Abstract High-fluoride groundwater along coastal plains is frequently documented. The typical geological processes of seawater intrusion and soil salinization both result in conditions of high Na+, HCO3−, Cl−, TDS (Total Dissolved Solid), and the previous related researches argue high-fluoride groundwater is favored in such conditions, together with low Ca2+. But the fluorosis in soil salinization area contrarily does not occur although more seawater mixes, and the region is characterized by higher Na+, Cl−, TDS. Thus, groundwater in Kenli District of soil salinization area and Buzhuang Town of seawater intrusion area was analyzed and compared, to reveal the potential groundwater fluoride evolution. Generally, the groundwater fluoride levels in Kenli District (0.19–1.27 mg/L) are within but those in Buzhuang Town (0.65–9.87 mg/L) are beyond the safe limit. Geochemical properties indicate a direct cation exchange in Kenli District but a reverse cation exchange in Buzhuang Town. The reverse cation exchange results in the decreasing of groundwater Ca2+(Mg2+) and under-saturation of CaF2, MgF2, CaCO3, MgCO3, CaSO4‧2H2O, MgSO4 minerals, which permits more fluoride dissolving and causes high-fluoride groundwater in seawater intrusion areas. However, the direct cation exchange increases groundwater Ca2+(Mg2+) levels to the saturation of CaF2, CaCO3 and MgCO3 minerals, and the high Ca2+ restricts the fluoride dissolution and causes low fluoride groundwater in soil salinization areas. Moreover, the calculated Ca2+, Mg2+ originating from cation exchange (△(Ca2++Mg2+)C) occupies a large proportion because of the high Na+ levels in Kenli District, and is positively correlated with the degree of mixed seawater. The difference of cation exchange processes and orientations in the two areas is confirmed to be the key to groundwater fluoride levels. Moreover, extraordinarily high levels of groundwater Na+ is found to contrarily decrease groundwater fluoride levels because of cation exchange, which is different from the previous observation that Na+ always enhances groundwater fluoride. A new insight into the function of Na+ on groundwater fluoride evolution was proposed. These findings provide some information about groundwater fluoride evolution along coastal plains.

中文翻译：

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全球沿海平原地下水氟化物演化地球化学过程——来自海水入侵区与土壤盐渍化区对比的证据

摘要 沿海平原的高氟地下水经常被记录在案。典型的海水入侵和土壤盐渍化地质过程都导致了高 Na+、HCO3-、Cl-、TDS（总溶解固体）的条件，之前的相关研究认为在这种条件下，高氟地下水有利于高氟地下水，同时低Ca2+。但土壤盐渍化区虽然海水混入较多，但反之并未发生氟中毒，且该区以较高的Na+、Cl-、TDS为特征。因此，对土壤盐渍化区垦利区和海水入侵区布庄镇地下水进行了分析比较，揭示了地下水氟化物的潜在演化规律。总体而言，垦利区地下水氟化物水平（0.19-1.27 mg/L）在布庄镇（0.65-9. 87 mg/L) 超出安全限值。地球化学性质表明垦利区为直接阳离子交换，而步庄镇为反向阳离子交换。反向阳离子交换导致地下水Ca2+(Mg2+)减少，CaF2、MgF2、CaCO3、MgCO3、CaSO4‧2H2O、MgSO4矿物欠饱和，使更多的氟化物溶解，导致海水侵入区地下水出现高氟化物。然而，直接阳离子交换使地下水Ca2+(Mg2+)含量增加至CaF2、CaCO3和MgCO3矿物的饱和，高Ca2+限制了氟化物溶解，导致土壤盐渍化地区地下水含氟量低。此外，由于垦利区Na+含量高，计算出的Ca2+、Mg2+来源于阳离子交换(△(Ca2++Mg2+)C)占很大比例，且与海水混合程度呈正相关。两个地区的阳离子交换过程和取向的差异被证实是地下水氟化物水平的关键。此外，由于阳离子交换，发现异常高水平的地下水 Na+ 会相反地降低地下水氟化物水平，这与之前观察到的 Na+ 总是增强地下水氟化物不同。提出了对 Na+ 对地下水氟化物演化作用的新见解。这些发现提供了一些关于沿海平原地下水氟化物演化的信息。由于阳离子交换，发现异常高水平的地下水 Na+ 会相反地降低地下水氟化物水平，这与之前观察到的 Na+ 总是增强地下水氟化物不同。提出了对 Na+ 对地下水氟化物演化作用的新见解。这些发现提供了一些关于沿海平原地下水氟化物演化的信息。由于阳离子交换，发现异常高水平的地下水 Na+ 会相反地降低地下水氟化物水平，这与之前观察到的 Na+ 总是增强地下水氟化物不同。提出了对 Na+ 对地下水氟化物演化作用的新见解。这些发现提供了一些关于沿海平原地下水氟化物演化的信息。