Ponding-test procedure for assessing the infiltration capacity of storm-water basins, Nassau County, New York

A procedure to obtain field data for assessing performance of storm-water basins having high infiltration rates is described. The basin selected is divided into test plots, and treated municipal water from a fire hydrant is ponded to the desired depth so that infiltration rates and flow patterns in the unsaturated zone can be determined by standard methods. The study was made at a storm-water basin excavated in glacial outwash deposits in Nassau County, Long Island, N.Y. in 1971-72. The types of interpretations and evaluations that can be made from test data are described, and applications of the procedure as well as guidelines for developing a similar testing program are given. Observations of special significance were: (1) Infiltration rate of the basin at low water stage is about 1.5 feet per hour but increases with increased water level; (2) flow through the unsaturated zone is essentially vertical; (3) infiltration rate is controlled by a surface stratum --in this case a gravelly, sandy loam; and (4) the position of the water table has little effect on infiltration rate provided it is below the lower boundary of the controlling zone. These observations can be used in evaluating a storm-water basin's suitability for supplemental recharge with treated wastewater, in comparing procedures for increasing a basin's infiltration capacity, or in the design of new storm-water basin systems.