The Utility of the Chondrite Plot In Oilfield Contamination Investigations
Authors: Thimm H.F. , Hill D. 
Published: March 2010

It is shown that the natural abundance of rare earth elements in a soil sample, normalized in a chondrite plot against the abundance of rare earth elements in the meteoritic mineral chondrite, has a number of uses in oilfield contamination investigations. The shape of this chondrite plot is a function of the crystallization history of the minerals in the soil or rock sample, and it is this property that lends itself to interpretation of certain environmental data. As an example, it was shown that a larger area, initially thought to be contaminated by mercury as a consequence of oil and gas operations, was in fact only marginally contaminated. The chondrite plot was used to show that naturally deposited cinnabar (mercuric sulphide) brought from a 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, could be avoided.

Introduction

The contamination of soils by trace elements has, in recent years, increasingly been regulated by means of setting concentration standards for individual elements in soils. For example, in Alberta the so-called Tier 1 guidelines have been enforced, and similar guidelines have been introduced in British Columbia (Levels 1,2, and 3). Usually, as in Alberta's Tier 1(1) and the Level 1 standards of the Canadian Council of Ministers of the Environment (CCME)(2), the standard set for a given element is some high percentile of the distribution of the element in the agronomic surface soils within much of the jurisdiction. The CCME guidelines extend the concept of setting limits on individual contaminants, salts, and metals in soils to commercial use and industrial land use (Levels 2 and 3 respectively), which represent some raising of the allowable concentrations cited in Level l and an implied consideration of risks.

While it is often safe to assume that a soil concentration of some metal in excess of standards is due to contamination, exceptions due to much higher than normal naturally occurring concentrations do exist. The most common exceptions of this kind occur in non-agronomic soils, soils well below the plough layer that have been brought to surface during construction(3), and soils that have been formed from the sorting of certain minerals during their deposition.

An example of the last category are river flats of high gravel and sand content. These represent a relatively high energy depositional environment during flood stages. The settling processes will therefore favour solids of high diameter and solids of relatively high density. Thus it is no surprise that certain high density minerals tend to occur in coarse sedimentary deposits. A selection of the higher density minerals that might be expected in river flat deposits is given in Table 1, [data taken from Reference (4)]. It follows, therefore, that the elements constituting these minerals, to the extent that they are controlled as potential contaminants, may exceed the guidelines for a particular jurisdiction by reason of natural occurrence.