THREE DECADES OF AQUITARD HYDROGEOLOGY: WHAT HAVE WE LEARNED?

Half to two-thirds of all sediments are low-permeability clays, shales, tills and similar media. Together with more permeable aquifer and reservoir units, these "tight" formations are responsible for the huge range in subsurface permeability (exceeding 17 orders of magnitude) and the distinctive permeability architecture of the crust that controls groundwater flow. Low-permeability units make regional flow possible, store and release immense amounts of water and dissolved chemical mass, mediate responses of groundwater systems to disturbance, protect aquifers from contamination, and preserve evidence of past geologic processes. Despite this, systematic investigation of low-permeability hydrogeology is relatively recent, limited to perhaps the last 30 or so years. What have we learned in that time? We have learned much but left some questions unanswered while raising new and difficult ones. We now know a great deal about measuring low permeability and how small clay and shale permeabilities can be, but the applicability of Darcy’s law is still uncertain in some cases and measurements of low-permeability are still troublesome and rare. It has become clear that “exotic” phenomena, such as water flow under chemical potential gradients, can be important; unfortunately, well-documented examples do not exist. Clear evidence has been uncovered that secondary permeability is important in some aquitards and not in others, but the factors governing this heterogeneity in permeability scale-dependence are not understood. Fluid pressure anomalies have been discovered in aquitards in many different environments and convincingly linked to causative processes, but low-permeability pressure measurements are still infrequent and plagued by errors, artifacts, and overinterpretation. There is now little doubt that aquitards give up prodigious quantities of water where aquifers are exploited, but attempts to quantify this storage loss are limited by uncertainties in aquitard properties. By any measure, progress in low-permeability hydrogeology has been impressive. Nevertheless, it clearly still poses fundamental questions that must be addressed to understand more fully many aspects of hydrogeologic systems.