Groundwater depletion from confining layers

Depletion of storage in low-permeability confining layers is frequently overlooked or ignored in assessing regional groundwater budgets. This makes effective management of groundwater resources difficult by masking how much water has been derived from storage and making predictions of system behavior inaccurate. Analyzing changes in confining layer storage is viewed as troublesome because of the additional computational burden and because the hydraulic properties of confining layers are poorly known.
A new simplified method for computing estimates of confining layer depletion, as well as procedures for approximating confining layer hydraulic conductivity (K) and specific storage (Ss) using geologic information, offers an approach where minimal data are available or when scoping calculations are needed. The technique is demonstrated by application to two aquifer systems-the Dakota artesian aquifer system in the semi-arid climate of South Dakota, USA, and the coastal plain aquifer system in the humid climate of Virginia, USA. In both cases, depletion from confining layers was substantially larger than depletion from the aquifers.
For example, about 20 km3 of water was discharged from wells in the Dakota in the 100 years since development started. Approximately 2 percent of total withdrawals was derived from depletion within the sandstone aquifer itself, whereas 76 percent or more of the withdrawals was derived from depletion in shale confining units. In the Virginia Coastal Plain aquifer system, about 4.5 km3 of groundwater was withdrawn from wells between 1891 and 1980. Approximately 4 percent of total withdrawals were derived from depletion within the sand and gravel aquifers, whereas about 78 percent of the withdrawals were derived from depletion in unconsolidated clay and silt confining units.
The main source of uncertainty in these estimates is related to uncertainty in the value of the specific storage of the low-permeability materials (which often might be known only within an order of magnitude). We believe that these two examples are representative of many other similar systems and conclude that depletion of storage in low-permeability confining layers is the source of much of the groundwater produced from many confined aquifer systems. Over time scales of decades, groundwater depletion implies the ultimate transfer of large volumes of water from the continents to the oceans. Worldwide, the cumulative magnitude of groundwater depletion from storage in the subsurface may already be large enough to constitute a small but measurable contribution to sea-level rise during the 20th century.