*5.7. Discussion*

Based on the results of the water balance and the model output, the upstream confined aquifer received lateral recharging and vertical leakage recharging from the unconfined aquifer, and the downstream confined aquifer only received lateral recharging from the upstream confined aquifer, which was consistent with the hydrochemical and isotope analysis. In addition, the unit flux between the cells was apparently influenced by the water source exploitation. For example, the unit E received the recharge from unit e and unit C was 7.47 × <sup>10</sup>6m3/a, the discharged through lateral was 6.10 × 106m3/a. Because of over extraction, the difference between the inflow and outflow to unit E was −3.49 × <sup>10</sup>6m3/a. Compared with previous research, we identified the recharge sources and their relative contributions to the confined aquifer, and the data of the extraction were available, the quantitative water circulation model were reasonable, and the results were reliable.

#### **6. Conclusions**

According to the geological and hydrogeological data of the studied area, the recharging and discharging relationship between the unconfined aquifer and the confined aquifer was determined. Based on the hypothesis and principles of unit dividing, and the qualitative recharging and discharging relationship, a mixed-unit model was established to study the hydrochemical characteristics of the confined aquifer in the west of the studied area. Results showed that the confined aquifer was significantly affected by unconfined aquifer, and the confined aquifer received lateral recharging from upstream of the unconfined aquifer and vertical recharging from the upper unconfined aquifer. In the east of the studied area, the downstream confined aquifer no longer received the recharging vertically from unconfined aquifer, but mainly received the lateral recharging from the upstream confined aquifer.

The δD and δ18O of unconfined aquifers in the studied area were both at the upper left of the Global Meteoric Water Line, and their slope and intercept were both smaller than those of the global atmospheric precipitation line, indicating that the unconfined aquifer was significantly evaporated. The isotopic compositions of unconfined aquifer and confined aquifer in the west of the studied area were basically the same, indicating that there was a hydraulic connection between these two water bodies, and the confined aquifer received lateral recharging from the upstream unconfined aquifer and vertical leakage recharging from the upper unconfined aquifer. The δD and δ18O distributions of the unconfined aquifer and confined aquifer in the east of the studied area were relatively discrete, indicating that the hydraulic connection between these two water bodies was weak, and the downstream of confined aquifer in the east mainly received lateral recharging from the upstream confined aquifer. These results were consistent with the hydrochemical analysis.

Based on the mixed-unit model, the calculation results showed that the total recharged volume received by the confined aquifer in the studied area was 21.48 × 106 <sup>m</sup>3/a, in which the lateral recharging was 11.48 × <sup>10</sup><sup>6</sup> m3/a, accounting for 53.45% of the total, and the vertical recharging was 10.00 × <sup>10</sup><sup>6</sup> m3/a, accounting for 46.55% of the total. The vertical recharging amount in the southeast was very small and negligible. The total discharging volume was 36.87 × 106 m3/a, including lateral discharging whose amount was 15.67 × 106 m3/a, accounting for 42.5%, and the artificial pumping amount was 21.2 × <sup>10</sup><sup>6</sup> <sup>m</sup>3/a, accounting for 57.5%. The upstream confined aquifer received lateral recharging and vertical leakage recharging from the unconfined aquifer, and the downstream confined aquifer only received lateral recharging from the upstream confined aquifer. The confined aquifers in the entire region were in a state of negative balance, and this state was continuously accumulated from the upstream to the downstream.

**Author Contributions:** Conceptualization, J.H. and Y.G.; methodology, J.H.; software, J.H.; validation, J.H., Y.G. and S.L.; formal analysis, J.H.; investigation, J.H.; resources, Y.G.; data curation, Y.G.; writing—original draft preparation, J.H.; writing—review and editing, J.H. and Y.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by grants from the National Fund Program (No. U1603243), and the Key Laboratory of Geodynamic Processes and Metallogenic Prognosis of the Central Asian Orogenic Belt (No. 2020–004).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
