Water Resources, Floodplains, and Drainage Assessment for the North Meadows Extension to US 85 and Interstate 25

Transcription

1 Water Resources, Floodplains, and Drainage Assessment for the North Meadows Extension to US 85 and Interstate 25 March 2010 Prepared for: Town of Castle Rock Douglas County Colorado Department of Transportation Federal Highway Administration Prepared by: Felsburg Holt & Ullevig 6300 S. Syracuse Way, Suite 600 Centennial, CO CDOT Project CC Douglas County Project CI FHU Project No

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3 TABLE OF CONTENTS North Meadows Extension to US 85 & I-25 Page LIST OF ACRONYMS iii 1.0 Introduction Water Quality Regulations Municipal Separate Storm Sewer Systems (MS4) Safe Drinking Water Act Colorado Basic Standards for Groundwater Regulation Existing Conditions Impervious Areas Surface Water Stormwater Groundwater Floodplains Floodplain Regulations East Plum Creek Floodplain Water Quality Impact Evaluation Methodology for Impact Evaluation No-Build Alternative Impacts Common to the Build Alternatives Refined Alternative Refined Alternative Impact Summary Mitigation Water Quality Ponds Riprap Nonstructural BMPs Construction Mitigation Permits References Page i

4 LIST OF FIGURES North Meadows Extension to US 85 & I-25 Page Figure 1 Study Area Figure 2 Bedrock Aquifers in the Study Area Figure 3 East Plum Creek Looking North from Meadows Parkway Bridge Figure 4 Driscoll Model Results Prior to BMP Implementation LIST OF TABLES Table 1 Existing Impervious Roadway Areas Within the Study Area Table 2 Existing East Plum Creek Water Quality Table 3 Floodway Widths at Roadway Crossings Table 4 Floodplain Impacts Table 5 Floodplains and Drainage Related Impacts and Mitigation Table 6 Drainage Area Summary Table 7 Precipitation Data Table 8 Roadway Stormwater Runoff Concentrations for Analytes Evaluated Table 9 Common Impervious Surface Related Surface Water Quality Impacts Table 10 Comparison of New Impervious Areas and WQCV Table 11 Expected Removal Rates for Water Quality Constituents APPENDIX APPENDIX A DRISCOLL MODEL Page ii

5 North Meadows Extension to US 85 & I-25 LIST OF ACRONYMS BFE Base Flood Elevation BMP Best Management Practice BNSF Burlington Northern Santa Fe Railway CDOT Colorado Department of Transportation CDPHE Colorado Department of Public Health and Environment CDPS Colorado Discharge Permit System CFR Code of Federal Regulations CLOMR Conditional Letter of Map Revision Cu Copper EA EIS Environmental Impact Statement EPB CDOT Environmental Programs Branch FEMA Federal Emergency Management Agency FHWA Federal Highway Administration FHU Felsburg Holt & Ullevig FIRM Flood Insurance Rate Map ft feet gpm gallons per minute I-25 Interstate 25 MGD Million Gallons per Day MS4 Municipal Separate Storm Sewer Systems LOMR Letter of Map Revision NEPA National Environmental Policy Act NPDES National Pollutant Discharge Elimination System NPS Nonpoint Source P Phosphorus SDWA Safe Drinking Water Act SFHA Special Flood Hazard Area TSS Total Suspended Solids UPRR Union Pacific Railroad US 85 US Highway 85 Page iii

6 North Meadows Extension to US 85 & I-25 USACE USEPA USFWS USGS WQCD Zn US Army Corps of Engineers US Environmental Protection Agency US Department of Interior Fish and Wildlife Service US Department of Interior Geological Survey Water Quality Control Division Zinc Page iv

7 1.0 INTRODUCTION North Meadows Extension to US 85 & I-25 The National Environmental Policy Act (NEPA) of 1969 established a mandate for federal agencies to consider the potential environmental consequences of their proposed actions, to document the analysis, and to make the information available to the public for comment prior to implementation. In accordance with NEPA and related regulations, the Federal Highway Administration (FHWA), as the Lead Agency, in cooperation with the Colorado Department of Transportation (CDOT) as a Joint Lead Agency, has prepared this (EA) for a proposed extension of North Meadows Drive from The Meadows area to US Highway 85 (US 85) and to Interstate 25 (I-25) in Douglas County, Colorado. The proposed North Meadows Extension would cross the Burlington Northern Santa Fe (BNSF) Railway, East Plum Creek, and the Union Pacific Railroad (UPRR) (Figure 1 Study Area). The project is sponsored by the Town of Castle Rock and Douglas County. The US Department of the Interior Fish and Wildlife Service (USFWS) is a Cooperating Agency. Page 1

8 North Meadows Extension to US 85 & I-25 Figure 1 Study Area Page 2

9 2.0 WATER QUALITY REGULATIONS North Meadows Extension to US 85 & I-25 Water resources within the study area are managed through federal, state, and local regulations that establish the standards and management actions necessary to protect their physical, chemical, and biological integrity. The primary regulations governing surface water and groundwater resources in the study area are the Clean Water Act (CWA) and Safe Drinking Water Act (SDWA). The Colorado Department of Public Health and Environment Water Quality Control Commission (CDPHE WQCC) has the authority to establish and enforce water quality standards within the State of Colorado. 2.1 Municipal Separate Storm Sewer Systems (MS4) The primary water quality concern associated with the project results from the discharge of stormwater to receiving waters. As part of the CWA, entities with stormwater discharges are regulated under the National Pollutant Discharge Elimination System (NPDES) permit program. MS4s that are owned and maintained by municipalities and CDOT are required to obtain Colorado Discharge Permit System (CDPS) permits for stormwater discharges. The permit requires the permittee to develop and implement a stormwater management program to maintain and protect water quality conditions from their stormwater discharges. An MS4 permit authorizes new or existing discharges composed entirely of stormwater to an MS4. The Town of Castle Rock, Douglas County and CDOT each hold an MS4 permit from CDPHE. The project area receives stormwater from each of the three permitted areas (i.e., Town of Castle Rock property, Douglas County property, and CDOT right of way). Therefore, during the design phase, permanent water quality best management practices (BMPs) must be designed such that they comply with all three MS4 permits, as well as other CDPHE-WQCC rules and regulations. Coordination between the three permit holders will be required. Generally, terms and conditions of an MS4 permit require the permittee to develop and implement six stormwater management programs to reduce the discharge of pollutants from the MS4 to the maximum extent practicable: Construction Sites Program New Development and Redevelopment Program (also known as Post-Construction) Illicit Discharges Program (also known as Illicit Discharge Detection and Elimination) Industrial Facilities Program Public Education and Public Involvement Program Pollution Prevention and Good Housekeeping Program The New Development and Redevelopment Planning Program is the primary stormwater management program element for a planning level document. According to the MS4 permit, the program requires that new projects and significant modifications are reviewed for the need to include permanent stormwater BMPs, both structural and non-structural, to protect surface water. A permanent structural BMP can be a detention pond, a grassy swale, or an artificial wetland. Permanent non-structural BMPs can be a designated work practice that protects water quality or other non-structural components (CDOT, 2004). Page 3

