DECREASES IN GROUNDWATER STORAGE: THE PROBLEM OF CONFINING LAYERS

Extensive exploitation of groundwater in sedimentary terrains began well over a century ago and the pace of extraction has steadily accelerated since then. Although attention has focused on the resulting water level decreases and reductions in groundwater storage in aquifers, a significant volume of water may derive from clay-rich confining layers. Shales comprise more than half of all sedimentary media and typically are much more storative than carbonate- or sand-rich aquifers. Thus, to get a truer picture of global changes in groundwater storage, one may need to account for contributions of confining layers. Here I discuss a first-order approach to estimating decreases in confining layer storage using aquifer drawdown history and confining layer lithology. In the past 20 years we have gained a reasonably good grasp of clay and shale hydraulic properties; hydraulic conductivity and specific storage can be estimated from lithology and degree of compaction or lithification. If aquifer drawdown history is also known, then penetration of “drawdown” into the confining layer, and the resulting volume of fluid released from storage, can be calculated using an analytical expression for the confining layer response to the head change in the aquifer. Application of this approach to the Dakota Aquifer over an area of 1.3 x 10⁶ km² in the Rocky Mountain foreland indicates that roughly 4 x 10¹⁰ m³ of water have been released from storage by confining shales, as compared to roughly 8 x 10⁸ m³ of water from the aquifer itself. In other words, changes in groundwater storage in the aquifer appear to be a minor portion of the total. Although there is significant uncertainty in this estimate, it does indicate that confining layer contributions probably cannot be ignored when estimating total changes in storage. In addition, although rates of drawdown in the Dakota Aquifer have slowed, suggesting that a new steady state is being approached, confining layer hydrodynamics shows that release of water from the confining layers will persist for many thousands of years. It is also worth noting that, in many cases, storage changes in confining layers cannot be reversed because compaction in clays and shales is largely unrecoverable. Thus, even if withdrawals are stopped and the prior steady state regained, there will still be a net decrease in storage.