In January, Enbridge realized that it had vastly underestimated the amount of water needed for temporary trench dewatering during construction.
Dewatering is necessary when constructing a pipeline through wet areas. Excess water must be drawn out from the trench to protect workers and stabilize the soil. The water is later discharged back into the ecosystem. The appropriation of groundwater for dewatering requires a DNR (Minnesota Department of Natural Resources) permit.
The DNR originally granted Enbridge permission to draw about 510.5 million gallons for construction trench dewatering. During winter construction, Enbridge encountered more trench water than expected. This photograph shows an example of trench water along Construction Spread 5 within one hour of excavation this past winter:
Additionally, the DNR and PCA (Minnesota Pollution Control Agency) requested Enbridge consider using additional well point systems along the project to reduce the amount of sediment being discharged with the water. Although using the wells help produce cleaner discharge, dewatering using wells requires more groundwater than other methods.
On June 4, 2021, the DNR authorized Enbridge to appropriate an additional 4,472 million gallons of water for temporary trench dewatering for the remaining 144.5 miles of construction. This is nearly a ten-fold increase from the 510.5 million gallons originally granted.
We may not be able to directly observe groundwater appropriation activity, which will often occur in areas inaccessible to monitors. But it is useful to keep an eye on water levels in nearby wetlands, which could also be impacted by the current drought. Documentation of changing water levels over time could be helpful if we are able to gather enough data, including baseline conditions.