*2.3. Hydrogeological Conditions*

On the whole, 48 hydrogeological profiles were prepared with the collected data of over 1300 boreholes, dynamic groundwater monitoring data of 20 years and the water level data in 4 periods from 2009 to 2010, illustrating the hydrogeological conditions of the study area and laying the conditions for analyzing high-As groundwater.

The shallow aquifer of the HT Plain in the piedmont alluvial-pluvial fan in the north comprises Holocene and Epipleistocene proluvium with gravel and medium-coarse sand containing gravel, 10–130 m thick [23]. The water level varies from 20–40 m at the top of the fan group to 3–5 m at the front edge. The alluvial lacustrine plain of the Yellow River in the south primarily covers Holocene-Epipleistocene alluvial lacustrine facies medium-fine sand, fine sand and silty-fine sand, which is dominated by semi-confined water with local phreatic water [31]. The thickness exhibited by the aquifer increases from 20–80 m in SE to 100–240 m in NW. The burial depth of groundwater level is shallow, generally ranging from 3–10 m. On the whole, the shallow groundwater is recharged by the lateral runoff, the underground flow in the valleys, the atmospheric precipitation, and the infiltration of irrigation with water from the Yellow River in the northern bedrock mountain area. In the northern piedmont fan zone, the groundwater is largely supplied by the north-south runoff, and the runoff conditions deteriorate in the foreland depression. In the alluvial plain of the Yellow River in the south, the groundwater flows as SW-NE runoff. The shallow groundwater forms a drainage zone by complying with the main drainage line and eventually flows as runoff eastward to the Wuliangsuhai Lake [38–40].

The shallow aquifer in the SHH Plain mostly comprises Holocene and Epipleistocene medium-fine sand and silty-fine sand [41,42]. The local river channel covers sandy gravel. From the north to the bank along the Yellow River, the thickness of the aquifer tends to decrease from 30–40 m to 20–30 m, and the groundwater depth declines from 5–10 m to 1–3 m. The groundwater is primarily recharged by the surface runoff, the subsurface flow, atmospheric precipitation and irrigation infiltration while flowing from NW to SE as runoff to the east of Xisanzui. It flows as runoff from the northeast to the southwest to the east of Baotou, which is mostly discharged by mining and lateral discharge to the Yellow River [41,42].

The shallow aquifers in the eastern and western regions in the HB Plain exhibit different characteristics attributed to a wide range of sedimentary environments. The aquifer in the eastern region is the phreatic water-micro-confined aquifer of the piedmont plain of the Dahei River. The compositions vary from proluvium and alluvium gravels, pebbles and sandy gravels gradually to alluvial gravels, medium-coarse sand and fine silty sand to the lake basin. From the west to the east, the groundwater level is altered from nearly 15 m to approximately 5 m, and the thickness of the aquifer is up-regulated from 10 m to 27 m [43]. The aquifer in the western region, composed of Holocene and Epipleistocene medium-fine sand and silty-fine sand, refers to the phreatic water-microconfined aquifer of the alluvial lacustrine plain of the Yellow River [44]. The thickness of the aquifer varies from 80–100 m in the northwest to 20–40 m in the east. The burial depth of water level is 5–10 m close to the front of the mountain and 2–5 m generally following the

Yellow River and in the eastern region [45]. Laterally, the groundwater is recharged by the bedrock fissure water in some sections, subsurface flow of river valleys and surface runoff, as well as precipitation and irrigation infiltration in the mountainous area. Furthermore, the groundwater flows as runoff from the northeast to the southwest in the eastern region and flows as runoff from the northwest to the southeast and is discharged through mining, evaporation and lateral discharge to a canal for water release of Hasuhai Lake and the Yellow River.

The shallow aquifer of the SBYR Plain covers Holocene-Epipleistocene proluvium and alluvial lacustrine silty-fine sand, fine sand and silty sand [26,27]. The thickness of the aquifer rises from less than 50 m in the south to 50–100 m in the north, and it exceeds 100 m in local areas. In most areas, the burial depth of the groundwater level is less than 5 m, generally 5–15 m in the front of hills. The groundwater is largely recharged by atmospheric precipitation, lateral infiltration in the hilly southern area, infiltration of the valleys, and infiltration of the irrigation canals for water diversion from the Yellow River. The groundwater is mainly recharged from precipitation, runoffs from hills in the south, and water in irrigation channels from the Yellow River. It is discharged by evaporation, manual mining, and lateral seepage to the Yellow River [40–42].
