateral Ya, which then flows into Drain 7 via Lateral Yb; however, this arrangement is for the interim and all Lateral Ya flows would ultimately be redirected to the new South Drain pipeline in Woodward Avenue in the future. The South Drain pipeline would serve as the main stormwater conveyance facility for Drainage Zone 36. Portions of the South Drain pipeline in Woodward Avenue have been constructed or are currently under construction, including the South Drain pipeline from Atherton Drive to Main Street. For the South Drain pipeline, a 140 cubic foot per second (cfs) pump station and 54-inch diameter pipe would be required, as well as two 36-inch diameter force mains to convey flows from the pump station under SR 120 to the French Camp Outlet Canal. The City plans to phase construction of the South Drain pump station to provide pumping capacity on an as-needed basis. Future drainage improvements, including new laterals and pump stations in the project area, are illustrated in Figure

3 Figure City of Manteca Storm Drain Pump and Monitoring Stations Project Site Source: City of Manteca, Storm Drain Master Plan, March

4 Figure City of Manteca Storm Drain Sheds Project Site Source: City of Manteca, Storm Drain Master Plan, March

5 Figure Future Drainage Improvements Project Site Source: City of Manteca, Storm Drain Master Plan, March

6 Drain 8 would continue to serve the existing developed areas in Drainage Zone 36, and would continue to flow by gravity directly to the French Camp Outlet Canal. For the remaining currently undeveloped areas in Drainage Zone 36, the City proposes to use several existing laterals as collector facilities for the South Drain pipeline. Laterals Dd and Tbb are proposed as dual-use collector facilities for the areas east of SR 99, and Laterals W and X, as well as their sub-branches, are proposed as the dual-use collector facilities for the areas south of SR 120 and west of SR 99. Lateral X would run through the proposed project site, along the northern border of the Atherton Homes at Woodward Park I development area. A fundamental goal of the City s previous storm drain master plans was to minimize the use of SSJID s laterals for conveyance of stormwater runoff to SSJID s drains. Accordingly, previous master plans specified the construction of a separate stormwater conveyance network that by passed the laterals and transported storm drainage directly to the drains. Once the City s stormwater reached the drains, the drains would continue to provide conveyance to the French Camp Outlet Canal and to the San Joaquin River. In the 2013 Storm Drain Master Plan, however, the City recognizes the opportunity to minimize infrastructure costs for all parties by abandoning the concept of separate conveyance systems and instead expanding the use of SSJID s laterals. Laterals that are targeted to convey both stormwater and irrigation water to drains are called dual use facilities. In order for Laterals X and Y to function as dual-use collector pipes, two new junction structures would be required to divert stormwater from the laterals to the South Drain pipeline. In addition, four water level monitoring stations would be required throughout Drainage Zone 36 to prevent flooding during operation of the South Drain system. According to the City s 2013 Storm Drain Master Plan, the expanded use of SSJID laterals to convey City stormwater to SSJID drains is viable. Water Quality Water is essential to drinking, recreation, the viability of agriculture, and the development of housing, commerce, and industry, as well as the maintenance of high-quality fish and wildlife habitats. Land uses and activities that the City must consider in protecting the quality of the City s water include construction activities, agricultural land uses, dairies, and urban runoff. The City is classified as a Phase II city by the State Water Resources Control Board (SWRCB). Accordingly, the City and any new development within is required to comply with the water quality limitations specified in the SWRCB s State-wide General Stormwater National Pollutant Discharge Elimination System (NPDES) permit for Phase II cities. The City also requires developers to use water quality treatment principles and Best Management Practices (BMPs) in the design of stormwater facilities, including Low Impact Development (LID) principles and techniques in compliance with the City s NPDES permit. Use of LID principles and techniques would help to improve water quality, reduce peak discharges to SSJID laterals, help meet the requirements of the City s Stormwater NPDES Permit, and is recommended in the Manteca General Plan

