Seasonal Changes in Chemical and Microbiological Contamination of Rural Groundwater: A Case Study from Dalgo Pole, Bulgaria

Abstract One of the main EU priority is to ensure clean water for all its residents. However, challenges persist, particularly in rural regions lacking proper infrastructure, where groundwater from individual wells is basically used. The present study examines seasonal changes and potential pollution sources in five groundwater wells G1, G2, G3, G4 and G5 in the village of Dalgo Pole, Bulgaria. The results obtained by standard methods reveal consistent exceedances of NO3- concentrations, particularly notable in autumn, reaching 246 mg/dm3 in well G5. NH4+ concentrations exceed permissible levels only in summer, with values in wells G2 and G5 over 5 mg/dm3. NO2- levels are higher than allowable for drinking water by two times in the autumn sample of well G4. The nitrate pollution index and NO3-:Cl- ratio suggest sewage runoff and fertilization as predominant pollution sources, with seasonal variations. Microbial contamination persists year-round, especially mesophilic aerobic and facultative anaerobic microorganisms at 22 ○С with reduced levels in autumn, and E. coli viable cells peaking in summer and autumn. Others such as Enterococcus faecalis, Pseudomonas aeruginosa, and sulphite-reducing bacteria show highest concentrations in spring-autumn seasons. Principal component analysis indicates consistent positive relationships between NH4+ and microbiological concentrations during summer and autumn, particularly in wells G1, G2, and G4, highlighting seasonal trends and potential contamination sources. Hence, more efforts are needed to improve groundwater quality in vulnerable rural areas of Bulgaria, by implementing enhanced monitoring and control of well waters, mirroring practices in other EU countries.