A study on geochemistry and sources of colloidal fractions in coastal groundwater from different lithologies

The study on colloids in groundwater has been gaining great importance in addressing contamination transport. The objective of this study is to determine the size and nature of groundwater colloids and its association with heavy metals. Groundwater samples were collected from two bore wells representing hard rock and sedimentary aquifers in the coastal region during post- and pre-monsoon seasons. The colloids of 1.2, 0.8, 0.45 and 0.22 µm sizes were studied from these samples for mineral assemblages, geochemistry and metals such as Zn, Fe, Mn, Ni, Cu, Cr and Co. The mineral assemblage was studied by X-ray diffraction and the results show predominance of kaolinite in clayey sand formation, in all size fractions. The dominance of muscovite and illite also was observed in larger fractions obtained from the groundwater of charnockite formation. The X-ray fluorescence results of the colloidal fractions show most of the clays have higher SiO 2 and lower Al 2 O 3 values. Few size fractions were noted to have a high content of Fe 2 O 3 and MgO indicating the dominant source basic rock compositions. The larger fraction (1.2 µm size) indicated the muscovite, 0.8 µm size fraction showed chlorite composition and the smaller colloidal fractions (0.8 and 0.22 µm) reflected illite composition in the hard rock terrain. However, all the size fractions showed the predominance of kaolinite composition in the groundwater samples collected from sedimentary formation irrespective of the season. The heavy metal analysis result shows that the Fe and Mn were predominant in all the size fractions of both locations and in both seasons. Cr and Co were higher in larger fractions of both locations. In general, it is observed that the sedimentary formation well has higher heavy metal concentration except Co. Hence, the study proves that the metal concentrations present in finer size fraction of colloids also contribute to the transport of contaminants. Highlights Lithology plays a major role in groundwater colloids chemistry in the coastal aquifer. X-ray diffraction (XRD) and x-ray fluorescence (XRF) results resemble the variation of minerals predominance in different size fractions of groundwater colloids. The heavy metal association with groundwater colloids also vary with respect to different colloidal fractions.