Assessment of spatial and seasonal variations in water quality in Loukkos Estuary (Atlantic Coast of Morocco)

Loukkos estuary is a strategic ecosystem of major ecological and socio-economic importance. This study aims to provide new insights into the estuary's water quality through a detailed analysis of hydrochemical parameters combined with the use of pollution indices. Water samples were collected during wet (WS) and dry seasons (DS) from six stations along the Loukkos estuary. Concentrations of major ions revealed the following order of abundance Cl - > Na + > SO 4 2- > Ca 2+ > Mg 2+ > HCO 3 − > K + , which were higher during DS compared to WS. Spatial distribution revealed a well-marked decreasing gradient from downstream to upstream, attributed to marine influence downstream against more marked continental influence upstream. Nutrients were dominated by PO 4 2- followed by NO 3 - and NH 4 + . Metal concentrations of Fe, Pb, Zn, Cd, Cr and Cu showed mean values of 30.38, 18.61, 4.35, 4.6, 3.37 and 2.81 µg/L, respectively. Dominance of metals can be observed as Fe > Pb > Cd > Cr > Cu > Zn during WS and Pb > Fe > Zn > Cd > Cr > Cu during DS. HPI values ranged from 4.95 (ST1 in WS) to 268.07 (ST4 in DS) and MI from 0.53 (ST6 in WS) to 2.63 (ST3 and ST6 in DS). Based on the results of these indices, sampled stations were classified as unpolluted during WS, while during DS water quality deteriorated at all stations, especially at ST4, ST3 and ST6. PI results indicated Pb (1.58) and Cd (2.72) as most problematic metals, and likely adjacent domestic, industrial and agricultural wastewater were the main pollution sources, especially during DS. These results highlight the areas most affected by pollution and the major toxic pollutants, requiring regular monitoring and priority actions to mitigate their adverse effects on the aquatic ecosystem and human health.