Defining the Limits of Contamination
The current practical laboratory quantification limit (PQL) would be nice, however this is often a very low number and the compound(s) of concern may never reach BDL at these levels. Needless to say, it is not easy to answer the question as to how far to go, but somewhere near the human health limit or primary drinking water standard would be a good starting place.
Now let's suppose this limit is set at 5 mg/L for compound "X" and the laboratory detection limit is 1 mg/L (BDL). Now let's suppose the regulatory agency has asked you to define "X" to the MCL limit (5 mg/L). Does this mean having a "fence" of wells every 10 feet apart around the plume where the samples have been analyzed to show 5 mg/L or less? Of course not. Every 50 feet, 100 feet? In some cases, the last outermost well may be 10 or 20 mg/L and reasonable assures can be made that the end or the 5 mg/L limit may be very near this well, depending upon what the concentration trend is for that area. If the concentration of compound "X" is dropping off logarithmically, i.e., 1000, 100, 10 mg/L, etc. over every 20 feet from the center of the plume, then the limits can be extrapolated with some relatively good assurance that the 5- mg/L limit is just outside of the 10 isoconcentration contour and additional wells probably are not necessary. Ground water models extrapolate all the time and the results are generally accepted.
The ability to estimate the limits of the plume are in part dependent upon the real distribution of source areas (i.e., one source or many sources), concentrations of the compounds and the degree of homogeneity of the site's hydrogeology.