1. The proposed project would require paving a substantial portion of the substation site, which would reduce infiltration and slightly increase the amount and rate of runoff. The existing impermeable surface area at the site is 6,620 square feet. PG&E would add 34,205 square feet of impermeable surfaces including a 20-foot wide asphalt road, gunite swale to direct surface runoff, concrete pads for electrical equipment, the oil containment berm around the transformer pads and the concrete oil containment structure and skimmer structure. Following development, the total impermeable surface area would be 40,825 square feet (PG&E, 1997b). The increased impermeable surface area would increase runoff from the site. As the existing site is partially covered with impermeable surfaces and most of the soils are compacted, the increase in runoff at the site from the above noted added impermeable surfaces would not be directly proportional to increase in impermeable surface area. However, the anticipated runoff would increase sufficiently to require construction of a detention basin on the site to prevent exceedance of the capacity of the Town's storm drain into which site runoff would be directed.

    PG&E's design guidelines for runoff are for a storm with a ten-year recurrence interval and five minute duration. At the site this equates to rainfall of 4.32 inches per hour (PG&E, 1997b). Using this precipitation intensity factor applied to the increased impermeable surface area, runoff at the site following development is estimated to be 9.5 cubic feet per second (cfs). This rate of runoff would exceed the capacity of the Town's storm drain, which is designed to accept 7.1 cfs from the Vasona site for the same design storm. Therefore, following its design guidelines, PG&E has proposed construction of a 20708 square foot detention storage area at the north end of the site. The drainage swales, if filled, would overflow into that area to prevent excessive discharges to the storm drain provides storage that would allow PG&E to control runoff for the design storm such that a discharge of 2.5 cfs would be directed into the Town's stormwater collection system (PG&E, 1997b). Because of the proposed stormwater collection system and detention pond, the impact of the project would be less than significant. Additional mitigation is not required.
  1. The project site is not within either (1) a zone of potential flood hazard as defined by the Federal Insurance Program and illustrated in the Safety Element of the Town General Plan (Los Gatos, 1994), or (2) the inundation boundaries of the Vasona or Lexington Reservoir failure inundation maps (PG&E, 1997a); thus, no impacts related to flooding are present.
  1. Stormwater discharges during construction may contain high concentrations of pollutants, including total suspended solids. Erosion is expected to be minor because the site is flat. Sediments entrained in runoff during construction on the site would be captured by the existing runoff control ditches and existing runoff catchment pond on-site (which would also be modified as part of the project). Sediment would be generated by ditch excavation for laying the distribution feeder line, particularly if the excavated soil is piled adjacent to the ditch where it would be exposed to rainfall. While the construction period for the open trenching is short, the construction is proposed to occur in the winter when rain is a common event and there is a potential hazard of generating large amounts of silt that would be entrained in runoff and discharged into storm drains. Were a storm to occur during the construction, substantial sediment could be directed into the storm drains, clogging them. Mitigation of the impact is possible through application of "best construction practices", which at a minimum should include the mitigation measures noted below. As a result of these measures, little sediment is likely to be discharged into the Town's storm drains. As the project includes proposed construction activity that would disturb less than five acres of land, PG&E need not obtain a permit from the Regional Water Quality Control Board.

    Surface water runoff from the site after construction is expected to contain minor concentrations of a variety of pollutants typical of electrical substations (e.g., automobile fluids, suspended solids, metals, and organics), but it is not expected to occur in concentrations that would be acutely toxic to aquatic life because the volume of construction-related traffic would be small.

