**5. Calculation of Charging and Discharging of Confined Aquifer with Mixed-Unit Method** *5.1. Division of Mixed Units*

Because the studied area is located in the plain of alluvial–diluvial fine-soil particles, the conceptual model of the mixed units was established accordingly as shown in Figure 5. The mixed units of the unconfined aquifer and confined aquifer were, respectively, marked as a, b, c, d, e, f, and g and C, D, E, F, and G. Units a and b are located in the alluvial–proluvial slope gravel plain as a single-structure unconfined aquifer and are the recharging source of confined aquifer. All other units are in the alluvial–diluvial fine-soil-grain plain. Units G and F are discharging units. The transformation relationship between units of the aquifer is shown in Figure 7.

**Figure 7.** The transformation relationships between units.

#### *5.2. Hydrochemical Characteristics*

The unconfined aquifer is widely distributed in the studied area, and its salinity in the upper stream was about 0.6 g/L, and pH was 8.4 with an HCO3·SO4–Mg·Catype water. It was 1.3–3 g/L in the middle stream with water types of SO4·Cl–Na·Mg and SO4·Cl–Na·Ca·Mg, and pH was 8.3. The salinity in the west of the downstream study area was 3.7 g/L, pH was 8.4, and the salinity in the east was 9.0 g/L, pH was 8.2, with water types of Cl·SO4–Na and Cl–Na. The water-chemistry type of unconfined aquifer units a→c→e→f changed from HCO3·SO4–Mg·Ca, to SO4·Cl–Na·Mg, to SO4·Cl–Na·Ca·Mg, to Cl·SO4–Na (or Cl–Na. That of unconfined aquifer b→d→g changed from HCO3·SO4–Mg·Ca to Cl·SO4–Na (or Cl–Na) and SO4·Cl–Na·Mg (or SO4·Cl–Na·Ca·Mg).

The salinity of confined aquifer from the middle-upper stream to the downstream of the Aksu River Basin did not change significantly with a salinity of 1 g/L, pH of 8.1 and water types of SO4·Cl–Ca·Na·Mg, SO4·HCO3·Cl–Mg·Na·Ca and Cl·SO4–Na·Ca in the downstream. Water types of the confined aquifer unit C→E→F changed from SO4·Cl–Ca·Na·Mg or SO4·HCO3·Cl–Mg·Na·Ca to Cl·SO4–Na·Ca. From units D→G, it changed from SO4·Cl–Ca·Na·Mg or SO4·HCO3·Cl–Mg·Na·Ca to Cl·SO4–Na·Ca. These data showed that, along the flowing path of the confined aquifer, in the middle and upper steams of the west, the confined aquifer receives a large amount of recharging laterally from the unconfined aquifer (a→C→E), and a small amount of recharging vertically from the unconfined aquifer (c→C→E). When the water exchange between the confined aquifer and the unconfined aquifer was reduced (e→E and f→F), evaporating concentration and cation-exchange adsorption (e→f) occurred in the unconfined aquifer, and cation exchange adsorption occurred in the confined aquifer (E→F). The middle and upper streams in the east receive a large amount of recharging water vertically from unconfined aquifer (d→D), with a small amount from unconfined aquifer laterally (b→D). Its downstream receive a large amount of water recharging laterally from confined aquifer (D→G). with a small amount vertically from unconfined aquifer (g→G). The unconfined aquifer has significant evaporating concentration (d→g), and the confined aquifer mostly has cation exchange adsorption (D→G). The water chemistry characteristics in this basin are shown in Figure 8.

**Figure 8.** Water chemistry characteristics.

#### *5.3. Distribution Characteristics of Isotopes*

As shown in Figure 9, after the precipitation line slope of the aquifer of the studied area was compared with the Global Meteoric Water Line (GMWL) proposed by Craig [32], the slope and intercept were both smaller than the global precipitation line and δD and δ18O were all located at the upper left of GMWL. These results showed that strong evaporation had taken place in the whole of the aquifer.

**Figure 9.** Relationship between δD and δ18O.

The isotopic compositions of the unconfined aquifer(a→c→e→f) and the confined aquifer(C→E→F) in the west were basically the same, indicating that unconfined aquifer and confined aquifer had a certain hydraulic connection. As shown in Figure 10a, in the middle and upper steams of the west, because of the influence of evaporation, the δD and δ18O in the unconfined aquifer increased gradually, while the δD and δ18O in the confined aquifer increased simultaneously. These results showed that the unconfined aquifer recharged the confined aquifer vertically (c→C, e→E). In the downstream, the δD and δ18O of the unconfined aquifer f and the confined aquifer F were significantly different, indicating that the confined aquifer was less replenished by the unconfined water ( f→F). The <sup>δ</sup>D and <sup>δ</sup>18O of unit C→E→F were closer to unit c→e→f than to unit a, indicating that the confined aquifer received a large amount water recharging laterally from the unconfined aquifer (a→C→E→F).

**Figure 10.** Relationship between δ18O and relative distance.

The <sup>δ</sup>D and <sup>δ</sup>18O values of the unconfined aquifer (b→d→g) and the confined aquifer (D→G) in the east were significantly different, indicating that the hydraulic connection between the two water bodies was weak. As illustrated in Figure 10b, from upper to lower in the east, because of the influence of evaporation, the δD and δ18O were enriched along the way and in the confined aquifer, the δD and δ18O decreased. These results showed that the confined aquifer in the middle and upper streams received a large amount of water recharging vertically from the unconfined aquifer (d→D) and the confined aquifer in the lower streams received a small amount of water recharging vertically from unconfined aquifer (g→G).The <sup>δ</sup>D and <sup>δ</sup>18O of unit D→G were closer to Unit d→g than to the mountain exit stations, indicating that the confined aquifer D→G received a large amount water recharging laterally from unconfined aquifer (b→D→G).

#### *5.4. Recharging Relationships between Unconfined Aquifer and Confined Aquifer*

According to the hydrochemical and isotope distribution characteristics of the studied area, the confined aquifer in the alluvial–diluvial plain area received water recharging from the loose rock porous unconfined aquifer and upper porous unconfined aquifer of the Quaternary in the upper alluvial–diluvial slope plain, which was discharged through artificial and lateral downstream. The recharging and discharging relationship between units in the unconfined aquifer and confined aquifer are shown in Figure 11.

**Figure 11.** Groundwater circulation in the unconfined aquifer and the confined aquifer.
