What's in Cambridge's Drinking-Water Source? Road Salt, Nutrients, Pesticides, Caffeine

The Cambridge drinking-water source area: the Hobbs Brook (Cambridge Reservoir) and Stony Brook watersheds, and the aqueduct that carries their water to Fresh Pond. USGS.

The study

From 2005 to 2008, the U.S. Geological Survey (USGS) and the Cambridge Water Department sampled 11 tributaries that feed Cambridge's drinking-water reservoirs, in dry weather (base flow, which is mostly groundwater) and in storms. They measured:

• sodium and chloride
• nutrients (nitrogen and phosphorus)
• commonly used pesticides
• caffeine

The goals: describe water quality now, set a baseline for the future, track trends, and see how the watersheds' land use shapes the water — to guide how the source area is managed.

The partnership dates from 1997. It began with a study of the three reservoirs, went on to a monitoring network on the streams that feed them, and by 2015 ran water-quality stations on every major tributary and all three reservoirs.

Cambridge's water supply

• The department delivers about 15 million gallons a day to more than 100,000 customers.
• Water comes from three reservoirs in series: the Cambridge (Hobbs Brook) Reservoir, the Stony Brook Reservoir and Fresh Pond, spread over parts of Cambridge, Lexington, Lincoln, Waltham and Weston.
• They drain a source area of 23.7 square miles, crossed by Interstate 95 and Routes 2, 2A, 20 and 117, with large paved industrial, commercial and residential areas. Cambridge owns only about 5% of this land.

Water leaves the Cambridge Reservoir into Hobbs Brook, which joins Stony Brook about 1.6 miles downstream; Stony Brook flows about a mile more into the Stony Brook Reservoir. Storm drains from roads and parking lots also empty into both reservoirs. From Stony Brook Reservoir an aqueduct carries water to Fresh Pond, where it is stored before treatment; overflow goes to the Charles River in Waltham.

Findings

Road salt: highest where there is the most pavement

Sodium and chloride are a particular worry because ordinary treatment does not remove them. Chloride affects the taste of drinking water; sodium contributes to high blood pressure in susceptible people. Both occur naturally, but here the big source is road salt and other deicers, which dissolve and reach streams through runoff and groundwater.

• Concentrations varied widely, most in small, developed subbasins.
• They rose in summer, when flow is low, and fell in higher flows — a sign that groundwater carries much of the salt.
• In base flow, median chloride in five tributaries along Interstate 95 exceeded the U.S. Environmental Protection Agency's (EPA) 250 mg/L secondary standard.
• Sodium throughout the source area, and even at the raw-water intake at Fresh Pond, routinely exceeded the Massachusetts 20 mg/L drinking-water guideline.
• Concentrations rose with road density.
• Each year about 2,600 metric tons of sodium and 4,800 metric tons of chloride reached the Stony Brook Reservoir. In a typical year about half of that water — and so half the salt — goes on to Fresh Pond to become drinking water.

Median base-flow chloride by tributary subbasin, 2005–8: the highest concentrations lie along Interstate 95. USGS.

Nutrients: above reference levels

In these reservoirs phosphorus is the nutrient that limits plant growth, so more of it could cause eutrophication — too much algae and plant growth, leading to low oxygen, metals released from sediment and harm to aquatic life.

• Phosphorus comes from rocks and soils, fertilizers, septic systems and sewers — and from eroding soil and winter road sand washed off pavement.
• Nitrogen comes from rain, car exhaust, decaying organic matter, fertilizers and wastewater.

Nearly every stormflow sample exceeded the phosphorus reference; base-flow samples were near or below it. Phosphorus levels were similar across the four tributaries despite their different land uses.

Total phosphorus in base flow and stormflow at five USGS stations, 2005–8. USGS.

Pesticides: many, but at trace levels

• 25 pesticides were found in the source area, and 6 in raw water at the Fresh Pond intake. Most are in over-the-counter lawn and shrub products.
• Only nine turned up in 15% or more of samples. The insecticide imidacloprid and the herbicides norflurazon and siduron were in about 24–41% of samples.
• Concentrations were typically under 1 microgram per liter — below EPA drinking-water guidelines and other health standards, and far below levels lethal to local fish. The exception: some imidacloprid samples exceeded long-term guidelines for aquatic life.
• At station 01104475, which drains a residential area beside the Stony Brook Reservoir, imidacloprid was found in 94% of samples.

Pesticide concentrations, water years 2005–8, against the level that kills half of bluegill sunfish in 96 hours, EPA drinking-water limits and a Canadian long-term aquatic-life guideline. USGS, after Smith (2013).

Caffeine

Caffeine usually signals wastewater, though spilled drinks and litter can add it too.

• It was found in about 64% of 171 samples, at 0.003–1.82 micrograms per liter, more often in storm samples.
• Base-flow caffeine tracked parking-lot area.
• The yearly caffeine load reaching the Stony Brook Reservoir equaled about 6,500 twelve-ounce cups of coffee (assuming about 346 mg of caffeine per liter of coffee).

Measuring discharge at USGS streamgage 01104415 near Lexington. USGS.

Managing the source area

Water quality here is shaped by natural factors, such as drought and storms, and human ones, such as paved area and pesticide use. With so many land uses, many different substances could reach the water supply. Low-salt zones and structural best management practices could cut what gets in.

Continuous monitoring is what makes management possible: as of 2015, the USGS and the Cambridge Water Department ran 11 stations measuring both the quantity and quality of water. Without accurate information on past and present conditions, management strategies can be neither carried out nor judged.

The weir at USGS streamgage 01104475 near Weston. USGS.

The outlet pond below the Stony Brook Reservoir, at USGS streamgage 01104460 near Waltham. USGS.

Sources

Based on Kirk P. Smith and Marcus C. Waldron, "Water quality in the Cambridge, Massachusetts, drinking-water source area, 2005–8," U.S. Geological Survey Fact Sheet 2015–3030 (2015), prepared with the Cambridge Water Department; a work of the United States government in the public domain. The maps, charts and photographs are taken from the fact sheet's PDF.