Science for the Stewardship of Cape Cod's Groundwater

A USGS fact sheet from 2014; studies described as current were under way then.

Groundwater is Cape Cod's only source of drinking water and a major source of fresh water for homes, industry and farms. Water flowing out of its aquifers also feeds freshwater ponds, streams and coastal wetlands.

Cape Cod has six separate groundwater-flow systems, or lenses. Of the roughly 450 million gallons a day that rain adds to them, about 69 percent flows directly to the coast, about 24 percent to streams, and almost 7 percent is pumped by public-supply wells. In most places the water in the sand-and-gravel aquifers is shallow and easily contaminated by human activity and by salt water. Continued building and population growth have raised fears that drinking water will become scarcer and that the water reaching ponds, streams and the coast will keep getting worse.

The USGS has studied Cape Cod's ground and surface water for more than 50 years.

Simulated water-table contours for 2000–2005. The water table forms a series of mounds, about 6 to 70 feet above sea level, and groundwater flows outward from each mound toward the coast.

Where the wells' water comes from

Cape Cod's year-round population more than doubled between 1970 and 2010, straining water supplies and raising worries about water quality. Managing the water starts with knowing where it comes from. In an unconfined aquifer — one whose upper boundary is the water table — each well, stream, pond or stretch of coast draws from its own recharge area at the water table. Pumping and recharge shape those areas; change them, and flow directions, recharge areas, travel times and the quality of the water arriving can all change.

Simulated recharge areas for 2000–2005. Their size and shape change as pumping and recharge change.

USGS studies have mapped recharge areas for public-supply wells and natural discharge areas, and measured how pumping affects groundwater and pond levels, streamflow and discharge to the coast. Most built regional groundwater-flow models for current and possible future pumping. They show that small shifts in hydraulic gradients, from changes in pumping or recharge, can substantially change flow directions and so the sources of water to wells and natural discharge areas. Water managers, state and local regulators and others use the models to weigh changes in water supply and wastewater disposal.

Local models look closely at particular wells, ponds and contaminant plumes, many near Joint Base Cape Cod (formerly the Massachusetts Military Reservation), in support of the Department of Defense's environmental cleanup. In 2014 the USGS was building an updated regional model of western Cape Cod that includes the base's contaminant plumes, which could show how cleaning them up affects the areas feeding neighbouring towns' wells.

A simulated groundwater flow path from a known soil contamination area on Joint Base Cape Cod toward the Cape Cod Canal. The coloured bands show confidence intervals.

Nitrogen from wastewater

Much of the densest development is near the coast, putting coastal waters at risk of eutrophication. Nitrogen is the nutrient of greatest concern: too much of it degrades water quality and clarity, makes harmful algal blooms more frequent and destroys marine habitat. Excess nitrogen in groundwater can also harm drinking water.

Algal growth in Nauset Bay, Cape Cod National Seashore, Eastham. Septic systems are a major source of the excess nitrogen in groundwater flowing into the bay. Photograph by John Colman, USGS, 2005.

Wastewater is the largest nitrogen source in developed areas. Many densely built towns rely on septic systems, and some are weighing sewers and central treatment. Regional USGS models of central and western Cape Cod have traced travel times from wastewater sources to wells, streams, ponds and coastal waters, estimated how much reaches them, and tested disposal scenarios. They generally show that good wastewater planning can cut nitrogen reaching sensitive waters — but because groundwater can take more than 100 years to travel from where it enters to where it discharges, results can be a long time coming.

Local studies follow nitrogen to individual salt marshes and estuaries, including protected areas of Cape Cod National Seashore, and examine the chemical reactions that raise or lower nitrogen along the way. In 2014 the USGS and the EPA were jointly studying septic nitrogen reaching several saltwater embayments in Falmouth.

Climate change

Cape Cod's long coastline and fresh groundwater sitting on salt water make it especially exposed. Estimates put sea level about 1 to 6 feet higher by 2100, and northeastern precipitation up by as much as 20 percent. Much attention goes to shoreline erosion, storm flooding and inundation; less to what changing sea level and recharge will do to coastal aquifers, including well inland:

• saltwater intrusion that could ruin drinking-water supplies;
• a higher water table flooding infrastructure from below, septic systems included;
• harm to freshwater habitats in low-lying areas.

On outer Cape Cod, fresh groundwater floats on saline groundwater; closer to the mainland it reaches down to bedrock and the freshwater-saltwater boundary lies near the coast.

Preliminary regional modelling indicates that rising seas will raise water tables inland, increase groundwater discharge to streams and push salt water into coastal aquifers. Better climate projections will sharpen these assessments, which matter for planning adaptation to sea-level rise.

A field laboratory: the wastewater plume

The USGS Cape Cod Toxic Substances Hydrology Research Site lies by the former wastewater-treatment plant on Joint Base Cape Cod, whose infiltration beds were used from 1936 through 1995. The plume of treated wastewater extends nearly 4.5 miles through the sand-and-gravel aquifer, a complex mix of phosphate, nitrate, metal ions, detergents and other organic compounds — an ideal natural laboratory for studying how contaminants move underground.

The nitrate plume from 60 years of treated-wastewater disposal, shown 55 feet below sea level: about 4.5 miles long and near coastal discharge areas in 2007.

More than 30 years of research there by the USGS and others is used widely by managers, regulators, educators and scientists, and its methods have been applied elsewhere on Cape Cod: other plumes on the base, pharmaceuticals and personal-care products in groundwater, and nutrient-rich groundwater flowing into ponds and coastal waters.

A USGS team samples nitrogen in shallow groundwater discharging to Eel Pond, Falmouth. Photograph by Denis LeBlanc, USGS, 2013.

Fifty years of water levels

Long-term water-level records show how the system behaves and how groundwater and ponds change over time. Where pumping and wastewater do not interfere, they can show whether groundwater is responding to climate; they give the lows and highs to expect in droughts and wet spells, which has helped in designing septic systems; and they are essential for calibrating the flow models used to predict responses to changes in land use and climate.

Water table at USGS observation well MA–PZW 78, Provincetown, 4,300 feet from Cape Cod Bay. Levels have risen about 0.15 inch a year on average since 1975, possibly because of sea-level rise.

Since the mid-1960s the USGS, the Cape Cod Commission, the Association to Preserve Cape Cod, the Massachusetts Departments of Environmental Protection and of Conservation and Recreation, and the National Park Service have measured water in wells and ponds. About 13 wells in the mid-1960s grew by the mid-1970s with 12 ponds read by the Association's volunteers (to about 2000) and 40 wells monitored by the Commission (ongoing); in the early 2000s the USGS added 5 wells and 1 pond with support from the base's cleanup agencies. Levels are measured from monthly to continuously and published on Groundwater Watch.

Sources

• Barbaro, J.R., Masterson, J.P., and LeBlanc, D.R., 2014, Science for the stewardship of the groundwater resources of Cape Cod, Massachusetts: U.S. Geological Survey Fact Sheet 2014–3067. https://pubs.usgs.gov/publication/fs20143067 — the figures and USGS photographs come from its PDF; an aerial photograph used by permission is left out. Funding came from the National Park Service, the U.S. Air Force and Army National Guard, the Massachusetts Departments of Environmental Protection and of Conservation and Recreation, and the Cape Cod Commission.
• Masterson, J.P., and Walter, D.A., 2009, Hydrogeology and groundwater resources of the coastal aquifers of southeastern Massachusetts: U.S. Geological Survey Circular 1338.
• Rewritten in hubnx's own words.