Hydrochemical and isotopic characteristics of groundwater in the Ndop plain, northwest Cameroon: resilience to seasonal climatic changes

摘要

Shallow groundwater (＞30 mbgl) is an essential source of drinking water to rural communities in the Ndop plain, northwest Cameroon. As a contribution to water management, the effect of seasonal variation on the groundwater chemistry, hydrochemical controls, drinking quality and recharge were investigated during the peaks of the dry (January) and rainy (September) seasons. Field measurements of physical parameters were preceded by sampling 58 groundwater samples during both seasons for major ions and stable isotope analyses. The groundwater, which was barely acidic (mean pH of 6) and less mineralised (TDS ＜ 272 mg/l), showed no significant seasonal variation in temperature, pH and TDS during the two seasons. The order of cation abundance (meq/l) was Na~+ ＞ Ca~(2+) ＞ Mg~(2+) ＞ K~+ and Na~+ ＞ Mg~(2+) ＞ Ca~(2+) ＞ K~+ in the dry and rainy seasons, respectively, but that of anions (HCO_3~- ＞ NO_3~- ＞ Cl~- ＞ SO_4~(2-) ＞ F~-) was similar in both seasons. This suggests a negligible effect of seasonal variations on groundwater chemistry. The groundwater, which was CaMgHCO_3 and NaHCO_3, is chemically evolved rainfall (CaMgSO_4Cl) in the area. Silicate mineral dissolution and cation-exchange were the main controls on groundwater chemistry while there was little anthropogenic influence. The major ions and TDS concentrations classified the water as suitable for human consumption as per WHO guidelines. The narrow cluster of δ~(18)O and δD of same groundwater from both seasons between the δ~(18)O and δD values of May-June precipitation along the Ndop Meteoric Water Line indicates meteoric origin, rapid recharge (after precipitation) and timing of recharge between May and June rainfall. Diffuse groundwater recharge mainly occurs at low altitudes (＜ 1,400 m asl) within the plain. Besides major ions and TDS, the similar δ~(18)O and δD of groundwater from both seasons indicate a consistent groundwater recharge and flow pattern throughout the year and resilience to present day short-term seasonal climatic variations. However, controlled groundwater abstraction is recommended given the increasing demand.