Pest House Pond Water Quality Management is Underway

Pest House Pond is small, brackish wetland approximately ¾ acre in size located along the shoreline of Nantucket Harbor between Monomoy and Shimmo. The pond and surrounding property totaling 3.6 acres was donated to the Foundation in 1973 by the Shimmo Association, with an additional 1-acre abutting parcel donated by Mrs. I. H. Burnside in 1979.

You may wonder – how Pest House Pond get its peculiar name? It was named for the “Pest House” that once existed in this vicinity in the 1760’s when there was a smallpox epidemic taking place on the island. Anyone exhibiting symptoms or suspected of being exposed was sent to this facility for treatment and to prevent the rest of the population from becoming infected with this serious, highly contagious disease.

For the past several years, our Science and Stewardship Department has been investigating the cause of consistent algal blooms in this small wetland, which is surrounded by seasonal residential development. We worked with researchers from UMass Boston in 2004 – 2008, who collected multiple years of water quality monitoring data. Results indicated that leachates from lawn fertilizer use and septic systems on surrounding properties were the most likely sources of the excess nutrients responsible for the algal growth.

Public outreach and education efforts were initiated to encourage the neighbors to alter their landscaping practices and regularly service their septic systems. With their cooperation, the situation improved somewhat in that the algal blooms no longer covered the entire surface of the pond during the late summer, as was once the case. Nonetheless, in recent years there has still been substantial algae populations present in the pond during the late spring, summer and fall months.

For over 70 years, Pest House Pond has been connected to Nantucket Harbor via an underground pipe that drains onto the beach between the high and low tide lines just north of the Cathcart Road access. Although the reasons for installing the pipe are unknown, it was likely a historic public works project aimed at draining the pond for mosquito control. Both the Foundation and the Town of Nantucket have had ongoing concerns about water quality impacts to Nantucket Harbor from this pond discharge.

In order to gain a better understanding of the condition of the pond and options for managing the persistent algae, we contracted with SWCA Environmental Consultants in Amherst, MA to conduct an environmental assessment. In June 2017, SWCA biologists visited the site and recorded pond water and sediment cross section (bathymetry) data, collected algae samples for laboratory identification, characterized bordering vegetation communities, inspected the outflow pipe and made additional observations. These data were presented in an assessment report that also included an analysis and approximate costs of possible management options to improve the water quality and aesthetics of the pond.

Laboratory analysis documented large infestations of two algae types: Cladophora sp. and Microcystis sp. The former species formed a floating mat that covered approximately 15% of the pond surface. The later species, which is classified as a Cyanophyta-Blue-Green algae, was present in the sub-surface of the entire pond and was recorded within the water column at extremely high levels. Microcystis, which appears to have recently established in the pond, is a potential toxin producer and can be harmful to humans exposed via physical contact or inadvertent ingestion or inhalation. Both types of algae potentially host many types of bacterium, which can include species harmful to humans. Because algae were observed exiting the pond into Nantucket Harbor via the outlet pipe, the Foundation immediately reported this information to the Town of Nantucket’s Natural Resources and Health Departments. An order to remove all sources of the algae from the outflow pipe was subsequently issued by the Nantucket Health Department.

To address this issue, we incorporated the recommendations from SWCA Environmental Consultants into a management plan for Pest House Pond. Removing the outfall pipe was not a feasible option because it is deeply buried under well-established vegetation. Therefore, we applied for and received permitting from the Nantucket Conservation Commission to install a flow control valve in the pond-side end of the pipe, which was put in place in June 2018.

In order to monitor water levels in the pond and predict if flooding of nearby adjacent properties may take place during periods of heavy rainfall, we placed a stilling well within the pond and installed a data logger that records water level readings every 4 hours. Data from the logger is transmitted remotely so that it can be downloaded and reviewed anytime without the need for a site visit.

Since the valve was installed and closed last June, we have had to open it only three times (for 2-3 days each time) in order to prevent flooding on adjacent property. Otherwise, the valve has remained closed and there has been no pond water input into Nantucket Harbor. This is a huge improvement over the previous situation when the pipe was continuously discharging pond water containing algae and high levels of nutrients into the harbor.

Once we had the ability to isolate the pond from the harbor, we applied for and received permitting to move forward with algae and excess nutrient control within the pond in early August. Also included in this permit was permission to treat a small population of Phragmites (common reed) – a non-native, invasive species – with targeted herbicide. SWCA Environmental Consultants completed one round of treatments in mid-September of this year after the 30-day permit wait period was over, with encouraging results.

Following many years of planning, permitting and working with stakeholders, we are excited to finally be able to move forward with on-site management to address the poor water quality issues in this pond. Now that the permitting component of this project has been completed, we can hit the ground running next year and apply treatments early in the season, as soon as any sign of algae is detected with a goal of preventing future toxic algae blooms in the pond. This project, which is the first of its kind on Nantucket,  will also serve to provide insight into the effectiveness of these management strategies and inform future pond management projects on the island.

The Nantucket Conservation Foundation is a private, non-profit land trust that depends contributions from our members to support our science projects, conservation property acquisitions and land management efforts. If you are not already a member, please join us!  www.nantucketconservation.org