Differentiating Sources of Groundwater Plumes in Urban Environments

Potable groundwater resources in urban areas are often threatened by various plumes of mobile contaminants.  Increasingly, groundwater professionals are being called upon to determine the source of individual plumes and to apportion the contributions of multiple sources within a plume.  Although the principal contaminants of concern are commonly volatile organic compounds or heavy metals, often the sources can be more definitively identified through analysis of other trace constituents in the plumes. There are several analytical and interpretive techniques that can be used to differentiate plumes and their likely sources. 

 The minor ions bromide (Br) and iodide (I) can be particularly diagnostic because they behave conservatively.  Cross-plots of I/Na versus Br, or Cl/Br versus Cl can often distinguish the source of contaminants from landfills, septic systems, animal wastes, and road salting.   The ratio of stable isotopes of hydrogen, nitrogen, and oxygen may indicate sources derived from the evaporation of water in lagoons or due to process waters derived from a different aquifer than through which the plume is traveling.  Similarly, ¹⁵N to ¹⁴N ratios can be used to differentiate dissolved ammonia or nitrate in a plume attributable to a sewage-related source.  

 In special cases, the radioactive isotope of hydrogen, tritium (³H), can also be used due to its highly documented relationship with atmospheric testing of nuclear weapons during the 1950’s.  Tritium has also been found at elevated levels in disposal facilities containing gaseous tritium lighting devices (GTLDs) such as self illuminating exit signs and luminescent paints.

 Finally, methyl tertiary butyl ether (MTBE) has been used to provide a time-line marker for the release of petroleum-related contaminants into the groundwater.  MTBE was added to gasoline as an oxygenate under federal mandate beginning around 1980, only to be banned in the late 1990’s due to its high mobility and recalcitrant properties in groundwater.