Abstract: Summary A hydrochemical-isotopic investigation of the Laizhou Bay Quaternary aquifer in north China provides new insights into the hydrodynamic and geochemical relationships between freshwater, seawater and brine at different depths in coastal sediments. Saltwater intrusion mainly occurs due to two cones of depression caused by concentrated exploitation of fresh groundwater in the south, and brine water for salt production in the north. Groundwater is characterized by hydrochemical zonation of water types (ranging from Ca–HCO3 to Na–Cl) from south to north, controlled by migration and mixing of saline water bodies with the regional groundwater. The strong adherence of the majority of ion/Cl ratios to mixing lines between freshwater and saline water end-members (brine or seawater) indicates the importance of mixing under natural and/or anthropogenic influences. Examination of the groundwater stable isotope δ18O and δ2H values (between −9.5‰ and −3.0‰ and −75‰ and −40‰, respectively) and chloride contents (∼2 to 1000meq/L) of the groundwater indicate that the saline end-member is brine rather than seawater, and most groundwater samples plot on mixing trajectories between fresh groundwater (δ18O of between −6.0‰ and −9.0‰; Cl<5meq/L) and sampled brines (δ18O of approximately −3.0‰ and Cl>1000meq/L). Locally elevated Na/Cl ratios likely result from ion exchange in areas of long-term freshening. The brines, with radiocarbon activities of ∼30 to 60 pMC likely formed during the Holocene as a result of the sequence of transgression-regression and evaporation; while deep, fresh groundwater with depleted stable isotopic values (δ18O=−9.7‰ and δ2H=−71‰) and low radiocarbon activity (<20 pMC) was probably recharged during a cooler period in the late Pleistocene, as is common throughout northern China. An increase in the salinity and tritium concentration in some shallow groundwater sampled in the 1990s and re-sampled here indicates that intensive brine extraction has locally resulted in rapid mixing of young, fresh groundwater and saline brine. The δ18O and δ2H values of brines (∼−3.0‰ and −35‰) are much lower than that of modern seawater, which could be explained by 1) mixing of original (δ18O enriched) brine that was more saline than presently observed, with fresh groundwater recharged by precipitation and/or 2) dilution of the palaeo-seawater with continental runoff prior to and/or during brine formation. The first mechanism is supported by relatively high Br/Cl molar ratios (1.7×10−3–2.5×10−3) in brine water compared with ∼1.5×10−3 in seawater, which could indicate that the brines originally reached halite saturation and were subsequently diluted with fresher groundwater over the long-term. Decreasing 14C activities with increasing sampling depth and increasing proximity to the coastline indicate that the south coastal aquifer in Laizhou Bay is dominated by regional lateral flow, on millennial timescales.

Abstract: The hydrochemistry of minor elements bromide (Br), boron (B), strontium (Sr), environmental stable isotopes (18O and 2H) together with major-ion chemistry (chloride, sodium, calcium) has been used to constrain the source(s), relative age, and processes of salinization in the Continental Terminal (CT) aquifer in the Saloum (mid-west Senegal) region. Seventy-one groundwater wells which include 24 wells contaminated by saltwater and three sites along the hypersaline Saloum River were sampled to obtain additional information on the hydrochemical characteristics of the groundwater defined in previous studies. Use of Br against Cl confirms the Saloum River saline water intrusion up to a contribution of 7% into the aquifer. In addition to this recent intrusion, a relatively ancient intrusion of the Saloum River water which had reached at least as far as 20 km south from the source was evidenced. The high molar ratio values of Sr/Cl and Sr/Ca indicate an additional input of strontium presumably derived from carbonate precipitation/dissolution reactions and also via adsorption reactions. The variable B concentrations (7–650 μg/L) found in the groundwater samples were tested against the binary mixing model to evaluate the processes of salinization which are responsible for the investigated system. Sorption of B and depletion of Na occur as the Saloum river water intrudes the aquifer (salinization) in the northern part of the region, whereas B desorption and Na enrichment occur as the fresh groundwater flushing displaces the saline waters in the coastal strip (refreshening). In the central zone where ancient intrusion prevailed, the process of freshening of the saline groundwater is indicated by the changes in major-ion chemistry as well as B desorption and Na enrichment. In addition to these processes, stable isotopes reveal that mixing with recently infiltrating waters and evaporation contribute to the changes in isotopic signature.