7 Construction Activities Construction grading, which exposes bare soil, could impact water quality. Rainfall on bare soil could cause erosion and sedimentation into nearby water bodies. Unstable soil could be washed or wind-blown into nearby surface water. Construction activities could also result in petroleum products and other pollutants from construction equipment entering nearby drainages. Agricultural Land Uses Water running off of irrigated agricultural fields may contain nutrients, sediments, fertilizers, and pesticides. Improper use and disposal of farm chemicals could contaminate surface and groundwater resources. Agricultural procedures could also result in erosion of unstabilized soil, especially during conversion of vegetation. Aerial spraying could also drift into nearby water bodies. Dairies Wastes from confined animals, including nitrate, ammonia, bacteria, total dissolved solids (TDS), and oxygen demanding constituents, could cause problems in nearby surface and groundwater. The nearest dairy is located nearly four miles northwest of the project site along Airport Way, north of Yosemite Avenue. Urban Runoff Urban runoff includes household chemicals (including pesticides, herbicides, and paints), as well as petroleum products from automobiles and landscaping equipment. Municipal sources of pollution include government yards where transportation, fueling, and maintenance activities take place. Groundwater Groundwater supply for the City of Manteca consists of 38 square miles of the Eastern San Joaquin County Groundwater Basin, a sub-basin of the San Joaquin Valley Groundwater Basin. The Eastern San Joaquin County Groundwater Basin is bounded by the Mokelumne River to the north, the Stanislaus River to the south, the San Joaquin River to the west, and bedrock to the east. The City operates and maintains its own groundwater system of wells interconnected with a transmission/distribution pipe system. The groundwater supply is indirectly affected by annual rainfall, and a multiple-year drought could decrease groundwater supplies. According to the Manteca Municipal Services Review, despite the possibility, groundwater supplies have been available at a consistent level. Water levels in the area are maintained by the proximity of the Delta channels to the west of the City limits. Groundwater recharge comes from irrigation of agricultural lands surrounding the City and infiltration from streams flowing west out of the Sierra Nevada. Such groundwater recharge occurs in areas with permeable materials that allow the infiltration of water along streams, alluvial fans, and foothill areas. The Manteca General Plan area includes a variety of 4.8-7

8 soil types that provide percolation to groundwater; however, streams or alluvial fan conditions do not exist within the Manteca General Plan area. Thus, notable groundwater recharge areas are not identified within the General Plan area. Groundwater levels are relatively high throughout the City, and according to the Manteca General Plan EIR, most municipal domestic wells produce groundwater that meets or exceeds the State of California drinking water quality standards. As the groundwater quality in the City is very good, minimal treatment is required. It should be noted that groundwater supply and quality are further addressed in Chapter 4.11, Public Services and Utilities, of this Draft EIR. Flooding The Federal Emergency Management Agency (FEMA) categorizes flood prone areas based on the frequency of occurrence. The primary flood hazard in the area is the San Joaquin River, located west of the City limits, nearly six miles west of the project site, and its tributaries including Walthall Slough along the southwestern border of the City s study area. A levee under the jurisdiction of Reclamation District 17 running from Williamson Road east to Airport Way provides flood protection for the land north and east of Walthall Slough. The project site is located approximately three miles northeast of the levee. Areas subject to inundation in the event of dam failure would generally coincide with the 100- year floodplain area. As shown in Figure 4.8-4, the project site is within Flood Hazard Zone X (unshaded), which is described by FEMA as an area of minimal flood hazard, usually above the 500-year flood level REGULATORY SETTING The following is a description of federal, State, and local environmental laws and policies that are relevant to the review of hydrology and water quality under CEQA. Federal Federal Clean Water Act The Federal Clean Water Act establishes the basic structure for regulating discharges of pollutants into surface waters of the U.S., and sets water quality standards for all contaminants in surface waters. Water quality standards are intended to protect public health, enhance the quality of water, and serve the purposes of the Clean Water Act. The Act defines water quality standards as federal or state provisions or laws that designate the beneficial uses of water and establish water quality criteria to protect those designated uses

