How microgravity can be used with a pumping test for the identification of horizontal anisotropy in unconfined aquifers.
8ª Asemblea Hispano Portuguesa de Geodesia y Geofisica, Évora, Portugal, 2014
How microgravity can be used with a pumping test for the identification of horizontal anisotropy in unconfined aquifers
Estimation of hydraulic parameters of an aquifer is usually performed via pumping test interpretation. This invasive method requires drilling pumping and observation wells. As the process is expensive, only a pumping well and one or two observation wells or piezometers are drilled. The interpretation is usually done assuming aquifer isotropy and homogeneity. In many aquifers, however, horizontal anisotropy in hydraulic conductivity greatly affects the flow regime and its disregard may lead to important miss interpretations, especially for environmental impact assessments. When a pumping test is performed in an anisotropic aquifer, the drawdown cone shape differs from the radial symmetric pattern characteristic of isotropic conditions and the conventional configuration with one or two observation points is unable to capture it. Since the 1990’s there has been a rapid growth in the use of geophysics to achieve a better hydrogeological interpretation. Gravity is a geophysical technique sensitive to underground mass variations. The new gravimeters allow monitoring the changes caused by water being pumped from unconfined aquifers. This paper studies, based on numerical simulations performed in synthetic models of pumping tests in media with different hydraulic conductivity anisotropy ratios, the capabilities of gravimetry in the identification of anisotropy. The procedure has been innovatively integrated in a unique finite element simulation code. The results show how a microgravity survey could be an adequate and relatively cheap monitoring tool for the identification of anisotropy. This is valuable information that can be used in the decision making process for performing or discarding additional studies.