Hydraulic properties of ignimbrites: matrix and fracture permeabilities in two pyroclastic flow deposits from Cimino-Vico volcanoes (Italy)

Abstract

Hydrological properties of ignimbrites are known in detail only for some areas, although these rocks cover large areas with considerable thicknesses in many volcanic regions of the world. This study investigates hydrological properties of two ignimbrites of Latium (Central Italy), different in age, composition, and origin. The dual porosity of the ignimbrites was examined through laboratory tests, pumping tests, and outcrop surveys. The degree of welding, composition, and stratigraphy of the two ignimbrites are the main factors determining their hydrological properties. The two ignimbrites share a low matrix permeability, showing a different fracturing degree. The more welded ignimbrite is characterized by lower porosity of the matrix and higher fracturing degree, while the less welded ignimbrite is characterized by higher porosity of the matrix and lower fracturing degree. Hydraulic conductivity and storage capacity of the highly welded ignimbrite mainly depend on the denser network of discontinuities. The hydraulic conductivity of the younger ignimbrite, less welded, mainly depend on the sparser network of discontinuities and on the layer of unconsolidated coarse pyroclastic deposits at its base, while the storage capacity depends on the more porous matrix. Should the pyroclastic rocks be used as aquifer for water supplies, or, in other cases, as substratum of waste disposal sites, the dual porosity of the ignimbrites must necessarily include different scales of analysis in order to evaluate the role of matrix and fractures on the permeability of rock mass. In any case, the results of laboratory and on-site tests are to be interpreted taking into account the stratigraphy of the ignimbrite.

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