10 North Meadows Extension to US 85 & I-25 Following general guidance in the New Development and Redevelopment Program (CDOT, 2004), the BMPs identified for the North Meadows Extension project will offer collection and passive treatment of stormwater that is currently being directly discharged into existing untreated stormwater systems. In addition, the BMPs may also provide protection to receiving waters from chemical spills that may occur in the study area. 2.2 Safe Drinking Water Act Public drinking water supplies (systems serving more than 25 people) from both groundwater and surface water sources are regulated by the Safe Drinking Water Act. These sources include lakes, rivers, reservoirs, springs, and groundwater. Under the Safe Drinking Water Act, USEPA and the Congress established national health-based standards for drinking water contaminants specified as having known adverse human health effects. As with the CWA, USEPA has delegated regulatory authority of the Safe Drinking Water Act to the CDPHE Water Quality Control Division (WQCD). Section 3.4 Groundwater includes information about public water supply wells in the study area. 2.3 Colorado Basic Standards for Groundwater The Colorado Basic Standards for Groundwater (Regulation No. 41) and Site-specific Water Quality Classifications and Standards for Groundwater (Regulation No. 42) were established pursuant to the Colorado Water Quality Control Act. Regulation No. 41 establishes statewide standards and a system for classifying groundwater to protect existing and potential beneficial uses of groundwater in the state, while Regulation No. 42 establishes site-specific classifications for groundwater areas throughout the state (CDPHE, 2004a; CDPHE, 2006a). Groundwater throughout the state is expected to comply with the narrative and numeric standards established within Regulation No. 41. The narrative standards require waters to be free of toxic, carcinogenic chemicals that are a danger to the public health. Narrative standards apply to Domestic Use-Quality, Agricultural Use-Quality, Surface Water Quality Protection, and Potentially Usable Quality. Groundwater in selected areas of Colorado does not meet these standards due to natural and manmade conditions. These areas are identified under Regulation No. 42 and alternative uses and standards are specified. The Town of Castle Rock well field (discussed in Section 3.4) is defined in Regulation 42 as all confined and unconfined aquifers within the saturated zone. Groundwater in this area is classified as both domestic and agricultural use. Water quality standards are the same as in Regulation 41 (CDPHE, 2006). 2.4 Regulation 73 Phosphorus is a pollutant that is derived from various sources such as sewage, fertilizers, soap and stormwater runoff. It is known to promote algae blooms, cause undesirable smells and tastes, and raise the levels of other toxic elements in water. In order to control the levels of phosphorus loading in the Chatfield Reservoir, CDPHE developed Regulation 73. Regulation 73 sets a total maximum annual load (TMAL) of phosphorus into the reservoir. The TMAL applies to both point-source dischargers such as wastewater treatment plants, and non-point source discharges such as stormwater runoff. This regulation requires the Chatfield Watershed Authority to implement a program of BMPs for control of nonpoint sources. Page 4

11 3.0 EXISTING CONDITIONS North Meadows Extension to US 85 & I-25 This section presents the existing conditions of the surface water, stormwater, and groundwater within the study area. The effects of Refined Alternatives 6 and 7 will be discussed in subsequent sections. 3.1 Impervious Areas Water quality is affected by impervious areas. Impervious areas increase the volume, velocity and contaminant amount of runoff that reach waterbodies such as lakes and streams. The combination of these factors can create erosion, sedimentation, and contamination of waterbodies. Generally, the less impervious surface area that is present, the less impact to water quality. About 55 percent of the existing impervious roadway area within the study area is located within the I-25 corridor. A summary of the existing impervious areas within the study area are presented in Table 1 Existing Impervious Roadway Areas within the Study Area. Table 1 Existing Impervious Roadway Areas Within the Study Area Road Segment Existing Impervious Area (ac) 1 I Atrium Drive 0.57 US N. Meadows Dr Total Impervious areas within right-of-way The impervious roadway areas in the table above are only those within the study area. It is important to note that a significant amount of the East Plum Creek drainage basin upstream of the study area is developed and thus has impervious surfaces. 3.2 Surface Water The North Meadows Extension study area is located within the Plum Creek Watershed, a subbasin of the Chatfield Watershed. The Chatfield Watershed contains 2,150 surface acres of water, and approximately 280 miles of streams, and is used for recreation, fisheries, drinking water supplies, and other beneficial uses. The Plum Creek sub-basin, approximately 300 square miles in area, is more densely developed than other portions of the Chatfield watershed, and is thought to contribute a majority of the nutrient loading, such as phosphorus, into Chatfield Reservoir. The main stream within the study area is East Plum Creek, which flows north/northwest just west of US 85 in the study area. East Plum Creek originates at the Palmer Divide within the Pike National Forest and flows north through the Town of Castle Rock. East Plum Creek eventually drains into West Plum Creek near Sedalia, which ultimately discharges into Chatfield Reservoir. One unnamed tributary to East Plum Creek crosses under I-25 and US 85 near Castlegate Drive North. Page 5

12 North Meadows Extension to US 85 & I-25 Plum Creek has experienced some erosion over the years due to increasing development in the area. Meanders in the stream are becoming more channelized. The existing water quality within East Plum Creek is not pristine. A summary of the existing water quality is presented in Table 2, Existing East Plum Creek Water Quality. Table 2 Existing East Plum Creek Water Quality Parameter Value Alkalinity, milligrams per liter as CaCO3 100 Hardness, milligrams per liter as CaCO3 160 ph 6.8 Temperature, degrees Celsius 12.4 Dissolved Oxygen, milligrams per liter 9.5 SOURCE: USGS Stream Gauge , Plum Creek at Titan Road, East Plum Creek is classified by CDPHE as Upper South Platte River Segment 10a (CDPHE 2008). According to the Colorado Regulation 38, Classifications and Numeric Standards for South Platte River Basin (Amended 1/14/2008, Effective 3/1/2008), the WQCC has established the following designated uses for East Plum Creek (CDPHE, 2008): Recreation 1a waters suitable for human contact, in which primary contact uses (e.g. swimming) have been documented or are presumed to be present. Class 1 warm water aquatic life capable of sustaining a wide variety of warm water biota including sensitive species or could sustain such biota but for correctable water quality conditions. Agriculture waters suitable or intended to become suitable for irrigation of crops usually grown in Colorado and which are not hazardous as drinking water for livestock. Water supply waters suitable or intended to become suitable for potable water supplies. All tributaries to the East Plum Creek system, including the tributary in the study area, are classified as Upper South Platte River Segment 11a (CDPHE, 2008). The designated for Segment 11a include: recreation 1a, agriculture, and Class 2 warm water aquatic life. These designated uses have their own unique water quality standards. Streams that do not meet established water quality standards ( impaired streams ) are placed on the Colorado 303(d) List. Within the study area, East Plum Creek and the tributary to East Plum Creek do not currently have any water quality impairments (CDPHE, 2008). Phosphorus As previously mentioned, phosphorus is a concern in the Chatfield Basin, which includes East Plum Creek. The number one source of phosphorus to Chatfield Reservoir has been identified as nonpoint sources (NPS). Nonpoint sources of phosphorus include rainfall or snowmelt moving over and through the ground. Phosphorus is also a typical highway runoff constituent, typically used in roadside fertilizers. Page 6

