Origin of groundwater salinity and hydrogeochemical processes in the confined Quaternary aquifer of the Pearl River Delta, China

SummaryThe Pearl River Delta (PRD), located in the coastal area of South China, contains a large confined Quaternary aquifer. This aquifer has been identified to have high concentrations of total dissolved solids-ranging from 1 g/l in the inland area to 26.8 g/l near the southeast shoreline. This work aims to investigate the origin of groundwater salinity and the major hydrogeochemical processes controlling the groundwater evolution in the confined basal aquifer of the PRD. A total of 40 boreholes were drilled in the PRD during 2007-2009, and groundwater samples were collected for major ion (Ca2+,Mg2+,Na+,K+,NH4+,Cl-,SO42- and HCO3-) and isotope (δ18O, δ2H, 3H, δ13C and 14C) analyses. These analyses demonstrate that groundwater in the confined basal aquifer is recharged by local rainwater and seawater, and seawater is the major salinity source for groundwater. The contribution of seawater varies from less than 1% in the inland areas to 74% near the shoreline. SO42- reduction and methanogenesis occur in this reducing confined aquifer. The results of ion exchange show that Mg2+ exchanges mainly with the NH4+ ion, which is derived from the mineralization of sedimentary organic matter. Ion exchange between Na+ and Ca2+ and groundwater facies suggest that the brackish aquifer in the northwest inland area is undergoing freshening. 3H analysis suggests that infiltrated rainwater is younger than seawater. 14C analysis indicates that the seawater most likely infiltrated into the confined basal aquifer during the Holocene transgression period.

► Groundwater from a Holocene-Pleistocene aquifer-aquitard system was studied. ► Salinity was caused by palaeo-seawater intrusion. ► Seawater can be kept in such an aquitard-aquifer system for a geologically long time. ► Mg2+ exchanges with NH4+ derived from a geological source.