Page 114 - Tallinn University

Tallinna Ülikooli üliõpilaste 2015/2016. õppeaasta PARIMAD TEADUSTÖÖD / Artiklite kogumik HARIDUSTEADUSED
most every hydrodynamic aspect. e latter was particularly evidently exempli ed by the fact that the groundwater, which was more similar to the water of the Nabala-Rakvere aquifer than the water of the Tuhala River, occasionally discharged in some of the lower springs of the WW group.
e karst formations have apparently developed on the basis of a network of intersecting NW-SE and NE-SW fractures. e main absorption points in the recharge area (the grike zone of ämmaauk, the Kirsutauk ponor in the Virulase valley) are related to the NW-SE oriented features. e section of Tuhala River, stretching from the Kata dam until the Virulase valley, is able to absorb approximately 1.4-1.6 m3/s of water, the rest is absorbed in the numerous ponors of the Virulase valley. e hydraulic gradient and NE-SW oriented features determine the ultimate ow direction of the karstic groundwa- ter towards the Pirita bedrock valley in the NE.
e distribution and thickness of poorly permeable Quaternary glacial till is one of the main determi- nants of the formation and functioning of springs in the discharge area of the TKA. As the groundwa- ter is able to discharge to the surface in places where the poorly permeable Quaternary cover is thin or absent, the bottoms of su osion sinkholes and bedrock escarpments are usually preferred locations. e WW is situated in the bottom of a gentle depression formed in Quaternary deposits. In order to establish over ow in the WW, the piezometric GWL 56.39 m asl has to be overcome. In the recharge area, the corresponding threshold conditions for the WW over ow are WL= 59.23 m asl and Q=3.97 m3/s in the Kata dam monitoring point (the southwesternmost monitoring point in Fig. 1). e ini- tiation of over ow in the WW primarily depends on achieving a su cient water level and discharge in the recharge area, however the saturation level of the adjacent aquifer may have an considerable impact on this phenomenon. e summarized Q of the WW spring group during over ow was meas- ured to be approximately 1 m3/s.
A direct hydraulic connection between the recharge and discharge area of the karst system was con- rmed by three groundwater tracer tests carried out in the TKA. e results show that karst ground- water travels at rapid velocities (Vmax=500-673 m/h) at relatively modest hydraulic gradient (0.003- 0.004) indicating that karstic groundwater ows primarily through well-developed conduits. e breakthrough curves also revealed that in addition to the main conduit(s), numerous smaller tribu- tary ow paths might exist in the system. e latter suggests that the TKA karst system is in essence a network of hydraulically interconnected fractures and joints (a fracture zone), among which, bigger conduits function as preferential ow paths.
Tracers injected in the ämmaauk, the Virulase cave and the Kirstuauk ponor, were all detected in the Veetõusme spring group, giving assurance that this is the preferred ow trajectory during me- dium- ow conditions (up to WL=55.5 m asl and Q=2.4 m3/s). e groundwater physico-chemical parameters and water level correlations indicated that the WW is permanently hydraulically linked to the karst system. However, during all three tracing tests carried out, tracers were not detected in WW spring group. e reason for this was the fact that all three tracing tests were conducted during medium- ow conditions and the prerequisite ow threshold piezometric GWLs (55.5-56.39 m asl) were not exceeded in the TKA karst system.
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