13 3.3 Stormwater North Meadows Extension to US 85 & I-25 The two build alternatives would disturb more than one acre; therefore they must comply with the requirements of both local MS4 programs, detailed in Section 2.0 Water Quality Regulations. Exact MS4 requirements will be finalized during final design. Currently, in the study area, major impervious surfaces are from The Meadows development area and the Castle Pines Commercial area. Existing stormwater BMPs within the study area include: Three detention ponds on the northeast side of US 85/ Meadows Parkway that provide stormwater control from the Castle Pines Business Park and the Factory Outlet Shops. One detention pond located at the Castle Villas/The Pines apartment complex. 3.4 Groundwater The study area is situated above the consolidated bedrock aquifers of the Denver Basin and the unconsolidated shallow alluvial aquifers associated with East Plum Creek. The Denver Basin consists of four principal sedimentary rock aquifers (each named after their corresponding geologic formation) that underlie the study area, including the Dawson, Denver, Arapahoe, and the Laramie-Fox Hills aquifers (see Figure 2 Bedrock Aquifers in the Study Area). Figure 2 Bedrock Aquifers in the Study Area SOURCE: Colorado Geological Survey, 2003, Fig. 6-1, p. 86 The Town of Castle Rock currently relies on groundwater from the deep Denver Basin aquifers for 100 percent of its water supply (Douglas County 2006). Groundwater from the aquifers is brought to the surface via wells and provides a water supply for multiple uses. The Town of Castle Rock has developed a strategic plan to increase water supply from additional sources, with potential alternatives including: Increasing the local alluvial groundwater supply from East and West Plum Creek Importing surface water Implementing direct and indirect reuse systems Adding surface reservoir storage The Town of Castle Rock has three existing well sites located within the project impact area. Two of the sites are located adjacent to the existing North Meadows Drive. The other site is Page 7

14 North Meadows Extension to US 85 & I-25 located between East Plum Creek and US 85, near an existing drilling business. According to the Town of Castle Rock, there are a total of six Denver Basin aquifer wells at the three sites. The 1996 Safe Drinking Water Act (SDWA) requires source water assessments be completed for public drinking water sources. The purpose of the assessment is to analyze the potential susceptibility of each drinking water source to contamination. Potential contamination is analyzed in two different methods: Physical Setting the ability of groundwater to provide a sufficient buffering capacity to mitigate potential contamination Source Threat potential of a contaminant source to provide a sufficient amount of contaminant to the source water to pose a health risk to consumers CDPHE completed a Source Water Assessment Report for the Town of Castle Rock in At the time of the assessment, the Town of Castle Rock had 25 active groundwater wells, 20 of which were found to have moderately low susceptibility (CDPHE, 2004). Well locations are not given in the report for security reasons. Approximately 35 percent of all public drinking water sources in the state have moderately low susceptibility. The report also states that 19 of the Town of Castle Rock s water sources have moderately high or high susceptibility to contaminants from roads. Page 8

15 4.0 FLOODPLAINS North Meadows Extension to US 85 & I-25 This section presents a summary of the floodplain associated with East Plum Creek. Local regulations, existing conditions, impacts and mitigation strategies are also discussed. 4.1 Floodplain Regulations Executive Order Floodplain management (U.S. DOT Order ; 23 CFR 650, Subpart A) directs all federal agencies to avoid to the extent practicable and feasible all shortterm and long term adverse impacts associated with floodplain modification. Also, they are to avoid direct and indirect support of development within 100-year floodplains whenever there is a reasonable alternative available. Projects that encroach upon 100-year floodplains must be supported with additional specific information. The U.S. Department of Transportation order , titled Floodplain Management and Protection, prescribes policies and procedures for ensuring that proper consideration is given to the avoidance and mitigation of adverse floodplain impacts in agency actions, planning programs and budget requests. The purpose of the Flood Disaster Protection act (42 USC ; dot Order , 23 CFR 650 Subpart A and 23 CFR 771) is to identify flood-prone areas and provide insurance. The Act requires purchase of insurance for buildings in special flood-hazard areas. The Act is applicable to any federally assisted acquisition or construction project in an area identified as having special flood hazards. Projects should avoid construction in or develop a design to be consistent with Federal Emergency Management Agency (FEMA) identified flood hazard areas. The Town of Castle Rock participates in the National Flood Insurance Program administered by FEMA. In conjunction with this program the Town regulates development and construction activities within floodplains. FEMA requires revision of the Flood Insurance Rate Map (FIRM) for any construction or development within the floodplain that results in an increase in regulatory base flood elevations (BFEs), or an increase in floodplain boundaries. When this is anticipated by a proposed project, a Conditional Letter of Map Revision (CLOMR) must be obtained from FEMA before the project is constructed. After the project is completed, a Letter of Map Revision (LOMR) must be obtained from FEMA to finish the revision of the FIRM. More specifically, Conditional Letter of Map Revision (CLOMR) FEMA's comment on a proposed project that would affect the hydrologic or hydraulic characteristics of a flooding source. The letter does not revise an effective map, it indicates whether the project, if built as proposed, would be recognized by FEMA. Once a project has been completed, the community must request a Letter of Map Revision (LOMR) to the Flood Insurance Rate Map (FIRM) to reflect the project. "As-built" certification and other data must be submitted to support the revision request. Letter of Map Revision (LOMR) The LOMR process is FEMA s process to modify an effective Flood Insurance Rate Map (FIRM), or Flood Boundary and Floodway Map (FBFM), or both. LOMRs are generally based on the implementation of physical measures that affect the hydrologic or hydraulic characteristics of a flooding source and thus result in the modification of the existing regulatory floodway, the effective Base Flood Elevations (BFEs), or the Special Flood Hazard Area (SFHA). Page 9

