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Tallinna Ülikooli üliõpilaste 2015/2016. õppeaasta PARIMAD TEADUSTÖÖD / Artiklite kogumik HARIDUSTEADUSED materialS and methOdS
Surface water level (WL) points and groundwater level (GWL) monitoring points were set up at the key recharge and discharge points of the TKA (Fig. 1). e manual WL-GWL monitoring took place on average three times a month during the study period of October 2014-December 2016. In all the WL monitoring points, discharges (Q) were measured during di erent ow conditions for six times. e ow velocity or Q measurements were carried out with General Oceanics 2030 current meter, SonTek FlowTracker ADV (Acoustic Doppler Velocimeter) and a boat mounted SonTek RiverSurveyor S5 ADP (Acoustic Doppler Pro ler). Based on the measured Qs and WL monitoring data, the WL-Q rating curves were compiled for each monitoring point. During the study period, three quantitative groundwater tracer tests with sodium chloride (NaCl) (TT-1 & TT-2) and uorescent dyes (TT-3) were conducted. YSI 556 MPS, YSI Professional Plus and YSI 600XLM eld multimeters were used to monitor electrical conductivity readings during TT-1 & TT-2. Fluorescent tracers were detected from grab samples and charcoal bags. e quantitative analysis of uorescent dye samples were carried out in the biochemistry laboratory of Tallinn University. e Estonian Weather Service and Estonian Land Board provided meteorological, hydrological and cartographical data. e Geological Survey of Estonia provided additional survey equipment.
reSultS and diScuSSiOn
During the period of October 2014-December 2016 (extended, originally until February 2016 in the master thesis), three high- ow periods with several ow peaks were observed. During the observed period, the Q of the Tuhala River varied between 0.035 and 4.5 m3/s and the mean Q was 1.1 m3/s. As indicated by the synchronous uctuations of WL/GWLs, a direct hydraulic link exists between the recharge and discharge areas of the karst system. In less than an hour, the slightest WL changes in the recharge area re ected in the water levels of the discharge area situated approximately 1-1.2 km downstream.
During medium to high ow conditions, the input Q in the recharge area of the karst system was higher than the output Q (on average 70-80% of the input) at the discharge area (Fig. 2). e Q imbal- ance could be attributed to the threshold-controlled dynamic throughput capacity of the karst system as it observed that the recharge area (the Virulase valley) was to become inundated when the threshold Q (2-2.4 m3/s) was exceeded. e throughput capacity was primarily determined by the summarized output Q capacity of the ow paths of the Veetõusme perennial spring group and the WW intermittent spring group, of which the latter was activated only during high ow conditions. e total throughput capacity of the karst system could reach approximately 3.2-4 m3/s. When speci c threshold WL in the recharge area was exceeded, the excess recharge was to partially bypass the karst system along the Kuie River dry valley.
During low ow conditions, the output Q could be two times higher than the input Q (October 2015 in Fig. 2), indicating to recharge originating from the adjacent aquifer. e groundwater exchange with the adjacent aquifer was also con rmed by the fact that during recession periods (periods when the adjacent aquifer was gradually depleting the saturated dynamic storage), some springs were dis- charging groundwater, which di ered signi cantly from the karstic groundwater originating from the Tuhala River. Although the operation of the karst system is primarily dependent on the hydrological conditions in the Tuhala River, a signi cant interaction with the adjacent aquifer is perceivable in al-
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