Lake Hopatcong Phoslock Treatment

To prevent harmful algal blooms (HABs) in New Jersey’s largest lake, Princeton Hydro implemented a clay-based nutrient inactivating technology called Phoslock in Lake Hopatcong during the summer of 2020.  This was the largest Phoslock treatment to occur in the Northeastern U.S. at the time.  

Lake Hopatcong suffered from large-scale and persistent HABs over the course of the 2019 summer season, where local and county health agencies closed off all beaches and issued advisories over large sections of the lake. These unprecedented conditions had significant negative impacts on the ecological, recreational, and economic resources of the lake and region. So, to combat HABs in the 2020 summer season, the Lake Hopatcong Commission has partnered with the Lake Hopatcong Foundation, four municipalities (Jefferson, Hopatcong, Mt. Arlington, and Roxbury), two counties (Morris and Sussex), and our team members at Princeton Hydro to develop both short- and long-term management strategies for the lake. 

This project involved treating 50 acres of the southern end of Lake Hopatcong with Phoslock, a clay-based product that inactivates phosphorus in both the water column and the sediments, making this critical nutrient unavailable for algal growth. The Phoslock treatment, which requires proper permitting by NJDEP, is applied as a slurry distributed from a boat. 

After Phoslock is applied, it sinks through the water column, binding phosphate as it moves towards the sediment. Once settled at the bottom of the lake, it forms a very thin layer and continues to bind phosphate released from the sediment, thus controlling the release of phosphorus into the lake. One pound of phosphorus has the potential to generate up to 1,100 lbs of wet algae biomass. However, 1.1 tons of Phoslock is capable of removing 24 pounds of phosphorus — that’s over 26,000 lbs of wet algae biomass not growing in the lake for every 1.1 ton of Phoslock applied. In turn, Phoslock’s ability to suspend biologically available phosphorus is therefore a major step towards improving a lake’s water quality.