Abstract: Variation in the Connecticut River Stage and Its Relationship to Contaminant Migration in an Underlying Bedrock Aquifer

Donald M. Maynard, Alan R. Liptak, Eric R. Hanson

Hydrogeologic and chemical tests of a contaminated bedrock aquifer indicate that a river is not a barrier to subsurface contaminant migration.

Three bedrock water supply wells located in the Connecticut River Valley in Vermont contain dissolved trichloroethene, a dense, chlorinated solvent. The overlying gravel aquifer west of the Connecticut River is not measurably contaminated. Both the surficial and the bedrock aquifers in New Hampshire, east of the river, are contaminated with trichloroethene.

An aquifer pumping test demonstrated a hydraulic connection between all three contaminated wells. Maps of bedrock fractures and bedding revealed east-west trending preferential flow pathways. Calculations based on bedrock aquifer hydraulic head maps predicted flow directions and gradients along the pathways.

Bedrock aquifer water levels collected over three weeks characterize two flow regimes; a valley regime, and an upland regime. Valley regime flow is influenced by river stage and changes daily. Upland regime flow is primarily controlled by seasonal recharge. The river alters from a "gaining" to a "losing" stream on a seasonal basis. This causes shifts in the position of the hydrologic boundaries and the groundwater divide.

The hydrogeologic data and seasonal changes in bedrock aquifer water quality indicate that the contaminated wells are in the valley flow regime and that the contamination source is to the East, beneath or beyond the Connecticut River.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York