By Hydrosimulatics INC  

Dewatering Impact Analysis Problem

SUMMARY: Dune Shores LLC is proposing to construct a 17 ft deep boat marina near Lake Mishigamaa at the mouth of the Gitchi River. But they are running into fierce opposition from the local community and environmental activists who argue that the excavation of the new marina will cause the surrounding water table to decline, including in the area of several important interdunal wetland ecosystems. You be the judge … would the proposed plan by the developer work? Should permits be issued by the Army Corps of Engineers and the DEQ?

 

Background, Arguments & Regulations

Dune Shores LLC is proposing to construct a 17 ft deep boat marina near Lake Mishigamaa at the mouth of the Gitchi River.  But they are running into fierce opposition from the local community and environmental activists. The community thinks that the excavation of the new marina will impact the local groundwater hydrology, pulling water from around the site into the marina basin and causing the surrounding water table to decline, including in the area of several important interdunal wetland ecosystems.

Dunes Shoes, however, disagrees. They argue that environmental protection does not mean a complete restriction on human activities or development.  In this case, Dune Shores believes they can design the development to co-exist harmoniously with the natural ecosystem, with no long-term impact on the groundwater table in the wetland areas. In particular, the developer proposes building a marina basin with a clay liner at the bottom and sheet steel penetrating 30 ft deep along the perimeter of the basin, which will seal the basin from the outside groundwater environment, eliminating any potential long-term impact.

Citizens countered that, even if the basin is sealed off to eliminate long-term impacts, the temporary impacts of dewatering / construction would be devastating for the wetland ecosystems. In response, Dune Shores proposed adopting a very aggressive construction schedule to minimize or eliminate any short-term impacts, reducing the initially planned 60-day dewatering / construction process to 7 days: 5 days of dewatering, two days of clay liner construction, and 2 days of refilling by opening up the basin to the Gitchi River. They argue that the water table decline during this temporary process is well within the natural fluctuations of the water table.

The US Army Corps of Engineers requires that the short-term fluctuations be limited within natural fluctuations. Data shows that the groundwater table in the area fluctuates naturally in response to Lake Mishigamaa water level fluctuations, where the natural all-time high and low differ by 5 to 6 ft.

The state Department of Environmental Quality (DEQ) requires that the long-term impact be limited to less than 6 inches.