Spatial Variability and Contamination Levels of Fresh Water Resources by Saline Intrusion in the Coastal Low Lying Areas of the Douala Metropolis-Cameroon

Abstract

A study based on the spatial variability and contamination levels of fresh water resources by saline intrusion was conducted in the Douala coastal area. The study was aimed at highlighting the associations between major ions in ground water from which cause-effect relationships could be inferred. Water samples were collected from 19 stations. 3 stations were selected from the mangrove area and 16 stations were selected from the rest of the area partitioned into four transects (coastal transect, inner transect 1, inner transect 2 and inner transect 3). Sampling was done repeatedly during the dry season and these samples were analysed for physico-chemical parameters. Results show that the samples were acidic (pH: 4.7 - 6.7). Total dissolved solids (TDS) and electrical conductivity (EC) values ranged between 70.3 - 3703 mg/L and 136.4 - 7333 μS/cm respectively indicating medium salt enrichment and brackish waters. High temperatures (T°C) and low dissolved oxygen (DO) values of 26°C - 30.3°C and 0.96 - 3.9 mg/L respectively were observed and this could be interpreted as the acceleration of biological and chemical processes of ground water resources. Major ions were within the WHO limits except for Ca2+ (20.3 - 85 mg/L) and Mg2+ (6.6 - 49.6 mg/L) respectively whose concentrations were slightly higher. The leading cations were Ca2+ > Na+ > Mg2+ > K+ while were the leading anions. The multivariate analysis approach (MAA) values obtained for water variables showed that F1, F2 and F3 accounted for 61.6%, 21.5% and 9.9% respectively of total variance with strong loadings and these were considered to account for the ground and surface water quality of the area. The main water types were 63.2% , 26.3% Ca2+-Cl- and 10.5% Na+-Cl-. 89.5% of the water types had secondary salinity implying that anthropogenic sources were the source of salinity. The water was not fit for drinking. The values calculated for percentage sodium and sodium absorption ratio to determine its suitability for agricultural purposes revealed that 63.15%, 15.5%, 5.3% and 15.8% of sampled water were excellent, good, doubtful and unsuitable respectively for irrigation purposes. The need for water resource monitoring and forecasting regarding deterioration in quality is imperative before hazards of the rise in sea level arise.