Groundwater Recharge Process in Tuul River Watershed,
Ulaanbaatar, Mongolia
201121279
Kohsuke Tomimatsu
1. Introduction
Ulaanbaatar (UB), capital city of Mongolia, located in semi-arid region has shown a significant
economic growth. Sugita (2003) mentioned approximately 90 % of population in UB depends on
groundwater for domestic usage. Davaa et al. (2002) reported, during winter season, groundwater table in
the floodplain area declines declined due to over pumping and frozen river. In the end of spring, δ18O and
δD in the groundwater in UB shows clear decreasing according with that of Tuul River. This result might
be influenced by snow melting. Ikeda (2011) mentioned Tuul River, flowing through UB as a major river,
is dominant source of the groundwater in the alluvial plains in summer. Also, a certain precipitation in
winter might affect the groundwater in UB in summer. However, the mechanism of the groundwater
recharge by river water in snow melt season has not been investigated. Therefore, the objective is to
evaluate a role of river water in groundwater recharge in spring season at Ulaanbaatar.
2. Methodology
We performed field survey in UB city from 29 th on April to 6th on May 2013. In this survey, 49 locations
(Wells: 40, Springs: 2, River: 7) were investigated and Electric Conductivity (EC), Oxidation Reduction
Potential (ORP), Dissolve Oxygen (DO), pH, Water Temperature, Static Groundwater Level (SGWL) and
Dynamic Groundwater Level (DGWL) were measured. And, 46 water samples were taken for the analysis
of inorganic solute constituents and stable isotopes of δ18O and δD.
3. Results and Discussion
The groundwater level in the flood plain in Tuul River was lower in the spring 2013 than that observed
in summer 2009 and 2010, suggesting a recharge from the river water was dominant due to high hydraulic
gradient in the spring season. Also, the stable isotopic ratio of
18
O and deuterium was higher in the
groundwater during the spring than that of summer in 2009 and 2010, whereas that of river water in the
spring 2013 was lower than that of summer 2009 and 2010.
End member mixing analysis (EMMA) was applied to estimate contribution rate of groundwater in the
floodplain area on May. Tuul River, a well in the South area and a well in the North area were used as End
member and δ18O and HCO3- were used as tracers. The contribution ratio of Tuul River in spring,
groundwater in the North Mountain, and a well in the South Mountain to floodplain groundwater was
estimated to be ranging from 20.3% to 69.8%, 4.9% to 28.3% and 1.7% to 68.3%, respectively.
Additionally, contribution ratio of Tuul River in summer, that of spring, groundwater in North Mountain
and groundwater in South Mountain to the groundwater in the floodplain area in summer was estimated to
be 60.4% to 94.0%, 0% to 15.0%, 1.7% to 31.9% and 0% to 23.6%, respectively. δ18O, HCO3- and SiO2
were used as tracers.
4. Conclusions
The role of groundwater in the south mountain was dominant in the groundwater of the floodplain
during melting season in 2013, whereas the role of Tuul River water in the spring and summer seasons was
dominant in recharge of the groundwater in the floodplain of Tuul River during summer season 2009 and
2010.
5. Reference
Davaa, G., Oyunbaatar, D., Klaus, F., (2002) Interaction of groundwater and surface waters revealed by
stable isotopes in the Tuul river basin, Mongolia
Ikeda, K., (2011) Investigating Interaction Between River and Groundwater around Ulaanbaatar, Mongolia
Sugita, M., (2003) Interaction between hydrological cycle and biological cycle. Issue 5, pp. 559-562.
1320m
1315m
1310m
1315m
1310m
1305m
1305m
1300m
1300m
1295m
1290m
1285m
<August 2009>
1295m
1290m
1285m
<May 2013>
Fig. 1 Spatial distribution of hydraulic head of groundwater
(May 2013 and August 2009)
Fig. 2 Spatial distribution of end member mixing Fig. 3 Spatial distribution of end member mixing
analysis (May 2013)
analysis (August 2009)