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Renata Kadlecova, Czech Geological Survey (Czech Republic)
Frantisek Buzek, Czech Geological Survey (Czech Republic)
Harald Oster, Spurenstofflabor (Germany)
Jiri Bruthans, Charles University (Czech Republic)
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The dynamics of the groundwater circulation were studied between 2004 and 2007. First part of the study, took place in two catchment areas in the crystalline rocks and in another two catchment areas the marginal part of the Bohemian Cretaceous Basin (BCB) located in central part of the Czech Republic. Both studied regions have been affected by extensive agriculture activities leading into nitrate release to groundwater. Therefore the nitrogenous contents in the groundwater were monitored as well.
Studied areas have various geological settings. However, unifying element in both areas is a presence of single shallow aquifers in subsurface rock weathering zone. These aquifers have dominant fracture porosity and a free groundwater level. The catchment areas in crystalline rocks are drained from 32 % by artificial drainage system allowing the study of water circulation dynamic in unsaturated zone. Obtained oxygen stable isotopes data enable an estimation of proportion of infiltration water versus groundwater entering into the drainage system. Groundwater nitrate concentration together with CFCs, SF6, oxygen stable isotopes and tritium activity were used to estimate the water mean residence time and/or mixing ratio modern and older water components in the artificial drainage system and wells.
The oxygen stable isotopes data were used for infiltration calculation in the discharge, estimate of the catchment recharge and model fitting. Transit infiltration time to the artificial drainage system is approximately one year highly depending on infiltration and discharge dynamics in the crystalline rocks. Groundwater contribution formed a significant part of drainage discharge (from 65 to 98 percent). Nitrate content and nitrate stable isotopes data were used for specification of nitrate flux and nitrate source in the drainage discharge. The same two-component model was used for nitrate balancing.
The artificial drainage system composes of about 80 % groundwater with mean residence time 5-20 years based on CFCs, SF6 data and tritium tracers and about 10-20 % shallow groundwater flow with residence time 5-13 months based on monthly monitoring of oxygen stable isotopes variation.
The mean residence time in discontinuous remnants of sedimentary fill in the BCB is between 20 and 35 years (including non saturated zone). The groundwater dating indicates that the contemporary nitrate content is the result of extensive agriculture activities in seventies and eighties.
The research is supported by Ministry of Agriculture of the Czech Republic (Project No. QF4062) and Ministry of the Environmental of the Czech Republic (Project No. SP/2e7/229/07).
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