If you happen by Hummock Pond on July 16, you’ll see thousands of dollars worth of chemicals being unloaded into the water. Payment for the drop-off comes courtesy of the Nantucket Conservation Foundation, the Nantucket Pond Coalition, and abutters of the pond, the largest on the island after Sesachacha. The chemicals are PAC, short for polyaluminum chloride, and they inactivate the mineral phosphorus—a nutrient in the pond that, if left unchecked, allows for the growth of blue-green algae, which is toxic to both people and pets and can create a harmful environment for fish and other wildlife.

People making recreational use of water that has harmful algal blooms—swimming or coming into contact with the water when getting in or out of a boat—can end up with skin or eye irritation. Inhaling a spray of contaminated water can cause asthma-like symptoms, and more sustained contact can cause severe neurological problems.

Hard figures for incidents of algal bloom poisoning on Nantucket are lacking, but in 2021, the latest year for which figures are available, the Centers for Disease Control and Prevention said there were more than 100 reports of people across the country becoming sick from exposure and more than 2,700 incidents of animal illness. The Bureau of Climate and Environmental Health reports almost 2,500 incidents of animal death. Of course, all of these numbers represent only information shared, not illness and deaths that went undocumented. A dog can get sick and even die just licking its own fur after algal bloom contact.

That’s why it’s so important to bind up the phosphorus in the water with PAC. Phosphorus provides “a feeding frenzy for algal systems,” according to the Town’s water quality specialist, Emma Morgan. It reaches the pond largely via runoff from fertilizer and septic systems, meaning its overabundance in the island’s waters is a new problem, geologically speaking. Nantucket’s 80-plus ponds, representing about 5 percent of the island, are 6,000 years old, and the island has been populated by people of European descent for 400 years—but only within the last 100 did the degradation of the island’s ponds really begin, says Bob Williams, founder and president of the Nantucket Pond Coalition. “The Wampanoag didn’t have septic,” he says. “They didn’t have 100,000 people in the summer.”



While the Town can’t close the ponds, those frequenting a pond at which algal blooms have been confirmed will notice newly installed signs warning people to stay out of the water and to not let their pets step into it or drink it.