Hydrogeological constraint on nitrate and arsenic contamination in Asian metropolitan groundwater

A total of 243 groundwater samples from five Asian metropolitan areas (Bangkok, Jakarta, Manila, Seoul, and Taipei) were analysed to determine the status and mechanism of groundwater nitrate (NO3) and arsenic (As) pollution. The samples were divided into three groups based on the [NO3] versus [As]: high [NO3] (10–150 mg/l as NO3) with low [As] (0–5 µg/l), low [NO3] (0–10 mg/l) with high [As] (5–120 µg/l), and low [NO3] (0–10 mg/l) and [As] (0–5 µg/l). Groundwater from the Seoul hard‐rock and Jakarta unconfined aquifer belonged to the first group, while that of the confined aquifer of Jakarta, Manila, and Bangkok belonged to the second group and the Taipei groundwater belonged to the last group. Comparison of the hydrogeological parameters and groundwater geochemistry among cities demonstrated that the redox conditions defined the behaviour of contaminants. In groundwater from Manila and Bangkok, where impermeable marine clay deposits comprise a large proportion of the surface geology, NO3 was depleted due to denitrification, while the As levels were higher as a result of the reducing conditions of the aquifer. Conversely, the relatively oxic environments in the Jakarta unconfined aquifers and the Seoul crystalline rock aquifer resulted in less denitrification and As dissolution. The results of this study suggest that [NO3] versus [As] diagrams are a useful method of evaluating the pollution types and contamination processes among the different urban groundwater systems while considering the hydrogeological settings. Copyright © 2011 John Wiley & Sons, Ltd.