The paper overviews the research material related to occurrence of sinkholes and change of their water quality in the conditions of increasingly natural environment and minimal anthropogenic impact. Sinkholes create favourable conditions for interaction of surface and groundwater. Identification of possible sources of sinkholes pollution is very important in solution of groundwater safety problems. Water quality investigations are carried out in 4 sinkholes, which are of different age and have differently overgrown slopes. Sinkhole water quality may worsen due to accumulation of organic materials (increasing peat content) or natural changes of environment (abrasion of slopes, overgrowth, etc.), while anthropogenic impact is minimal. Water quality in peat-filled sinkholes and young sinkholes with mineral slopes is different. Water of peat-filled sinkholes has bigger amounts of N-NO3, PTotal, SO4 and BOD7 than water in young sinkholes. The highest N-NO3 and SO4 concentration is observed in the sinkhole that is overgrown with trees and shrubs.

The most urgent environmental problem in cultivated land areas of Lithuanian karst region is the protection of subsurface water from technogenic pollution. In terms of subsurface water pollution, the ambience of sinkholes is considered to be particularly dangerous. The work was carried out in 2004-2006 in the district of Birzai. Having analyzed the regime of sinkhole water formed in the vicinity of sinkholes as well as the regime of ground water of their vicinity it was determined that this regime is different in respect of the local hydrogeological conditions. A common regularity was observed: during the snow thaw period and rainy period sinkholes are fed with water from ambient soil and with shallow ground water, during the dry period water accumulated in peat of the failures of sinkholes flows into the ambience, and during the particularly dry periods subsurface water abates in sinkholes and they start draining the ground water very intensively. Analyzing the interrelations of water quality indices it was determined that in most cases the quality of ground water in sinkholes and in their vicinity is closely related. Sinkholes with water level higher than the ground water level and with higher chemical compounds concentrations make no adverse effect on ground water quality due to comparatively low runoff into the environment. However, such sinkholes are considered to be potential pollution sources of lower aquatic layers. Nitrate nitrogen is one of the most harmful pollutants getting into the sinkholes from agricultural fields and farming land plots where mineral fertilizers are applied. Ground water of higher level polluted with the nitrogen is obviously polluting the subsurface water of the sinkhole as well.

The paper gives the data about the mineralization process (sum of ions) and the amount of nutrient ions (nitrogen, phosphorus) contained in water of two streams (basin areas 1.63 square km and 2.02 square km) located in soils of different texture in the karst zone. As the study results have shown, higher amounts of DIN (dissolved inorganic nitrogen) were determined under the conditions of higher humus content in the soil (more humus (0.8-3.0%) was observed in glacial lacustrine soils), uneven runoff distribution (when its formation process is interrupted), and increasing DIN concentrations in silt. N-NO3 purification coefficient in stream water was higher when the stream was flowing via non-drained wood-covered area, also under the conditions of higher air temperatures. TP (total phosphorus) concentrations contained in stream water mostly depended on large amount of precipitation (especially in summer) as well as on phosphorus contained in silt: higher P2O5 concentrations contained in silt determined lower TP concentrations in water (determination coefficient of the relation r**2=0.24-0.30).

Trying to find the best solution for the ecological problem of karst zone (i.e. protection of surface and ground water from agricultural activity) it is important to determine the migration extent of plant nutrients leached from the soil into the streams and ground water depending on the type of land use. The paper presents the results of the studies carried out in two vicinities of intensive karst zone in Birzai district. The obtained results show the influence of drainage water on the quality of stream water in drained areas. It has also been determined that drainage water quality is influenced by soil forming rocks as well as by the type of land use. In the failures of karst zone, as well as in arable land the quality of ground water is worse, especially when ground water is found near the ground surface.