Groundwater quality assessment using the Water Quality Index (WQI) and Pollution Load Index (PLI) in Derna, Libya, following Cyclone Daniel

Abstract

Groundwater is the primary source of drinking water in the city of Derna, eastern Libya. However, the catastrophic flooding caused by Tropical Storm Daniel has raised serious concerns about the degradation of groundwater quality and its suitability for human consumption. This study aims to assess the impact of Tropical Storm Daniel on groundwater quality through the application of the Water Quality Index (WQI) and the Pollution Load Index (PLI). Groundwater samples were collected from several wells in the city of Derna after the disaster. The physicochemical and microbiological parameters analyzed included pH, electrical conductivity (EC), total dissolved solids (TDS), nitrates (NO₃⁻), nitrites (NO₂⁻), ammonium (NH₄⁺), ammonia (NH₃), Escherichia coli (E. coli), and total coliforms. Water quality assessment was conducted in accordance with World Health Organization (WHO) recommendations and Libyan standards. The results revealed significant spatial variability in groundwater quality among the wells studied. Water quality index (WQI) values ​​ranged from 30.55 to 127.78, indicating quality classes from excellent to poor. The Kouriya and Bab Tobrouk wells exhibited the best water quality, with WQI values ​​of 30.55 and 44.00, respectively. In contrast, the Al-Jubaila and Al-Masouri wells recorded the highest values ​​(127.78), indicating a significant decline in water quality. This deterioration is primarily linked to high concentrations of electrical conductivity (4550 µS/cm), TDS (2520 mg/L), nitrates (66.4 mg/L), ammonium (0.6 mg/L), and ammonia (0.6 mg/L). PLI values ​​ranged from 0.211 to 1.703, indicating that the majority of wells were unpolluted, while some showed significant contamination. Furthermore, severe microbiological contamination was observed in some wells, with E. coli concentrations exceeding 100 CFU/100 mL. The results demonstrate that Storm Daniel had a significant impact on groundwater quality, promoting salinization, nitrogen enrichment, and localized microbiological contamination. Continuous monitoring, along with appropriate management and treatment measures, is therefore essential to ensure the safety and sustainability of groundwater resources in the Derna region.