Comparative analysis of shallow and deep groundwater chemistry, focusing on spatial distribution and assessment of associated health risks at the south-central coastal areas of Bangladesh

ABSTRACT This study analyzed 44 groundwater samples – 30 from shallow (<200 m) and 14 from deep (>200 m) aquifers – to assess the drinking water quality in the Barguna district. Parameters such as temperature, electrical conductivity (EC), and pH were measured on-site and major ions (Na, Ca, Mg, K, HCO3, Cl, and SO4), As, Fe, and Mn were analyzed in the laboratory. The spatial distribution of ions shows higher levels of Ca, Fe, and HCO3 in shallow tubewells (STWs) in the north, and elevated levels of Cl, Na, and K in the south. In deep tubewells (DTWs), only HCO3 is higher compared with STWs. STWs are mainly NaCl-type, while DTWs are characterized by Ca-HCO3 and Na-HCO3 types. The high EC values and the presence of Cl and Na in STWs indicate brackish to saline water, likely due to ancient saline pockets or saline water intrusion. Meanwhile, the prevalence of HCO3 in DTWs, along with low Na and Cl, suggests relatively fresh groundwater not affected by saline intrusion. According to health risk guidelines, shallow groundwater is highly vulnerable to saline and As contamination, which poses significant health risks. Conversely, deep groundwater is considered safe for drinking as it appears free from As contamination.