Abstract

Current procedures for the reclamation of evaporation ponds and other industrial lagoons require the analysis of pond sediments for a variety of trace elements specified by the Canadian Council of Ministers of the Environment (CCME). In general, trace element limits for this CCME list are set by the provinces (e.g. Tier 1 & 2 in Alberta, Levels A, B, and C in British Columbia). Failure of the specifications usually results in a requirement for risk assessment.

A problem often encountered with risk assessment Ls that the speciation of trace elements exceeding the standards in tile CCME list cannot be deduced from the analyses alone, because the CCME or government approved analytical procedures measure total concentrations, without regard for the chemical form in which an element of interest exists. This chemical speciation, however, can very strongly affect toxicity, solubility and leachability, and availability to plants of the element.

In many cases, the favoured speciation of trace elements in evaporation ponds can be calculated from geochemical thermodynamics, using pH and the reduction potential as master variables to obtain stability diagrams. These techniques require the analysis of tile pond water and pond sludge centrifugate. The practical use of the technique is illustrated in an example from an industrial facility in Alberta, where it could be shown that certain contamination had reacted to form stable and innocuous materials.

Introduction

The decommissioning of ponds and pits at gas plants and other facilities bas been a relatively high priority item for petroleum operators in the last two years. In addition to problems that may arise with respect to salinity and organic contamination, trace elements may usually be expected to be present in pond sediments. The guidelines of the Canadian Council of Ministers of the Environment (CCMEJ require analysis for a variety of trace elements, a1thougb the setting of critical values is left to the provinces. Failure to meet the criteria set will often result in a requirement for risk assessment.

Unfortunately, the information to be gained from routine analysis of pond sediments can be misleading in this respect The guidelines refer to total concentrations, and the analytical techniques for measuring these are recommended in the appropriate government publications. These methods will usually determine elements sucn as lead without regard for the original speciation of the metals. Speciation, however, is of critical importance when risks are being evaluated. Failure to obtain this information reduces the risk assessment to an almost meaningless activity, because it forces the use of default toxicities, solubilities, leachabilities, plant uptake potentials and other disposable parameters to such an extent that the final risk figure bears little resemblance to reality.

This problem is partly overcome by the use of secondary extraction procedures, such as the Canadian General Standards Board's [CGSB 1acelie acid - acetate extraction method at pH 5. While such empirica1 methods are useful to a degree, they do suffer from serious limitations. The CGSB method, for example, gives no information regarding liability at pH below 5.

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