Methods for determining the in situ hydraulic conductivity of shallow aquitards - an overview

Shallow clay-rich aquitards limit groundwater recharge to underlying aquifers, but they also protect the aquifers from contamination. The bulk hydraulic conductivity of such shallow aquitards can range from less than 1 mm/year to more than 100 m/year and may be much greater than the hydraulic conductivity of small intact samples of the aquitard material. This enhanced hydraulic conductivity diminishes the qualities of the aquitards for the protection of underlying aquifers but allows a higher rate of recharge. For aquifers that are overlain by aquitards, management and protection of groundwater resources may be critically dependent on reliable determinations of aquitard permeability. A variety of methods for determining bulk hydraulic conductivities of shallow clay aquitards is available; each has drawbacks and advantages, and each is based on simplifying assumptions. These methods include slug tests, pumping tests, response of the aquitard to mechanical loading, and analysis of natural pore-pressure fluctuations. Several of the commonly used methods require an independent measurement of specific storage. Laboratory methods for determining specific storage are probably not representative of in situ conditions and may lead to overestimation of aquitard permeability. Much of the theory developed to date depends on the assumption that horizontal displacement of the solid material is negligible, and this may not be a valid assumption for highly deformable media such as clay aquitards. However, with judicious selection of the most suitable methods for a particular site, good test design, careful instrumentation, and respect for the underlying assumptions, reliable determinations of aquitard permeability can be obtained.