Master Plan and Water Quality Management

Lake Mohawk

Lake Mohawk ManagementLake Mohawk, a 770-acre lake located in northwestern New Jersey, had a history of serious blue green algae blooms. The frequency and durations of these blooms steadily increased through the 1980s despite increases in the intensity and magnitude of copper-based algaecide treatments. In the early 1990s Princeton Hydro was contracted by the Lake Mohawk County Club (LMCC) to conduct a detailed water quality and trophic state assessment of the lake. We used the data to prepare a comprehensive lake management master plan. The key element of the plan involved the use of alum to reduce and control the lake’s sizable internal total phosphorus load and the phosphorus originating from stormwater and other external sources. This innovative nutrient control program was the first of its kind in New Jersey.

To decrease the lake’s internal phosphorus load 90,000 gallons of alum was applied by Sweetwater Technology, under the direction of Princeton Hydro, to limit the recycling of sedimentary phosphorus during periods of hypolimnetic anoxia. At the same time, working in concert Princeton Hydro, Sweetwater Technology and General Environmental Systems (GES) then designed and installed a metered alum dosing/aeration system. The system’s dual purpose was to maintain the water column in a destratified, fully aerated state while simultaneously injecting relatively small quantities of alum to control the lake’s daily, external phosphorus load. The original system installed in 1992 was subsequently redesigned and expanded by Princeton Hydro in 2001 and further upgraded in 2010. The current system configuration consists of two pump houses and 24 diffusers equipped with low-volume alum emitters. These diffuser/alum units are strategically placed throughout the lake. Princeton Hydro’s limnologists continuously track and modify the daily alum dose, as based on both the measured and computed influx of phosphorus entering the lake from storm sewers, septic systems, lawn runoff, and/or atmospheric deposition.

This innovative use of the alum, together with the mixing achieved through the destratification aeration system, has successfully controlled the lake’s phosphorus load, reducing the amount of biologically available phosphorus by 60%. This decreased the frequency and magnitude of blue-green algae blooms thereby alleviating the need for whole lake copper sulfate treatments. Princeton Hydro’s management efforts resulted in a five-fold increase in lake clarity, a rebound in the lake’s herbivorous zooplankton community, and the re-establishment of a diverse phytoplankton community no longer dominated by blue-green algae. These sustainable improvements in the lake’s overall water quality and ecology meet the lake user community’s recreational demands without resorting to the frequent or excessive use of algaecides.

The management plan also set the stage for the construction/installation of 24 stormwater basins, the installation of two flow activated storm sewer alum dosers, the adoption of septic management and no-phosphorus lawn fertilizer ordinances, and an aggressive public education and outreach effort focusing on the importance of phosphorus control and an integrated ecosystem approach to the lake’s management.

The success of this program was recognized by the USEPA through an Environmental Excellence Award, by the NJDEP through an Environmental Initiative Award and by the North American Lake Management Society through a Technical Merit Award.