RPS was contracted to provide a quantitative assessment of potential release scenarios to determine the downstream movement and behavior of drilling fluids within the Mackenzie River for the horizontal directional drill (HDD) crossing of the Enbridge Line 21 Segment Replacement Project in the Northwest Territories. The pipeline crossing is approximately 10 km upstream (to the east) of the Village of Fort Simpson freshwater intake and upstream of the confluence with the Liard River. The primary investigation is the potential for the entrainment of released drilling fluid into the Fort Simpson fresh water intake. Additional concerns resulting from an inadvertent return include impacts to fish and fish habitat.
Sediment transport modeling was then conducted for 2 load rates, at 3 sites along the HDD crossing, for 3 river discharge conditions. Each of the 18 model runs provided a 3-dimensional prediction of sediment concentrations through time. An analysis at the drinking water intake provided the range of concentrations to be expected and the associated timing.
Enbridge Mackenzie River Water Quality (WQ) Modelling
The crossing would be constructed using Horizontal Directional Drilled (HDD) technology. In the drilling phase of construction, a drilling fluid composed of a number of non-hazardous constituents and bentonite would be used to lubricate the drill bit and carry drill cuttings back out the bore hole. RPS was tasked with modelling an uncontrolled release (i.e. “frac-out”) of the drilling fluid into the river. The amount of time and potential concentrations of bentonite at the Fort Simpson drinking water intake were assessed under the range of site-specific and season-specific conditions.
RPS used two modeling systems to bound the range of predicted downstream movement and behavior of bentonite in the Mackenzie River. The BFHYDRO boundary conforming, general-curvilinear grid system hydrodynamic model was used to characterize the river flows. The SSFATE Lagrangian particle model, was used to simulate the downstream transport and fate of sediment particles (e.g. clay, silt, sand). A river discharge analysis was conducted to determine the range of conditions in the Mackenzie River and the 5th, 50th, and 95th percentile flow rates were used in the modeling. The inadvertent return load (in MT/hr) was determined along with the duration of the hypothetical release.