Nitrogen Island
The invisible ecological pressure underneath Nantucket
Burton Spruce Balkind
Miacomet Pond stretches inland from Nantucket’s south shore, where decades of accumulated nutrient loading, groundwater flow and surrounding development now converge in one of the island’s most closely watched ecological systems. Large-scale dredging operations are planned to begin here this fall as part of a long-term effort to reduce internal nitrogen buildup and stabilize water quality.
Don Costanzo • May 10, 2026
Nantucket’s environmental future is often discussed from the shoreline, where erosion, storms and rising seas are visible. But one of the island’s most significant ecological pressures moves underground, through sandy soil and groundwater, from septic systems, lawns, roads and developed land toward ponds and harbors already struggling to absorb the load.
Nitrogen now connects many of the island’s most contentious environmental and political debates. It links sewer expansion, fertilizer restrictions, pond dredging, eelgrass decline, shellfish habitat, development pressure and Town Meeting debt into one environmental system. It helps explain why some neighborhoods are being pushed toward sewer connections, why pond restoration has become increasingly expensive and why landscaping practices are no longer treated as purely private decisions.
The clearest current example is Article 15, the approved Somerset sewer expansion. The project will extend sewer infrastructure through part of the Hummock and Miacomet watershed area, including nearly three miles of gravity sewer infrastructure, thousands more feet of low-pressure sewer and a new pump station. The project carries a price tag of roughly $44.8 million, reflecting how expensive nitrogen correction becomes once ecological decline reaches infrastructure scale. Town officials frame the project as environmental protection, public health and infrastructure modernization, because sewer systems reduce nitrogen flowing from septic systems into groundwater before it reaches sensitive ponds and coastal waters.
That environmental argument is legitimate, but it also carries a second consequence that has become increasingly difficult for Nantucket to ignore. Sewer removes some of the physical limitations septic systems once imposed on development, allowing more bedrooms, more year-round intensity, more pools and more groundcover in areas previously constrained by wastewater capacity. The environmental fix can therefore become a growth mechanism unless the island decides exactly what problem it's trying to solve and what kind of development should follow.
Most of Nantucket’s nitrogen problem originates on land before eventually appearing in water. Wastewater leaves homes through septic systems and enters the ground. Fertilizer applied to lawns and landscapes washes into soil during rain events or irrigation. Stormwater carries nutrients from roads and developed property. On a mainland watershed, those nutrients may disperse across large river systems. Nantucket functions differently because the island is porous, groundwater-dependent and surrounded by shallow ponds and embayments with limited ability to absorb excess nutrients.
What enters the ground rarely stays there for long. Nitrogen moves through sandy soils and groundwater toward the island’s ponds, marshes and harbors, where the ecological consequences eventually become visible through algae blooms, declining water clarity, low dissolved oxygen and habitat loss. Because groundwater can move slowly through Nantucket’s aquifer over many years, some of today’s water-quality problems may partly reflect development patterns and land-use decisions made decades earlier. The town already regulates fertilizer because it formally recognizes that lawns and landscaping are part of the island’s water system.
Nantucket’s fertilizer regulations prohibit application within 100 feet of many wetlands and water bodies, limits application rates, and prohibits fertilizer use during part of the year because the island has already acknowledged that private landscaping decisions can become public environmental problems downstream.
Burton Spruce Balkind
Hummock Pond runs through one of Nantucket’s most sensitive watersheds, where development, groundwater, irrigated landscapes and nitrogen runoff now converge in a shallow ecosystem with limited capacity to absorb additional environmental pressure.
The Ponds
The Town’s 2024 Island-Wide Water Quality Report reflects how uneven conditions have become across the island. Nantucket Harbor and Madaket Harbor are showing signs of improvement and are approaching some restoration targets, while Hummock Pond and Miacomet Pond remain significantly impaired. Those ponds now function as visible indicators of what decades of cumulative nitrogen loading can do to a shallow coastal ecosystem. On an island with a year-round population of roughly 14,000 that can swell several times over during the summer season, cumulative environmental pressure compounds quickly.
Miacomet Pond may be the clearest example because it combines water quality decline, public expense and environmental management in one place. Town reports have described the pond as highly eutrophic, meaning nutrient-rich enough to fuel recurring ecological imbalance. In some recent monitoring years, nitrogen concentrations in Miacomet Pond exceeded 0.9 milligrams per liter, roughly double the range generally associated with healthy estuarine systems. Poor water quality and habitat impairment have become recurring conditions rather than isolated events.
Once a pond reaches that stage, the problem becomes far more difficult and expensive to reverse. Nutrients already trapped in sediments can continue feeding algae growth even after outside sources are reduced. Hummock Pond studies have identified internal nutrient release from bottom sediments as an ongoing contributor to water quality decline, meaning the pond itself can become part of the cycle.
