An Investigation Of Trace Element Contamination Of Sour Gas Plant Pond Sludges
Authors: Thimm H.F. , Hoffman E.L. 
Published: March 2010

The sour natural gas processing operations in the Pincher Creek area of southwestern Alberta have been the subject of considerable public controversy. This was in part fueled by earlier reports of pond water and sludge contamination by a variety of trace metals, including lead, cadmium, mercury and chromium. The work reported here was designed to (1) check the validity of the earlier report of contamination, and (2) test the hypothesis that metals might have been solubilized by frequent exposure of plant site soils to acid forming materials such as elemental sulphur, been transported to the ponds by surface run-off, and been trapped there by precipitation with sulphide or carbonate ions. The use of neutron activation analysis is described/or elemental analyses of some lanthanides (lanthanum, cerium, neodymium, samarium, europium, terbium, ytterbium, lutetium) and actinides (thorium, uranium). The relative abundances of these and some additional elements do not support the above metal migration ypothesis. Instead, the data are consistent only with contamination by chromium, and possibly mercury.

Introduction

In 1982, Gulf Canada Resources Inc., in order to permit the closure of its sour gas processing plant at Pincher Creek in southwestern Alberta, commissioned a consultant's report to investigate the measures necessary for clean-up and reclamation of the plant site.

That report presented trace element analysis for pond sludges, drain sediments, flare pit sediments and solids from other low-lying areas, which suggested serious contamination of these areas by trace elements(1). Results are given for the Pond 6 area (Fig. 1) in Table 1, together with data for the surface soils of the region, a more complete list of which is published elsewhere(2).

The data in Table 1 contributed to a major public controversy, despite the fact that other studies of trace element content of stack gases(3) and process streams(4) could not detect significant or even measurable amounts of these metals. With the obvious exception of cooling tower blowdown as a source for chromium, the sour gas processing operations (physical liquid-gas separation, condensate stabilization, amine absorption of H2S and CO2, water and hydrocarbon dewpoint control, sulphur recovery by the Claus process) are not associated with major use of chemicals containing these metals.

Because the original number of samples in reference 1 was small (3–4, Pond 6) compared to the number of soil data available (60 for Cd, 199 for others), the apparent pond sediment contamination was re-investigated.

Also, in view of the long-term storage of elemental sulphur on the plant site, it was thought possible that metal enrichment of ponds and low-lying areas might have occurred slowly, over a twenty-year period, by the following mechanism:sulphur is oxidized microbiologically to sulphuric acid,on the way to the drainage areas, metals that exist as oxides, basic carbonates or as exchangeable species in soil are leached by the sulphuric acid,because the pond levels only very rarely reach the overflow channel levels, and because the pond waters are usually high in dissolved sulphide at pH 7.6 – 8.6(1, 5), the metals precipitate as sulphides or carbonates, and accumulate in the pond sediment.