Summary and conclusions, Miljøstyrelsen

Biologiske effekter af toksiske stoffer i regnbetingede udløb

Summary and conclusions

In recent years, hazardous chemical substances has become a central issue in Danish environmental policy and regulation. With regard to the environmental impacts of such substances, especially the problems related to discharges of urban and industrial wastewater into the aquatic environment have received attention.

Until now, toxic chemical substances in urban stormwater and road runoff has not been a matter of concern although the Danish Environmental Protection Agency (Danish EPA) actually in 1997 published a study on heavy metals and organic micro-pollutants in surface runoff from urban areas. However, the possible adverse biological effects of such discharges were not assessed in the mentioned study. Therefore, the present study was initiated to indicate the risk of such effects to occur.

In the autumn of 2000, a limited field study was conducted in which road runoff from two selected areas was sampled and tested. The areas studied were a motorway section and a road system in an adjacent residential area, both located in northern Copenhagen. Some chemical characterisation of the samples was made but the emphasis was put on ecotoxicological testing by means of a test battery with daphnia, algae and bacteria.

Firstly, it was attempted to elucidate whether the degree of toxic effect and traffic density are correlated and, secondly, if a correlation between toxic effects and types of rain events exists. In addition to this, the toxicity of runoff water contaminated with municipal wastewater, and the toxicity of road runoff after sedimentation of particulate matter were investigated by use of manipulated road runoff samples.

Finally, sediments from two retention basins at two major motorways in the Copenhagen area were analysed and tested to give an indication of the possible toxicity of the particulate phase in surface runoff.

The results of the investigations and tests can be summarised as follows:

The water samples from the most heavily trafficked area tended towards a higher concentration of pollutants than the residential area samples. However, exceptions to this, especially for copper, were observed.

The tested road runoff water was toxic in standard laboratory tests with algae. One sample was also daphnia toxic.

It was not possible to establish an unambiguous correlation between toxicity and traffic density. Neither was there any distinct connection between toxicity and the types of rain events.

After 24 hours of sedimentation, the runoff samples showed significantly reduced levels of all substances that due to their physico-chemical properties have an affinity to fine particles such as clay or organic matter.

A concomitant reduction of toxicity in the samples was not observed, probably because the toxicity mainly is associated with dissolved substances or colloids that do not settle easily.

Mixing the road runoff with municipal wastewater (5:1) resulted in reduced toxicity but similar or increased levels of the investigated pollutants.

Pore water from retention basin sediments was toxic to algae. One sample was even highly toxic. The result indicates that the particulate phase in runoff from impervious surfaces cannot just be regarded as environmentally safe. The chemical analyses demonstrated that the sediments were significantly contaminated with heavy metals and toxic organic substances.

In the ecotoxicological tests, daphnia and algae were able to detect toxic components in both water and sediment samples whereas the bacterial test was not sufficiently sensitive to serve as a test organism.

In conclusion, the investigation has demonstrated that road runoff can be toxic to aquatic organisms in laboratory tests. It has not been possible to confirm the expectation that the samples from the area with the highest traffic load would also be the most toxic. Still, it is likely that such a correlation can be demonstrated if a larger programme is carried out. However, the present study has shown that significant exceptions to the general rule should be anticipated.

It is obvious that many activities taking place in and around private homes result in accidental spills and discharges of contaminated water and other liquids into the runoff pipeline system. Hence, despite a low traffic density, incidences will occur where high concentrations of chemicals can be found in stormwater and surface runoff from residential areas.

It is, however, not likely that the risk of biological effects of runoff discharges can be represented by simple parameters or a limited number of specific analyses. Hence, it appears to be irrelevant to require such analyses if the objective is to quantify the risk of effects in the receiving waters.

For direct discharges of wastewater contaminated stormwater into surface waters, the most relevant parameters to apply for regulatory purposes appear to be the hydraulic load or the contents of general organic matter and nutrients.

The results of this limited investigation indicate that measures to reduce the amount of suspended matter before discharge of road runoff into water bodies should be considered. Sediments which are significantly affected by suspended matter in road runoff can be toxic to aquatic organisms and contain significant amounts of heavy metals and toxic organic substances.