That reality helps explain why Nantucket increasingly finds itself paying for dredging projects, monitoring programs, engineering studies and watershed management plans that would have been unnecessary decades ago. Miacomet Pond is now moving toward one of the largest ecological interventions in recent island history. After years of permitting and environmental review, the Town has formally put the project out to bid for dredging scheduled to begin this fall. Current plans involve removing roughly 80,000 cubic yards of nutrient-laden sediment from the pond bottom, with some scenarios potentially exceeding 100,000 cubic yards depending on final engineering conditions.
The dredging project has already received a Conservation Commission Order of Conditions, a MEPA Environmental Impact Report Certificate, a 401 Water Quality Certification from MassDEP and a Chapter 91 Waterways Permit, reflecting years of state and local review. The work is prohibited between April 1 and August 31 because of protections for nesting shorebirds and sensitive seasonal habitat around the pond, effectively limiting construction to the fall and winter months.
That timing has introduced another layer of ecological complexity because some species that overwinter in pond sediments, including turtles, amphibians and benthic organisms, may now overlap with the construction window itself. The project includes broad environmental safeguards and mitigation requirements, but the dredging effort also reflects the increasingly difficult tradeoffs Nantucket faces as it attempts to restore ecosystems already altered by decades of accumulated nutrient pressure.
The island has entered the expensive phase of environmental correction. The cheaper phase involved allowing septic systems, landscaping and development to expand while assuming the ponds and harbors could absorb the pressure indefinitely. The current phase involves sewer expansion, fertilizer enforcement, stormwater infrastructure, pond openings, dredging and long-term environmental monitoring, all while residents debate how much development the island can continue supporting without overwhelming the systems being restored.
Divers and marine researchers work to restore eelgrass in Nantucket Harbor, where decades of nitrogen loading, declining water clarity and physical disturbance have contributed to major habitat loss. Recent harbor studies show eelgrass coverage has fallen sharply over the past several decades, transforming a once expansive underwater ecosystem into one increasingly dependent on active restoration and management.
The Harbor
The harbor systems reveal another side of the nitrogen problem because the impacts extend beyond water clarity into habitat itself. Eelgrass beds, once common throughout many coastal areas, are especially sensitive to nitrogen pollution because excess nutrients fuel algae growth that blocks sunlight from reaching underwater vegetation. Eelgrass is not simply decorative marine plant life. It stabilizes sediment, improves water quality and provides habitat for juvenile marine species that support the broader harbor ecosystem.
State and local environmental reports repeatedly connect nitrogen loading to eelgrass decline, nuisance algae and degraded benthic habitat in Nantucket’s embayments. Madaket Harbor and Long Pond studies identify septic systems, fertilizer use, runoff and groundwater contamination among the major controllable nitrogen sources contributing to ecological stress. When nitrogen levels rise high enough, the harbor changes physically because algae growth increases, dissolved oxygen declines and the underwater habitat supporting shellfish and marine life becomes less stable.
Recent harbor studies show how much broader the problem has become beyond nitrogen alone. Nantucket’s first Eelgrass Management Plan, finalized in 2025, documents significant long-term decline in eelgrass coverage throughout Nantucket Harbor, particularly in areas with poorer circulation and heavier human use. Harbor monitoring presented in 2025 estimated eelgrass coverage had fallen to roughly 14 percent, even as the rate of decline has slowed in recent years.
Researchers and local officials now identify nutrient pollution, warming waters, increased turbidity and direct physical disturbance from traditional moorings among the major pressures affecting the harbor ecosystem. In response, the Town and Nantucket Land & Water Council have begun exploring eelgrass-friendly mooring systems designed to reduce chain scarring across the harbor bottom, a sign that Nantucket is increasingly moving from passive conservation toward active ecological intervention.
That’s where the nitrogen issue stops being abstract chemistry and becomes a direct environmental story about how Nantucket’s waters are changing. The decline of eelgrass, the deterioration of ponds and the growing need for engineered environmental management are all connected to the same invisible movement beneath the island’s surface.
The Correction Phase
Nitrogen pollution on Nantucket is rarely the result of a single catastrophic event. It's the slow accumulation of thousands of ordinary decisions involving wastewater, landscaping, fertilizer use, and development density. In an island of shallow watersheds and growing development pressure, the environment has gradually lost its ability to absorb that load.
Today, Nantucket is in an expensive correction phase. Through sewer expansions, fertilizer restrictions, pond dredging, and restoration projects, the island is attempting to reverse decades of ecological decline. But those efforts also expose a growing tension: infrastructure designed to reduce nitrogen can simultaneously support additional growth and density.
The central question has shifted. It's no longer simply whether Nantucket can reduce environmental damage, but whether these interventions can meaningfully restore the island’s ecosystems, or merely slow further decline while development continues.
On an island this small, the line between private decisions and public consequences has become increasingly thin. What enters the ground in one backyard eventually reaches the harbor