RESIDENTIAL DEVELOPMENT AND THE EFFECT ON SEDIMENTATION IN ECHO LAKE, CHARLES RIVER, EASTERN MASSACHUSETTS

Echo Lake is a drinking water reservoir for the city of Milford, Massachusetts. It has been a protected water supply since its creation in 1882. Its shoreline and a 500-meter wide buffer zone have remained largely unaltered since that time. The reservoir and its buffer zone occupy approximately 40% of its 3.9 km² watershed. Residential subdivision construction began in 1970 in the upper reaches of the watershed and by 1999 had altered 39% of the watershed. To determine the effect of construction on the water quality in the lake over that time, a 43-cm long sediment core was collected from Echo Lake and sectioned at 5-mm intervals. The date of deposition of each interval was determined using gamma spectroscopy and the radioisotopes ²¹⁰Pb, ²¹⁴Pb, and ¹³⁷Cs.

The radiometric dating of the sediments established the total flux of sediment and metals into the lake. It was found that despite the large buffer zone surrounding Echo Lake, residential subdivision construction significantly changed the amount of sediment deposited, the concentrations of several metals, and the physical character of the lake. Over the period 1970 to 1999 the average rate of sediment deposited in Echo Lake increased by more than four times the rate of deposition before 1970. The concentration of metals such as Pb, Hg, and Cd also increased due to aerial deposition on an expanding road network and possible entrainment in storm water. The most important finding by radiometric dating was that bottom rooted plants that had been present since the lake’s creation in 1882, were exterminated in 1984, perhaps due to an increase in sedimentation and turbidity. The extinction of bottom vegetation coincided with the largest input of sediment into the lake until that time. The change in the lake’s character in 1984 indicated pending problems in Echo Lake’s water quality that occurred several years later. Although Echo Lake was surrounded by an unaltered 500 meter buffer zone and the work was more than 1 km distant in most cases, the construction of roadways from 1970 to 1999 adversely impacted the drinking water supply.