9 Figure FEMA Map for Project Vicinity Project Site Source: FEMA, November

10 National Pollutant Discharge Elimination System (NPDES) The National Pollutant Discharge Elimination System (NPDES) permit system was established in the federal Clean Water Act (CWA) to regulate municipal and industrial discharges to surface waters of the U.S. Each NPDES permit contains limits on allowable concentrations and mass emissions of pollutants contained in the discharge. Sections 401 and 402 of the CWA contain general requirements regarding NPDES permits. Section 307 of the CWA describes the factors that the Environmental Protection Agency (EPA) must consider in setting effluent limits for priority pollutants. Nonpoint sources are diffuse and originate over a wide area rather than from a definable point. Nonpoint pollution often enters receiving water in the form of surface runoff, but is not conveyed by way of pipelines or discrete conveyances. As defined in the federal regulations, such nonpoint sources are generally exempt from federal NPDES permit program requirements. However, two types of nonpoint source discharges are controlled by the NPDES program nonpoint source discharge caused by general construction activities, and the general quality of stormwater in municipal stormwater systems. The 1987 amendments to the CWA directed the federal EPA to implement the stormwater program in two phases. Phase I addresses discharges from large (population 250,000 or above) and medium (population 100,000 to 250,000) municipalities and certain industrial activities. Phase II addresses all other discharges defined by EPA that are not included in Phase I. Section 402 of the CWA mandates that certain types of construction activities comply with the requirements of the National Pollutant Discharge Elimination System (NPDES) stormwater program. The Phase II Rule, issued in 1999, requires that construction activities that disturb land equal to or greater than one acre require permitting under the NPDES program. In California, permitting occurs under the General Permit for Stormwater Discharges Associated with Construction Activity, issued to the SWRCB, implemented and enforced by the nine Regional Water Quality Control Boards (RWQCBs). As of July 1, 2010, all dischargers with projects that include clearing, grading or stockpiling activities expected to disturb one or more acres of soil are required to obtain compliance under the NPDES Construction General Permit Order DWQ. This General Permit requires all dischargers, where construction activity disturbs one or more acres, to take the following measures: 1. Develop and implement a Storm Water Pollution Prevention Plan (SWPPP) to include a site map(s) of existing and proposed building and roadway footprints, drainage patterns and storm water collection and discharge points, and pre- and post- project topography; 2. Describe types and placement of Best Management Practices (BMPs) in the SWPPP that will be used to protect storm water quality; 3. Provide a visual and chemical (if non-visible pollutants are expected) monitoring program for implementation upon BMP failure; and 4. Provide a sediment monitoring plan if the area discharges directly to a water body listed on the 303(d) list for sediment

11 To obtain coverage, a SWPPP must be submitted to the RWQCB electronically and a copy of the SWPPP must be submitted to the City of Manteca. When project construction is completed, the landowner must file a Notice of Termination (NOT). Construction Site Runoff Management In accordance with NPDES regulations, in order to minimize the potential effects of construction runoff on receiving water quality, the State requires that any construction activity affecting one (1) acre or more must obtain a General Construction Activity Stormwater Permit. Permit applicants are required to prepare a Stormwater Pollution Prevention Plan (SWPPP) and implement Best Management Practices (BMPs) to reduce construction effects on receiving water quality by implementing erosion and sediment control measures. Federal Emergency Management Agency (FEMA) The Federal Emergency Management Agency (FEMA) is responsible for determining flood elevations and floodplain boundaries based on U.S. Army Corps of Engineers (USACE) studies. The boundaries of the 100-year floodplain are delineated by FEMA on the basis of hydrology, topography and modeling during predicted rainstorms. Areas designated as flood zones are shown on published Flood Insurance Rate Maps (FIRMs), which FEMA is also responsible for distributing, that are used in the National Flood Insurance Program (NFIP). These maps identify the locations of special flood hazard areas, including the 100-year floodplains. The NFIP requires owners of property within designated flood zones to purchase flood insurance. FEMA allows non-residential development in the floodplain; however, construction activities are restricted within the flood hazard areas, depending upon the potential for flooding within each area. Federal regulations governing development in a floodplain are set forth in Title 44, Part 60 of the Code of Federal Regulations (CFR). These standards are implemented at the State level through construction codes and local ordinances; however, these regulations only apply to residential and non-residential structure improvements. Although roadway construction or modification is not explicitly addressed in the FEMA regulations, the California Department of Transportation (Caltrans) has also adopted criteria and standards for roadway drainage systems and projects situated within designated floodplains. Standards that apply to floodplain issues are based on federal regulations (Title 23, Part 650 of the CFR). At the State level, roadway design must comply with drainage standards included in Chapters of the Caltrans Highway Design Manual. CFR Section 60.3(c)(10) restricts cumulative development from increasing the water surface elevation of the base flood by more than one foot within the floodplain

