3. Assessment of Heavy Metals in Sediments from Exploratory Wells for Riverbank Filtration Sites Impacted by Extreme Environmental Conditions Using Principal Component Analysis

Heavy metal contamination of sediments is often observed as a result of the expansion of industrial sectors and agro-economic systems, especially in developing regions. The high pollution of the river Yamuna in India, especially the substantial heavy metal contamination not only to the river but also increasingly to groundwater in Mathura and Agra regions is of concern for the production of drinking water. Consequently, this study focused on a prognosis of the risk of heavy metal contamination during the investigations for a new riverbank filtration (RBF) site in the cities of Mathura and Agra. Twenty sediment samples were taken at each site during the drilling of an exploratory well for RBF up to a depth of 30 m. The heavy metals As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were analysed in soil and aquifer sediments and water from the exploratory wells. Principal component analysis (PCA) was subsequently performed for the heavy metal concentrations in the soil and sediment samples. In general, the heavy metal concentrations found in the aquifer sediments were significantly lower compared to concentrations in riverbed material reported in literature, lower than WHO limits for agricultural soils (except Cd and Pb in Agra) and lower than values in other literature sources. While the heavy metal concentrations found in the exploratory well water in Mathura were generally found to be within the WHO drinking water guideline limits, the mean concentrations of Cd and Pb in the exploratory well in Agra significantly exceeded the WHO guideline values. The study concluded that the risk of leaching of heavy metals and consequent contamination to groundwater by the vertical movement of irrigation water is expected to be significantly greater compared to the movement of infiltrated river water through the riverbed during the RBF. Consequently, caution should be exercised when selecting flood-plain areas for new RBF sites that have been irrigated in the past with surface water impacted by heavy metals. The major sources of heavy metals identified by PCA were mainly natural and to a certain extent anthropogenic, especially in the upper layers of the soil/aquifer and is also indicative of a lesser risk of heavy metal contamination during RBF. Nevertheless, for RBF to be effective at new sites impacted by extreme environmental conditions, well-head and source-protection zones have to be implemented to avoid contamination of the aquifer from above ground anthropogenic activities. Furthermore, frequent water quality monitoring for not only heavy metals, but also other parameters in the RBF well(s), river and ambient (landward side) groundwater is important.