Deponie 03
deponie
Deponie 02

Areas of Work
Disposal sites and isotope methods

In dense populated and industrialized areas, it is often very complex to accurately identify the origins of hazardous compounds and contaminants, which reached the groundwater. Commonly applied hydrochemical investigation technologies can be used only in unambiguous situations, for example in which the origin, the input conditions, the contaminant plumes, and the biological transformation processes are known. Under more complex hydrogeological conditions and in case of multiple sources, the validity of hydrochemical investigation technologies is limited. In these cases, isotope analyses (Tritium, Deuterium, Carbon-13 etc.) of the compounds and contaminants in groundwater can help to evaluate the situation.

Background

Most chemical elements have isotopes. For example, hydrogen has two stable isotopes, 1H (99,99 %) and 2H (Deuterium, 0,01 %), and one radioactive isotope 3H (Tritium). Analyzing stable isotopes, no absolute values, but isotope ratios are determined. These ratios are specified in reference to an international standard. By physical, chemical and biological processes the isotope ratios can be changed. These isotope effects lead to a spatial and temporal marking of different element cycles. In case of the radioactive isotope tritium, the radioactive decay is measured and specified by tritium units (TU).

Initial situation

The content of a municipal waste landfill effects the environment mainly by the discharge of gas and leachate. In most instances, the isotope signatures of CH4 and CO2 in the leachates of municipal waste landfills differ significantly from natural isotope ratios. Experience has shown that isotope methods are a very useful tool for the determination of the processes occouring in disposal sites).Thus, dispersing leachates and their contact to groundwater can be identified).

Deuterium and Oxygen-18 in water

Plotting ð2H against ð18O in a diagram, the values of meteoric water are on the so-called meteoric water line. If the precipitation water experiences evaporation, which is characteristic for stagnant water bodies under warm climate conditions, or by irrigation of effluents, 18O and 2H in water becomes enriched and, therefore, shift rightward of the meteoric water line. In disposal sites with organic burials (e.g. municipal waste landfills) water is formed by the decomposition of the organic material. In this process, hydrogen (H2) and methane (CH4) are formed, which causes isotope fractionation between H2/CH4 and water. The heavy hydrogen 2H is favored to be incorporated into the water molecule. Therefore, water formed in a disposal site is enriched in 2H in comparison to meteoric water. The enrichment in 2H and a potential enrichment in 18O due toirrigation or evaporation leads to a characteristic isotope signature. This is shown in the figure by an upward and rightward shift from the meteoric water line. Therefore, a determination of 18O and 2H signatures can be used to identify biological processes occurring in a disposal site and the percentages of the formed water. By comparing the isotope signature of the water formed in a disposal site with the isotope signature of groundwater, potential mixing processes can be detected.

Tritium in water

For decades, the aerial nuclear bomb tests between 1950 and 1963 were the main source of the anthropogenic tritium abundant in the environment. Since the termination of these tests, the tritium content continuously decreases because of radioactive decay of tritium. Experience has shown that leachates from disposal sites have a tritium content of 10 to 200 000 TU. The mean value of 800 TU for leachates clearly exceeds the tritium values of mean precipitation in Europe (approx. 8 TU), shallow groundwater and deep groundwater.´The origin of the high tritium content of leachates can not be explained clearly for the most disposal sites. Potential sources are waste from industry which has produced watches or fluorescence substances. Other sources might by waste from hospitals. However, if a disposal site is marked with 3H, it is an ideal tracer for the dispersion of leachates, because tritium is not influenced by chemical and microbial processes.

Carbon-13 in DIC

Dissolved inorganic carbon (DIC) in water is mostly available as CO2 and hydrocarbonate (HCO3-). Hydrocarbonate is formed predominantly by the dissolution of carbonates due to the reaction with CO2. If CO2 is formed organically under anaerobic conditions, the ð13C-value is in the range of the organic material. However, if CO2 is formed under anaerobic conditions concurrently with methane (CH4), CO2 is enriched in 13C. The formation of hydrocarbonates leads to a further enrichment of 13C in hydrocarbonate compared to CO2. The HCO3- and therefore the DIC of leachates from disposal sites are therefore highly enriched in 13C. Additionally, leachates have very high concentrations of DIC compared to most of the groundwater. This method allows a detection of small percentages (in %-range) of leachates in groundwater.