Occurrence of Escherichia coli as an Indicator of Pollution in Groundwater Wells in Tobruk City, Libya

Abstract

The issue of water scarcity has become increasingly severe, particularly in arid and semi-arid regions like those in the Arab world, where water resources are limited. Tobruk, a city facing such challenges, also grapples with water shortages due to the limited capacity of its desalination plant.  This study aims to detect the presence of E. coli as an indicator of pollution in groundwater wells in the city of Tobruk. Water samples were collected from groundwater wells in the city of Tobruk during the summer (June - July) in the year 2025. A total of 17 samples were collected from wells in different areas of the city, and the presence of E. coli bacteria was discovered using the method: The multiple fermentation tube method, endorsed by both the US Environmental Protection Agency (US-EPA) and the American Public Health Association, which will be employed for E. coli detection. This method provides both qualitative and quantitative analysis, identifying the type and number of bacteria in a sample. Out of the seventeen analyzed samples, bacterial growth was detected in thirteen samples (76.5%), while four samples (23.5%) showed no detectable bacterial growth. E. coli was isolated from seven samples (41.2 %), making it the most prevalent microorganism identified in the present study.  The relatively high proportion of E. coli -positive samples suggests significant fecal input, potentially originating from sewage leakage, surface runoff, or improper waste disposal practices in the studied areas. Mixed bacterial growth was observed in six samples (35.3%), where E. coli co-occurred with Coliform bacteria, Klebsiella spp., or Proteus spp. The presence of polymicrobial contamination is environmentally significant, as it reflects multiple contamination sources and prolonged exposure to organic pollution. Single bacterial isolates were identified in seven samples (41.2%), including Acinetobacter calcoaceticus, Serratia odorifera, Proteus spp., Citrobacter freundii, and Pseudomonas spp.These organisms are widely recognized as environmental and opportunistic bacteria capable of surviving in aquatic ecosystems enriched with organic matter.  Overall, the results indicate that well depth is an important but insufficient factor on its own in determining groundwater microbiological safety. Structural integrity of wells, hydrogeological settings, proximity to pollution sources, and land-use practices collectively influence the occurrence and distribution of microbial contaminants. The findings confirm that several groundwater sources in the study area are microbiologically compromised, with varying degrees of E. coli contamination. Moreover, the presence of other bacterial species in E. coli-negative samples highlights the necessity of comprehensive microbiological assessment rather than reliance on a single indicator organism. Continuous monitoring, improved sanitation management, and appropriate water treatment measures are essential to ensure public health protection.