4.2.1. Hydrochemical Types

According to the Piper classification method [40], there were 13 hydrochemical types in the SHH Plain and 19 types in each of the other three hydrogeological units (Figure 4). However, they showed different hydrochemical characteristics. The major hydrochemical type of high-As groundwater in the HT Plain and the SHH Plain referred to the HCO3·SO4·Cl type, followed by the HCO3·Cl type for anions, and mainly Na·Mg·Ca and Na·Mg for cations. The hydrochemical type of high-As groundwater in the HB Plain was largely the HCO<sup>3</sup> type, followed by the HCO3·SO4·Cl type and HCO3·Cl for anions, and mainly Na and Na·Mg·Ca for cations. The hydrochemical type of high-As groundwater in the SBYR Plain is mainly the HCO<sup>3</sup> type, the HCO3·SO4·Cl type, the HCO3·SO<sup>4</sup> type, and the HCO3·Cl type, all of which are approximately equally for anions, as well as Na, Na·Mg, and Na·Mg·Ca for cations.

**Figure 4.** Piper diagram of high-arsenic shallow groundwater quality in four hydrogeological units. (**A**) HT Plain, (**B**) HB Plain, (**C**) SBYR Plain, and (**D**) SHH Plain.
