## 2 Materials and Methods  
### Background and Geographical Framework of the Zhem Artesian Basin  
The Zhem Artesian Basin is located in the eastern part of the Caspian depression and is bordered in the northeast by the South Pre-Ural Basin, in the east and south by the Dongystau-Predmugalzharsky Basin, in the southwest by the North Caspian basin, and in the northwest by the Syrtovsky Basin (Figure 1). At 104,000 km\({}^{2}\), the basin's area is administered as follows: 85,000 km\({}^{2}\) or 82% belong to the Aktobe district's northwestern part, 15,000 km\({}^{2}\) or 14% belong to the sparsely populated territory of eastern Atyrau, and 4000 km\({}^{2}\) or 4% belong to the Western Kazakhstan district. Absolute elevations range from 50-75 mAS in the southwest to 300-420 m in the east-northeast. The basin is part of the Ural-Emben plateau, which is dissected by numerous shallow depressions demonstrating a ridged hilly morphology owing to a dense erosion network that is related to the Khobda, Uil, Sagyz, Temir, and Emba river valleys' drainage, combined with remnant table hills and ridges as high as 80-100 m above the valley plateau datum. The mean annual precipitation is 217 mm, with minimum and maximum values of 114 mm and 402 mm, respectively, gradually increasing in the northeast direction. While the Khobda and Emba rivers have natural flow patterns, the rest are disturbed by hydraulic structures ( dams). The mean annual flow rate of the rivers varies from 0.5 m\({}^{3}\)/s to 6.4 m\({}^{3}\)/s, depending on the catchment area and the amount of precipitation. From a geological and structural composition perspective, the Zhem Basin belongs to the inner bordering part of the Pre-Caspian Depression, which is a deep structure in the vast Russian foundation platform. The Pre-Paleozoic folded foundation depth changes stepwise from 5 to 12 km from the border to the center, covered by a sedimentary cover with four structural stages [24; 31]. The most promising Zhem Artesian Basin water supply resource is the sedimentary cover's third structural stage, consisting of Oligocene sandy  
Figure 1: The locations of the Zhem Artesian Basin. 1—basin boundary; 2—meteorological station and its number; 3—hydrological station and its number; 4—observation well and its number. argillaceous and Triassic-Carbonaceous sediments with a total thickness of 2-3 km that are confined to stratigraphic and lithologic facial sediment complexes (Figure 2). The hydrogeological cross-section of the Zhem Artesian Basin exploitable aquifer complex (Figure 3) consists of the following sections. Figure 2: Hydrogeological sketch of the Zhem Artesian Basin. 1—Upper Quaternary alluvial–deltaic- and overlying aeolian sediments aquifer; 2—Upper Quaternary–Modern alluvial sediments aquifer; 3—Middle-Upper Quaternary alluvial sediment aquifer; 4—Lower Quaternary alluvial sediment aquifer; 5—Quaternary marine sediment aquifer; 6—Neogene aquicudo; 7—Pliocene Syrt sediment aquifer; 8—local Paleogene aquifer; 9—Paleogene aquicudo; 10—Paleogene clastic sediment aquifer; 11—Upper Cretaceous aquifer; 12—Upper Cretaceous and Paleogene aquicudo; 13—local Upper Cretaceous aquifers; 14—Lower Cretaceous-Cenomanian sediment aquifer complex; 15—Jurassic aquifer complex; 16—carboniferous aquifer of fractured coal rocks; 17—Silurian–Devonian aquifer; 18—Magmatic aquifers; 19—Kazakhstan border; 20—hydrogenological cross-section line; 21—Zhem Basin boundary; 22—rifts; 23— basin divide lines; 24—Artesian basins: I—Zhem Basin; II—South Pre-Ural Basin; III—Dongystau-Predmugalzharsky Basin; IV—North Caspian Basin; V—Syrtovsky Basin; 25—brackish and saline groundwater recharge area 3–10 g/L; 26—natural groundwater head. Upper Quaternary to Modern (aQm-v) and Middle-Upper Quaternary (aQII-III) alluvial sediment aquifers are located in the river valleys and confined to the alluvial deposits of floodplains. The first and second terraces above the floodplain are represented by heterogeneous, sometimes gravelly sands interbedded with light loams and clay.