*3.1. Collection of Samples*

The samples were collected from the HT Plain and the SHH Plain in September and October 2009 and from the HB Plain and the SBYR Plain from June to August 2010. A total of 974 sets of samples were collected in total from the shallow groundwater, including 450 from the HT Plain, 56 from the SHH Plain, 278 from the HB Plain, as well as 190 from the SBYR Plain. The sampling points were evenly distributed (Figure 2). 40 mL arsenic samples, 250 mL iron samples, 250 mL trace element samples and 1500 mL samples for full analysis were collected from the respective sampling points. After water temperature, electrical conductivity (EC), pH, and Eh were stable, groundwater samples were taken. All samples were membrane filtered (0.45 µm) in the field. The samples used for As speciation analysis were stored in new but pre-rinsed HDPE bottles (Nalgene) after adding 0.25 MEDTA. The filtered samples were then acidified to pH 1 by the addition of ultra-pure HCl for major and trace element analysis. Protective agents were also added to other samples, i.e., 2.5 mL of 1:1 sulfuric acid solution and 0.5 g of ammonium sulfate to iron samples, as well as 2.5 mL of 1:1 nitric acid solution to trace element samples. Except for the samples intended for full analyses, the other samples were stored in a freezer with the temperature regulated at 4 ◦C and analyzed within 5 days.

**Figure 2.** Arsenic content distribution of groundwater samples in the Hetao Basin.
