VOLUME 48 (1)/April 2019

Groundwater resources on the territory of Bulgaria are unevenly distributed in both spatial and temporal aspects. The effective usage of these valuable assets is of paramount importance, since any over-exploitation would eventually lead to their depletion. Remote sensing data and satellite images have increasingly been used in groundwater exploration and management. An integrated approach was applied in the present study in order to delineate potential groundwater recharge zones on the territory of Bulgaria. Data from various sources were used to prepare different thematic layers. These layers were then transformed into raster data of 1×1 km. Lineament and drainage density maps of the research area were made with the help of GIS technology. In addition, a map was made for the annual total precipitation for the period from 1931 to 1985. DEM (Digital Elevation Model) data on a global scale at 90 m horizontal resolution were used for the slope analysis. A groundwater potential map was produced, which integrates several thematic maps, such as annual rainfall, geology, lineament density, land use, slope, soils, and drainage density. The thematic maps were then converted into a raster graphic format in order to be easily integrated into a GIS platform. The raster maps of these factors were then allocated a fixed score and weight-computed. The weights of those factors contributing to the groundwater recharge were derived by using the following components: geological map, lineament-length density map, land cover data base, soil data base, drainage-length density map, and slope gradient map. Subjective weights were assigned to the respective thematic layers, and they were overlaid in a GIS platform for the identification of potential groundwater recharge zones within the study area. These potential recharge zones were then categorized as being very good, good, moderate, poor, and very poor.

A detailed investigation into the calcareous nannofossils from the upper Campanian–Maastrichtian deposits of the Kladorub Formation (NW Bulgaria) has been carried out in order to examine their taxonomic content and test the applicability of cosmopolitan zonation schemes for this stratigraphic interval in the country. The Kladorub Formation is composed of silty to fine-sandy marlstones and rare marly limestones, occasionally interbedded with sandstone layers. The recovered nannofloras are abundant, taxonomically diverse and exhibit predominantly moderate preservation, which allowed precise taxonomic identifications and biostratigraphic analysis to be made. As a result, the presence of two previously undocumented, biostratigraphically significant taxa has been recorded (i.e., Eiffellithus parallelus and Ceratolithoides kamptneri). Consequently, the studied Upper Cretaceous sediments have been assigned to subzone UC15d^TP (pars.)–subzone UC20d^TP; in the uppermost 2 m of the section, the presence of zone NP1 has also been indicated, which is in concordance with previous authors’ data. Due to the lack of proper chronostratigraphic framework for the Kladorub Formation, top Uniplanarius trifidus and base Lithraphidites quadratus have, respectively, been used to approximate the Campanian/Maastrichtian and lower/upper Maastrichtian boundaries. The Cretaceous/Palaeogene boundary, however, could not be drawn with precision, because it falls within a 6.5-m interval of non-exposure. The resulting biostratigraphic framework offers higher stratigraphic resolution than previously used local zonation schemes and allows correlation with coeval sedimentary successions from other parts of the Tethyan and Boreal realms.