Alum (aluminum sulfate) is a nontoxic material commonly used in water treatment plants to clarify drinking water. In lakes, a low dose alum treatment is used to strip the water column of phosphorus. At higher doses, it is often used to inactivate phosphorus in the bottom sediments. This is accomplished by applying an aluminum salt to the lake which reacts with the water to form an insoluble aluminum hydroxide solid (floc). This floc falls through the water column removing phosphorus and then settles to the bottom forming a "blanket".

The phosphorus provides food for the algae. Thus excessive phosphorus and nutrient loads can be problematic. 

In some cases, a higher level of alum is used to seal areas of sediment that are releasing excessive nutrients prior to or in place of sediment removal. At this time only a low dose of alum is being used at Indian Lake. There are many improvements have been made in the watershed to help reduce phosphorus and nutrients. There is no specific data at this time that shows that the sediment is contributing significantly to the nutrient load but this continues to be studied. 

The practice of using Alum in surface waters has been around for 50+ years and has been utilized successfully on numerous projects in Massachusetts and around the country to lower phosphorus levels and reduce the frequency and severity of nuisance algae blooms.

The last 3 out of 4 years, we began the season with an alum treatment to ensure that everything possible was being done to be pro-active. 

Algae testing is also in full swing both with local volunteers and a second environmental company, ESS Group, Inc. Additionally, phosphorus and nutrient levels are being monitored by the Worcester Department of Public Works & Parks Lakes and Ponds Program all season long.