16 North Meadows Extension to US 85 & I-25 In order to participate in the National Flood Insurance Program and thereby allow citizens to acquire federal flood insurance, the Town of Castle Rock has adopted floodplain management requirements as a part of their municipal ordinances. The floodplain administrator must issue a floodplain development permit for any construction within the floodplain. Existing conditions, impacts and mitigation measures are described below for the East Plum Creek floodplain. 4.2 East Plum Creek Floodplain East Plum Creek is a perennial stream with a shallow, well defined channel that meanders through the majority of its length. There are over 20 small tributaries to East Plum Creek, with Haskins Gulch, which crosses US 85 approximately ¼ mile southeast of Happy Canyon Road, being the closest in proximity to the two alternatives (see Figure 3 East Plum Creek looking North from Meadows Parkway Bridge). Figure 3 East Plum Creek Looking North from Meadows Parkway Bridge The area surrounding East Plum Creek includes riparian open space, with ground cover mainly consisting of native plants. There is some erosion and minimal debris in the channel and in surrounding areas. The floodplain supports wetland, riparian and aquatic communities. East Plum Creek has had several flooding events in the past, more specifically from overflow of East Plum Creek in May 1864, June 1921, and September 1933, when flood discharge in East Plum Creek was estimated at 5,500 cfs. On June 16, 1965, an extreme rainfall event within the upper portions of the East Plum Creek and Cherry Creek basins created major flooding in portions of both watersheds. More than 12 inches fell near Castle Rock in 4 hours. East Plum Creek crested at 126,000 cfs, and large cut banks were left after land was washed away. The recurrence interval for this event was estimated at 500 years. Neither of the build alternatives would avoid impacts to the floodplain. It is anticipated that the East Plum Creek floodplain will be impacted as a part of each alternative due to the placement of fill for the proposed roadway alignment, and placement of piers in the floodway. Refined Alternative 6 would also require a proposed access structure over East Plum Creek. The width of the Plum Creek floodway at the proposed roadway crossing is presented below in Table 3 Page 10

17 North Meadows Extension to US 85 & I-25 Floodway Widths at Roadway Crossings, which identifies the encroachment type of each of the proposed alternatives. The CDOT NEPA Manual defines four different levels of floodplain encroachment that are determined by modeling the build alternatives with hydraulic software to determine the rise in the level of the floodplain: Significant Encroachment May result in high probability of loss of human life, will likely cause future damage that could be substantial in cost or extent (including interruption of service or loss of vital transportation facilities), or will cause a notable adverse impact on natural and beneficial floodplain services. Minimal Encroachment There is floodplain involvement but the impacts on human life, transportation facilities, and natural and beneficial floodplain services are not significant and can be resolved with minimal efforts. No Encroachment There are floodplains in the vicinity of the proposed alternatives, but no floodplain encroachment. No Involvement There are no floodplains in the vicinity of the proposed alternatives. Table 3 Floodway Widths at Roadway Crossings Refined Alternative 6 Refined Width of Floodway at Roadway Crossing 434 ft at Bridge 331 ft at Access Structure Encroachment Type Minimal Minimal Alternative 7 1 Encroachment type is considered minimal if encroachment can be minimized or eliminated by incorporating scour/erosion mitigation measures, selecting the appropriate pier alignment and improving channel banks during final design. Design should be based on a detailed hydraulic analysis of the stream for several storm events (including 500-year). Redundancy of revetment may also be required. Floodplain impacts for each alternative are presented in Table 4 Floodplain Impacts. These impacts have been calculated based on the planned footprint of the alternative (including roadway elements, bridge abutments, piers and water quality ponds) within the floodplain plus a 10 foot buffer. Table 4 Floodplain Impacts Refined Alternative 6 Refined Alternative 7 Permanent Impacts (ac) It is CDOT s policy to bring transportation facilities up to current drainage standards at the time that improvement or reconstruction projects are undertaken. Consistent with this policy, US 85 in the vicinity of Refined Alternative 6 would be subject to drainage improvements at either the time of the planned US 85 widening in this area, or at the time of implementation of Refined Alternative 6 (if selected). Such drainage improvements could include a larger cross drainage structure (which may require coordination with the UPRR), raising of US 85, improving channel conveyance, or a combination of these measures. Such drainage improvements would lessen the floodplain impacts of Refined Alternative 6. During preliminary and final design, coordination will occur with FEMA, the Town of Castle Rock, the Urban Drainage and Flood Control District, and Douglas County to ensure that the Page 11

18 North Meadows Extension to US 85 & I-25 design meets floodplain requirements. All FEMA requirements will be met, as outlined in FEMA s CLOMR and LOMR process. The impacts and mitigation of Refined Alternatives 6 and 7 on East Plum Creek are presented below in Table 5 Floodplains and Drainage Related Impacts and Mitigation. Table 5 Floodplains and Drainage Related Impacts and Mitigation Impacts Direct Impacts East Plum Creek 100-year floodplain Indirect Impacts Increased Impervious area Temporary Construction Impacts Construction activities within East Plum Creek Floodplain Mitigation - Follow FEMA Standards - Detailed design (Protect Channel/Bridge from flooding and high velocities) - Detention / Water Quality Ponds - Adhere to Town of Castle Rock, Douglas County and CDOT Erosion and Sediment Control Requirements Page 12

19 5.0 WATER QUALITY IMPACT EVALUATION North Meadows Extension to US 85 & I-25 This section describes the impacts of the No-Build Alternative and Refined Alternatives 6 and 7. The methodology for impact evaluation is discussed, followed by a discussion of the impact of each of the alternatives. 5.1 Methodology for Impact Evaluation For each alternative, surface water quality impacts were established by evaluating the total impervious surface area, changes in impervious surface area, and application of the Driscoll model, which is explained below. The total impervious surface area of each alternative was evaluated as a way to estimate water quality impacts in the absence of permanent BMPs. Driscoll Model The Federal Highway Administration (FHWA) developed the Driscoll Model as a technique to estimate the water quality impacts of a stream or lake that directly receives highway stormwater runoff (FHWA, 1990). The Driscoll Model has been applied as part of the impacts evaluation for this EA. The modeling approach described herein is consistent with FHWA guidance, and is used as a screening tool to compare pollutant mass loading between the No-Build Alternative and Refined Alternatives 6 and 7. The constituents analyzed using the Driscoll methodology for the proposed project include dissolved zinc (Zn), dissolved copper (Cu), total suspended solids (TSS) and total phosphorous (P). These constituents were selected because they are a good indicator of stormwater runoff quality from pavement and vehicles, and because of phosphorous sensitivities within the watershed. Potential sources of dissolved Zn and dissolved Cu in roadway runoff are from brake and tire wear. Phosphorus can come from roadside fertilizers and erosion of sediment. The Driscoll Model requires the following three main inputs to complete an analysis of stormwater loading: Drainage area characteristics Rainfall characteristics Pollutant concentration Each of the inputs is discussed below. A summary of the input values and results for the Driscoll model are presented in Appendix A Driscoll Model. Drainage Area Characteristics The Driscoll Model requires input of the area of the highway segment that contributes runoff to the receiving water. Both the area of the pavement and the total right-of-way area are considered when determining the percent impervious area. A summary of the drainage areas for each alternative in this analysis is presented in Table 6 Drainage Area Summary. Page 13

