Assessment of stormwater impacts on an urban stream with a detention pond.

Impacts of urban development on a small creek with an on-stream stormwater pond, which was built to mitigate the effects of a 13-ha commercial plaza on creek flows, were assessed by investigations of the creek-pond system hydrology, water and sediment chemistry and toxicity, and benthic communities. The hydrology and conventional pollutant loadings of this system were dominated by the creek catchment (4.5 km2), which contributed more than 95% of the total flow; the rest was contributed by the plaza runoff which was less polluted than typical residential runoff. Conventional bioassays (Daphnia magna, Microtox) did not indicate any confirmed acute toxicity in the creek flow, plaza drainage, or pond outflow. However, sediments accumulated in the pond were rather polluted and several sediment bioassays (including Sediment Microtox) indicated their severe toxicity. In benthic community assessments, taxa richness and total counts of benthic organisms did not change much when moving from upstream to downstream of the pond. Thus, the pond accumulates sediments and toxicants and thereby prevents further degradation of the creek condition downstream of the plaza drainage outfall.