Mobility of arsenic in mine impacted sub-arctic lake sedimentsThursday, November 21, 2019 - 9:00am to 9:20am Theatre Two
Sediments are widely used as environmental archives to reveal historical trends in contaminant deposition at lakes surfaces. However, some elements, such as arsenic (As), will not remain stable in the sediment after their initial deposition. Their redistribution is controlled by diagenesis, which represents physical mixing, biological processes and chemical reactions that transform sediments after deposition. In order to obtain a reliable historical record from sediment cores, the effect of diagenesis must be first quantified. To this end, pore water concentration profiles represent sensitive indicators of diagenesis that can be interpreted to estimate the extent to which diagenesis alters solid-phase distributions.
The main objective of this project is to use the sediments as environmental records to determine the extent and history of As contamination caused by the activities of the Giant Mine, near Yellowknife. We first determined the extent of As diagenesis in 8 lakes along an 80 km north-west transect away from a single point source of atmospheric As emission. We visited the lakes in June 2018 and May 2019 to collect a sediment core and pore water samples. Pore water was extracted directly from the cores using Rhizon lysimeter with a 0.2 mm membrane and dissolved As concentrations measured by inductively coupled plasma with mass spectrometry (ICP-MS).
Diagenetic modelling of the As concentration profiles in the pore water revealed that As fluxes ranged from 39,5 µmol/m2∙yr close to the mine to -27 µmol/m2∙yr at 80 km, decreasing with distance. At the 40 km mark, the sediment switches from a source to a sink of As to the water column. Maximum concentration of As reached in the pore water of each lake studied shows a decrease in concentration with the distance from the mine.
Correction for diagenesis revealed that up to 46% of the historical As had been remobilized by diagenesis. To our knowledge, this is the largest correction reported so far in the literature and is in line with previous evidence that interpreting sedimentary records in terms of historical deposition must be done after accounting for diagenesis. Reconstruction of the atmospheric inputs shows a peak in As concentration in 1950, which coincides with the timing of elevated As emission from Giant Mine operations.