12 State State Water Resources Control Board The State Water Resources Control Board (SWRCB) and the RWQCB are responsible for ensuring implementation and compliance with the provisions of the federal CWA and California s Porter-Cologne Water Quality Control Act. The project site is situated within the jurisdiction of the Central Valley Region of the RWQCB (Region 5). The CVRWQCB has the authority to implement water quality protection standards through the issuance of permits for discharges to waters at locations within the CVRWQCB s jurisdiction. Water quality objectives for the waterways within the CVRWQCB are specified in the Water Quality Control Plan for the Sacramento River Basin and San Joaquin River Basin (Basin Plan), which was prepared in compliance with the federal CWA and the Porter-Cologne Act. The Basin Plan establishes water quality objectives, and implementation programs to meet stated objectives and to protect the beneficial uses of water in the Sacramento-San Joaquin River Basin. Because the project site is located within the CVRWQCB s jurisdiction, all discharges to surface water or groundwater are subject to the Basin Plan requirements. Local The following are the local government environmental goals and policies relevant to the CEQA review process pertaining to the hydrology and water quality aspects of the proposed project. City of Manteca Public Facilities Implementation Plan (PFIP) The City of Manteca PFIP (1993) identifies a series of specific improvements required to accommodate drainage of the existing urban areas of the City. In addition, the plan identified other measures that apply to future development. The PFIP requires the following: Wherever possible, the land ultimately required for each improvement (included in the Drainage Master Plan) will be preserved before development occurs in an area; and Storm drainage and flood protection facilities should be constructed when each new development begins. Manteca has predominantly relied upon the lateral and drain facilities of SSJID for terminal drainage. In concert with development, the SSJID facilities have been realigned into rights-ofway and/or structurally upgraded to be compatible with the land conversion from agricultural to urban use. As in the past, drainage improvements would be constructed in a timely manner in order to maintain the level of service standard. This is accomplished by constructing the storm drainage and flood protection facilities for each new project, and by having the necessary facilities in place at time of occupancy. Manteca presently administers a variety of regulations designed to prevent flooding and address stormwater management. These include a flood ordinance, various provisions of the zoning

13 ordinance and subdivision ordinance, and construction codes for residential and non-residential developments. City of Manteca 2013 Storm Drain Master Plan The City s 2013 Storm Drain Master Plan provides guidance for the planning and design of all site-specific drainage projects within the City. As part of the requirements of the agreement that authorizes the City to discharge stormwater runoff into SSJID facilities for ultimate disposal to the San Joaquin River, the City requires all new development to attenuate its runoff in a storage facility before pumping into SSJID s facilities. In addition, the City and all new development are required to comply with the State s NPDES permit. City of Manteca Storm Water Management Program (SWMP) The purpose of the City Manteca s SWMP (2003) is to limit to the maximum extent practicable the discharge of pollutants into the waters of the United States, as required by the EPA, as part of the NPDES. The SWMP includes BMPs intended to reduce to the Maximum Extent Practicable (MEP), the quantity of storm water and the discharge of pollutants to the storm water system. The SWMP would be reviewed on an annual basis and any changes or modifications would be described and submitted to CVRWQCB. Manteca General Plan The following goals and policies of the Manteca General Plan are applicable to the hydrology and water quality aspects of the proposed project. Water Quality Policy RC-P-11 Policy RC-I-22 Policy RC-I-23 Minimize pollution of waterways and other surface water bodies from urban runoff. Maintain a buffer area between waterways and urban development to protect water quality and riparian areas. Utilize cost-effective urban runoff controls, including Best Management Practices (BMPs), to limit urban pollutants from entering the water courses. Groundwater Resources Goal RC-1 Goal RC-2 Minimize the consumption of water to reasonable levels consistent with a high level of amenities and quality of life for City residents and visitors. Maximize the beneficial uses of water by recycling water for irrigation and other non-potable uses

