See the US EPA documents on bio-assay using small minnows and daphnia. This is good solution if you do not have the ability to analyze each of the many pollutants found in storm water run-off. As you have already discovered the list of pollutants is quite extensive and analyzing the toxicity of multiple pollutants is daunting. However, the bio-assay will provide information about LD50 and carcinogens. You samples will kill so many of the daphnia population and fish population and the livers of the fish readily show cancer. Review the EPA publication for details.