Effect of mine dewatering on peatlands of the James Bay Lowland: the role of bioherms

Abstract

The James Bay Lowland host one of the largest wetland complexes in the world in part due to the low permeability of marine sediments that suppress groundwater seepage losses. Dewatering of an open-pit diamond mine in the area has depressurized the regional bedrock aquifer. Bioherms, fractured limestone outcroppings formed from ancient coral reefs that protrude to the peatland surface, lack this mantle of low-permeability sediments and provide a direct connection between the peatland (surficial) and the regional (bedrock) aquifers. Well transects and piezometer nests were installed around seven bioherms in the depressurized zone and one in a non-affected zone (control) to monitor the water table drawdown and change in hydraulic gradients around the bioherms. Water tables in the affected bioherms decreased between 2 and 4 m in the first 4 years of dewatering. The drawdown in the bioherms caused a localized water table drawdown in the peat surrounding the bioherms that extended to approximately 30 m from the edge of the bioherm during a dry period. Under wet conditions, drawdown was similar to that at the control site. Hydraulic gradients in the peat (which typically are very small) increased over the field seasons and in a few locations exceeded 1. These gradients represented significant losses to the local, near bioherm, system as at many of the locations surrounding the bioherms vertical seepage losses ranged between 1 and 4 mm/day, which are similar to the seasonal average evaporative water loss of~3 mm/day. The bioherms are acting as efficient drainage nodes; however, their influence is localized to the peat immediately (~<30 m) surrounding them.The distance from the edge of the bioherm is reported as the +XX value followed by the depths (m) of the piezometer midpoints used to calculate the gradient. BIOHERMS AND THEIR ROLE IN MINE DEWATERING

Effect of mine dewatering on the peatlands of the James Bay Lowland: the role of marine sediments on mitigating peatland drainage

“…The James Bay Lowland is typified by a complex arrangement of bogs and fens, which comprise the majority of the landscape (60%) and when combined with open water and other wetland types, make up >90% of the landscape (Riley, ). Peat deposits range in thickness from 0 m where exposed bedrock is present (Cowell, ; Whittington and Price, ), to ~4 m (Glaser et al ., ; Sjörs, ).…”

“…With evidence that depressurization is occurring (Whittington and Price, ), the assumption of no vertical flow in these large peatland systems surrounding the mine is now invalid. Therefore, the impermeable nature, as well as the thickness of the MS, will be assessed.…”

“…Discovery of kimberlite (diamondiferous) deposits in an area of the James Bay Lowlands has led to open‐pit diamond mining which requires substantial groundwater pumping to dewater the mine thus causing depressurization of the regional bedrock aquifer beneath the MS. (Whittington and Price, ) found that this depressurization caused a localized water table drawdown in the peat around exposed or sub‐cropping bedrock features (ancient coral reefs called bioherms (Cowell, )) which was limited to ~30 m from the edge of the bioherms where there are little or no MS. In a 12 km radius around the mine, approximately 100 bioherms can be found occurring in three distinct lateral bands (Whittington and Price, ), suggesting that their cumulative impact may be larger. Beyond this zone, the presence of MS was expected to play a role in protecting the peatland from excessive (vertical) recharge and desiccation (HCI, ) but has yet to be properly assessed.…”

Impacts and prognosis of natural resource development on water and wetlands in Canada’s boreal zone

“…The extent of water table decline depends on hydraulic properties of subsurface materials and pumping rates. For example, in wet areas, water losses in wetlands from mine dewatering can be significant and irreplaceable for the duration of the mine (Whittington and Price 2012).…”

Microclimatic response to increasing shrub cover and its effect on Sphagnum CO2 exchange in a bog

“…Drying of the HBL peatlands may result from climate warming if enhanced evapotranspiration (due to warmer temperatures) is not offset by greater precipitation, although coastal areas of the HBL may also be subject to flooding with sea level rise (Tarnocai, 2006). Drying may also be a potential consequence of resource development such as mining that involves groundwater pumping (Whittington and Price, 2012). Our understanding of peatland C cycling processes in the HBL is therefore critically important due to the potential for large feedbacks to the climate system in response to climate change or other types of disturbances.…”