2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 9
Presentation Time: 10:15 AM


HON, Rudolph1, DILLON, Peter1 and MERRITT, Neal2, (1)Department of Geology & Geophysics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467, (2)Norwell Water Department, Town of Norwell, 345 Main Street, Norwell, MA 02061, hon@bc.edu

Two adjacent catchment basins located in the coastal drainage system of southeastern Massachusetts represent semi-urban settings typical of the NE USA serviced by public water supplies with no municipal sewer systems. The basins drain approximately 3.63 and 3.35 square miles of forested land with numerous wetlands developed over shallow postglacial depressions surrounded by glacial till deposits. One catchment, Wildcat Brook, lies within the Town of Norwell where population and housing densities are 181/sq.km and 61/sq.km respectively. The Third Herring Brook catchment straddles the line between Towns of Norwell and Hanover (population density 326/sq.km, housing density 110/sq.km).

Both towns apply de-icing chemicals on the road surfaces during the winter seasons. Significant portions of the resulting briny solutions are mixed with precipitation that recharges ground water in the catchment basins. Our study includes data from a decade and half of monitoring of water quality from public water supply wells and from water quality monitoring at a number of surface water sampling stations. Data from a well located near the stream yields significantly higher sodium and chloride concentrations (up to 220 ppm of NaCl) than water in the well located further away (up to 120 ppm of NaCl) suggesting that infiltration from the streams is a significant contributor to the aquifer water budget.

Surface water geochemistry corroborates the NaCl values of ground water. Two observations about geochemical characteristics of the surface waters are relevant. (1) Ca, Mg, and Na in surface waters are highly correlated over a wide range (factor 4) of absolute concentrations suggesting a codependence of Ca and Mg on Na concentrations -- dissolved NaCl. Analyzed samples of collected de-icing chemicals confirmed stoichiometrically balanced NaCl. (2) A more significant observation in surface waters relates to a deficiency of Na needed to compensate Cl to match the stoichiometric ratio of NaCl. The unbalanced Cl over Na appears to be compensated by 10 to 20 molar % of Ca, Mg, and K from cation exchange equilibria between NaCl briny solutions from the road runoff and the soils of the catchment basins. Forced leaching of nutrients by NaCl concentrated road runoff and consequently their loss by stream drainage could lead to undesirable ecological changes.