Summary and conclusions, Miljøstyrelsen

Undersøgelse af kulbrintenedbrydning ved naturlige processer

Summary and conclusions

The topic of this report is natural attenuation of a gasoline contamination at a former gasoline retail site located on Nykøbingvej 295, Radsted (Lolland). A contamination, primarily gasoline, was identified in the unsaturated zone around an underground storage tank on the location. The contamination has spread to the saturated zone.

The scope of this project has been to compare the maximum extent to the theoretical extent of the pollution plume. Furthermore to investigate the development in concentration of BTEX over time/distance, to investigate the development of redoxsensitive parameters in- and around the pollution plume as well as to investigate the possibility of calculating the first order degradation rates of benzene, toluene, ethylbenzene and xylenes. Finally the scope has been to evaluate analytical parameters and monitoring frequency for evaluation of natural attenuation.

The data used in this report arises from earlier investigations conducted on the location, primarily performed for OM (Danish Petroleum Industry's Association for Remediation of Retail Sites) and a monitoring program paid by the Danish EPA under the Program for Development of Technology (TUP). Furthermore different projects performed by students of the Technical University of Denmark have contributed.

On the basis of the theoretical review and the data observed, the following conclusions about investigation and monitoring of natural attenuation can be made:

Documentation and monitoring of natural attenuation is difficult in aquifers with strong heterogeneous conditions. Heterogeneous geology can cause large variations in hydraulic parameters during the yearly cycle where precipitation changes.

When the geology is very heterogeneous a large number of wells is needed in order to produce at clear picture over the piezometric surface pressure of the aquifer. It is doubtful whether a good estimate of the overall groundwater flow direction can be made upon the pressure level of groundwater in just 3 wells. The resulting vector of the groundwater flow in the aquifer controls the spreading of pollution over time. This means that the pollution plume reacts somewhat "slow" in comparison to the rapid changes in groundwater flow direction. Changes in the redox conditions are occurring even slower, as a large buffer capacity is known to exist in the redox environment (e.g. a large pool of ferric iron and sulphate).

Fluctuations up to 1.5 meters in the groundwater level have been observed in the course of one year.

The horizontal extent of the pollution plume changes during a yearly cycle, but is considered to be in the order of 30-40 meters downgradient from the source area. For comparison, the overall movement of water particles is assessed to be about 1400 meters during the 26 years (as a minimum) the pollution has been present in the aquifer.

On the location in question, it is assessed that the pollution with hydrocarbons is being naturally degraded under aerobic, nitratereducing and ironreducing conditions. This assessment is primarily based on changes in the redoxchemistry, which coincides with the delineation of the pollution plume. It is somewhat more difficult to see this coincidence in the lower part of the pollution plume, because it is believed that the overall conditions change to more reducing conditions in the transition-zone from semi-confined to confined conditions in the aquifer.

It has been difficult to quantify mass reduction due to natural degradation processes. The reason is that it has not been possible to take dilution into account. No tracer tests were performed during the investigations and TMB degraded even faster than benzene etc. Therefore TMB could not be used as a tracer. A precise calculation of the flux through several cross-sections of the plume in different distances to show mass reduction has neither been possible. Yet a preliminary flux calculation suggested that the effects of dilution could not solely cause the decreased level of the contaminants.

By comparing the delineation of the contamination in the aquifer with two calculations using the risk-management model (JAGG) from the Danish EPA it is obvious, that the model is well suited to conduct a preliminary assessment of the spreading of contaminants. The risk management model though seems to have difficulty handling complex information, such as varying redox conditions and multiple 1. order degradation constants. Yet the primary scope of the model is to predict if a contamination poses a risk to the groundwater resource based on presumptions about e.g. conditions for degradation. If use of the model predicts that natural attenuation processes can eliminate a risk to the groundwater resource, this should be documented through more intense investigations through calculation of the site-specific 1. order degradation constant.

The essence of the results from this projects is, that it is difficult to document natural attenuation of carbon hydrates in strongly heterogeneous aquifers on a level corresponding to what is described in the theory (chapter 2).

A clear reduction in concentration of hydrocarbons as a result of increasing distance to the contamination source is shown and the contamination plume is assessed to be around 30-40 meters long. According to the present regulations the plume is within the initial treatment zone (the distance corresponding to the groundwater transport of one year or maximum 100 meters) /18/.

Recently a number of methods have been introduced, that can help document the effects of natural attenuation. Some techniques are qualitative, such as isolation of specific degraders (micro-organisms) in the contamination plume and correlating these with the composition of micro-organisms outside the contaminated area. Other methods are more quantitative, such as isotope fractionation, where changes in the ratio between different isotopes of different molecules can be directly related to preferential degradation.

To let natural attenuation processes of hydrocarbons be a part of investigation and cleanup of contaminated sites is widespread in a number of countries, e.g. the U.S.A. The methods for documentation of natural attenuation have been tested abroad through almost a decade and it is evident that a number of considerations can still be added in order to make the documentation more robust and trustworthy, as this project clearly shows. This case also implies the need for guidelines for the "use" of natural attenuation in cases with carbon hydrate contamination in Denmark.