14 Goal RC-7 To protect water quality in the San Joaquin River and in the area s groundwater basin. Policy RC-P-1 Policy RC-P-2 Policy RC-P-4 The City shall continue to implement water conservation standards for all commercial and industrial development, and for all existing and new residential development. The City shall explore potential uses of treated wastewater when such opportunities become available. The City shall require water conservation in both City operations and private development to minimize the need for the development of new water sources. Development of private water wells within the city limits shall be allowed only where the City makes a finding that municipal water service is not readily and feasibly available, and such private well systems shall only be allowed to be used until such time as City water service becomes available. Flooding Goal S-3 Goal S-4 Prevent loss of lives, injury, and property damage due to flooding. Pursue flood control solutions that minimize environmental impacts. Policy S-P-7 Policy S-P-8 Policy S-P-9 Policy S-P-10 Policy S-P-11 Policy S-P-12 Regulate all uses and development in areas subject to potential flooding through zoning and other land use regulations. Cooperate with other agencies in the pursuit of a regional approach to flood issues. Combine flood control, recreation, water quality, and open space functions where feasible. Ensure that any existing structures subject to the 100-year flood provide adequate protection from flood hazards. Ensure that the impacts of potential flooding are adequately analyzed when considering areas for future urban expansion. New residential development, including mobile homes, shall be constructed so that the lowest floor is at least one foot above the 100-year flood level

15 Policy S-P-13 Non-residential development shall be anchored and floodproofed in accord with the Federal Emergency Management Agency (FEMA) standards to prevent damage or causing damage due to a 100-year flood or, alternatively, elevated to at least one foot above the 100-year flood level. When improvements to existing development are made costing at least 50 percent of the current market value of the structure before improvements, the structure shall be brought into compliance with FEMA standards IMPACTS AND MITIGATION MEASURES Standards of Significance An impact is considered significant, consistent with Appendix G of CEQA Guidelines, if the proposed project would result in any of the following: Substantially alter the existing drainage pattern of the site or area; Create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff; Violate any water quality standards or waste discharge requirements, or otherwise substantially degrade water quality; Interfere substantially with groundwater recharge; Place housing within a 100-year floodplain, as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map; Place within a 100-year floodplain structures which 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; or Inundation by seiche, tsunami, or mudflow. It should be noted that the proposed project s impacts associated with groundwater supply is further addressed in Chapter 4.11, Public Services and Utilities, of this Draft EIR. Method of Analysis The information contained in the Hydrology and Water Quality chapter of this Draft EIR was derived primarily from the Manteca General Plan and associated EIR, the Manteca Municipal Services Review, and the City s 2013 Storm Drain Master Plan. Determinations of significance were made based on the project s modifications to existing or planned conditions, and the existing infrastructure s ability to accommodate the proposed project

16 Project-Specific Impacts and Mitigation Measures The following discussion of impacts is based on the implementation of the proposed project in comparison with the standards of significance identified above Impacts related to the drainage pattern of the site and surface runoff. Development of the proposed project would result in new impervious surfaces that currently do not exist on the site. As a result, additional runoff would be generated during storm events, which would contribute to the flood potential of receiving drainage facilities if adequate capacity does not exist within the facilities. However, the Atherton Homes at Woodward Park I and II sites include detention basins that would detain stormwater accumulated on the sites during major storm events. The detention basins are required to be designed in accordance with the City s 2013 Storm Drain Master Plan, particularly Section 3.7, including, but not limited to, the following: Sized adequate to hold a 10 year, 48 hour duration storm runoff volume resulting from 3.56 inches of rainfall occurring over the entire contributing area; Include a positive shut-off control; Include water quality treatment sufficient to meet stormwater NPDES permit requirements; Be multi-purpose facilities when practical; and Provide volume without allowance for percolation or outlet facilities. In addition, the proposed project would include construction of storm drain lines in the proposed streets, which could collect and convey stormwater runoff to the detention basins for temporary storage as well as water quality treatment. From the detention basins, stormwater runoff would be conveyed via a new on-site pump station to SSJID s Lateral X. According to the City s 2013 Storm Drain Master Plan, the developer would be required to expand the existing Lateral X from 36 inches to 42 inches along the northern boundary of Atherton Homes at Woodward Park I site in order to be a dual-use lateral, as identified in Figure It should be noted that the DeJong Property portion of the proposed project is being considered at a program-level. As such, the stormwater drainage system for the DeJong Property has not yet been designed. As discussed above, the proposed project would implement the requirements of the City s SWMP, including BMPs to maximize stormwater quality, and would be consistent with the City s NPDES Phase II Stormwater Permit. However, because the stormwater drainage system has not yet been designed for the DeJong Property, and the design of the required stormwater facilities on the Atherton Homes at Woodward Park I and II sites have yet to be determined at a subdivision-level basis, runoff from the project site could potentially contribute to an exceedance of the capacity of the City s stormwater facilities. Therefore, this impact is considered potentially significant

