Study Of Groundwater Hydrochemistry Characteristics Between The Mangrove And Bald Beach Transects In Dongzhaigang National Nature Reserve

The groundwater has a direct impact on the mangrove ecosystem, the condition ofgroundwater quality affects the rootage, growth and development of mangroves and thediversity of mangrove wetland ecosystem. Investigating the groundwater quality inmangrove wetlands may provide scientific guide for protection and management ofmangroves. This paper analyzed and compared the hydrochemistries of the groundwatersamples from the mangrove transect (M-M’) and the bald beach transect (B-B’) inDongzhaigang National Nature Reserve. And multivariate statistical analysis (includingprincipal component analysis and cluster analysis) was used to analyse the source of thechemical composition, according to the differences of groundwater hydrochemistrycharacteristics between the two transects, the possible factors limiting mangroves toestablish in transect B-B’ were confirmed. The results indicated the followings.The hydrochemistry of groundwater from transects M-M’ and B-B’ belongs toNa-Mg-Cl type. There were significant differences between the two transects in physicaland chemical parameter values of groundwater. For example, the average values of pH,salinity, TDS of groundwater from wells in M-M’ were less than those of B-B’, and theORP in M-M’ was greater than0, indicating oxidizability, but in B-B’ the ORP was lessthan0, indicating reductibility. The dissolved oxygen DO in M-M’ was less than that ofB-B’. The groundwater temperature in mangrove transect M-M’ was higher than that of thebald beach transect B-B’. The values of physical and chemical parameters of groundwatervaried with depth in the mangrove transect, but remained almost constant in the bald beachtransect. This indicated that there were more complex biogeochemical orhydrogeochemical processes in the groundwater and soil substrate within the scope ofmangrove transect than in the bald beach transect. The average concentration ofconventional ions (Na+, K+, Ca2+, Mg2+, Cl-, SO42-, Br-), and the element Si and heavymetals V, Fe, Sr in groundwater from M-M’ were smaller than those of B-B’. The averageconcentration of Na+was4.62g/L for of B-B’, and3.99g/L for M-M’. The averageconcentration of anionic Cl-was15.34mg/L and12.93mg/L in B-B’ and M-M’ transects,respectively, and the heavy metal Fe,5.73mg/L in B-B’ and3.68mg/L in M-M’. But theaverage concentrations of the heavy metals Ba, Mn, Pb and Sb in M-M’ were significantlyhigher in M-M’ than in B-B’, e.g., the average concentration of Mn was0.83mg/L inM-M’ and0.27mg/L in B-B’.Principal component analysis and cluster analysis showed that the conventional ions (K+, Na+, Ca2+, Mg2+, Cl-and SO42-) and V, Sr in groundwater in two transects mainlycome from seawater intrusion. According to cluster analysis of the physical and chemicalparameters, we speculated that the source of other heavy metals was the river carryingaccumulation and dissolution of rock mineral through the leaching effect of groundwaterand surrounding rock. Considering the surrounding environment of transect B-B’, thegroundwater quality of B-B’ was also closely related to the anthropogenic activity.Comprehensive analysis indicated the followings: the possible factors preventing themangroves from their rootage, growth and development in the bald beach included tidaleffects, groundwater quality—salinity, lack of inland fresh water and the anthropogenicactivity, among which the groundwater salinity, capacity of inland fresh water recharge,and human activities are the main factors.