Abstract

Current procedures for the reclamation of evaporation ponds and otherindustrial lagoons require the analysis of pond sediments for a variety oftrace elements specified by the Canadian Council of Ministers of the Environment (CCME). In general, trace element limits for this CCME list are setby the provinces {e.g. Tier I & 2 in Alberta, Levels A, B, and C in British Columbia}. Failure of the specifications usually results in a requirement forrisk assessment.

The use of certain geochemical techniques, such as complete inorganicidentification, rare earth fingerprints, and the use of chondrite plots, can beused to augment the CCME investigations. They permit the separation ofcontaminated elements from elements that occur naturally above CCME standards.Conclusions can also be drawn about the soil stratification around the lagoon.These techniques are established in geochemistry, but have not, to date, beenwidely applied in petroleum environmental investigations. In an example from agas plant in Alberta, the use of these techniques is illustrated. Theexamination of chandrite plots from pond bottom samples suggested thatstratification was not horizontal, thus introducing considerable heterogeneityinto analytical results. The non-horizontal stratification was confirmed bydrilling and well log elimination. As a consequence, a number of elements, although technically in exceedance of the CCME guidelines, could be assigned tonatural background, with an according reduction in environmentalliability.

Introduction

In addition to problems that may arise from salt and organic contaminationduring pond decommissioning studies, trace elements are often detected in pondsediments. Analysis is required for a variety of trace elements specified bythe guidelines of the Canadian Council of Ministers of the Environment [CCME].Critical values for the elements of concern are set by individual provinces.This fact bas given rise to a set of criteria such as Alberta Tier I and British Columbia Levels A, B and C, which are summarized in Table r. Both the Tier I and B.C. Level A criteria in essence represent natural backgroundvalues, to be expected in most of the agricultural soils of the provincesconcerned. Failure of these criteria, depending on land use, often results in arequirement for risk assessment.

Applications

The application of the new guidelines for trace elements, having regard for thevery severe methods of extraction in common use, can create problems with datainterpretation. For example, if arsenic is found at 11 ppm, it is unclearwhether it arose from a contaminant or from one of the common arsenateminerals, which are widely distributed and very stable. One is therefore facedwith preparing risk evaluations without the benefit of information concerningchemical identity, an important shortcoming because toxicity is usuallystrongly dependent on chemical speciation and stability. This situation forcesthe use of default values with respect to acute toxicity, plant uptake, leachability and other properties, resulting in a risk figure that probablybears little resemblance to actual risks.

A second, very common problem arises from the fact that the Tier I or Level Ametal criteria represent background in the topsoils.

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