Groundwater Contamination in Semi-Arid Zones: Assessing Organophosphorus and Organochlorine Pesticide Risks from Agricultural Intensification in the Guir Watershed

This study evaluates pesticide contamination in groundwater downstream of the Kaddoussa Dam (Guir watershed, Morocco) and investigates the influence of agricultural activities on water quality. Nine sampling stations were strategically selected during November 2023 (post-agricultural season) to analyze spatial patterns of 18 pesticide residues: 7 organophosphorus and 11 organochlorines. Identification and quantification were performed via gas chromatography method, targeting both compound classes. Key findings reveal moderate yet localized contamination. The total concentration of organophosphorus pesticides (∑POPs) ranged from 0 µg/L (S8) to 0.191 µg/L (S4), with peak concentrations at stations S3 (0.190 µg/L) and S4 (0.191 µg/L), correlating spatially with intensive agricultural zones. Otherwise, the total concentration of organochlorine pesticides (∑POCs) showed lower levels (0–0.060 µg/L), with maxima at S4 linked to endosulfan and HCH isomers.  Notably, none of the detected organochlorine concentrations exceeded 0.06 µg/L, indicating relatively low levels of contamination. Central stations (S2–S6) exhibited co-occurrence of both pesticide groups, dominated by organophosphorus pesticides (0.135–0.191 µg/L), while peripheral sites (S1, S7, S8, S9) displayed negligible or undetectable residues. Despite sub-regulatory thresholds, the persistent detection of pesticides underscores ecological and public health risks, particularly in arid regions with heightened vulnerability due to limited healthcare access and water scarcity.  Even at low concentrations, organophosphorus and organochlorine pesticides pose significant threats to aquatic ecosystems through bioaccumulation, while also presenting acute health risks to farmworkers and local communities dependent on contaminated groundwater. This study highlights the urgent need for integrated pesticide management strategies to mitigate long-term environmental and socio-economic impacts in agriculturally intensive, water-stressed regions.