Abstract of the proposal

Relative to aquifers, little research has been directed at low permeability units, known as aquitards, even though in many countries aquitards typically overly aquifers used for drinking water. Excluding shale aquitards, many important aquitards globally are formed of clayey Quaternary deposits. Although the international literature presents comprehensive studies of groundwater flow and hydrochemistry of several aquitards, little pertains to Quaternary aquitards of marine origin. This study is focused on the extensive clayey aquitard, generally 10-30m thick, in the Pearl River Delta (PRD). This aquitard overlies a regional sand aquifer (basal aquifer). The geology of this aquitard has previously been studied in detail indicating that it has two distinct units of marine origin, an upper one of Holocene age and a lower one of Pleistocene age with a thin discontinuous sand aquifer in between. The basal aquifer has spatially variable natural water quality and there has been only minimal aquifer exploitation but this is changing due to pollution of surface water and greater water demand. We hypothesis that the hydraulic, geochemical and solute transport characteristics of the two aquitard units have important influences on the flow and hydrochemistry in the basal aquifer and also that the aquitard has preserved evidence of past geologic process due to extremely slow groundwater flow in some zones. We propose to apply systematically a large number of different but complementary investigative methods to examine the similarities and differences between the two aquitard units and their hydrologic and hydrochemical interactions with the two aquifers. The work will be conducted by an experienced interdisciplinary team with expertise in hydrogeology, hydrogeochemistry, Quaternary geology and numerical modeling using multiple techniques including coring and core hydrochemical analyses, multi-level piezometers, environmental isotope tracers, X-radiography, high-resolution groundwater hydraulic, chemical and temperature profiling, and numerical flow and transport modeling. In addition to providing valuable insights concerning the groundwater resources of the PRD, the findings of this project will have broad scientific importance because this comprehensive combination of investigative techniques has not been used previously in aquitard research and therefore determining the merits of this approach has relevance to many other aquitard investigations, particularly complex marine aquitards in coastal areas. Aquitard units with some similar characteristics to those in the PRD likely also occur in the Yangtze River and Yellow River deltas and elsewhere in Asia.