17 Mitigation Measure(s) Implementation of the following mitigation measure would reduce the above impact to a less-than-significant level Prior to the recording of any Final Map, the applicant shall submit a master drainage plan, subject to the review and approval by the City Engineer. This plan shall address the following requirements: Calculations of pre-development runoff conditions and postdevelopment runoff scenarios, using appropriate engineering methods, to evaluate potential changes to runoff through specific design criteria and account for increased surface runoff; Assessment of existing drainage facilities within the project area and an inventory of necessary upgrades, replacements, redesigns, and rehabilitation; A proposed maintenance program for the on-site drainage system; Phasing standards for drainage systems to be installed on a project- /parcel-specific basis; and Improvement of Lateral X to a dual-use lateral consistent with the City s Storm Drain Master Plan. Drainage systems, including any detention basin(s), shall be designed to be implemented from the master drainage plan shall provide for no net increase in peak stormwater discharge relative to current conditions, ensure that 100-year flooding and its potential impacts are maintained at or below current levels, and ensure that people and structures are not exposed to additional flood risk. Prior to issuing a grading permit, the City shall require the project applicant to demonstrate that the portion of the project subject to the grading permit is consistent with the recommendations and conclusions of the master drainage plan and shall implement the measures identified in the plan. If the plan does not adequately address the drainage impacts of the specific development, the City shall require the applicant to prepare additional analysis and incorporate measures consistent with the scope and performance standards associated with the plan to ensure that drainage and flooding impacts are avoided Short-term construction-related impacts related to water quality. Project development would involve the construction of approximately 706 single-family units, which would require grading, excavation, and other construction-related activities that could cause soil erosion at an accelerated rate during storm events. All of these activities have the potential to affect water quality and contribute to localized violations

18 of water quality standards if stormwater runoff from construction activities enters receiving waters. Construction activities such as grading, excavation, and trenching for site improvements would result in the disturbance of on-site soils. The exposed soils have the potential to affect water quality in two ways: 1) suspended soil particles and sediments transported through runoff; or 2) sediments transported as dust that eventually reach local water bodies. Spills or leaks from heavy equipment and machinery, staging areas, or building sites also have the potential to enter runoff. Typical pollutants include, but are not limited to, petroleum and heavy metals from equipment and products such as paints, solvents, and cleaning agents, which could contain hazardous constituents. Sediment from erosion of graded or excavated surface materials, leaks or spills from equipment, or inadvertent releases of building products could result in water quality degradation if runoff containing the sediment or contaminants should enter receiving waters in sufficient quantities. Impacts from construction-related activities would generally be short-term and of limited duration. Because the proposed project would require construction activities that would result in a land disturbance greater than one acre, the applicant would be required by the State to obtain a General Permit for Discharges of Storm Water Associated with Construction Activity (Construction General Permit), which pertains to pollution from grading and project construction. Compliance with the Permit requires the project applicant to file a Notice of Intent (NOI) with the SWRCB and prepare a Storm Water Pollution Prevention Plan (SWPPP) prior to construction. The SWPPP would incorporate Best Management Practices (BMPs) in order to prevent, or reduce to the greatest extent feasible, adverse impacts to water quality from erosion and sedimentation. However, as the proposed project could potentially result in short-term impacts to surface water quality related to construction, impacts would be considered potentially significant. Mitigation Measure(s) Implementation of the following mitigation measures would reduce the above impact to a less-than-significant level To reduce or eliminate construction-related turbidity or sediment water quality effects, the City shall require the project contractors to comply with the requirements of the City s SWMP. In addition, the City shall require the project contractors to obtain coverage under the General Construction Permit before the onset of any construction activities, where the disturbed area is one acre or greater in size. A SWPPP shall be developed by a Qualified SWPPP Developer (QSD) in accordance with the CVRWQCB requirements for NPDES compliance and implemented prior to the issuance of any grading permit before construction. The SWPPP shall be kept on-site during construction activity and shall be made available upon request to representatives of the CVRWQCB

