Trace Element Speciation in Evaporation Pond Reclamation Assessments

Abstract

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 1 & 2 in Alberta, Levels A, B, and C in British Columbia). Failure to meet the specifications usually results in arequirement for risk assessment. A problem often encountered with risk assessment is that the speciation oftrace elements exceeding the standards in the CCME list cannot be deduced fromthe analyses alone, because the CCME or government approved analyticalprocedures measure total concentrations, without regard for the chemical formin which an element of interest exists. This chemical speciation, however, canvery strongly affect toxicity, solubility and leachability, and availability toplants of the element. In many cases, the favoured speciation of trace elements in evaporationponds can be calculated from geochemical thermodynamics, using pH and thereduction potential as master variables to obtain stability diagrams. Thesetechniques require the analysis of the pond water and pond sludge centrifugate.The practical use of the technique is illustrated in an example from anindustrial facility in Alberta, where it was shown that certain contaminantshad reacted to form stable and innocuous materials. Introduction The decommissioning of ponds and pits at gas plants and other facilities hasbeen a relatively high priority item for petroleum operators in the last tenyears. In addition to problems that may arise with respect to salinity andorganic contamination, trace elements may usually be expected to be present inpond sediments. The guidelines of the Canadian Council of Ministers of the Environment (CCME) require analysis for a variety of trace elements, althoughthe setting of critical values is left to the provinces. Failure to meet thecriteria set will often result in a requirement for risk assessment. Unfortunately, the information to be gained from routine analysis of pondsediments can be misleading in this respect. The guidelines refer to totalconcentrations, and the analytical techniques for measuring these arerecommended in the appropriate government publications. These methods willusually determine elements such as lead without regard for the originalspeciation of the metals. Speciation, however, is of critical importance whenrisks are being evaluated.