Siting Criteria and Considerations. Different regulatory organizations have attempted to improve the likelihood of an infiltration pond functioning properly by developing criteria that must be met before a pond location is selected. The common thread in these criteria is that infiltration ponds function best when the soil is highly permeable, the slope of the infiltration surface is low, and the pond is high above the water table. Specifically, the Department of Ecology requires the soil infiltration rate to be at least 0.5 in/hr (1.3 cm/hr), the slope to be no greater than 15 percent, and the bottom of an infiltration facility to be a minimum of 5 feet (1.5 meters) from the seasonal high water mark (WDOE, 2001). The provision for the minimum distance to the water table is in place as an effort to avoid the low hydraulic gradients that develop when the wetting front from the infiltration pond reaches the water table. These types of requirements are similar to those used in other states and jurisdictions. For example, the design guidelines prepared by the State of Maryland (2000) state that the bottom must be 4 feet (1.2 meters) from the water table, the infiltration rate may not be lower than 0.52 in/hr (1.3 cm/hr), and the slope must be less than 20 percent. The approach used to site infiltration ponds developed by the Swedish Association of Water and Sewage Works is based on a point system (Stahre and Xxxxxxx, 1990). Points are assigned to potential sites according to the ratio between the tributary connected impervious area and the infiltration area, the nature of the surface and underlying soils, the slope of the infiltration surface, the vegetation cover, and the degree of traffic on the surface. Sites with high scores are considered suitable, whereas sites that score low are not considered.
