Comparison between the hydraulic properties of two low-permeability alluvial systems supported by geophysical methods

The mechanisms for vertical flow through clayey alluvial deposits are poorly understood. This type of deposits are usually studied as sets of separated permeable (aquifers) and less permeable (aquitards) strata assuming water flow is negligible over the short to medium term. Nevertheless, in some clayey deposits this simplification proves to be inadequate as there is evidence of vertical groundwater flow. This study was aimed at improving the understanding of hydrological mechanisms for vertical flow, determined by the degree of homogeneity or heterogeneity of the alluvial deposit. In order to study the vertical hydraulic connection through clay-silt dominated deposits, near surface and borehole geophysics have been combined with other hydrological techniques at two different sites with a contrasting degree of heterogeneity in the Liverpool Plains within the Namoi River catchment (NSW, Australia). At the Breeza DPI farm site, situated in an area of groundwater extraction for irrigation studies indicates active deep drainage through heterogeneous clayey strata and hydrochemical changes occurring in the underlying aquifer systems. By contrast, at the Cattle Lane site, located upstream in a dryland farming area with no groundwater extraction, there appears to be little significant vertical flow through a more homogeneous clayey deposit.