    The proposed electrical transformer banks would contain up to 12,280 gallons of inert mineral oil. The transformer would be installed on sealed concrete foundations, and the surface of the substation would be constructed of hard materials that would direct any leaks into an on-site Spill Prevention Control and Countermeasure Pond (SPCC). The gunite-lined SPCC pond would be designed to contain all of the oil from the largest piece of equipment plus the amount of rainwater associated with a two-year storm. Gunite is a construction material composed of cement, sand, crushed slag and water, mixed and pressure sprayed onto a surface. It forms a cement-like surface. An over/under weir system (essentially a dam-like structure that controls water flow and allows passage of flows from a lower level in the ponded water while restricting passage of the upper water) with a manually operated gate valve is proposed to ensure separation of the oil and water and any oil released would be retained in the SPCC pond until it can be collected and transported to an approved disposal site. The skimmer would separate oil from the runoff water; the latter would be discharged into the Town's storm sewer through a 15-inch reinforced concrete pipe. In heavy storm periods, the SPCC pond would be monitored by PG&E staff inspections for containment and proper transport of storm discharge. This should be adequate to prevent unplanned releases and overflows. Pursuant to Environmental Protection Agency (EPA) requirements, the equipment and spill containment area are inspected by PG&E staff on a monthly basis. Storm water runoff from other portions of the yard which are not included within the drainage ditch that would receive runoff from the transformer pad would be separately discharged to a storm drainage pipe system. This storm drainage pipe system would discharge to an existing storm water collection system (in Winchester Boulevard). Because of the proposed runoff containment and discharge design, no developed portion of the site would discharge directly to Los Gatos Creek. All discharges would be directed into existing Town storm drainage facilities.


The following mitigation measure would reduce the potential impact of sediment generation discharged into storm drains to a less-than-significant level.

Mitigation Measure IV.c-1: During grading and construction of the site, PG&E shall require the construction crew or contractor to:

Mitigation Measure IV.c-2: During construction of the distribution feeder line, PG&E shall require the construction crew or contractor to:

  1. There are no water bodies present on the site. The project would result in creation of a storm drainage system that would contain water only intermittently. The level of increase in offsite discharge from the project would be within the design requirement for the Town's storm drains and would not result in a significant change in the amount of water in any water body (see {a}, above).
  1. No water course is present on site. The proposed project would have no effect on the course or direction of surface waters.
  1. The proposed project is located in the Santa Clara County Groundwater Basin, which is managed by the Santa Clara Valley Water District. The ground water basin has experienced significant draw down historically. The project site is located on an alluvial terrace at an elevation of between 288 and 295 feet above sea level. The groundwater table in this area is not known, but the elevation and surrounding topography indicate that it is well below the ground surface. Neither the site nor the slope at the eastern side of the site have any springs or indications of conditions that suggest high groundwater conditions on the site.

    Ground water recharge facilities are located along Los Gatos Creek approximately 5,000 feet downstream of the project site. The recharge facilities are located at a maximum elevation of approximately 250 feet, approximately 38 feet lower than the proposed project site surface. The project would not require any removal of groundwater, either during construction or operation. The project would increase the impervious surface area of the site which would produce some decrease in infiltration of precipitation. However, the site is unlikely to contribute significantly to groundwater recharge because of its small area, the absence of a creek or water body on site and the existing, relatively compacted condition of the surface soil. As a result, the project would reduce the potential for groundwater recharge by a negligible amount. Therefore, there would be no impact related to any change in the quantity of groundwater.
  1. The project would not require any removal of groundwater, either during construction or operation, nor would it intercept a groundwater aquifer during construction (see discussion under checklist item IV.f). The project would not include any deep cuts or other features which would intercept or impede the flow of groundwater. The shallow trenching required for laying the distribution line is not likely to affect groundwater conditions. Therefore, the project would have no impact on the direction or rate of flow of groundwater.
  1. The compacted fills and impervious surface areas would prevent infiltration of contaminants into the soils. The roposed SPCC pond would be lined to prevent infiltration of contaminants from the pond into the subsurface soils. Run off or percolation from the proposed project would not be expected to affect groundwater quality in the area. Minor pollutants from construction would be entrained in runoff and would not pose a hazard to groundwater conditions (See also the discussion under checklist item IV.c).
  1. The proposed project would not use groundwater or affect a local aquifer. Therefore the project would have no impact on the availability of groundwater for public water supply.
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