Stable isotope methods in groundwater engineering

During the past two decades there has been a major expansion in the number of environmental studies making use of stable isotopes as tracers, either of sources of material or of processes. Some of these studies have produced techniques, approaches or findings of interest and practical application in the engineered management of contaminated land or groundwater. Perhaps the most important application of stable isotopic techniques in this context is their ability to distinguish contaminant mass loss owing to beneficial subsurface biodegradation from decreased contaminant concentration resulting from dilution or sorption, processes that do not lead to a reduction in the total contaminant burden. This arises from changes in the carbon isotopic composition of organic contaminants that accrue during biodegradation. There are analogous changes in the isotopic composition of electron acceptors used to drive microbial respiration during contaminant biodegradation. These can be used to ascertain the microbial processes responsible for biodegradation and constrain the biodegradation potential. Stable isotopes can also be used to ‘fingerprint’ different sources of solutes and enable discrimination of contaminant source and flow pathways.