This study aimed to evaluate groundwater quality in Bac Lieu province, Vietnam using multivariate statistical methods and groundwater quality indices (GWQI). Eleven groundwater quality parameters including pH, chloride (Cl^-), total dissolved solids (TDS), nitrate (N-NO₃⁻), ammonium (N-NH₄⁺), sulfate (SO₄²⁻), iron (Fe), manganese (Mn), Arsenic (As), hardness and coliforms were collected at seven monitoring sites in May 2020. These parameters were compared with the national technical regulation on groundwater quality (QCVN 09-MT:2015/BTNMT). Cluster analysis (CA) and principal component analysis (PCA) were used to elaborate on the groundwater quality variation and pollution sources. The results indicated that groundwater in the study area was polluted by N-NH₄⁺ while other parameters were within the national regulatory limits. The high concentration of N-NH₄⁺ could be attributed to intensive agricultural practices, especially fertilizer usage. CA results divided the monitoring sites into three clusters by the parameters of pH, N-NO₃⁻, Cl^-, TDS, SO₄²⁻, Fe, Mn, and hardness. The results of PCA revealed that the groundwater quality variation could be caused by four potential sources. The main parameters that influenced groundwater quality were pH, Cl^-, TDS, N-NO₃^-, N-NH₄⁺, Fe, and Mn. The GWQI values were in the range of 2.0 – 12.6, which means that groundwater quality at all studied sites is of excellent quality. Preventive measures should be strictly implemented to avoid groundwater pollution since this water source is more pivotal under the effects of surface water pollution and climate change.

Keywords: ammonium, cluster analysis, groundwater, principal component analysis