A paleolimnological approach for interpreting aquatic effects monitoring at the Diavik Diamond Mine (Lac de Gras, Northwest Territories, Canada)

Abstract

Korosi JB, Thienpont JR, Eickmeyer DC, Kimpe LE, Blais JM. 2020. A paleolimnological approach for interpreting aquatic effects monitoring at the Diavik Diamond Mine (Lac de Gras, Northwest Territories, Canada). Lake Reserv Manage. 36:297–313. A paleolimnological assessment of Lac de Gras (Northwest Territories, Canada) showed pronounced aquatic ecological and biogeochemical changes occurring since at least circa 1950, well before diamond mining operations began in 2000. These changes are likely a response to regional climate warming, which is confounding the interpretation of an Aquatic Effects Monitoring Program (AEMP) intended to identify early-warning indicators of diamond-mining impacts to water quality. In the latest AEMP report, action level exceedances based on 3 years of monitoring in Lac de Gras were reported for chlorophyll a and strontium, yet sediment cores collected from 3 different sites in the lake exhibited notable increasing trends in these parameters since pre-1950. Increases in the small, centric diatom Discostella pseudostelligera have also been occurring since pre-1950, which we infer to be a response to climate warming. Recent (post-1996) D. pseudostelligera increases observed from aquatic effects monitoring of nearby small lakes have previously been linked to nitrogen fertilization from diamond mining. Thus, our paleolimnological results clearly indicate that parameters predicted to respond to mining impacts are also responding similarly to regional climate warming. Based on this, AEMP adaptive management strategies need to consider the potential additive or synergistic effects of mining and climate change when establishing action level exceedances for water quality and ecological indicators. Using our paleolimnological data, we calculated background (premining) rates of change in key geochemical parameters that can provide a benchmark for evaluating ongoing changes in the current mining period, and for establishing AEMP significance thresholds.