20 North Meadows Extension to US 85 & I-25 Table 6 Drainage Area Summary Alternative No-Build Area of ROW (acres) 40.6 Pavement Area (acres) 19.2 Refined Alternative 6 Area of ROW (acres) 71.6 Pavement Area (acres) 42.0 Refined Alternative 7 Area of ROW (acres) 76.9 Pavement Area (acres) 49.6 The areas of impervious pavement were estimated from concept drawings. Existing land use conditions within the project study area is partially developed; development or land disturbance activities may impact the current water quality. Rainfall Characteristics FHWA s Driscoll Model design procedure documentation provides initial estimates of rainfall statistics required for the model. These estimates are presented for large regions of the U.S.; the regions are so broad that portions of the region can have drastically different rainfall characteristics than the rest of the region. FHWA recommends obtaining long-term site specific precipitation data from a local rain gauge where available to refine the rainfall statistics to be used in the model. Local precipitation data was obtained from a gauge located at the former Stapleton Airport. The data used in this analysis was collected from 1948 to The winter months of November 15- March 15 were not included because the Driscoll Model is designed to predict stormwater runoff in the form of rain events, and not snowmelt. The Stapleton rain gauge data is considered to be more site-specific than the values suggested by the Driscoll model documentation for the Denver area, and thus were used in the Driscoll analysis. Table 7 Precipitation Data presents the precipitation data from the Stapleton rain gauge for use in the Driscoll Analysis. Table 7 Precipitation Data Stapleton Rain Gauge Rainfall Depth (in.) Duration (hrs.) 5.26 Intensity (in/hr) Time between storms (days) 3.86 Pollutant Concentrations The stormwater runoff concentration data for the constituents analyzed using the Driscoll model were obtained from the I-70 Mountain Corridor Tier 1 Draft Programmatic Environmental Impact Statement (I-70 PEIS) (CDOT, 2004). The I-70 PEIS study team conducted multi-year stormwater sampling events in an effort to characterize the stormwater discharged from I-70. Page 14

21 North Meadows Extension to US 85 & I-25 Two types of runoff data were collected in the I-70 PEIS, snowmelt and rainfall. The project team determined that the rainfall data was the most appropriate and applicable data for this project; the data was considered to better represent the type and frequency of precipitation in the study area. The I-70 PEIS data is the most recent and accurate stormwater data from roadways in Colorado. Other stormwater data from roadways in Denver are available; however, these data may not be representative of current conditions because they were collected more than 30 years ago. The runoff concentration data used in this analysis are presented in Table 8 Roadway Stormwater Runoff Concentrations for Analytes Evaluated. Table 8 Roadway Stormwater Runoff Concentrations for Analytes Evaluated Constituent Minimum (mg/l) Maximum (mg/l) Mean (mg/l) Median (mg/l) Number of Samples Copper Zinc Total Suspended Solids Total Phosphorus SOURCE: CDOT, All constituents are dissolved concentrations mg/l = milligrams per liter The results of the Driscoll model are presented in Figure 4 Driscoll Model Results Prior to BMP Implementation. The results are given in terms of loading in pounds of contaminant per storm event. For example, the result for zinc is the total amount of zinc that would be washed off of both the impervious and pervious surfaces into East Plum Creek during an average storm. Generally, the higher amount of impervious surfaces are associated with an alternative, the higher amount of contamination in the stormwater that would be discharged into East Plum Creek. It is important to note that these are only estimated values and the actual loading will vary with the time of year, severity of the storm, and the time in between storms. Page 15

22 North Meadows Extension to US 85 & I-25 Figure 4 Driscoll Model Results Prior to BMP Implementation Event Mass Load Pounds (lbs) Copper Zinc No-Build Refined Alternative 6 Refined Alternative 7 Event Mass Load Pounds (lbs) Phosphorus No-Build Refined Alternative 6 Refined Alternative 7 Page 16

23 North Meadows Extension to US 85 & I-25 Figure 4 Driscoll Model Results Prior to BMP Implementation (cont d) Event Mass Load Pounds (lbs) TSS No-Build Refined Alternative 6 Refined Alternative 7 The figure above graphically displays the Driscoll model results for each alternative. Constituent loading is measured in pounds of constituent leaving the roadway per a median rainfall event. The relationship between the alternative s loading is the same for every constituent analyzed in the Driscoll model, only the magnitude of the loading changes. The loads for the No-Build Alternative are used as a baseline comparison to Refined Alternatives 6 and 7. The No-Build Alternative has the lowest predicted constituent loading when compared to the build alternatives. Since the Driscoll model is a screening tool that differentiates impacts among alternatives, the results should not be used to determine if water quality standards are expected to be exceeded. The loading information from the Driscoll model is used to comparatively estimate which alternative may have more water quality impacts. It can be assumed that an alternative with a higher predicted load (i.e., a greater quantity of constituent leaving the road) would have more water quality impacts than another alternative prior to the application of BMPs. Alternativespecific discussion of the Driscoll model results are presented later in this section. Mitigation of the impacts of each alternative is discussed in Section 6 Mitigation. Page 17

24 5.2 No-Build Alternative North Meadows Extension to US 85 & I-25 The No-Build Alternative has about half of the impervious roadway surface area as compared to the build alternatives. Existing water quality in the study area is not pristine under the No-Build Alternative. As previously mentioned, the study area lies within a developing urban environment that has already experienced impacts from man-made activities. The quality of stormwater runoff in the study area would be dependent on the implementation of BMPs associated with other projects covered under an MS4 permit. Projects over one-acre in size that are located within MS4 areas will typically require BMPs, thereby reducing impacts from increased impervious surface area. Based on the results of the Driscoll Model, the No-Build Alternative has the lowest predicted contaminant loading in each of the modeling situations, when compared to Refined Alternatives 6 and 7. Since Refined Alternatives 6 and 7 would increase the impervious surface, they would create higher contaminant loading than the No-Build Alternative. It is key to note that even though the No-Build Alternative has the least amount of predicted loading, this only represents the area of proposed roadway features. Development in the study area will continue, which will create additional impervious surfaces, and thus additional loading not represented by the results of the Driscoll Model. Groundwater Groundwater impacts are not expected as a result of activities associated with the No-Build Alternative. There are no sole source aquifers in the study area. 5.3 Impacts Common to the Build Alternatives In the absence of permanent BMPs, direct effects on surface water quality common to Refined Alternatives 6 and 7 would result from the addition of paved impervious surfaces, primarily the addition of roadway surfaces. Impervious surface areas closest to waterbodies would have the greatest impacts to water quality. Additionally, the build alternatives would add vehicle travel and thus increase contaminant load. Impacts common to the No-Build Alternative and Refined Alternatives 6 and 7, in the absence of permanent BMPs and proposed mitigation, are included in Table 9 Common Impervious Surface Related Surface Water Quality Impacts. Page 18

