IMPROVED UNDERSTANDING OF POND-AQUIFER INTERACTION ON THE BASIS OF STABLE ISOTOPES OF WATER

Stable isotopes of oxygen and hydrogen were used to improve the understanding of pond-aquifer interaction at South Pond of Lake Cochituate and Walden Pond, kettle-hole lakes in urbanized settings in eastern Massachusetts. Surface water dominates the inflow and outflow of South Pond, which has a relatively short water-residence time of 7 months. In contrast, Walden Pond has no surface-water inlet or outlet and a relatively long water-residence time of 5 years.

A public-supply wellfield, consisting of three active wells, is adjacent to South Pond. Water levels from South Pond and the underlying aquifer indicate flow was from the direction of the pond to the wellfield. The isotopic composition of pond water varied seasonally and was distinctly different from the isotopic composition of ground water upgradient of the pond. 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mixing model based on the average stable isotope values of pond water and ground water indicated that 64% of the water withdrawn at the wellfield was derived from the pond, +/-15% at the 95% confidence interval; pond water accounted for most of the uncertainty in the result. The rate of pond water infiltrating into the aquifer and discharging to the wellfield was 1.5 ft3/s at the average pumping rate of 2.4 ft3/s.

An isotope mass-balance method was used to quantify ground-water inflow as part of a water and nutrient budget investigation of Walden Pond. The isotopic composition of pond water varied little seasonally and was distinctly different from the isotopic composition of ground water. The pond isotopic values plot on a graph of hydrogen against oxygen isotopes to the right of the local meteoric water line because evaporation from the pond cause an increased enrichment of the oxygen isotopes relative to the hydrogen isotopes in the pond water. An average ground-water inflow of 0.44 ft3/s was estimated based on the isotope mass-balance method. This estimate compares favorably to that resulting from a method that used the product of the rate of precipitation recharge and the size of the contributing area of the pond.