Yalei LIU, Menggui JIN, Xing LIANG China University of Geosciences, Wuhan mgjin@cug

1. 46th IAH Congress, 2019 #396 Groundwater salinization mechanism in an arid inland basin :the Manas River Basin in the Northwest China YaleiLIU, Menggui JIN, Xing LIANG China University of Geosciences, Wuhan mgjin@cug.edu.cn

2. Outline Introduction Groundwater salinization mechanism Soil evaporation and salts accumulation Conclusions

3. Manas River Basin (MRB) • TypicalMountain-Oasis-Desert inland basin • Continental arid climate • Hot and dry summer and chilly winter Mean annual air T=6.5 ºC Mean annual P: 100-200mm Mean annual E: 1500-2000mm （Liu, Jin et al, HP, 2018） 2/17

4. Continuous decline of GWL and GW salinization Declined rate of GW level increased from upstream to downstream: • 0.74 m/yrin piedmont plain; • 0.59~3.07 m/yr& 0.35～3.98 m/yrfor middle and deep confined aquifers in fine soil plain. Total dissolved solids (TDS) of GW are increasing

5. Objectives • In order to rational use of water resources and protection of soil salinization and the environments in the arid water-stressed basin, GW salinization mechanism and the interactions between soil evaporation and salts accumulation processes were investigated based on systems analysis of regional hydrogeology, hydrogeochemistryand water stable isotopes in aquifers and aquitards. • Field monitoring, bromide tracing, soil column experiments and numerical modeling were applied to study the interactions between soil evaporation and salts accumulations.

6. Groundwater flow • GW generally flows from the southern piedmont plain to the northern fine soil plain and then to the terminal Manas lake. • The Manas lake is the final discharge area in the inland basin.

7. GW TDSs decrease with the well depth. ZK-01 The three different sizes of circles and squares mean three salinity gradient concentrations: small ones stand for fresh water (0~1g/L); middle ones for brackish water (1~3g/L) and large ones for saline water (>3g/L). （Liu, Jin et al, HP, 2018）

8. Groundwater salinization(GWS) • The deuterium excess（dex=δ2H-δ18O*8）indicate the GW salinization of local flow system is mainly resulted from evaporites dissolution and evaporation; but dissolution in regional system.

9. Groundwater salinization(GWS) Shallow GWS were affected by evaporation and has the lowest remaining coefficient (0.84) and the largest increased salinity (1.97g/L ) for shallow groundwater.

10. Pore water in Aquitards • TDS of pore water in aquitardswere 1.6-52.2g/L. • Calcium sulphate crystals were founded through boreholes.

11. Salinization mechanisms • Cl concentrations and δ18O of GW in aquifers and pore water in aquitardsindicate that dissolution and evaporation are main processes for shallow GW salinization, while vertical mixing with aquitardpore water contributed to the salinization of middle and deep GW. Groundwater in aquifers Pore water in aquitards

12. （Liu, Jin et al, 2018, HP） dex=17.1‰，S=0.19g/L GWSM • GW overflows and forms springs near Jiahezi, which is a local flow system with fresh groundwater; andabout 38 yrs of age. • Irrigation return flow and evaporation in the Oasis Plain are main processes for the GW salinization; and GW ages are 36yr -200yr. • From mountain area to Manas Lakemain horizontal flow is regional flow system and GW ages are 9.7 kyr-32.3 kyr. The GW are salinized mainly due to dissolution processes.

13. Evaporation estimated by bromide tracer • The results show that the evaporation rates (ER) vary from 0.04-2.26 mm/d during May to August, averaged 0.52 mm/d.

14. Interaction between evaporation and salts accumulation Silt column experiments were carried out to understand the interactions between evaporation and salts accumulation: temperature and humidity were control to be nearly constant.

15. Evaporation rates (ER) and electric conductivity (EC) • Evaporation rates decreased and EC of top soil increased with time.

16. Interaction between evaporation and salts accumulation • Evaporation resulted in salts accumulation & formation of salt crust on top soil. • And then the evaporation rates (ER) decreased until the salt crust broken down.

17. Conclusions • Dissolution processes during GW flow, evaporation and evaporites dissolved by irrigation return flow in the Oasis Plain are main processes for the GW salinization. • Saline water released from aquitards to aquifers are important components for GW salinization in the cone of depression areas. • Phreatic evaporation rates in the fine soil plain are 0.04~2.26mm/d during May to August.

18. Conclusions • Evaporation induces the salts accumulation and precipitates on soil surface. The velocities of salt crust growth and the resistance of evaporation increased at the early stage of evaporation, and then evaporation rate slow down mainly due to the salts crust (7.5mm/d).

19. Acknowledgements • National Natural Science Foundation of China (U1403282) • Colleagues of Xinjiang Agricultural University for field work assistant: Professors Jinlong Zhou, Xinguang Dong, Ruiliang Jia, Mr. Jisheng Zhang and others. • Colleagues and students of CUG for cooperation: Yanfeng Liu, Zhang Wen, Bin Ma, Jianjun Wang and others.

20. 谢谢！ Thank you very much for your attention！

21. 地下水咸化 GW samples of stable isotopes distributed left to the GMWL indicate that the GW originates from Tianshan Mountains （Liu, Jin et al, HP, 2018）

22. Groundwater salinization(GWS) dex=δ2H-δ18O*8 The deuterium excess（dex=δ2H-δ18O*8）indicate the GWS in local GW flow system is due to evaporites dissolution and evaporation; but dissolution in regional system. （Huang & Pang, 2012） Isotope and TDS of source streams