Construction resumed Aug. 13 on a project at Naval Support Facility Dahlgren that is designed to remove pollutants storm water runoff, protecting the Chesapeake Bay watershed. The Willow Oaks Constructed Storm Water Wetland, located on a portion of the former Willow Oaks Golf Course, was partially excavated when work was halted in April.
When work is finished, the project will complement the completed Upper Machodoc Constructed Storm Water Wetland on the opposite side of Sampson Road and bring NSF Dahlgren into compliance with the Environmental Protection Agency’s new maximum daily load rules. The rules, which limit the amount of pollutants that enter the Chesapeake Bay in storm water, came into effect in 2012 and become progressively stricter until 2028. Dahlgren’s storm water wetlands will put it very close to meeting the 2028 requirement.
A wet spring and challenging soils proved troublesome for construction workers. “Equipment was sinking,” said Brenna White, storm water program manager for NSF Dahlgren. “Digging out there became difficult due to the water content and the soil type. It was not only difficult to excavate, but impossible to build any kind of stable slope.”
Marine clay, also known as “fatty” clay, was discovered on a portion of the six-acre site; approximately four feet below the ground surface, the clay concealed a layer of water. In the process of excavating and reshaping the sediment basin as part of the constructed wetland, the water seeped out, combined with the clay to form slurry and made work impossible.
The summer months facilitated draining and drying of the site, allowing construction to begin again. “Going forward, the plan is to do the far side of the wetland that is closest to housing first,” said White.
That portion of the site has soil that is easier to dig and shape, though the rest of the area is expected to be workable soon thereafter. A portion of the fatty clay will be relocated to provide a liner for the wetland, helping it hold the water. Topsoil recovered from the excavation will also be incorporated into the wetland during the final phases of construction to help native plants establish.
Plants along high marsh, low marsh and deep pools of the wetland are the key to removing harmful pollutants from storm water. Nitrogen and phosphorous that are found naturally in soils and in commercial fertilizers enter the Chesapeake Bay in unchecked storm water runoff. The elements are beneficial to plants on land, but can cause excessive algae growth in bodies of water. Algae blooms in the Chesapeake Bay typically deplete oxygen levels in the water, forming “dead zones” that harm aquatic life as well as the economic prospects of local fishermen and watermen.
Wetlands remove those pollutants by slowing the flow of storm water and allowing plants to harvest nitrogen and phosphorous, which they metabolize. Once the storm water has meandered through the wetland, it enters Upper Machodoc Creek free of the pollutants.
While weather will always be a factor in the construction of a wetland, the groundwater table is lower in the summer than it was in the spring; a lower groundwater table is desirable when excavating large volumes of soil to create wetlands. “The rest of the excavation should not be too weather-dependant,” said White. “A little bit of moisture is good to help keep the dust down.”
For an idea of what the finished wetland will look like, the Upper Machodoc Constructed Storm Water Wetland, completed last year, is illustrative. Arrowhead, pickerelweed and vetch are in full bloom. Young bald cypress, hickory, redbud and serviceberry trees are slowly but surely growing.
“The redbud and serviceberries are good forage for the deer,” said White.
The wetlands’ process of maturation will unfold over several years. Sun-loving vetch and clover will give way to shade-loving plants and tall trees. Wildlife has already made good use of the new ecosystem, with blue herons, wood ducks and ospreys being the most common visitors.
“It really is the habitat we were hoping it would be,” said White.
White encouraged base residents and employees to use a gravel path around the Upper Machodoc Constructed Storm Water Wetland to take in the plants, animals and evolving landscape. “It’s a continuous process,” she said. “The constructed wetlands were built not only to treat storm water, but also as an education and outreach tool that the whole community can enjoy.”