Naturlig nedbrydning af miljøfremmede stoffer i jord og grundvand

English Summary

Background
In the Danish Environmental Protection Agency’s "Guidelines on Remediation of Contaminated Sites" (draft 1997) a principle for risk assessment of a groundwater resource has been laid down. Based on the contamination strength a calculation is made establishing whether contamination as a result of sorption and dispersion will exceed the groundwater quality values and consequently put the groundwater resource at risk. In that case, a calculation of the significance of a possible natural degradation of the dissolved substances is made to assess whether the contamination still presents a risk. First order degradation is used.

In the light of this, the Danish Environmental Protection Agency has initiated this literature study with the purpose of comparing first order degradation rate constants for substances in soil and groundwater and suggesting methods for establishing degradation potential on a specific site as well as control measures and parameters for documentation of the projected degradation.

The project comprises the compounds for which the Danish Environmental Protection Agency has laid down soil or groundwater quality values. However, heavy metals, fluorides, cyanides and mixed products (e.g. petrol) are not included.

Information has been collected through systematic search in 12 international data bases comprising scientific writings from 1992-98 (search performed by the State Library Århus) and from the Danish Geotechnical Institute Library and the Danish Institute of Environmental Science and Engineering Library.

….unsaturated zone
The literature study did not produce any articles about degradation rate constants for contamination components during porewater transport through the unsaturated zone. A major problem is that it has been impossible to find any material giving a thorough description of monitoring methods in relation to contamination components in the unsaturated zone. On the present showing, risk assessments including degradation in the unsaturated zone cannot be performed.

….saturated zone
In relation to the saturated zone, only a limited number of first order degradation rate constants relevant for natural Danish conditions have been found. All relevant degradation rate constants are mentioned in the report. Redox environments are also included in the report in so far as documentation has been found in the literature.

Most of the data which have been collected are related to BTEXs and chlorinated solvents. Only limited information about the remaining compounds exists and as regards many of the compounds (pentachlorophenol, acetone, isopropyl alcohol, styrene, ether and diethyl ether) no relevant information has been found. For PAHs and DDT only half-life periods in soil are mentioned.

From site to site but also on the same site especially the BTEXs but other compounds too show differences of several magnitudes in the listed first order degradation rate constants.

It is estimated that the listed first order degradation rate constants for the saturated zone form the basis of applying degradation in connection with risk assessment of most of the compounds for which limit values exist; especially the BTEXs and the chlorinated solvents.

In connection with risk assessment the listed first order degradation rate constants cannot be used to make a conservative risk assessment but only to make an approximate calculation of whether the groundwater quality values are observed. Alternatively, following a determination of the actual degradation rate on the specific site a risk assessment based on the actual degradation rate can be made.

If a risk assessment shows that the effect of natural degradation according to calculations is that soil contamination does not constitute a risk to the groundwater resource it must be proved that degradation takes the expected course.

Estimation of site-specific degradation rate
Based on the literature study the report points out how it can be proved that degradation in the saturated zone takes place and how the actual degradation rate on the specific site can be determined by applying a tracer, e.g. an organic substance such as TMB (trimethylene benzene), an inorganic substance (total chloride) or total ethene.

The directions are based on American protocols for natural degradation ("intrinsic remediation"/"natural remediation") and the methods listed are not commonly applied in Denmark. However it is estimated, that the principles can be transferred to Danish conditions relatively easily.

Suggestions for future projects
In the conclusion suggestions for future projects are made. The suggestions relate to projects designed to improve the basis of applying degradation in connection with risk assessment. It is, among other things, concluded that there is a need to develop monitoring methods concerning contamination components in the porewater in the unsaturated zone.