Plumes of landfill leachate-impacted groundwater can be difficult to fully delineate in groundwater systems that have been widely contaminated by other sources, such as septic systems, water softener discharges, road salting, animal wastes, and agricultural activities.  Recent studies by Panno (2006) and others have shown that the minor ions, bromide and iodide, can often be utilized to differentiate landfill leachate plumes from plumes emanating from other sources.  The typically low levels of these ions in natural water systems require specialized analysis by the neutron activation method. Panno demonstrated that different sources of water can be identified by cross-plotting of chloride/bromide ratios versus chloride and iodide/sodium ratios versus bromide.  The viability of these methodologies is demonstrated in two case studies both involving the delineation of landfill plumes in a complex, multi-source, groundwater contamination settings.
It has also become increasingly clear in the last few years that the vast majority of municipal solid waste landfills produce leachate that contains elevated levels of tritium.  This tritium can serve as a highly-diagnostic tracer of landfill leachate in groundwater systems.  The authors recently conducted a study of landfills in New York and New Jersey and found that the mean concentration of tritium in the leachate from ten municipal solid waste landfills was 33,800 picoCuries/liter (pCi/L) with a peak value of 192,000 pCi/L. Studies in Pennsylvania, California, and the UK produced similar results. Background levels in most aquifers are usually in the 25 to 100 pCi/L range. The tritium found in MSW leachate is believed to derive principally from gaseous tritium lighting devices used in some emergency exit signs.  Other sources include tritium-containing compasses, watches, and even novelty items, such as “glow stick” key chains.
References
Panno, S.V., 2006.  Characterization and identification of Na-Cl sources in ground water.  Ground Water, 44:  176-187.