Mountain Block and Front Recharge to the aquifers in sedimentary basins in Kazakhstan: evidence from geothermal studies and water isotopes

The mountains often “subsidize” water resources to lowlands in arid and semi-arid regions. In the case of Central Asia (CA), the mountain cryosphere is the “water tower” for the population residing in the intermountain valleys and lowland plains. Much of the water research in CA is focused on surface water provision by mountain glaciers and snow such as rivers, lakes, and reservoirs, whilst groundwater recharge is much less investigated. The preliminary hydrogeologic models of deep (up to 3km) groundwater recharge for sedimentary basins (southern Kazakhstan) in the upper Ily and lower Syr Darya river valleys are derived from interpretations of stable isotopes of oxygen, hydrogen, and tritium. The results show that in these basins groundwater at different depths (both shallow and deep) bears depleted abundance of heavy stable-isotope species indicative of winter precipitation (snow). In Ily basin (Zharkent depression) the proportion of snow(melt) contribution increases with depth with water from very deep geothermal wells (~3000 m, the upper cretaceous aquifer) being more isotopically depleted compared to shallower geothermal wells (i.e. depths up to 650m) and shallow groundwaters. Additionally, formation water in the deep geothermal wells (Zharkent area) had no detectible tritium (< 0.71 TU) pointing to an absence of modern recharge (within the past ~ 70 years). Geothermal water from wells (shallow ground water and depth of ~ 1200 m) and a spring in the lower Syr Darya river and the North Aral area, also shows deleted stable water isotope imprints suggesting strong winter recharge contributions. This is surprising as the Aral Sea depression has very low precipitation (less than 200 mm/year) and very little snowfall. These isotope data suggest regional aquifers are recharged primarily by lateral groundwater flows via deep flow paths from mountain regions. This is further corroborated by low salinity (< 1 g/l) of most deep geothermal water samples pointing to dilution by snowmelt. We suggest the aquifers in these CA regions may be replenished mostly by Mountain Block and/or Front recharge mechanisms, but this hypothesis requires further investigation.