Paper No. 11
Presentation Time: 9:00 AM-6:00 PM


BESANCON, James1, HON, Rudolph2 and ANDERSON, Jacob2, (1)Department of Geosciences, Wellesley College, 106 Central St, Wellesley, MA 02481, (2)Earth & Environmental Sciences, Boston College, 140 Commonwealth Ave, Chestnut Hill, MA 02467,

Many urban water supplies such as those in Wellesley, Natick, and Norwell, Massachusetts experienced recent increases in sodium and chloride levels. Sodium content in one was 35 μg/mL in 1988 and stands at about 95 μg/mL today, far exceeding the state secondary drinking water standard of 20 μg/mL. Chloride ion shows a similar increase, approaching ecological and human limits. Road salt deicers account for these changes. We delineate pathways for salt moving into drinking water.

Modflow models show the main groundwater sources for Wellesley College wells are glacial outwash and glacial lake aquifers in three basins found to the north of state highway 135. All are heavily developed suburban areas. Monitoring of these basins shows significant differences in sodium content.

Jennings Pond stream water shows the same summer sodium content as it did in 1985 at about 45 μg/mL, but jumps in the winter as the adjacent major highway is deiced. Boulder Brook is similar to Jennings, with sodium content about 45 μg/mL. Bogle Brook has significantly higher sodium (98 μg/mL) even in summer. The three streams supply the aquifer after merging in a pond, whose surface water at 70 μg/mL sodium is a weighted average of the three streams.

Street drainage enters the streams through storm sewers over the entire area. All three basins have closely comparable street density and flowing stream density (meters per square meter). Differences that might explain the sodium levels include size and heavily salted highways. The high sodium basin (Bogle) is larger than the others and contains 14,700 meters of state and interstate highway, while Jennings has 4200 meters and Boulder only 1200 meters. Bogle basin is dominated by glacial outwash and till deposits, while Jennings has a large glacial lake deposit of lower permeability and Boulder is characterized by hills with thin soil and bedrock exposures.

The Norwell aquifer is also characterized by glacial outwash, though by contrast to Wellesley and Natick aquifers it has much higher levels of dissolved de-icing chemicals. Commonly obtained values for chloride concentrations are in the range of 150 to 250 μg/mL, a likely consequence of stronger de-icing chemical contamination from nearby 2 state highways, impervious surfaces associated with a mall, and a salt storage facility