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The Tracy area has also experienced well contamination problems due to seepage of septic tank wastewater. However, these problems are few and isolated and do not appear to be serious. Septic System Suitability. The soils in the study area vicinity are suitable for septic system usage (Kaufman, 1987). The soils tend to have high percolation rates and a gravelly phase in the soil column at six to ten feet depth. These soil characteristics plus the absence of near surface groundwater indicate the suitability of project area soils for septic usage. Nearby residences are currently on septic systems and have experienced no problems with effective sewage disposal. IMPACTS Drainage. The increase of impervious surfaces (residences, driveways, and roadways) in the study area would result in a corresponding increase in the volume of runoff generated. The amount of additional runoff generated from the study area and the project site has not been calculated. Due to the fairly level terrain of the study area, a minimal amount of grading and excavation should be necessary. The general overall natural drainage patterns will be affected by development. Runoff will be diverted toward streets and flow along curbs and storm drains into retention basins to the northeast. Drainage plans for the entire study area have not been finalized. Runoff from the project site will be retained in an existing dry pond located at the southeast corner of the property (see Figure 1-3). With the design and implementation of an appropriate storm drainage plan, it is unlikely that there would be any significant adverse impacts on drainage in the study area. Refer to Section 3.4, Public Services, Storm Drainage, for a discussion of the need for a public drainage system. Santa-Carbona Irrigation Canal. Development of the study area and project site will have little impact on the operation of the irrigation canal. Runoff from the study area will not enter the canal. Because the water level in the canal is regulated, water from the canal is not likely to overflow onto the study area or project site. The District emphasizes the need for fencing along the canal easement to prevent encroachment of people from adjacent residential areas (Terrill, 1987). Flooding. The study area and project site are not contained within a 100 year floodplain and are, therefore, not expected to be subject to flooding. Additional runoff created by the construction of impervious surfaces is not expected to cause significant on site or downstream flooding. Areas adjacent to storm water retention basins may be susceptible to flooding following major storm events. Any development proposed in the study area would be required to include provisions to ensure accommodation of increased runoff. Surface Water Quality. The potential exists for degradation of surface water quality due to urban runoff. An expanded road network and increased vehicle activity in the study area would result in an increase in the amount of urban contaminants contained in runoff from the site. These contaminants would include minor amounts of petroleum hydrocarbons, heavy metals, and rubber particles. The overall impacts to on site and off site water quality, however, are expected to be insignificant. 3.5-2