A regional investigation on natural background levels and contamination assessment of shallow groundwater contaminants in a coastal urbanized area.

Knowledge on natural background levels (NBLs) is indispensable for evaluating groundwater contamination at regional scales. This study used a combination of modified oxidation capacity method and the box plot method to assess NBLs of common contaminants including iron, nitrite, sodium, total hardness, fluoride, sulfate, and selenium in groundwater of a coastal urbanized area of China (Pearl River Delta [PRD]), discussed geochemical factors controlling NBLs of these related contaminants in various groundwater units via multivariate statistical techniques, and evaluated groundwater comprehensive contamination in the PRD on the basis of groundwater NBLs. Results showed that NBLs of iron, nitrite, sodium, total hardness, fluoride, and sulfate in Groundwater Unit I of the PRD were higher than those in other groundwater units. Conversely, NBLs of selenium in Groundwater Units II and III of the PRD were higher than that in other groundwater units. Both of NBLs of iron and total hardness in Groundwater Unit I were higher than their allowable limits in groundwater quality standards recommended by China, while others were not. The high NBL of iron in Groundwater Unit I was probably attributed to the reductive dissolution of iron minerals in Quaternary sediments induced by the mineralization of organic matters in overlaid marine sediments. By contrast, the irrigation of river water and the leaching of trapped seawater in overlying marine sediment were likely responsible for the high NBL of total hardness in Groundwater Unit I. Using a groundwater contamination evaluation method on the basis of NBLs shows that uncontaminated groundwater accounted for 17.6%, 32.4%, 45.9%, and 53.6% in urban lands, peri-urban lands, agricultural lands, and other lands of the PRD, respectively. Groundwater with moderate to extremely high contamination was mainly distributed in Foshan, Dongguan, southern part of Guangzhou, western part of Shenzhen, northern part of Zhongshan, southern part of Zhuhai, and downstream of these cities. The leakage of industrial wastewater and domestic sewage and the irrigation of contaminated river water were likely main factors controlling groundwater comprehensive contamination in the PRD. PRACTITIONER POINTS: Groundwater natural background levels (NBLs) of seven contaminants in a coastal urbanized area (Pearl River Delta) were assessed. Geochemical factors controlling groundwater NBLs of seven contaminants in the Pearl River Delta were discussed. Groundwater comprehensive contamination in the Pearl River Delta was assessed on the basis of groundwater NBLs.