Simeon Valtchev’s research while affiliated with Bulgarian Academy of Sciences and other places

Mercury and its compounds are listed as hazardous substances and prohibited by Bulgarian regulations for discharge in groundwater. Regional studies on the natural distribution of mercury in groundwater are scarce but in recent decades a significant amount of data has been accumulated in the National Environmental Monitoring System. We analysed groundwater samples from two different regions and the results showed Hg concentrations similar to published data and not significantly different from values measured in nearby surface waters, with no patterns found by host rock type. Correspondence between the published data and the results presented here indicate typical background Hg concentrations between 0.1 µg/L and 0.3 µg/L for most regions in Bulgaria. Additional studies with a larger geographical scope are needed for a more detailed characterization of the natural variations in background mercury contents in groundwater in Bulgaria.

Groundwater vulnerability to contaminant infiltration from the surface, and to state deterioration, is related to the spatial distribution of rocks with diverse porosities, flow characteristics and unsaturated zone thickness. For the present regional assessment of groundwater vulnerability, it is considered most appropriate to adopt a method based on spatial differences in the geological and tectonic, and related hydrogeological, conditions in Bulgaria. The objective of the present study is to compile a vulnerability map of the unconfined groundwater in the country. This groundwater type is very important for drinking water supply, and it is at the highest risk of contamination. The groundwater on the territory of the Republic of Bulgaria has been divided into seven vulnerability classes, based on the corresponding hydrogeological units. The map is based on the geological map of Bulgaria in scale 1:100 000. It shows that groundwater is of low vulnerability over a large part of the country’s area (57%). Medium to medium-high groundwater vulnerability is defined for approximately 31% of the total area, while high and very high vulnerability are defined for 7% and 5%, correspondingly.

Bulgaria is rich in sources of mineral water of temperatures in the range of 25 °C–100 °C and various chemical compositions due to diverse geological and hydrogeological conditions. A large portion of the reservoirs of low-mineralisation water is suitable for bottling. Most of them are situated in Southern Bulgaria and related to fractured hydrothermal systems, while in Northern Bulgaria mineral waters of low mineralisation are formed in the relatively shallow levels of artesian aquifers. In the last 30 years, the utilisation of bottled mineral water has increased and, currently, there are at least 18 bottling facilities, while 12 others have closed down for various reasons. The purpose of the present study is to characterise the chemical composition of the Bulgarian mineral waters utilised for bottling, in terms of geological setting, quality, compliance with regulations and possible problems associated with mineral deposition and corrosion. The water type largely depends on the host rocks. In most cases, sodium-type waters are utilized and, in the case of anions, bicarbonate and sulphate. The only exceptions are the waters formed in limestone, dolomite and marble, which are calcium-bicarbonate and calcium-magnesium type.

The regional hydrogeological conditions in Northern Bulgaria determine the existence of a large artesian basin consisting of layered aquifers. The groundwater temperature increase with depth and the conditions in the deeper layers are favourable for the formation of stratified hydrothermal reservoirs with potential for heat extraction. Although it was initiated in the 1980s, the utilization of this green energy resource is still in a pre-feasibility phase. The Upper Jurassic–Lower Cretaceous aquifer has the highest yield and it is the most prospective geothermal source in the multi-layered sedimentary complex in Central Northern Bulgaria. This study provides new understanding of the formation and potential for extraction of thermal energy from this stratum, which has all characteristics of a low temperature hydrothermal reservoir. The study includes delineation of its boundaries, evaluation of hydrogeological and thermal parameters and data on hydraulic connectivity. The flow and temperature fields, the elements of the water and heat balance, as well as the groundwater quality, have been assessed. The groundwater and thermal energy resources in the hydrothermal reservoir have been estimated on a regional scale, including a scenario with reinjection. Different options for potential utilization of the thermal energy using reinjection wells have been put forward.

The objective was to investigate how water infiltrated into waste dumps at a mine site. The electrical resistivity method of field geophysics was applied to produce 4D imaging of progressive water infiltration into the waste dump. The goal is to test a method for investigating how rain water infiltrates unconsolidated materials in mine waste dumps. This is an important problem when evaluating the water balance in waste dumps and understanding the conditions for contamination of the water flowing through the waste materials. The trial was carried out in one of the two large dumps at Elatsite mine, which are composed of rocks with various fragment size and diverse mineral composition. The investigation was undertaken by discharging salt solution into the waste dump and taking geophysical measurements on a rectangular electrode grid at certain time intervals. The grid consisted of 64 electrodes forming 10×5 m cells and covering a 70×35 m area. As a result, it was possible to record how the infiltration and dispersion of the salt solution developed in space and time. In the last one of the seven surveys, 40 hours after the start of the trial, it was established that the salt solution reached a depth of approximately 40 m. The results could be used for predicting the interaction between water and waste material.

Bulgaria is relatively rich in geothermal water of temperature in the range of 25ºC-100ºC and the total flow-rate exceeds 3,000 L/s. Generally, there are more than 170 geothermal fields: 102 of them are state-owned and the rest of them – belong to some municipalities. Up to 72% of the total known flow-rate from the reservoirs has a comparatively low temperature - up to 50ºC. Flow-rates for individual sources vary within 1-20 L/s for most of the reservoirs. The highest temperature (approximately 100ºC) is measured at the surface in Sapareva Banya near Rila Mountain. TDS (total dissolved solids) vary between 0.1 g/L and 1.0 g/L for most of the reservoirs in Southern Bulgaria, while in Northern Bulgaria it is significantly higher – the maximum is up to 150 g/L. The installed capacity amounts to about 97.5 MWt (for 2017), excluding the low grade energy use by ground source heat pumps (GSHP). Geothermal energy has only direct utilization - in balneology, heating of buildings, air-conditioning, greenhouses, geothermal ground source heat pumps, direct thermal water supply for industrial processes. Geothermal water is also used for bottling of potable water and soft drinks. Most of the hydrothermal sites are developed as mountainous or sea resorts. Electricity generation from geothermal water is not currently available in the country.