Oil and gas exploration in remote Arctic regions has many inherent challenges. Foremost among these is the need to develop strategies to dispose of wastes associated with the drilling of exploratory onshore hydrocarbon wells. The Mackenzie Delta in Canada’s western Arctic is underlain by significant discovered and predicted reserves of hydrocarbons, in particular natural gas, and exploration has been occurring in the Mackenzie Delta region since the 1960s. Waste products created during the drilling of deep, exploratory wells are disposed of in large sumps, which are large pits that are excavated into the permafrost . Drilling muds used during the drilling process include a variety of chemical constituents such as large quantities of potassium chloride, among others . Previous studies have shown some of these chemicals can be toxic to aquatic life .
Sumps were intended to act as a permanent containment location for relict drilling wastes; however, recent observational evidence suggests a number of sumps may be leaching contents [see image below, 3]. Since more than 150 drilling sumps have been constructed in the Mackenzie Delta region since the mid-1960s, many of them in the vicinity of small lakes and ponds, this represents a potentially significant stressor to this ecologically significant, water-rich region. In this study we use a combination of contemporary limnological sampling and inferences of past conditions using material preserved in lake sediments (the multidisciplinary study of paleolimnology ) to assess the impacts of drilling fluids on the freshwater ecosystems of the Mackenzie Delta uplands region.
Water samples were collected from approximately 100 lakes in the Mackenzie Delta uplands region, including: 1) 20 lakes directly in the vicinity of drilling sumps, and thus potentially impacted by saline drilling fluids; 2) 47 lakes with no visible disturbances; and 3) 34 lakes impacted by significant, naturally occurring permafrost degradation in the form of retrogressive thaw slumps. Retrogressive thaw slumps are a common form of thermokarst, which currently occurs on the shorelines of approximately 10% of lakes in the Mackenzie Delta uplands, and are increasing in size and growth rate as a result of recent warming . Thaw slumping is known to result in major changes to lakewater chemistry and biology [6-8], and thus these lakes were used to place water quality in drilling sump-impacted lakes in the context of lakes strongly impacted by an independent regional stressor.
In order to determine the timing of water quality changes related to sump failure and the resulting impacts on aquatic biota, subfossils of microinvertebrates, known as cladocerans, were analyzed in a sediment core taken from a moderately impacted lake, as well as two lakes with no drilling sumps in their catchments, and thus suitable as control locations. Cladocerans are an important component of the aquatic food web in lakes, and are important indicators of ecosystem health.
Results and Discussion
Lakes with drilling sumps in their catchment have significantly elevated chloride levels relative to all other lakes included in the survey. Chloride is an important chemical constituent of drilling fluids, while chloride from the glacially-derived surficial geology of the Mackenzie Delta region is naturally low. Thus, elevated chloride concentrations in lakes downstream of drilling sumps provided clear chemical evidence of the impacts of drilling sumps on the chemistry of nearby lakes. The degree of impact varied between sump-affected lakes related to the degree of impact from the nearby sump. This is not surprising as some sumps exhibit significant loss of containment, while others remain largely intact.
Using the subfossil remains preserved in lake sediments to infer limnological change over the recent past, we recorded an increase in the relative abundance of a saline-tolerant cladoceran corresponding to the timing of initial sump construction in the sump-affected site. This suggests that construction and abandonment techniques at, or soon after, sump establishment may have resulted in impacts to downstream aquatic ecosystems. No comparable change was observed in the two control locations studied.
With hydrocarbon development in the north predicted to expand in the coming decades, the use of sumps must be examined in light of the threat of accelerated permafrost thaw, and the potential for these industrial wastes to impact sensitive Arctic ecosystems. Given the continued interest in exploiting hydrocarbon resources in this and many other regions of the Arctic in the future (including the approved Mackenzie Gas Project) the results of this study and others on sump integrity are important for determining best practice strategies related to drilling waste disposal in these sensitive ecosystems.
Full study published in PLoS ONE, November 2013. Images and other details related to this study can be found at: http://post.queensu.ca/~pearl/mackenziedelta/macdelta.html
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