北海滨海多层地下水地球化学特征及季节性-年际动态OA

[Objectives]Revealing the seasonal-annual variations of groundwater geochemistry for coastal multiple-aquifers in Beihai.[Methods]This study investigates and monitors hydrochemistry,as well as C isotopes of dissolved inorganic carbon(δ13CDIC)and S isotopes of sulfate(δ34SSO4)in the multiple groundwater of Beihai.[Results]The results show groundwater pH values are 4.1～7.5(average 5.8±1.1,N=15),the background levels of Ca2+,Mg2+,Na+,Cl-,SO42-,and HCO3-are 0.5～10.0,<2.0,2.0～8.0,2.0～10.0,3.0～8.0,and<50.0 mg/L,respectively.δ13CDIC and δ34SSO4values are-15.4 ‰～-3.0 ‰(average-8.8±5.0 ‰,N=8)and 3.9 ‰～21.3 ‰(average 10.7 ‰±7.2 ‰,N=114),respectively.At the monitoring site weakly disturbed by anthropogenic activities,main elements in phreatic groundwater exhibit a seasonal trend,the loadings in the wet season are lower than in the dry season;while they show no clear seasonal-annual trend in confined groundwater;δ34SSO4values are constant(4‰).For the sites highly affected by surface mariculture and tidal effects,phreatic as well as shallow and moderate confined groundwater contains SO42-concentrations up to 1 240 mg/L,its main element loadings and δ34SSO4values display irregular and strong fluctuations(variation coefficient 33.6 ‰～103.6％),with no clear seasonal-annual tendency;however,δ34SSO4values of the deep confined groundwater(ca.140 m below land surface)vary slightly(4 ‰～6 ‰).[Conclusions]The formation of acidic groundwater is due to infiltration of acidic rainwater and oxidation of sulfides in the strata.Silicate weathering,inputs of saline water(seawater and surface mariculture pond water),organic matter degradation,and soil CO2 dissolution jointly account for DIC loadings and δ13CDIC values.Atmospheric deposition and sulfide oxidation are important sources of SO42-in groundwater,whereas inputs of saline water are mainly responsible for the observed high δ34SSO4values and high SO42-loadings.A small leakage of overlying and up-gradient saline groundwater is responsible for the slight fluctuations of δ34SSO4values in the deep confined groundwater.The high frequence intrusion of seawater caused by tidal effect and a seasonal dilution effect caused by rainfall are antagonistic.Consequently,it is difficult to observe seasonal dynamics of groundwater chemistry by routine monitoring for sites located at the coastal zones.Therefore,high frequency monitoring is necessary to evaluate diurnal variations of hydrochemistry induced by the tide.Our findings improve the understanding of geochemistry and dynamics of groundwater,and are of theoretical basis for water resource management in coastal areas.