25 Table 9 Sediment Anti-Icing / De- Icing Chemicals (Salt-Based Deicers) Metals Nutrients General Construction Activities Construction of new piers, culverts, etc. Increased Stormwater Velocity & Volume North Meadows Extension to US 85 & I-25 Common Impervious Surface Related Surface Water Quality Impacts Direct Impacts Proposed Mitigation 1 Harmful to aquatic life. Sedimentation directly degrades aquatic habitat. Suspended sediment increases turbidity and reduces aquatic plant life productivity. Suspended sediment can be fatal to aquatic species by reducing dissolved oxygen levels (Trombulak and Frissell, 2000). Potentially harmful to aquatic species, including plants. CDOT is conducting research to better understand the aquatic life effects. Toxic to aquatic life. Bio-accumulation. Metals that bind to suspended solids and decaying organic matter can persist in the environment for long periods of time. Contamination of drinking water supplies. Toxic to aquatic life. Excessive nutrients, primarily nitrogen and phosphorus, can cause extreme algal growth, which is toxic to certain aquatic organisms. Algal blooms and die-off causes large swings in dissolved oxygen levels and in extreme cases fish kills. Alters aesthetics. Can cause designated use impairments. Erosion. Harmful to aquatic life. Vegetation removal at construction sites increases stormwater runoff velocity and volume causing accelerated erosion. Riparian vegetation removal reduces stream bank stability, accelerates erosion, alters aquatic habitat and shading, and causes in-stream temperature changes. Construction vehicles deposit sediment onto surrounding roads, which is later mobilized during storm events. Erosion. Harmful to aquatic life. Alters streamflow within channel. Erosion/sedimentation upstream and downstream of structures. Reduces quality and quantity of aquatic habitat. Erosion. Harmful to aquatic life. Increased stormwater runoff velocity and volume causes stream channelization (i.e., straightening). Channelization increases surface water velocity and exacerbates erosion and sedimentation. Reduces quality and quantity of aquatic habitat. o Water Quality Ponds o Riprap o Nonstructural BMPs (continued decreasing use of salt and sanding) o Vegetation Restoration or improvements o Nonstructural BMPs (decreasing use of salt and sanding) o Water Quality Ponds o Well Abandonment o Nonstructural BMPs (Spill prevention plan during construction) o Water Quality Ponds o Construction BMPs 2 Minimize in-stream activities Stormwater Management Plan (silt fence, inlet protection, containerization of wastes, etc.) Revegetation and replacement of site, including riparian areas Spill Prevention Plan Construction Phasing o Riprap o Construction Phasing o Water Quality Ponds o Riprap (1) See Section 6.0 Mitigation for a description of proposed mitigation measures. (2) Construction activities of the size of the Refined Alternatives 6 and 7 currently are required by permit to implement these actions. Page 19

26 North Meadows Extension to US 85 & I-25 With the implementation of either of the build alternatives, the study area would have more total impervious surface areas than the No-Build Alternative. Maintenance activities can also impact water quality as a result of implementing and maintaining any of the refined alternatives. The primary maintenance impact would come from the application of traction sand during winter months. Temporary and Indirect Impacts The implementation of Refined Alternatives 6 or 7 would result in construction-related impacts at water body crossings if not mitigated. The majority of construction related impacts results from the demolition and/or construction of structures, rail lines, and roadway lanes. Constructionrelated impacts and the proposed mitigation to minimize these impacts are included in Table 9 Common Impervious Surface Related Surface Water Quality Impacts. Construction staging areas often include heavy vehicle traffic and the storage and use of hazardous materials. Without being managed appropriately, construction staging areas could contaminate nearby waterways. The proposed construction mitigation measures are summarized in Section 6.0 Mitigation and are required by permit and policy on most construction projects. Any additional land development would occur in areas covered by the Town of Castle Rock s MS4 Permit and thus, would have to comply with the conditions of that permit and likely institute temporary and permanent BMPs. Groundwater Build alternatives 6 and 7 would not directly impact existing well facilities. The status of groundwater well use will have to be determined during final design to identify the necessary course of action for each well. Active wells that would be directly impacted may need to be relocated. Any active and non-active wells to be abandoned would need to be plugged and sealed, as coordinated with the Town of Castle Rock Utilities Department. 5.4 Refined Alternative 6 Refined Alternative 6 has the lowest amount of impervious roadway area, thus the lowest loading of all contaminants when compared to the other build alternative. The impervious area and contaminant loading are approximately double that of the No-Build, but are only slightly less than other build alternative. This is because the Refined Alternative 6 alignment is more direct and requires less impervious surfaces. However, the difference between the loading results between the build alternatives is relatively small. Groundwater Groundwater impacts are not expected as a result of activities associated with Refined Alternative 6. This alternative is located the furthest from the wells located in the Study Area. 5.5 Refined Alternative 7 Refined Alternative 7 has the highest amount of impervious roadway area, thus the highest loading of all contaminants when compared to Refined Alternative 6. The impervious area and loading are slightly higher than the other build alternative. The primary reason for the increased loading is associated with longer length of roadway in Refined Alternative 7, as compared with Refined Alternative 6. Page 20

27 North Meadows Extension to US 85 & I-25 Groundwater Refined Alternative 7 would be located adjacent to Castle Rock water supply wells. This alternative is not anticipated to impact the wells. The wells are deep and completely sealed at the surface, thereby preventing potential impacts from project stormwater runoff. Consultation with the Town of Castle Rock indicated that the wells would not be directly affected by this alternative, thus no additional mitigation is needed. 5.6 Impact Summary Based upon the above discussion, the No-Build Alternative will have fewer water quality impacts than Refined Alternatives 6 and 7. This is based upon the fact that the build alternatives will add impervious surface area, increased vehicle travel, and subsequently have a higher contaminant load. It should be noted that the additional impervious surface from the Refined Alternatives 6 and 7 are very small when compared to the impervious surfaces added from proposed land developments. The additional impervious surface from Refined Alternatives 6 and 7 is not expected to result in substantial water quality impacts, when compared to existing conditions when considering that water quality BMPs (discussed in the next section) will be applied to reduce water quality impacts. Page 21

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29 6.0 MITIGATION North Meadows Extension to US 85 & I-25 This section describes the mitigation measures that have been identified to address impacts that would be expected with the Refined Alternatives 6 and 7. This discussion also includes mitigation measures to address temporary impacts associated with the construction and permanent impacts associated with the long-term operation of Refined Alternatives 6 or 7. If stormwater runoff is left unmitigated, all of the refined alternatives would have water quality impacts due to changes in stormwater characteristics from the addition of impervious surface area and increased vehicle travel. Other impacts could result from the demolition and construction of roadways and structures (e.g., bridges, culverts, piers, retaining walls) near East Plum Creek. To reduce the impacts to water resources, a combination of mitigation measures consisting of permanent structural, nonstructural, and temporary construction BMPs should be implemented in the Study Area. BMPs identified as part of Refined Alternatives 6 and 7 include water collection and passive treatment of stormwater. Permanent structural BMPs have been identified and included in the conceptual design. Permanent structural BMPs would remain in place and require routine maintenance to ensure their functionality. Water quality ponds and riprap outlet protection are examples of structural BMPs. In the areas where project crosses CDOT right-of-way, the mitigation must be consistent with CDOT s MS4 design criteria identified in the New Development and Redevelopment Program (CDOT, 2004). The performance criteria that have been selected for permanent structural BMPs within the Project Study Area should provide 100 percent water quality capture volume (WQCV) or 80 percent TSS removal. The removal efficiencies for these types of BMPs (e.g., extended detention basin) are 50 percent to 70 percent TSS and 30 percent to 60 percent total zinc (CDOT, 2004). Based on these removal efficiencies, it is assumed that they will reduce the amount of contaminants that reach surface waterbodies. Improvements to East Plum Creek may be required depending on the alternative layout selected to protect bridge abutments from potential scour. According to the East Plum Creek Watershed Master Plan Preliminary Design Report, channel stabilization may be required in the vicinity of the project. 6.1 Water Quality Ponds Water Quality Ponds have been identified as a structural BMP for this project. Potential locations of water quality ponds are on either side of the stream crossing at East Plum Creek and just west of US 85. Preliminary sizing information for the ponds will likely capture 100 percent of the Water Quality Capture Volume (WQCV). Additionally, detention is required for minor and major storm events per the Town of Castle Rock Storm Drainage Design & Technical Criteria Manual. These issues may increase the estimated size of the facilities shown and associated costs for each alternative. Coordination between MS4 permit holders will be required to determine pond locations, as well as maintenance agreements. During that time, coordination will be required to identify if separate ponds for each permit holder is appropriate. Final determination of water quality pond sizing will occur during final design. Page 23

