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Iv.G Water Resources

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Iv.G Water Resources IV.G WATER RESOURCES 1. INTRODUCTION This section addresses the potential impacts of the proposed Hidden Creeks Estates project on the existing hydrology conditions on the project site. Sources utilized in the preparation of this section include correspondence with personnel at the Bureau of Engineering of the City’s Public Works Department, as well as technical studies. A hydrology study for this project site was prepared by HMK Engineering, Incorporated in January 2007 and is provided in its entirety in Appendix IV.G of this Draft EIR. 2. METHODOLOGY Pre-Developed Condition and Post-Developed Condition Hydrology reports were prepared by HMK Engineering, Incorporated in January 2007. Using the information contained in these reports, as well as the City of Los Angeles Stormwater Program Standard Urban Stormwater Mitigation Plan requirements for 10+ housing developments, the California Stormwater Best Management Practices (BMP) Handbook created by the California Stormwater Quality Association (CASQA) and the Flood Insurance Rate Maps (FIRMs) provided by the Federal Emergency Management Agency (FEMA), an analysis of potential changes to surface water hydrology, surface water quality, groundwater levels, and groundwater quality as a result of project implementation was completed. 3. EXISTING CONDITIONS a. Surface Water Hydrology The project site consists of approximately 285 acres of undeveloped land located immediately west of the community of Porter Ranch and is situated between Browns Canyon Wash and Mormon Canyon Creek, immediately north of the confluence of these two creeks. The partial upstream watershed of Browns Canyon and Mormon Canyon starts at the high ridge of Oat Mountain, which is located north of the project site. The main mountain channels of both canyons flow a distance of approximately 13,200 feet and merge to a downstream confluence point at the southern portion of the project site’s boundary. This confluence point is located 7,000 feet north of the Ronald Reagan freeway (118 Freeway). The merged flow continues from the 118 Freeway to join the existing concrete channel of Browns Canyon wash. The total drainage basin area of the watershed is approximately 3.51 square miles. Impact Sciences, Inc. IV.G-1 Hidden Creeks Estates Draft EIR 860.01 April2008 IV.G Water Resources The overall topography of the site is sloped and the project site has a low chance of flooding. According to public flood maps provided by FEMA,1 the project site is within Zone C. A Zone C designation denotes an area of minimal flooding. The project site is not located within a 50- or 100- year flood area and would not be subject to harm caused by 50-year or 100-year storm events. Few structures exist on the project site. Known structures include buildings associated with ranching operations, structures associated with filming and production sets, and experimental housing structures. Due to the relatively undeveloped character of the site, impervious surfaces are minimal and do not interfere with the surface water hydrology. Table IV.G-1, Pre-Developed Condition Hydrology, shows the existing total watershed acreage of Browns Canyon and Mormon Canyon. Table IV.G-1 Pre-Developed Condition Hydrology 50-Year Storm Debris 50-Year Storm Drainage Watershed Undeveloped Production Undeveloped/Burned Description Area (Acres) (Cubic Feet per Volume (Cubic (CFS) Second) (CFS) Yards) Browns Canyon 1,284.5 2,858.55 3,204.87 64,225 Mormon 962.9 2,055.94 2,430.32 48,145 Canyon Total 2,247.4 4,914.49 5,635.19 112,370 Source: HMK Engineering, Inc. Pre-Developed Condition Hydrology Report. January 2007. b. Surface Water Quality According to the USGS soil survey for the site, slopes range from 2 percent to 50 percent. Erosional surfaces are the major landscape feature found throughout the property. Steep hillsides, naturally erodable and unconsolidated soils, and the long-term impacts of cattle grazing, such as loss of native vegetation, soil compaction, eroded streambanks, and decreased rainwater infiltration and increased overland surface flows, have all resulted in landscape features associated with high levels of erosion. These features include slumped hillsides, a number of ephemeral washes, and the formation of deeply eroded gullies with concomitant soil and vegetation loss." The current level of erosion coupled with the total potential debris volume of 112,320 cubic yards leads to surface water that is turbid and 1 Federal Emergency Management Agency. Flood Insurance Rate Maps. http://www.fema.gov/hazard/map/firm.shtm. Accessed on March 30, 2007. Impact Sciences, Inc. IV.G-2 Hidden Creeks Estates Draf t EIR 860.01 April 2008 IV.G Water Resources contains debris, but not necessarily polluted or contaminated. However, ranching operations on the project site and the operation of the existing equestrian center in the southern portion of the site do have uses that result in the potential for animal wastes to contribute to deterioration in surface water