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Lac Ste. Anne County AB

Environmental geochemistry and petrology of the recent sediments from lakes in the vicinity of the coal-fired power plants in central Alberta, Canada

Author(s): Sanei, H.

Year: 2005

This study utilizes geochemical and petrological approaches to characterize recent sediments and their porewaters from the central Alberta lakes to elucidate the possible impact from coal utilization in this region. A multi-elemental analysis of recent sediments in conjunction with other inorganic and organic geochemical approaches are applied to determine the sources, quantity, and processes involved in the temporal and spatial distribution of trace elements in the study region. Concentration versus depth profiles in the sediments and the associated porewaters suggest that geochemical processes impact the mobility and vertical distribution of trace elements in these sediments. Although inputs of trace elements to ecosystems have clearly been elevated by emissions from the coal-fired power plants, diagenetic processes and natural inputs cannot be ignored in determining the temporal and spatial distribution of lake sediments. A combination of various biogeochemical processes may control the distribution of elements and nutrients in sediment and porewater. However, because of the alkalinity and eutrophic conditions of the studied lakes, and in particular Wabamun Lake, the Ca-OM fraction plays the most important role as substrate for trace elements and nutrients (e.g., P). The higher input of calcareous fly ash in Wabamun Lake as compared to the other studied lakes may cause higher scavenging of trace metals. The size of fly ash particles tends to decrease towards the more recent part of the sediment profile indicating the effect of particle emission control measures adopted by the power plants. There is no evidence of fly ash particles in the sediments deposited prior to the commencement of coal-fired power plants in Wabamun Lake (before 1956). Fly ash particles can be also found in the post-industrial sediments of Isle Lake. However, the sediment of Lac Ste. Anne shows no evidence of fly ash deposition. The affinity of various rare earths elements (REEs) to the labile organic matter (S1-OM) in porewater causes preferential fractionation of REEs throughout the porewater profile during the diagenesis process. In contrast, the sediments show little fractionation as a result of high clastic input of REE-enriched clay minerals in the sediment that mask the patterns caused by diagenetic processes. The relationships between the temporal distribution of organic matter and concentrations of trace elements indicate that the significant positive correlation between TOC and metals in recent sediments arises mainly from the portion of organic matter related to the thermally labile compounds released during pyrolysis at 300°C (S1-compounds). In contrast, the higher molecular, kerogen-derived hydrocarbons show a consistently lesser correlation with trace elements. The strong affinity between temporal distribution of metals and thermally labile compounds is due to both the chemical reactivity and petrological characteristics of these amorphous compounds. The S1-compounds are derived mainly from 'bitumen stain-like' amorphous organic matter. The fluid-like nature of the S1-compounds provides surface coating for the sediments grains, which accounts for the strong grain surface adsorption of organic matter.