Isotope enrichment

Isotopic evidence for MNA- and ENA-remediation.

The ability to measure isotopic signatures is the key technology for monitoring the establishment of MNA and ENA remediation concepts. Heavy-isotope enrichment of contaminants provides evidence for natural attenuation or the successful implementation of in situ bioremediation. With only a few samples, we will be able to survey the extent of bioremediation within an aquifer. Attenuation amounts have been recorded in isotopic signatures since the initial pollution event.

The principle used for monitoring biodegradation is based upon "isotope fractionation", a term that can be used synonymously with isotope enrichment. Under the context of biological elimination of contaminants, molecules with stable light isotopes (i. e. 12C) are preferably removed. Thus, the relative amounts of heavy isotopes (i.e. 13C) will increase in the residual contaminant pool. We detect the isotope signatures at certain spots within the site. Spots with heavier isotopic values provide evidence for natural attenuation, while the spot with the lightest isotopic value typically represents the contaminant source.

Significant isotopic enrichment in groundwater can only be triggered by biodegradation. The enrichment is independent from contaminant concentrations, and therefore not affected by transport effects such as dilution or dispersion. There are currently no alternative monitoring tools (repeated measurement of concentration, column experiments, metabolite fingerprinting, molecular genetics) that provide comparable insights into biological processes.

Contaminant concentraitsions within a plume

Left figure:
Contaminant concentrations within a plume influenced by diffusion, dilution, sorption, dispersion and microbial degradation.

Right figure:
Isotope signatures of 13C reveal a gradient of degradation within the plume. Up to 99% of the contaminants have been degraded.

Isotope signatures of 13C

Since isotopic enrichment is proportional to biodegradation, natural attenuation can be quantified if biological and substrate specific constants are available. So-called quantitative isotopic monitoring is preferred at well-characterised sites, where hydrogeological parameters and source distributions are known. At these sites, amounts and rates of biodegradation can also be derived.

Isotopic enrichment factors for over twenty contaminants have been characterised and over 100 specific constants determined, mostly for carbon (see table Isofrac). However, isotope enrichment may only be quantified for compounds with less than 12 carbon atoms.

Depending on the specific site parameters, biodegradation amounts from 40% to 99% can be determined. Frequently, monitoring is successful and microbial activity in contaminated groundwater is wide-spread. A list of sites where isotopic monitoring of biodegradation was successfully applied is available in our research section, where you may also find the limitations and uncertainties associated with isotopic monitoring. Be aware that isotopic monitoring does not guarantee that biological degradation will occur at your site.