quality. c. Groundwater Level The project site belongs to the San Fernando Valley Groundwater Basin (SFVGB). The SFVGB is bounded to the north and northwest by the Santa Susana Mountains, to the north and northeast by the San Gabriel Mountains, to the east by the San Rafael Hills, to the south by the Santa Monica Mountains and Chalk Hills, and to the west by the Simi Hills. The surface is drained by the Los Angeles River and its tributaries. The annual precipitation in the San Fernando Valley ranges from 15 to 23 inches per year with an average of approximately 17 inches.2 Groundwater Level Trends Groundwater levels in the basin have been relatively stable between 1979 through 2000. During this period, depth to groundwater varied from approximately 5 to 40 feet in the west, approximately 40 feet in the south and north, and a variation of 80 feet in the eastern parts of the basin.3 Water levels tend to follow long-term precipitation patterns with drops during periods of low rainfall and rises during periods of high rainfall. Groundwater flows generally from the edges of the basin toward the center, then beneath the Los Angeles River Narrows into the Central Subbasin of the Coastal Plain of Los Angeles Basin.4 Mapped soils on the project site include Balcom, Gazos, and Lopez. Balcom has a moderate to moderately slow permeability and both Gazos and Lopez have a moderately slow permeability.5 2 California Department of Water Resources Planning and Local Assistance. California’s Groundwater – Bulletin 118. http://www.dpla2.water.ca.gov/publications/groundwater/bulletin118/basins/pdfs_desc/4-12.pdf. Accessed on June 6, 2007. 3 Ibid. 4 Ibid. 5 Impact Sciences. Jurisdictional Delineation Waters of the United States and Streambeds. July 2006. Impact Sciences, Inc. IV.G-3 Hidden Creeks Estates Draf t EIR 860.01 April 2008 IV.G Water Resources Groundwater Storage The groundwater storage capacity of the basin is approximately 3,670,000 acre-feet (af). Groundwater storage during 1998 was estimated to be approximately 3,049,000 af.6 The SFVGB supplies approximately 15 percent of the Los Angeles domestic water need, while also acting as a vast underground reservoir where water accumulates during years of abundant rainfall and is stored for future use.7 Groundwater Budget (Type A) Although the SFVGB is managed by adjudication, not enough data exist to compile a complete groundwater budget. A total of approximately 108,500 af of groundwater was pumped from the SFVGB during the 1997/1998 water year. Additionally, subsurface outflow of approximately 300 af to the Raymond Groundwater Basin and 404 af to the Central Subbasin of the Los Angeles Coastal Plain Groundwater Basin from SFVGB was estimated. To balance the extraction, a total of 61,119 af of native runoff water was diverted to spreading grounds for infiltration in the SFVGB.8 d. Groundwater Quality Characterization Groundwater in the basin varies from calcium bicarbonate character in the east to calcium sulfate- bicarbonate character in the western part of the basin.9 Total dissolved solids (TDS) content in the basin ranges from approximately 326 to 615 mg/L. Water sampled from 125 public supply wells showed an average TDS content of 499 mg/L in the basin and a range from 176 to 1,160 mg/L.10 6 California Department of Water Resources Planning and Local Assistance. California’s Groundwater – Bulletin 118. http://www.dpla2.water.ca.gov/publications/groundwater/bulletin118/basins/pdfs_desc/4-12.pdf. Accessed on June 6, 2007. 7 City of Los Angeles Department of Water and Power. Water Quality. http://wsoweb.ladwp.com/Aqueduct/historyoflaa/waterquality.htm. Accessed on June 6, 2007. 8 California Department of Water Resources Planning and Local Assistance. California’s Groundwater – Bulletin 118. http://www.dpla2.water.ca.gov/publications/groundwater/bulletin118/basins/pdfs_desc/4-12.pdf. Accessed on June 6, 2007. 9 Ibid. 10 Ibid. Impact Sciences, Inc. IV.G-4 Hidden Creeks Estates Draf t EIR 860.01 April 2008 IV.G Water Resources Impairments Contamination of volatile organic compounds (VOC) such as trichloroethylene (TCE), perchloroethylene (PCE), petroleum compounds, chloroform, nitrate, sulfate, and heavy metals have been found in the basin. TCE, PCE, and nitrate contamination has been found in the eastern part of the basin and elevated sulfate concentration has been found in the western part of the basin. During a public well sampling to test the water quality, as required under DHS Title 22 program from 1994 through 2000, a small percentage of wells tested had pollution concentrations above a Maximum Contaminant Level (MCL). Table IV.G-2, Water Quality in Public Supply Wells, shows how many wells had a concentration above an MCL and for which constituent group. Table IV.G-2 Water Quality in Public Supply Wells Number of Number of wells with a Constituent Group wells concentration above an sampled MCL Inorganics – Primary 129 6 Radiological 122 13 Nitrates 129 44 Pesticides 134 3 VOCs and SVOCs 134 90 Inorganics – 129 17 Secondary Source: California Department of Water Resources.
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