An assessment of the spatial and temporal distribution of nitrate and trace element concentrations in groundwater in coastal districts of Bangladesh

Groundwater is considered a significant source of drinking water around the world. However, the naturally occurring trace elements, mostly As, B, Fe, Mn, Cu, and Zn are proven to deteriorate the groundwater quality. This study aimed to evaluate the seasonal distribution of trace metals and $N{O}_3^{-}$ in groundwater and the associated risk to human health in the coastal region of Bangladesh. The result indicated that As exceeded the WHO and BDWS limits during wet and dry seasons in several coastal districts. Despite the abundant presence of Fe throughout the entire study area, it does not present any significant health risk. But alarming conditions of Mn have been observed all over the coastal area in both seasons. Aquifers with shallow depths showed to be more contaminated than deeper ones. The spatial distribution maps showed that $N{O}_3^{-}$ and Cr were found in high concentration in some similar areas during the dry season. The studied elements showed a pattern in exceeding of WHO permissible limits such as Fe > Mn > As > Cr > $N{O}_3^{-}$ in wet season and Mn > Fe > As > Cr > $N{O}_3^{-}$ in dry season. Therefore, high non-carcinogenic and carcinogenic risk was found among adult and children population via oral exposure. Most of the samples showed cancer risk at medium to very high. The principal component analysis observed the pollution sources, revealing that groundwater contamination in this region was mostly due to geogenic sources. This study clearly showed that the groundwater in coastal districts is heavily contaminated, which is a concerning issue. The aforementioned findings have given some clarity on the coastal region's groundwater quality state, which can be beneficial in formulating a plan safe water supply.