Assessment of Highly Polluted Fluvisol in the Spittelwasser Floodplain Based on National Guideline Values and MNA-Criteria (11 pages)

Abstract

Background, Goal and Scope

Nearly all large European rivers have been used as disposal pathways for several unwanted substances. Deposition of the contaminated solids occurs in regions of slack water and also during floods in adjacent floodplains. As a result of the recurrent input, most of the floodplains in central Europe are contaminated by manifold inorganic and organic pollutants, representing a risk for both plants and groundwater at the site, and for areas downstream. The aim of the present paper is to demonstrate and assess the sink and source functions of a highly contaminated 'fluvisol' (= alluvial soil) for inorganic and persistent organic pollutants. The extent of contamination is described on the basis of national guidelines. These results are further discussed on the basis of the 'monitored natural attenuation' (MNA) potential, i.e., with respect to the requirements for the cheapest problem solution for large scale contamination of floodplains. The MNA potential is assessed according to several lines of evidence such as contaminant burial, mechanical and chemical mobility, transformation to less toxic forms, and dilution due to dispersion.

Methods

The existing inorganic and organic pollutant status and the ecotoxicological potential of the Spittelwasser floodplain near Jeßnitz was documented by analysing several element and compound depth profiles of a typical alluvial soil site. Geochemical analyses were performed on solid material and soil solution according to standard procedures. In laboratory experiments monoliths were continuously irrigated with deionised water and the leachate was analysed weekly. In addition to the geochemical, the ecotoxicological status of the samples was studied using different biotests for the dissolved and the solid phase.

Results

The measured alluvial soil profile is severely contaminated with numerous inorganic and organic pollutants. Highest enrichment factors according to the catchment based, element specific geological background were found for Hg (402), Pb (8.9), Cr (7.1) and Cd (6.3). Assessment was made of the hazard potential of both the solid material and the soil solution according to the German Federal Soil Protection and Contaminated Sites Ordinance (BBodSchV). Relating to the respective maximum contents in the soil profile, the overstepping of BBodSchV trigger values is highest for Hg with a coefficient of 49.0 followed by Cd (14.3), Pb (12.2), Zn (7.3), Cr (5.9) and Cu (3.6). Pollutant concentrations in the soil solution exceed the threshold values for Cd, Cu and Zn by factors of 6.3 to 12. While DDT is retained in the topsoil, there is a considerable translocation of HCH towards deeper horizons.

Discussion

The algae test confirms a strong toxicity in the top layer of the soil profile. In contrast, no such toxic reactions were indicated by the luminescent bacteria test and the Daphnia test at this depth. However, the deeper layers (10 - 100 cm) show a toxic reaction by the Daphnia test, thus indicating a relevant translocation of contaminants.

Conclusions and Perspectives

The Spittelwasser floodplain represents a significant sink for inorganic and persistent organic pollutants. Simultaneously, the area is a relevant secondary source of dissolving contaminants like heavy metals and HCH. Although strict criteria in the lines of evidence for applying MNA are not fulfilled, it can be stated that the alluvial soil offers a high natural retention potential for As, Hg, DDT, and PCDD/F. These pollutants are firmly retained and are not found in the soil leachate. This meets the demands of the EC-WFD for pollutant retention close to their primary source. But for agricultural use and groundwater recharge the natural attenuation potential of the soils in the Spittelwasser area will not be sufficient to achieve acceptable conditions medium-term. The main problem for the dissemination of pollutants is not chemical stability but mechanical dispersion. This is particularly the case for PCDD/F, with concentrations of more than 10,000 ng/kg I-TEQ in the upper Spittelwasser soil layers. Dispersion processes can result in a wide spatial contamination of downstream floodplains.