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Bearbejdning af målinger af regnbetingede udledninger af Npo og miljøfremmede stoffer fra fællessystemer i forbindelse med NOVA 2003

Summary and conclusions

This project is concerned with collection, quality control and analysis of runoff concentrations during wet weather in combined sewer systems in Denmark. More than 150 parameters are analyzed, of which 146 are heavy metals and organic pollutants. The parameters are defined in the Danish Aquatic Environment Monitoring and Assessment Programme , normally known as NOVA 2003.

One of the important aims of the monitoring program is to assess the importance of different contributions to receiving waters. The primary goal of this project is to support the overall aim by establishing intervals of typical concentrations for as many of the 146 substances as possible.

A total of three catchments have been monitored. However, only in one of the three catchments the measurement campaign was successful. This catchment consists of both combined sewer systems and separate sewer systems and is thus not suited to be used as the only source of information. Further, only nine storm events have been analyzed.

From studies of nutrient pollutants it is known that there is a high variation in the concentration levels in the runoff and discharges from combined sewer systems during wet weather. The information from the national surveillance program must be supported by other studies, both from national and international sources. The other studies focus both on measurements directly in discharges from combined sewer overflows and on measurements on the three primary sources of pollution: rain runoff from roofs and streets, sewage, and resuspension of material in the sewers.

There are two main findings of the review of the different studies:

Generally the concentration levels for the same substance varies greatly, not only between catchment and countries, but also between different events. The aim of the study is therefore to establish concentration intervals for average discharges from a catchment, not to assess the overall variation.

Traditional mass balances are difficult to apply. For some of the heavy metals the mass balance can be checked, i.e. the contributions from the primary sources can be added and compared to the measured discharge. The discharge was in some cases significantly lower than expected. If mass balances are applied, greater care must be taken with respect to the behaviour of the substances, i.e. sorption, complex binding etc.

Concentration intervals were established for 24 substances, of which eight are heavy metals and 16 organic pollutants, see Table 1. The intervals are constructed in order to cover what is expected to be the variation of average concentrations in typical catchments in Denmark. The intervals are large and are thus difficult to apply on individual catchments. However, the intervals may be used to evaluate the importance of combined sewer overflow relative to other contributions. For the remaining 122 substances it has not been possible to construct concentration intervals as such; for most of these substances, however, some information has been found. This information may be used in scenario evaluations at the national level in order to compare to other sources of the particular substances.

Table 1
Estimates of concentration intervals for combined sewer overflows for substances where sufficient information is available.

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