Distribution Of Groundwater Rare Earth Elements In The Typical Region Of The North China Plain And Modeling Study On Their Complexation With Iron And Manganese

The chemically coherent properties of rare earth elements?REEs?have led to their utility as effective tracers for understanding geochemical processes in low-temperature aquatic systems.It has been commonly recognized that complexation has a huge impact on groundwater REEs.However,roles of iron and manganese complexation in the fractionation and mobilization of REEs in groundwater remain unclear.This investigation has focused on Quaternary groundwater from the typical region of the North China Plain?NCP?,aiming at studying the distribution of REEs in groundwater,influence of iron and manganese complexation on the mobility of REEs in groundwater,and hydrogeochemical processes fractionating REEs.This study has its significance in both theory and practice.The major conclusions are shown as follow:1.Concentrations of REEs in groundwater from the typical region of the NCP are at a level between pmol/L and nmol/L,which are similar to the values of groundwater from other countries and the Hetao basin?Inner Mongolia?and the Datong basin?Shanxi?.An overall decrease in REEs concentrations has been observed along the groundwater flow paths in the region nearest to the recharge areas,whereas REEs concentrations varies substantially with groundwater flow in the central and littoral plains,which may be related to human activity.2.Groundwater has Upper Crust Composite?UCC?normalized REEs patterns being enriched in Middle rare earth elements?MREEs?and Heavy rare earth elements?HREEs?relative to Light rare earth elements?LREEs?.This is evidenced by high UCC normalized ratios(?Gd/Nd?_UCCCC and?Yb/Nd?_UCC)?generally greater than 1?.In the piedmont,shallow groundwater normalized patterns are similar to those of deep groundwater,indicating that close hydrological interactions occur between shallow and deep aquifers.With groundwater flow,shallow and deep groundwater normalized patterns evolve into relatively flat,which may be attributed to the different geochemical reactions along groundwater flow path.3.In the typical regions,Ce anomaly?Ce/Ce*?shows an increase along the groundwater flow paths.Total dissolved iron?Fe_T?and manganese?Mn_T?concentrations are positively correlated to Ce/Ce*in general.This indicates that the development of Ce anomalies is closely related to chemical reactions involving Fe and Mn in groundwater.The lower Ce anomalies in the piedmont plain may be due to the relatively oxidizing conditions which favor oxidative scavenging of Ce resulting in its depletion in groundwater.With groundwater moving downgradient,groundwater conditions evolve into relatively anoxic,where Ce oxidation is disfavored or the oxidized Ce occurs in forms of colloids.This may explain the weakly negative and positive Ce anomalies observed in the central and littoral plains.4.Concentrations and speciations of REEs in groundwater are influenced by pH.Generally,REEs concentrations decrease with increasing pH.REE carbonate complexes?LnCO₃⁺and Ln?CO₃?₂^-?are predicted to have relatively higher concentrations under the higher pH.5.REE complexation with Fe and Mn oxyhydroxides??Fe_oH and?Mn_oH?is an essential control on the distribution of REEs in groundwater,whereas the impact of this sorption on REEs changes along the groundwater flow paths.Proportions of?Fe_oNd²⁺and?Mn_oNd²⁺?illustrated by Nd?range from<1%to 84%?average 9%?and from<1%to 99.5%?average 24%?,which generally increase along the groundwater flow path.Consequently,in the piedmont groundwater,where Fe and Mn manganese oxyhydroxides are less prevail(?Fe_oNd²⁺and?Mn_oNd²⁺are below 5%),carbonate complexation leads to MREEs and HREEs enrichment in groundwater.However,while in the central and littoral plains,competition of dissolved carbonate ions with Fe and Mn oxyhydroxides controls REE concentrations and fractionations.Therefore,Fe and Mn oxyhydroxides play an important role in the mobility of REEs in groundwater,especially in the downgradients.