5.8.1 Langdon Lake Internal Load Management Project

ProjectLangdon Lake Internal Load Management Project
DescriptionDesign and implementation of strategies to reduce internal phosphorus loading, including: feasibility study; aquatic vegetation survey update and management plan; fishery survey update and management plan; biomanipulation strategies that may include aquatic vegetation management, zooplankton community and fishery manipulation, and chemical treatment.
Need

The modeled watershed phosphorus wash-off load to Langdon Lake is not sufficient to explain the in-lake concentration of total phosphorus that exceeds the lake’s total phosphorus goal. An “unknown load” to Langdon Lake is attributed to internal loading, and is likely a combination of bottom sediments, aquatic vegetation, and model accuracy. An alum treatment was previously applied to Langdon Lake, which responded with a few years of improved water clarity but which soon returned to its pretreatment condition. A watershed and lake diagnostic / feasibility study will be required to confirm the internal and external phosphorous loading to Langdon Lake, as well as the necessary load reductions. In particular, the “unknown load” must be identified as either an external or internal load before specific strategies can be selected.

A variety of strategies would be investigated in the diagnostic and treatment study proposed in this plan. One potential strategy is an alum injection system to treat this loading on an ongoing basis. The feasibility study would determine the injection points and the extent of treatment required. The cost for such an improvement assumes there would be no land acquisition cost as the necessary land is currently publicly owned.

The project assumes the construction of the wetland detention pond described separately to serve as the floc settling pond, although that pond project can stand alone if necessary. There would be an ongoing annual cost to operate the system for the 75 percent removal required to achieve 55 μg/L. In addition, this project would identify and implement a suite ofstrategies to manage aquatic vegetation, the fishery, and zooplankton community to achieve water quality and clarity goals. The project includes an ongoing vegetation and fishery management plan. Management of invasive aquatic vegetation that contributes to lake water quality and usability issues would require annual treatment for three to five years.

OutcomeReduction in phosphorus load from internal sources estimated at 148 pounds annually; improved water clarity; more diverse aquatic vegetation community; improved aesthetics.
Estimated Cost and Funding
Investigation, permitting, cooperative agreement,
fish, vegetation, and zooplankton surveys,
construction. Funding source is the District capital levy.
Annual cost of injection system operation, funded
from the District operating levy
$82,700 Design
$537,800 Construction
$801,700 Total
$69,300
Annual operating cost
Schedule

2012 Fish, vegetation, and zooplankton surveys, development of management plans, cooperative agreement, design

2013 Construction