Origins and processes of groundwater salinization and pollution in urban coastal aquifer under the influence of serious industrialization and urbanization were determined using major and trace element/ion geochemistry, microbial and environmental isotope analyses, and statistical correlation and hierarchical cluster analyses of chemical parameters. Electrical conductivities (EC) of the groundwater in the coastal aquifer ranged from 487 to 4280 µS/cm. Average groundwater EC values measured in wet and dry seasons were 1130 ± 420 µS/cm and 1096 ± 526 µS/cm, respectively. Wells with high EC values were noticed not only near the gulf coastline (1799–2801 µS/cm) but also at the further inland (1709–4280 µS/cm). Statistical evaluation of the groundwater analyses showed that EC had a relatively good correlation with Cl (r = 0.85–0.90, p < 0.01). Cl–Br ion couple also exhibited a significant correlation with a r squared value of 0.9545, suggesting that salinity of the coastal aquifer was controlled dominantly by single source. Piper diagram showed that predominant cation and anion types in the coastal groundwater exhibited shifts generally from Ca to Na, HCO3 to Cl and to less HCO3 to SO4. A significant variation observed in hydrogeochemical facies of the coastal groundwater resulted from dissolution of calcite, dolomite and gypsum, mixing with seawater, formation water and sewage effluents, and cation-exchange processes. Evaluation of Cl/Br mass ratio along with total nitrogen and microbiological contents of the groundwater samples indicated that seawater intrusion was occurring in the coastal aquifer. However, other sources such as mixing with saline formation water (paleo-seawater) and sewage effluents also contributed groundwater salinization locally at the inland. High seawater mixing ratios (2.5–5%) were obtained from wells close to the gulf coastline, while inland wells generally exhibited low mixing ratios (mean 0.36% ± 0.3). Fecal contamination in coastal groundwater was at notable levels. Varying redox environments (oxic/suboxic/anoxic) were observed in the coastal aquifer. Seasonal variation in the redox conditions controlled the Mn, Fe, and As enrichments in the coastal groundwater above their MCL values. This study also showed when employed with the other pollutant indicators, Cl/Br mass ratio could be used effectively to delineate the sources of contamination and salinization in coastal groundwater exhibiting low seawater mixing ratios.