Lanthanide Fingerprinting And Acidification Of A Glacial Till Soil

Abstract

Abstract A glacial till soil from southwestern Alberta was examined in terms of its lanthanide, Th and Sc content in order to establish a lanthanide labelling technique or soil type. It is shown that this technique may also be applicable in acidic soils. The acidification of the soils tested showed that the soils were capable of at least partial, natural pH recovery. This behaviour was found to be due to the presence of two minerals, one of which war confirmed as monazite, a tertiary rare earth phosphate; calcite, dolomite or albite (feldspar) contributions to this pH control were not important. Some implications of this result with respect to environmental monitoring at sour gas plants are discussed. Introduction In a previous paper(1), we reported on the use of lanthanide (rare earth) element abundances as an investigative tool for estimating the degree of pollution of pond sediments at a sour gas plant. The original intent in that work was to use the lanthanides as markers for trace element migration in soils subject to the deposition of sulphuric acid precursors. Instead, the hypothetical migration could not be detected, and the lanthanide abundance data implied that even where there is a high acidification potential due to sulphur dusting, the elements may serve better as a fingerprint for the identification of soil strata. The ability to label soils and sediments by lanthanide abundance might have numerous applications, apart from the utility in environmental assessment during decommissioning of gas plants which we have already reported(1). The present work deals with the examination of soil data from the Pincher Creek area of southwest Alberta to confirm both the soil labelling ability and the implied lack of sensitivity to deposition of acids. Because it is known from the previous work that the metals in question are easily stripped off the soil with 2.5 M sulphuric acid, it is also necessary to know the soil pH range in which the lack of acid sensitivity can no longer be assumed. Finally, it is of interest to compare the ease of metal removal from glacial till soils with data from the acid precipitation work at Hubbard Brooks, New Hampsttire(2,3) and others(4,5). There have been reports of groundwater pollution by metals released from acidified soils(6). In Alberta, the occasional deficiency of copper in livestock feed has been speculatively linked to the deposition of acidic gases to soil(7). Study Area The area chosen for study (Fig. 1) is located in the extreme southwestern comer of Alberta. Two sour gas processing plants had been operating in this area for over 25 years at the time of soil sampling (1983). Emissions for both plants typically totalled 40 t/d SO2 with particulates at 200 kg/day. Prior to 1972, SO2 emissions were substantially higher, near 130 t/d. The area shown in Figure 1 may be divided into two major soil Types(8). Soils developed from glacial lake bottom material extend to the southeast of Pincher Creek townsite, and then south to the Waterton Reservoir.