impact of storm water runoff on a small urban stream

Background, aim, and scope In urban areas, storm water runoff often transports various pollutants, some of which settle and form sediments. In order to have the comprehensive view of the ecological state of storm water runoff recipients, both water and sediments of the stream must be assessed. In the Baltic Sea Area, the Water Framework Directive & HELCOM Recommendations aim to prevent or minimise pollution caused by harmful substances arising from storm water runoff, in order to promote the ecological restoration of the Baltic Sea--one of the most vulnerable seas. The aim of the study was to investigate the toxicity of bottom sediments of a small storm water runoff recipient focusing on the potential impact of successive discharges of urban storm water. Some storm water runoff quality parameters and the toxicity of bottom sediments of recipients was studied in this research. Materials and methods During 9 years, at four discharge points, minimum four grab samples per year at each discharge point were taken for chemical characterisation. General parameters (pH, SS, BOD₇, CODCr and TPH) in liquid phase samples were analysed according to standard methods. Annual limit values were taken from the Lithuanian EPA requirements for the management of storm water runoff with a focus on prevention and control of contamination. Eleven composite samples of stream bottom sediments, each consisting of ten sub-samples, were collected in 2006. Toxicity screening from sediments was performed using the plant Lepidium sativum according to modified I. Magone's methodology (Magone I, Bioindication of phytotoxicity of transport emission. In: Kachalova O-L, Zinatne (eds) Bioindication of toxicity of transport emissions in the impact of highway emissions on natural environment. Riga, pp 108-116, 1989). The level of toxic impact of Lepidium sativum (compared to control) was assessed according to the modified method of Wang (Rev Environ Contam Toxicol 126:88-127, 1992). Results The mean pH of urban storm water runoff does not vary much from neutral, but range values are quite different, from 4.0 up to 8.7. The highest concentration of SS reached 800 mg L⁻¹, TPH--2.4 mg L⁻¹, BOD₇--300 mg O₂ L⁻¹ and CODCr--1,400 mg L⁻¹. The SS was above the limit in 64% of total amount of grab samples, TPH--37%, BOD₇--41% and CODCr--55%. The toxicity analysis of the bottom sediments showed varying toxicity of bottom sediments along the stream. From nine analysed samples of bottom sediments, 30% had we