Characterizing groundwater hydrochemistry and stable isotopes in northern Namibia landscapes with an emphasis on seasonal variations

Groundwater is Namibia’s primary water source, but it is increasingly vulnerable to recharge fluctuations. This study assessed seasonal variations in groundwater hydrochemistry and stable isotopes across three contrasting landscapes – Okatjandja, Uukolonkadhi, and Okongo – over two years, covering wet and dry seasons. Most major ions met Namibia and WHO drinking standards, except Omaoipanga (B6), which exceeded fluoride (>1.5 mg L−1), sulphate (up to 450 mg L−1), and TDS (>1 000 mg L−1) standards. The water quality index revealed better quality during wet seasons, and Okongo had the best overall water quality. Significant seasonal differences were only observed for Na+ and K+. Hydrochemical facies were dominated by calcium bicarbonate (Ca-Mg-HCO3) and sodium bicarbonate (Na-HCO3) across seasons, with rock weathering as the main control. Stable isotopes confirmed meteoric recharge; Okongo showed depleted signatures, while Okatjandja exhibited isotopic enrichment signatures. These findings highlight the need for targeted groundwater management amid climatic stress.