Abstract
Assessing natural background levels (NBLs) of chemical components in groundwater is useful for the evaluation of groundwater contamination in urbanized areas. The present study assessed the NBL of total dissolved solids (TDS) in various groundwater units in the Pearl River Delta (PRD) where urbanization is a large scale and discussed factors controlling groundwater salinity contamination in the PRD. Results showed that the NBL of TDS in groundwater in the coastal-alluvial plain was more than 1.5 times that in other groundwater units because of the seawater intrusion in this groundwater unit. By contrast, interactions of water and soils/rocks were the main factors controlling the NBLs of TDS in other groundwater units. Groundwater salinity contamination in the PRD was positively correlated with the urbanization level. Wastewater from township-village enterprises and industrial wastewater were likely to be the main sources for groundwater salinity contamination in the PRD. Moreover, the wastewater leakage from sewer systems was one of the main pathways for groundwater salinity contamination in urbanized areas, because the proportion of groundwater salinity contamination in urbanized areas formed in 1988–1998 was more than 1.5 times that in urbanized areas formed in 1998–2006 regardless of groundwater units. Besides, sewage irrigation and leakage of landfill leachate were also important sources for groundwater salinity contamination in the PRD.
Highlights
Groundwater plays a crucial role in our livelihoods by making itself available for drinking
Note that a higher value of 3 meq/L was selected as the threshold value for oxidation capacity in comparison with previous studies because geogenic sources are sometimes responsible for high levels of SO4 in groundwater in the Pearl River Delta (PRD) [24]. en, total dissolved solids (TDS) concentrations in PS datasets in various groundwater units were tested by Grubbs’ test (α 0.01) until normal distributions were obtained, and the outliers in various groundwater units were deleted (Supplementary material, Section 3.2) [25]. e remaining datasets were denoted as residual datasets. e maximum concentrations of TDS in residual datasets in various groundwater units were extracted as natural background levels (NBLs)
Using oxidation capacity >3 meq/L and NH4-N concentrations >0.5 mg/L to identify groundwater samples with anthropogenic influence in this study, nearly 60% of groundwater samples with anthropogenic influence in original datasets were removed out (Figure 2), and the remaining datasets were denoted as PS datasets. en, TDS concentrations in PS datasets in various groundwater units were tested by Grubbs’ test (α 0.01), and outliers in PS datasets were eliminated (Figure 3). e remaining datasets were denoted as residual datasets. e maximum concentrations of TDS in residual datasets in various groundwater units are extracted as NBLs; thereby, NBLs of TDS in units A to D were 688 mg/L, 432 mg/L, 321 mg/L, and 99 mg/L, respectively
Summary
Groundwater plays a crucial role in our livelihoods by making itself available for drinking. Groundwater is often contaminated due to the large-scale urbanization and industrialization in urbanized areas [2, 3]; thereby, its quality in these areas is often deteriorated [4]. The Pearl River Delta (PRD) is a rapidly urbanized and industrialized area in China, and groundwater in this area is often contaminated by the salinity because of various anthropogenic sources such as high levels of salinity in wastewaters [3, 5]. Several studies reported that geogenic sources are mainly responsible for the origin of groundwater salinity in Quaternary aquifers in the PRD by using isotopic, hydrochemical, and microbial evidence [6,7,8]. The origins of groundwater salinity in fissured and karst aquifers as well as the spatial distribution of groundwater salinity contamination in the PRD are unclear. us, in order to improve the management and protection of groundwater resources, it is necessary to evaluate the status of groundwater salinity contamination and investigate the factors controlling groundwater salinity in the PRD
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