This study analyzed data that had been previously collected by the Occoquan Watershed Monitoring Laboratory under the National Urban Runoff Program, to obtain an understanding of the degree of treatment of stormwater runoff obtainable in retention (maintains a water pool) and detention (drains completely) basins. Three basins were studied, two retention and one detention, and a total of 191 storm events were monitored using automatic sampling equipment. The samples were analyzed by the laboratory for COD, all nitrogen forms, all phosphorus forms, total suspended solids, and heavy metals (iron, manganese, etc.). It was concluded that detention and retention ponds achieve substantial reduction in the pollutant loads carried by stormwater runoff from residential areas. Non-soluble settleable materials were removed to a comparable degree by both basin types. However, the pool of water characteristic of a retention basin offered a superior environment for the removal of soluble pollutant forms, particularly nitrogen and phosphorus. The treatment obtainable in detention and retention basins make them effective devices for the abatement of urban runoff pollution. Through the mechanisms of solids settling and microbial degradation, a basin can treat polluted runoff to a considerable extent and reduce damage to receiving bodies of water. / M.S.