Natural background levels in groundwater in the Pearl River Delta after the rapid expansion of urbanization: A new pre-selection method

Abstract

Establishment of natural background levels (NBL) of groundwater in urbanized areas such as the Pearl River Delta (PRD) is challenging. Pre-selection methods are the most common approaches for NBL assessment, but it will overestimate (or underestimate) contaminated groundwater in urbanized areas by using present pre-selection methods with empirical definite values because of complicated human activities. Unlike present pre-selection methods, this study aims to establish a new pre-selection method with the indicative of Cl/Br ratios to identify contaminated groundwaters with convincing evidences. Specifically, this new method consists of indicatives of the oxidation capacity and the Cl/Br ratio combining with contaminated-markers. In addition, factors controlling NBL of Cl and NO₃ in groundwater in various hydrogeological units in the PRD were also discussed. Main procedures of this new method: contaminated-markers in various hydrogeological units are extracted by a hierarchical cluster analysis, thereby determining threshold values of Cl/Br ratios and Cl concentration in various hydrogeological units for identifying contaminated groundwater; After that, groundwater chemical datasets was selected by the oxidation capacity, and then tested by Grubbs' test until normal distributions. Groundwater Cl and NO₃ concentrations in datasets before and after this new method are dependent and independent of urbanization levels, respectively, indicating that the new method is useful for groundwater NBL assessment in urbanized areas such as the PRD. Both the seawater intrusion and the diffusion of Cl from marine deposits are likely to be responsible for the much higher NBL-Cl in coastal-alluvial and marine aquifers than in other hydrogeological units. Groundwater Cl enrichment resulted from groundwater recharge and evaporation is mainly responsible for the higher NBL-Cl in alluvial-proluvial aquifers than in lacustrine aquifers, fissured aquifers, as well as karst aquifers. More than double times NBL-NO₃ in alluvial-proluvial and fissured aquifers than in other hydrogeological units is probably attributed to more oxidizing conditions of their vadose zones and groundwaters.

Keywords: Cl/Br ratios; Groundwater; Hydrogeological units; Natural background levels; Pre-selection method.