Qualitative, quantitative, and visual methods to assess natural attenuation

Monitored natural attenuation (MNA) is increasingly becoming accepted as a viable, cost effective option for managing the risks posed by contaminated groundwater in certain situations (Agency, 2000). In order for MNA to be approved as a viable option it has to be demonstrated that natural attenuation (NA) is occurring and that it is sufficiently effective to cause no unacceptable risk to receptors, and that remedial measures can be met within a reasonable timeframe. Natural attenuation is demonstrated using three lines of evidence (R&D 95). Primary lines of evidence involve the use of historical contaminant data to demonstrate a trend of reduced pollutant concentrations down-gradient of the source, along the groundwater flow path. This form of evidence shows that attenuation is taking place, but fails to establish if contaminant mass is being destroyed by biological or non-biological degradative mechanisms. Secondary lines of evidence involve measuring changes in chemical and geochemical analytical data to prove a loss of contaminant mass. Two approaches are available (Agency, 2000): • Using chemical and geochemical analytical data in mass balance calculations to show that decreases in parent contaminant and/or election acceptor/donor concentrations can be directly correlated to increases in metabolic by-products and/or daughter compounds; • Using historical chemical data, complemented, if necessary, by biologically recalcitrant tracer testing, to demonstrate that the plume is shrinking, stable or expanding at a rate slower than predicted by conservative groundwater velocity calculations. Tertiary lines of evidence use data from laboratory microbiological testing to show that indigenous bacteria are capable of degrading site contaminants. This line of evidence should be used when the first two are inconclusive. Optional line of evidence (ASTM, 2004) may be used to more rigorously interpret data developed as secondary lines of evidence. The clear presentation of data is an important component in setting out the evidence in support of natural attenuation (Agency, 2000). The data can be displayed qualitatively (e.g. graphically), quantitatively (e.g. mass balance calculations), or visually (e.g isopleth maps). This report details the methods available for presenting data to support natural attenuation for each level of lines of evidence. The methods presented are not exhaustive, but do represent the principal methods employed