30 North Meadows Extension to US 85 & I-25 The preliminary water quality pond sizes were estimated by determining the existing and proposed impervious areas within the disturbance area of each refined alternative. The WQCV is based on one-half inch of precipitation over the proposed impervious area plus 20 percent sediment accumulation. The difference from existing versus proposed impervious area results in the required WQCV. A summary of the estimated WQCV for each alternative is presented in Table 10, Comparison of New Impervious Areas and WQCV. Table 10 Comparison of New Impervious Areas and WQCV New Impervious WQCV Alternative Surface Area (acres) (acre-feet) No-Build 0 N/A Refined Alternative Refined Alternative In order to determine the effect of the water quality ponds on the water quality, it is necessary to look at the removal efficiency of the BMP. The CDOT New Development and Redevelopment Stormwater Management Program (CDOT, 2004) presents expected removal ranges in the Denver Area for a number of pollutants. The average expected removal rate was compared to the Driscoll results to determine the expected loading rate per storm event. The results of this analysis are presented in Table 11- Removal Rates for Water Quality Constituents. It should be noted that removal rates for copper were not provided in the above mentioned CDOT program document, and thus not considered in this analysis. Page 24

31 North Meadows Extension to US 85 & I-25 Table 11 Zinc Expected Removal Rates for Water Quality Constituents Driscoll Event Loading (lbs) BMP Percent Removal (%) Probable Event Loading (lbs) No-Build 0.09 N/A 0.09 Refined Alternative Refined Alternative Phosphorus Driscoll Event Loading (lbs) BMP Percent Removal (%) Probable Event Loading (lbs) No-Build 0.6 N/A 0.6 Refined Alternative Refined Alternative TSS Driscoll Event Loading (lbs) BMP Percent Removal (%) Probable Event Loading (lbs) No-Build 465 N/A 465 Refined Alternative Refined Alternative The resulting water quality for each of the build alternatives will contain lower amounts of TSS than the no-build with the addition of water quality ponds. 6.2 Riprap Riprap will be placed at bridge abutments, piers, and at critical portions of a channel or floodplain in order to avoid progressive or catastrophic failure of a structure. Riprap reduces water quality impacts by protecting stream systems from accelerated erosion and sedimentation processes that can occur around structures. The most effective method of stabilization at bridge abutments and piers is the use of riprap. Riprap that is correctly sized, is angular, and placed on a granular material or fabric, has a better record for erosion and scour protection than other methods such as vegetative cover. Despite its reliability, riprap must still be monitored and maintained. An energy dissipation device or material, such as riprap, will control postconstruction erosion near bridges. If riprap is used above the ordinary high water level of the river, it must be covered with topsoil and vegetated. Stream systems within the study area may also be affected by the design and size of culverts and bridges, as well as the application of associated riprap. These impacts are listed in Table 9 - Common Impervious Surface Related Surface Water Quality Impacts. Page 25

32 6.3 Nonstructural BMPs North Meadows Extension to US 85 & I-25 Nonstructural BMPs reduce or eliminate pollutant mobilization within stormwater runoff. Parking lot sweeping and spill containment measures are examples of nonstructural BMPs. Project construction phasing is another nonstructural BMP to be implemented to minimize potential water quality impacts. Phasing construction activities minimizes the effects associated with large areas of exposed ground and with soil compaction from heavy machinery use, both of which are commonly associated with transportation projects. Nonstructural BMPs include measures that can be incorporated into the winter maintenance practices. For example using liquid deicers instead of sand/salt mixtures has fewer impacts to surface water bodies. 6.4 Construction Mitigation There is potential for impacts to surface water bodies during the demolition and construction of roadways and structures (e.g., bridges, culverts, piers, retaining walls). Temporary construction BMPs should be implemented to reduce erosion associated in areas of ground disturbance during these activities. These measures remain in place until soil stabilizing vegetation has been reestablished. Silt fences, erosion log barriers, and temporary check dams are examples of temporary construction BMPs. When put into practice, the actions identified below will help minimize impacts: If cranes and other equipment are used for bridge demolition or construction within a stream or streambank area, the equipment will be kept out of the river to the greatest extent possible, and all work shall minimize temporary impacts to the river. The creation of a crane pad is necessary if cranes or other equipment cannot be kept out of the river. Sediment may not enter the river from land disruption and subsequent erosion. Therefore, construction BMPs will be implemented and maintained in compliance with the CDPHE general construction permit. Construction plans must develop and adhere to a stormwater management plan (Section 402, Clean Water Act; CDPHE Regulation 61). Caissons used to create bridge piers may require groundwater dewatering. A discharge permit and a possible treatment strategy may be needed before dewatering activities can occur. Vegetation or other erosion control techniques must be established to prevent sediment loading in compliance with the CDPHE general stormwater construction permit. Additionally, temporary diversions might be necessary at times to construct caissons and abutments. If these are needed, then the water will be allowed to flow around the construction area and allowed to return to the creek channel. If other regulated materials are present in staging areas, they must be removed and appropriately recycled or disposed of after construction activities. During construction, they must be properly contained. Typical materials include containerized regulated liquids such as paints, solvents, oil, grease, chemicals, pesticides, and herbicides, and chlorofluorocarbon (CFC) containing equipment (equipment must be emptied before equipment is removed) [Colorado Hazardous Waste Regulations (6 Colorado Code of Regulations [CCR] )]. If the above measures and other BMPs required to meet the CDPS construction permit are incorporated and maintained properly, then temporary exceedances of water quality standards should not occur. Page 26