19 Compliance and coverage with the SWMP and General Construction Permit shall require controls of pollutant discharges that use BMPs and technology to reduce erosion and sediments to meet water quality standards. BMPs may consist of a wide variety of measures taken to reduce pollutants in stormwater and other non-point-source runoff. Measures range from source control, such as reduced surface disturbance, to the treatment of polluted runoff, such as detention basins. BMPs to be implemented as part of the SWMP and General Construction Permit (and SWPPP) may include the following practices, or other BMPs identified in the California Stormwater Quality Association (CASQA) Construction BMP Handbook. Temporary erosion control measures (such as silt fences, staked straw bales/wattles, silt/sediment basins and traps, check dams, geofabric, sandbag dikes, and temporary revegetation or other ground cover) will be employed to control erosion from disturbed areas. Use a dry detention basin (which is typically dry except after a major rainstorm, when it will temporarily fill with stormwater), designed to decrease runoff during storm events, prevent flooding, and allow for off-peak discharge. Basin features will include maintenance schedules for the periodic removal of sediments, excessive vegetation, and debris that may clog basin inlets and outlets. Cover, or apply nontoxic soil stabilizers to, inactive construction areas (previously graded areas inactive for 10 days or more) that could contribute sediment to waterways. Enclose and cover exposed stockpiles of dirt or other loose, granular construction materials that could contribute sediment to waterways. Ensure that no earth or organic material will be deposited or placed where it may be directly carried into a stream, marsh, slough, lagoon, or body of standing water. Prohibit the following types of materials from being rinsed or washed into the streets, shoulder areas, or gutters: concrete, solvents and adhesives, thinners, paints, fuels, sawdust, dirt, gasoline, asphalt and concrete saw slurry, and heavily chlorinated water. Ensure that grass or other vegetative cover will be established on the construction site as soon as possible after disturbance. The City, its contractors, or the project applicant shall select a combination of BMPs that is expected to minimize runoff flows and remove contaminants from stormwater discharges. The final selection of

20 BMPs will be subject to approval by the CVRWQCB. The City shall verify that an NOI has been filed with the SWRCB and that a SWPPP has been developed before allowing construction to begin. The City shall perform inspections of the construction area, to verify that the BMPs specified in the SWPPP are properly implemented and maintained. The City shall notify contractors immediately if there is a noncompliance issue and shall require compliance. If necessary, the City shall require that additional BMPs be designed and implemented if those originally constructed do not achieve the identified performance standard Long-term operational impacts related to water quality. The project facilities (e.g., homes, paved driveways, and roads) would involve a substantial amount of new impervious surface, which could increase the amount of surface runoff, as well as convey non-point-source contaminants to surface waters via SSJID facilities during storm events. Additional runoff could result in an increase in the amount of pollutants entering waterways. Contaminated runoff waters could flow into French Camp Slough or ultimately to the San Joaquin River and could degrade the water quality of those water bodies. During the dry season, vehicles and other urban activities release contaminants onto the impervious surfaces, where they would accumulate until the first storm event. During the initial storm event, or first flush, the concentrated pollutants would be transported via runoff to stormwater drainage systems. Anticipated runoff contaminants associated with the proposed project could include sediment, pesticides, oil and grease, nutrients, metals, bacteria, and trash. It should be noted that such contaminants may already be in runoff from the existing project site. As shown in Figures 3-4 and 3-5 in Chapter 3, Project Description, of this Draft EIR, the project includes two detention basins an approximately 3.53-acre basin in Atherton Homes at Woodward Park I, and an approximately 4.3-acre basin in Atherton Homes at Woodward Park II - to detain stormwater during major storm events, and in part to remove pollutants from stormwater runoff. In addition, the proposed project would implement the requirements of the City s SWMP, which would include BMPs to maximize stormwater quality and be consistent with the City s NPDES Phase II Stormwater Permit. The BMPs would include a combination of source control, structural improvements, and treatment systems to the extent required in order to ensure compliance with applicable regulations. Because the proposed project has the potential to release urban pollutants in runoff that could enter and potentially pollute the local water systems, a potentially significant impact would occur. Mitigation Measure(s) Implementation of the following mitigation measures would reduce the above impact to a less-than-significant level

