*4.1. Content and Regional Distribution of Arsenic in Groundwater*

The high-As samples of shallow groundwater took up 47.33% of the total samples, and the average concentration of arsenic in the groundwater reached 45.58 µg/L. Only 18 samples had lower contents than the detection limit (<0.1 µg/L), and the detection rate reached 98.15% (Table 1). According to the above table, the high-As groundwater in the Hetao Basin was widely distributed. The high-As groundwater was extensively distributed in the four hydrogeological units, and it achieved the widest distribution in the HT Plain. The high-As groundwater samples accounted for 53.78% of the 450 sample sets. The maximal arsenic content was 916.70 µg/L, and the arsenic content in groundwater was 64.13 µg/L on average, which was the highest in the whole region. Only three groundwater samples contained arsenic contents lower than the detection limit (<0.1 µg/L), and the detection rate took up 99.33%. The SHH Plain was the minimum. Though there were only 56 sets of samples, the detection rate was 100%. The average arsenic content reached 40.64 µg/L, only smaller than the HT Plain. The average arsenic content in the groundwater of the SBYR Plain was the minimum (22.51 µg/L) among the four hydrogeological units. Furthermore, the number of high-As groundwater samples in the HB Plain accounted for 38.49% of the total number of samples, which was the minimum in the whole region.


**Table 1.** Statistical table of arsenic content in shallow groundwater of the Hetao Basin.

As revealed by the distribution of high-As groundwater points (Figure 1), high-As groundwater in the Hetao Basin was distributed throughout the region with an extremely uneven spatial distribution. The variability of arsenic contents in groundwater was analyzed by using SPSS (IBM, Armonk, NY, USA). The spatial variation coefficient of arsenic content was 2.07, and the variation coefficient of arsenic content in groundwater of the four hydrogeological units ranged from 1.57 to 2.18, which demonstrated that the spatial variability of arsenic content in groundwater was significantly large in the whole basin and the four hydrogeological units. There were three relatively concentrated distribution areas of high-As groundwater in the whole basin (Figure 3), with arsenic contents of underground water usually larger than 50 µg/L. The mentioned areas consisted of (1) an intersection zone of the front edge of the piedmont alluvial-pluvial fan of the HT Plain and the alluvial lacustrine plain of the Yellow River, (2) both sides of the modern river channel of the Yellow

River from the HT Plain to the SHH Plain and the HB Plain, as well as (3) the central region of the HB Plain, located in the middle east of Tumd Left Banner.

**Figure 3.** Contour map of high-arsenic shallow groundwater points and groundwater level elevation in Hetao Basin.