33 North Meadows Extension to US 85 & I-25 Permanent structural BMPs, nonstructural BMPs, and temporary construction BMPs must be regularly inspected and maintained to ensure functionality and efficiency. This includes inspections of proper BMP operation, outfall discharges and erosion protection, and detention pond sediment removal. Areas within CDOT Right-of-Way In construction areas within CDOT s right-of-way, including I-25 and US 85, the contractor must comply with CDOT specifications. CDOT s specifications for managing stormwater at a construction site (currently specifications and 208) will be followed in these areas. Groundwater Unidentified groundwater wells are not expected to be encountered within the right-of-way for any of the build alternatives. However, if any are found, the status and use of the wells will have to be determined prior to construction activities. Active wells in the final right-of-way would need to be relocated and all wells would need to be plugged, sealed, and abandoned. If groundwater is encountered during activities associated with excavations for caisson/retaining walls, the discharge of groundwater is authorized if the following conditions are met: The source is groundwater and/or groundwater combined with stormwater that does not contain pollutants in concentrations exceeding the State groundwater standards in Regulations 5 CCR and 42; The source is identified in the SWMP; Dewatering BMPs are included in the SWMP, and These discharges do not leave the site as surface runoff or to surface waters. A separate Clean Water Act Section 402 Construction Dewatering Permit or Individual Construction Dewatering Permit may be required to be obtained from CDPHE. Page 27

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35 7.0 PERMITS North Meadows Extension to US 85 & I-25 This section presents information on permits that will likely be required for any of the build alternatives. The following permits will likely be required based on final design of Refined Alternatives 6 or 7. Douglas County and Town of Castle Rock GESC Permit Douglas County and the Town of Castle Rock have a permitting program for grading, erosion, and sediment control on public and private construction projects within unincorporated limits of the County and town limits. These Grading, Erosion and Sediment Control (GESC) Manuals describe the permitting programs that have been adopted to promote environmentally-sound construction practices in the county and town. The development, implementation, and enforcement of the GESC Permit Programs is mandated by both the Federal Government and the State of Colorado. The Federal Clean Water Act s NPDES Stormwater Regulations require that stormwater discharges from certain facilities be authorized under discharge permits (40 C.F.R., ). The goal of the NPDES stormwater permits program is to reduce the amount of pollutants entering streams, lakes, and rivers as a result of stormwater runoff from residential, commercial, and industrial areas. Stormwater Discharges Associated with Construction Activity Construction activities produce many different kinds of pollutants which may cause stormwater contamination problems. The main pollutant of concern at construction sites is sediment. Grading activities remove grass, rocks, pavement and other protective ground covers, resulting in the exposure of underlying soil to the elements. Construction Sites that disturb one acre or greater, or are part of a larger common plan of development disturbing one acre or greater, are covered under Colorado s stormwater permitting requirements. Section 404 Permit Construction work in wetlands, streams, rivers, and other U.S. waters is regulated by Section 404 of the Clean Water Act. The U.S. Army Corps of Engineers (USACE) is the federal agency authorized to issue Section 404 Permits for certain activities conducted in wetlands or other U.S. waters. A 404 Permit may be required for this project depending on final design of bridge piers located in or adjacent to Plum Creek. Dewatering Construction of the Refined Alternatives 6 or 7 may require a dewatering permit if groundwater is encountered during construction. Dewatering permits are required for any groundwater discharges, regardless of the size of the project. Dewatering activities covered under this permit include, but are not limited to, pumping groundwater from wells used to temporarily lower the groundwater table and removal of groundwater, which may be mixed with stormwater runoff, from foundations and other excavations. Floodplain Development Permit Douglas County requires a floodplain development permit for any proposed development within the County s Floodplain Overlay District. Both of the build alternatives would have bridge piers located within the floodplain. Page 29

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37 8.0 REFERENCES North Meadows Extension to US 85 & I-25 Castle Rock Development Company, Habitat Conservation Plan for a Portion of the Meadows Property, Douglas County, Colorado. December. Chatfield Watershed Authority Annual Report. Chatfield Watershed Authority Master Data Spreadsheet. Chatfield Basin Conservation Network Green Infrastructure System: Conserving Connections for Nature and People Colorado Department of Public Health and Environment (CDPHE) Section 309 Report: A Study of Colorado Water Quality Classifications and Standard Issues under C.R.S. Section Water Quality Control Division. December. Colorado Department of Public Health and Environment (CDPHE). 2008a. Regulation No. 31, The Basic Standards and Methodologies for Surface Water (5-CCR ) amended 1/14/08, effective 5/31/08. Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE). 2008b. Regulation 38: Classifications and Numeric Standards for South Platte River Basin; Laramie River Basin; Republican River Basin; Smoky Hill River Basin (Amended 1/14/2008, Effective 3/1/2008). Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE) Regulation 41: The Basic Standards for Groundwater (Amended 1/14/08, Effective 5/31/08). Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE). 2006a. Regulation 42: Site- Specific Water Quality Classification and Standards for Groundwater. Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE) Regulation 73:Chatfield Reservoir Control Regulation). Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE) Regulation 93: Section 303(d) List Water Quality-Limited Segments Requiring Total Maximum Daily Loads (TMDLs). Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE) Regulation 94: Colorado s Monitoring and Evaluation List. amended 3/11/08, effective 4/30/08 Water Quality Control Commission. Colorado Department of Public Health and Environment (CDPHE) Section 305(b) Final Report: Status of Water Quality in Colorado Water Quality Control Division (WQCD). April. Page 31

38 North Meadows Extension to US 85 & I-25 Colorado Department of Transportation (CDOT) Erosion Control and Stormwater Quality Guide. Colorado Department of Transportation (CDOT). 2004a. Drainage Design Manual. Colorado Department of Transportation (CDOT). 2004b. I-70 Mountain Corridor Tier 1 Draft Programmatic Environmental Impact Statement. Colorado Department of Transportation (CDOT). 2004c. New Development and Redevelopment Stormwater Management Program. Colorado Department of Transportation (CDOT) Standard Specifications for Road and Bridge Construction. Colorado Groundwater Association Colorado Groundwater Atlas. Prepared by Andrea Aikin, E. Anderman, E. Harmon, S. Paschke, D. Plazak, and M. Riemann. Douglas County Public Works. 2009, Grading, Erosion and Sediment Control Manual (GESC), March 2004, Amended March East Plum Creek Watershed Master Plan Preliminary Design Report, February 2009 Federal Highway Administration (FHWA) Pollutant Loadings and Impacts from Highway Stormwater Runoff. Volumes I, II, and III. FHWA-RD ; FHWA-RD ; and FHWA-RD April. Town of Castle Rock, Storm Drainage Design and Technical Criteria Manual Town of Castle Rock, Stormwater Master Plan. January. Town of Castle Rock Grading, Erosion and Sediment Control Manual (GESC). March. Town of Castle Rock Water Quality Report. July. Transportation Research Board (TRB) National Cooperative Highway Research Program (NCHRP). Evaluation of Best Management Practices for Highway Runoff Control. NCHRP Report 565. United States Geological Survey Real-Time Water Data for the Nation. Online at: Retrieved 02/15/09 Page 32

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