SUMMARY: Big Dune Shores LLC is proposing to construct a 17 ft deep boat marina near Big Lake 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. Big Dune Shores thinks that a sealed marina bottom will eliminate any long-term impact on the water table in the wetland areas, and a quick dewatering operation will result in a temporary drawdown of the water table that is well within natural fluctuations. You be the judge … would the proposed plan by the developer work? Should permits be issued for construction?

Background, Arguments & Regulations

Big Dune Shores LLC is proposing to construct a 17 ft deep boat marina near Big Lake 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.

Big Dunes Shoes, however, disagrees. They argue that environmental protection does not mean a complete restriction on human activities or development.  In this case, Big 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, Big 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 regional natural resources department 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 Big Lake water level fluctuations, where the natural all-time high and low differ by 5 to 6 ft. The natural resources department also requires that the long-term impact be limited to less than 6 inches.

Proposed development near the mouth of the Gitchi River.

Objectives & Deliverable 

You be the judge … would the proposed plan by the developer work? Should permits be issued by the regional natural resources department?

Develop a model of groundwater flow at the site using the information provided below to come to a conclusion regarding this situation. Specific questions to address as part of your analysis include:

• What will be the long-term impact on the water table in the wetland areas?
• What will be the temporary impact on the water table in the wetland areas?

Prepare a 1-2 page report that summarizes your approach and findings. You should discuss your findings with regards to responsibility for the contamination. Include any detailed model results / graphics in support of your conclusions in an appendix.  

Given information

Site Hydrogeology

There are three types of major land systems within the study area: i) dune areas, with little vegetation, higher elevations and thick deposits of coarse- and fine-grain sands; ii) forested areas at medium elevations with thick deposits of sand intermixed with thin layers of fine-grains clay and silts of low permeability; and 3) coastal sands at lower elevations with thick deposits of coarse-grain sands. Analysis of well logs suggests that thicker fine-grained deposits underlie the proposed basin location. 

Construction process

The construction process can be separated into two-distinct phases: dewatering and refilling. The process of dewatering is expected to require excavating approximately 17 ft over a period of 7 days. Prior to dewatering, sheet pile walls will be installed to a depth of 30 ft on all sides of the boat basin. Dewatering will cause water levels in the basin to drop from an initial value of 0 ft to -17 ft. Immediately afterwards, the seal between the Gitchi River and the basin will be removed, and the basin stage will equal the Gitchi River Stage (see below).

Field Data

The information/data in the following graphic are available from the preliminary study. Also included below is the dewatering-refilling curve (water levels) for the proposed basin.

MAGNET/Modeling Hints:

• Use ‘Synthetic mode’ in MAGNET to create a model domain with the same dimensions as shown in the map.
• Overlay the provided SiteMap image file included in the problem description. Choose ‘Use Domain Extent’ to fit the image to the established domain size.
• Conceptualize the River and Stream features as constant head boundary conditions
• Treat the domain boundaries as ‘no-flow’ boundaries. The lateral boundaries are sufficiently far from the proposed marina to not be impacted by the dewatering and refilling operations. 
• Use zones features to assign specific parameters to the Forested Areas, Dunes Areas, Wetlands, and surface water bodies (Lake Mishigamaa and Gitchi River). The Coastal Sands areas can be parameterized with the domain attributes assigned in the Domain Attributes menu. 
• Use a zone feature to represent the Basin as a time-varying prescribed head (stage) feature.
• Use a 'skinny' zone along the Basin perimeter to represent the sheet pile by assigning an effective hydraulic conductivity of 7 ft/day. Check the box next to 'Zone Boundary Included' to make sure this zone is captured by the discretized model.
• Use a relatively large grid size (NX of 100 or 150). 
• Use monitoring wells to track the head dynamics in/near the Wetland areas during dewatering and refilling