Comparative analysis of groundwater fluorine levels and other characteristics in two areas of Laizhou Bay and its explanation on fluorine enrichment

Abstract

The groundwater characteristics in two areas, Tushan town (TS) in Laizhou County and Puzhuang town (PZ) in Changyi County, were compared to discuss the dynamics of fluorine enrichment. Groundwater has average fluorine levels of 2.33 mg/L in PZ town ranging from 0.65 to 9.87 mg/L and of 0.91 mg/L ranging from 0.27 to 1.25 mg/L in TS town. The geochemical indexes indicate seawater intrusion in the both areas, the more serious seawater intrusion in PZ town causes the higher pH, electrical conductivity (EC), total dissolved solids (TDS), HCO3− and Na+ than TS town, which can accelerate fluorine leach ability from rock/soil and result in higher fluorine than in TS town. So, the seawater intrusion is related to groundwater fluorine levels. Moreover, the groundwater shows the contrary process of cation exchange. That is, the exchange of Na+ to Ca2+ in TS town occurs and that of Ca2+ to Na+ occurs in PZ town during seawater intrusion. As a result, PZ town has relatively higher Na+ and lower Ca2+ than TS town, and the groundwater types transform from Ca-Cl type in TS town to Na-Cl type in PZ town with the ongoing seawater intrusion. Such a process decreases fluorine solubility in groundwater and contributes to lower fluorine levels in TS town. Therefore, the seawater intrusion and the special cation exchange explain well the different fluorine levels in the two areas, which should be considered when the fluorine enrichment is discussed in coastal regions.

Fluoride-leaching simulation of aquifer sediment and its influence on groundwater fluoride levels along coastal plains

“…The China Geological Survey has set the seawater intrusion standards by Cl À of 250 mg/L, Br À of 0.55 mg/L, and TDS of 1.0 mg/L (Wang et al 2015). The supplied water was characterized by non-intrusion.…”

“…Second, the NaF solubility is remarkably higher than CaF 2 or MgF 2 (Chae et al 2007;Chen et al 2012Chen et al , 2019Chen et al , 2020b. Meanwhile, the fluoride levels in groundwater are restricted by Ca 2þ levels because of the low CaF 2 solubility, and Ca 2þ -F À negative relationship in groundwater is commonly observed and documented (Ozsvath 2009;Wang et al 2015). The leached fluoride from rock (soil) and groundwater fluoride levels are also proved to increase with Na þ increasing and Ca 2þ decreasing by field investigations or simulation experiments (Krainov & Zakutin 1994;Gao et al 2007).…”

“…Groundwater Na þ increases because more Na þ mixes when seawater intrudes. While groundwater Ca 2þ generally decreases because of the alkaline condition, CaCO 3 precipitation and Na-Ca cation exchange (Gao et al 2007;Wang et al 2015;Chen et al 2020b). The geochemical characteristics of high Na þ and low Ca 2þ can greatly promote fluoride-leaching from soil or rock because of the following reasons: First, the Na-F combination is prior to Ca-F or Mg-F combination.…”

The Enrichment Process of Groundwater Fluorine in Sea Water Intrusion Area of Gaomi City, China

“…Na + –Ca 2+ cation exchange is the prominent process of sea water intrusion, including direct cation exchange (the groundwater Ca 2+ exchanges with the Na + in aquifer materials) and reverse cation exchange (the groundwater Na + exchanges with the Ca 2+ in aquifer materials). However, the reverse cation exchange was confirmed to occur at the initial stage of intrusion (Wu et al 1994; Wang et al 2015). Moreover, field investigation and simulation experiments in sea water intrusion areas also observed that Mg 2+ was less affected by cation exchange and that Na + –Mg 2+ exchange seldom occurs (Sajil Kumar et al 2014; Wang et al 2015).…”

“…However, the reverse cation exchange was confirmed to occur at the initial stage of intrusion (Wu et al 1994; Wang et al 2015). Moreover, field investigation and simulation experiments in sea water intrusion areas also observed that Mg 2+ was less affected by cation exchange and that Na + –Mg 2+ exchange seldom occurs (Sajil Kumar et al 2014; Wang et al 2015). Such different evolution processes of Ca 2+ and Mg 2+ can explain their different relationships with groundwater F − (Figure ).…”