The chondrite plot is a plot of the log concentration, normalized against theabundance in a meteoritic mineral, chondrite, of the rare earth elementsagainst atomic weight. The shape of this plot is a function of thecrystallization history of the minerals in the soil or rock sample. The plotmay therefore be used in a number of environmental investigations in thepetroleum industry. As an example, it was shown that a larger area, initiallythought to be contaminated by mercury as a consequence of oil and gasoperations, was in fact only marginally contaminated. The chondrite plot wasused to show that naturally deposited cinnabar (mercuric sulphide) brought froma river flat gravel source, greatly exceeded the contaminant elemental mercury.Since cinnabar is a non-toxic form of mercury, substantial clean-up costs, which would have been incurred as a result of conventional investigation, couldbe avoided.
The contamination of soils by trace elements has, in recent years, increasinglybeen regulated by means of setting concentration standards for individualelements in soils. For example, in Alberta the so-called Tier 1 uidelines havebeen enforced, and similar guidelines have been introduced in British Columbia(Level 1, 2 and 3). Usually, as in Alberta's Tier I and B.C. Level 1 standards, the standard set for a given element is some high percentile of thedistribution of the element in the agronomic surface soils within thejurisdiction. The B.C. guidelines extend this concept to commercial use andindustrial use soils (levels 2 and 3 respectively), which represent someraising of the allowable concentrations in each case; Alberta has yet to fallowthis lead.
While it is often safe to assume that a soil concentration of some metal inexcess of standards is due to contamination, exceptions due to much higher thannormal naturally occurring concentrations do exist. The most common exceptionsof this kind occur in non-agronomic soils, soils well below the plough layerthat have been brought to surface during construction1, and soilsthat have been formed from the sorting of certain minerals during theirdeposition.
An example of the last category are river flats of high gravel and sandcontent. These represent a relatively high energy depositional environmentduring flood stages. The settling processes will therefore favour solids ofhigh diameter, and solids of relatively high density. Thus it is no surprisethat certain high density minerals tend to occur in coarse sedimentarydeposits. A selection of the higher density minerals which might be expected inriver flat deposits is given in Table 1, (data taken from reference 2). Itfollows, therefore, that the elements constituting these minerals, to theextent that they are controlled as potential contaminants, may exceed theguidelines for a particular jurisdiction by reason of natural occurrence. It isimportant (to be able to distinguish cases of natural exceedance of guidelinesfrom cases of contamination, although this is not always easy.
In the current example, it is shown that a soil contaminated with elementalmercury could be distinguished from a river gravel, naturally enriched inmercury due to the probable presence of cinnabar, which had been brought to thesite in question as construction gravel some 40 years previously.