21 4.8-3 Prior to approval of any improvement plans, the project applicant shall prepare and submit a Stormwater Quality Plan to the City Engineer for review and approval. The Plan shall identify multiple BMPs to reduce or eliminate water quality effects from polluted runoff from the project in areas with a potential to drain into storm drainage systems or surface waters. The BMPs may include a combination of source control, structural elements, and treatment systems to include, but would not be necessarily limited to, the practices below. Grass buffer strips, high infiltration substrates, and grassy swales will be used where feasible throughout the project site to reduce runoff, serve as biofilters, and provide initial stormwater treatment. This type of treatment will apply particularly to parking lots. Physical devices will be placed at outlets of pipes and channels to reduce the velocity or the energy of exiting water. Outlet protection helps to prevent scour and minimize the potential for downstream erosion by reducing the velocity or energy of concentrated stormwater flows. Dry detention basins which are typically dry except after a major rainstorm, when they temporarily fill with stormwater will be created and designed to decrease runoff during storm events, prevent flooding, and allow for off-peak discharge. Basin features will include maintenance schedules for the periodic removal of sedimentation, excessive vegetation, and debris that may clog basin inlets and outlets. The City, contractors, or the project applicant shall select a combination of BMPs that is expected to remove contaminants from stormwater discharges. The final selection and design of BMPs shall provide maximum contaminant removal, represent the best available technology that is economically achievable, and explicitly identify the expected level of effectiveness at contaminant removal. The City shall conduct inspections following the construction to ensure that all identified BMPs have been properly installed. The project shall adopt a regular maintenance and monitoring schedule to ensure that these BMPs function properly during project operations. If necessary, the City shall require that additional BMPs be designed and implemented if those originally constructed do not achieve the identified performance standard Impacts related to groundwater recharge. Development of the proposed project would result in new impervious surfaces that currently do not exist on the site. Thus, an incremental reduction in the amount of natural

22 soil surfaces available for the infiltration of rainfall and runoff to the underlying aquifer would occur. In addition, the project site is only a small percentage of the total 38 square miles of the Eastern San Joaquin County Groundwater Basin utilized by the City, and would not be expected to substantially interfere with the overall recharge of the subbasin. The majority of runoff from the project site would drain to the local storm drainage system and eventually to the San Joaquin River. It should be noted that the proposed project would include landscaping, as well as park sites, that would simultaneously provide stormwater detention basins, which would continue to provide areas for potential groundwater recharge. Therefore, development of the proposed project would not be expected to substantially interfere with groundwater recharge, and impacts would be less than significant. Mitigation Measure(s) None required Impacts related to flooding. As discussed previously and shown in Figure 4.8-4, the project site is within Flood Hazard Zone X (unshaded), which is described by FEMA as an area of minimal flood hazard, usually above the 500-year flood level. Thus, development of the proposed project would not place housing or structures within a 100-year floodplain or expose people or structures to a risk of loss, injury, or death involving flooding, including flooding as a result of a failure of a levee or dam or inundation by seiche, tsunami, or mudflow. Accordingly, restrictions on development or special requirements associated with flooding are not required for the project. Therefore, the proposed project would result in a less-than-significant impact related to flooding. Mitigation Measure(s) None required. Cumulative Impacts and Mitigation Measures Cumulative impacts to hydrology and water quality within the City of Manteca. While cumulative development within the City of Manteca would result in additional stormwater runoff and entry of pollutants into receiving waters via construction and operation of future projects, each project is required to comply with the City s regulatory stormwater documents, standards, and requirements (including the City s NPDES permit). Compliance with such would ensure that each project provides adequate storage capacity for the additional stormwater runoff generated, as well as incorporates sufficient BMPs to successfully remove pollutants from site runoff during the construction and operational phases. As demonstrated above, the proposed project, with implementation of the mitigation measures set forth in this chapter, would not result in any significant impacts to hydrology (drainage and flooding) or water quality. As a result, the proposed project s incremental contribution to cumulative hydrology and water quality impacts would be less than significant

23 Mitigation Measure(s) None required. Endnotes 1 City of Manteca. Manteca General Plan 2023 Policy Document. October 6, City of Manteca. Manteca General Plan 2023 Environmental Impact Report. October 6, City of Manteca. Manteca Municipal Services Review. June 16, City of Manteca. Storm Drain Master Plan. March Federal Emergency Management Agency. Flood Insurance Rate Map Number 06077C0640F. October 16, Available at: Accessed November 25,