Distribution characteristics and causes of arsenic in shallow groundwater in the Pearl River Delta during urbanization

doi:10.13745/j.esf.sf.2022.1.26

Abstract

Abstract:

A high level of arsenic (As) in groundwater (ρ(As)>10 μg/L) is a potential threat to safe drinking water and the ecological environment. The sources of As in groundwater derived from coastal urbanized areas are complex, which mainly include various anthropogenic and geogenic sources. The rapidly urbanized Pearl River Delta was selected as the study area. The occurrence of and key driving factors for As enrichment in the shallow aquifers in areas with different urbanization levels in the Pearl River Delta were evaluated via mathematical statistics and principal component analysis. According to the results, the total dissolved As concentration in shallow groundwater ranged from below the detection limit to 420 μg·L^-1 in the study area, with As(Ⅲ) as the main form. ρ(As) was found to be generally higher in porous aquifers than in fractured or karst aquifers. The proportion of As in the groundwater of urbanized areas was more than five times that of non-urbanized areas. Among the 1567 groups of groundwater samples collected from the study area, 89 high As groups, or 5.7% of total, had high level of As (ρ(As) >10 μg·L^-1). Among them, 82 groups were from shallow porous aquifers, 4 groups from fissure aquifers, and 3 groups from karst aquifers, accounting for 7.8%, 0.8%, and 9.7% of total, respectively. Compared with historical hydrochemical data collected from 2005 to 2008, the average ρ(As) in shallow high As groundwater of newly added construction areas increased by 30% in 10 years. The chemical type of high As groundwater mainly included HCO₃-Ca and Cl-Na types, characterized by high pH, low redox potential (Eh), low $NO 3 -$ concentration, and positive correlation between ρ(As) and concentrations of $NH 4 +$ , Fe, and Mn or oxygen consumption. Under microbial degradation and mineralization of organic matter, reductive dissolution of As-bearing iron (oxygen) hydroxide in the Quaternary basement muddy sedimentary strata in the Delta plain was identified as the cause of As mobilization. The leaking and infiltration of As-containing industrial wastewater produced during the urbanization process were also important sources of As in groundwater in Shunde District, southern Foshan City. Due to the dual effects of the original sedimentary environment and human input, the neutral to weakly alkaline, closed to semi-closed reducing environment that is rich in organic matter formed in the Delta plain, was the main cause of the occurrence of high As in groundwater.

Key words: urbanization, arsenic in groundwater, distribution characteristics, driving factors, ionic ratios, Pearl River Delta

CLC Number:

X523

LÜ Xiaoli, ZHENG Yuejun, HAN Zhantao, LI Haijun, YANG Mingnan, ZHANG Ruolin, LIU Dandan. Distribution characteristics and causes of arsenic in shallow groundwater in the Pearl River Delta during urbanization[J]. Earth Science Frontiers, 2022, 29(3): 88-98.

Figures/Tables 11

Fig.1 Hydrologic map of the study area and distribution of groundwater with different levels of arsenic contamination

Fig.2 Hydrogeological profile of I-I' and II-II' (locations see Fig.1)

Fig.3 Distribution of high As groundwater in areas with different urbanization levels

Fig.4 Piper plots of groundwater classified by arsenic concentration

Fig.5 Eh-pH plots for high As groundwater in areas with different urbanization levels

Fig.6 Relationships between As concentration and pH or NH 4 +, COD, DO, NO 3 - and Eh concentrations in high As groundwater

Table 1 Arsenic statistics for groundwater in various aquifers and in areas with different urbanization levels

类别		样品组数	最小值/ (mg·L^-1)	最大值/ (μg·L^-1)	平均值/ (μg·L^-1)	中位值/ (μg·L^-1)	变异系数/ %	检出率/ %	高砷地下水率/%
含水层类型	孔隙水	1 054	ND	420	5.32	0.9	414	80.5	7.8
	裂隙水	482	ND	56	0.87	0.2	356	61.2	0.8
	岩溶水	31	ND	25	2.73	0.5	211	83.9	9.7
城镇化水平	2008年建设用地	383	ND	420	5.63	1.7	481	85.1	7.3
	2009—2018年新增建设用地	691	ND	210	5.08	0.6	361	77.1	8.1
	非城镇建设用地	493	ND	22	0.894	0.2	248	63.1	1.0
珠三角地区合计		1 567	ND	420	3.90	0.5	469	74.7	5.7

Fig.7 Variations of As concentration with the depth to water table in high As groundwater in various aquifers

Fig.8 The ρ(As) box diagram of groundwater in different periods and urbanization levels of high-arsenic groundwater

Fig.9 Gibbs diagrams for hydrochemical composition of high As groundwater in various aquifers

Table 2 Principal component loadings for shallow groundwater samples

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