In the main irrigation area of the Padthaway Proclaimed Wells Area, viticulture is the dominant form of agriculture, and has been since the early 1970s. Local groundwater, extracted from the underlying Quaternary unconfined aquifer, is used extensively for such irrigation. A high irrigation density and associated groundwater recycling has resulted in significant degradation in the quality of this groundwater. The groundwater flow in the unconfined aquifer is mainly through two Pleistocene geological formations; uppermost is the Padthaway Formation and underlying it, the Bridgewater Formation. In some places the two aquifers are separated by the Keppoch Clay Member of the Padthaway Formation. If present, the clay can act as a confining unit, hydraulically separating the greater unconfined aquifer into two subaquifers. Investigation wells drilled near Padthaway in the early 1970s intersected the Keppoch Clay Member and indicated that the salinities of the groundwater in the two subaquifers were much the same. Since then there has been a significant long-term increase in the groundwater salinity in the Padthaway Formation upper subaquifer. It remained uncertain, however, if there had been a similar salinity increase in the underlying Bridgewater Formation subaquifer. If better quality water were to exist at depth, it could be used as an alternative source for groundwater extraction. The results of a study into the viability of this option form the basis for this report. To assess the areal extent of the Keppoch Clay Member, an initial study was conducted using available bore log information. This was followed by a drilling program to assess the hydrogeological relationship between the two subaquifers. Results of the groundwater quality investigation suggest that in the main irrigation area, the confining unit is absent, and the two geological formations are in good hydraulic continuity, with both subaquifers having similar groundwater salinities and hydrogeological characteristics. To the west of the main irrigation area, where the confining unit is present, groundwater quality in the lower Bridgewater Formation subaquifer is slightly better than in the Padthaway Formation subaquifer, but the permeability of the Bridgewater Formation is significantly lower than in the main irrigation area. It is therefore concluded that the Bridgewater Formation subaquifer is not a potential source of better quality groundwater for more salt-sensitive irrigated crops.