Co-existence of industrial organic contaminant plumes with municipal water supply wells in fractured rock aquifers

Abstract

Long-term studies combining research and conventional methods are being conducted at DNAPL sites where the organic contaminant plumes co-exist in the same fractured sedimentary rock aquifer with municipal water supply wells. Two of the research sites overlay the horizontally layered, Silurian dolostone aquifer. The Cambridge site has a metolachlor pesticide plume detected nearly 1 km down-gradient at a municipal supply well 20 years after spills occurred. The Guelph site plume is TCE. The plumes were characterized in detail using high-resolution methods for identifying hydraulically active fractures and variable matrix conditions. The 3-D plume concentration characteristics have been monitored for more than a decade using depth-discrete multilevel systems. The plumes have evolved to nearly stationary position (steady-state) after three decades due to the combined effects of diffusion and sorption in the matrix with dispersion in a dense network of well-connected fractures. Degradation occurs in each of the plumes but is a secondary factor in terms of current plume extents. These research studies show that high resolution monitoring can be used to inform site conceptual models and decision-making by municipalities and regulatory agencies to allow these plumes to coexist within or proximal to municipal or private well fields without imposing excessive or arbitrary demands for subsurface source removal (i.e. remediation) or well abandonment.