Water-Resources Investigations Report 01-4040
By PAUL J. FRIESZ and PETER E. CHURCH
A U.S. Army facility on a peninsula in South Pond of Lake Cochituate was designated a Superfund site by the U.S. Environmental Protection Agency in 1994 because contaminated ground water was detected at the facility, which is near the Natick Springvale public-supply wellfield. The interaction between South Pond and the underlying aquifer controls ground-water flow patterns near the pond and determines the source of water withdrawn from the wellfield.
A map of the bathymetry and the thickness of fine-grained pond-bottom sediments was prepared on the basis of fathometer, ground-penetrating radar, and continuous seismic-reflection surveys. The geophysical data indicate that the bottom sediments are fine grained toward the middle of the pond but are coarse grained in shoreline areas. Natick Springvale wellfield, which consists of three active public-supply wells adjacent to South Pond, is 2,200 feet downgradient from the boundary of the Army facility. That part of South Pond between the Natick Springvale wellfield and the Army facility is 18 feet deep with at least 14 feet of fine-grained sediment beneath the pond-bottom. Water levels from the pond and underlying sediments indicate a downward vertical gradient and the potential for infiltration of pond water near the wellfield. Head differences between the pond and the wellfield ranged from 1.66 to 4.41 feet during this study. The velocity of downward flow from South Pond into the pond-bottom sediments, determined on the basis of temperature profiles measured over a diurnal cycle at two locations near the wellfield, was 0.5 and 1.0 feet per day. These downward velocities resulted in vertical hydraulic conductivities of 1.1 and 2.9 feet per day for the pond-bottom sediments.
Naturally occurring stable isotopes of oxygen and hydrogen were used as tracers of pond water and ground water derived from recharge of precipitation, two potential sources of water to a well in a pond-aquifer setting. The isotopic composition of pond water varied seasonally and was distinctly different from the isotopic composition of ground water. The isotopic composition of shallow water beneath and adjacent to South Pond near the wellfield corresponds to the temporal variation of pond water, indicating that nearly all water at shallow depths was derived from pond water. A two-component mixing model based on the average stable isotope values of the source waters indicated that 64 ±15 percent at the 95-percent confidence interval of the water withdrawn at the public-supply wells was derived from the pond; pond water accounted for most of the uncertainty in the result. The rate of infiltration of pond water into the aquifer and discharging to the wellfield was 1.0 million gallons per day at the average pumping rate.
Surface Geophysical Techniques
Bathymetry and Morphometry
Thickness of Fine-Grained Bottom Sediments
Natick Springvale Wellfield
Geohydrology of Wellfield Area
Temperature Profiles and Stable Isotope Techniques
Vertical Ground-Water Flow and Vertical Hydraulic Conductivity of Pond-Bottom Sediments
Isotopic Composition of Pond Water and Ground Water
Characterizing Interaction of Pond Water and Ground Water
Summary and Conclusions
Appendix A. Pond stage and water levels in piezometers and observation wells, South Pond of Lake Cochituate, Natick, Massachusetts
Appendix B. Water temperature in South Pond and in piezometers NCW83 and NCW85, September and October 1998, Lake Cochituate, Natick, Massachusetts
Appendix C. Temperature, specific conductance, delta deuterium, and delta oxygen-18 in water, February 1998 to July 1999, South Pond of Lake Cochituate, Natick, Massachusetts
(in back pocket)
1. Map showing bathymetry and thickness of fine-grained bottom sediments, South Pond of Lake Cochituate, Natick, Massachusetts.
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