GLACIGENIC SYSTEMS: IMPLICATIONS FOR SUBSURFACE CONTAMINANT MIGRATION AND WATER QUALITY

In North America, glaciation resulted in the formation of valley-fill deposits now serving as water supply aquifers for many areas of Canada and the northern United States. Such glacigenic aquifers are system-deposits constructed of stratified sediments associated with erosion and deposition during glacier advance and retreat. Most often, retreat sequence deposits are well-preserved, consisting of till overlain by ice-contact sands and gravels. They demonstrate an overall fining-upward of grain size, occasionally contain an upper unit of glacimarine or glacilacustrine silt or clay, and are frequently capped by postglacial fluvial and colluvial deposits. Conversely, a typical glacigenic advance sequence would demonstrate similar units, but in reverse order with an overall coarsening-upward of sediments to a layer of till. All units are normally discontinuous and heterogeneous when examined in three dimensions. Except for deposition in deep basins, the units expected beneath till are often missing from a sequence due to erosion from continued glacial advance. These sequences, lying between bedrock and till or till and overlain fine-grained sediments, are frequently interpreted incorrectly as confined aquifers.

Case studies from New Brunswick, Canada, demonstrate the consequences to environmental reclamation associated with acid mine waters, and to the contamination of water-supply aquifers by farm wastes and industrial chemicals. In these examples, aquifer leakage and contaminant dispersal can be attributed to one or more of three migration pathways that are common to glacigenic aquifers: (1) fracture openings from glacitectonic deformation and reactivated bedrock structures commonly underlying till, (2) discontinuous and heterogeneous sediment layers, and (3) the scouring of impermeable units along channel bottoms by deglacial or postglacial fluvial erosion, thus forming pathways between units. In most cases contaminant migration is also related to anthropogenic factors, including the volume of contaminant released, subsurface residence time and the rate of groundwater extraction.