Elsevier

Journal of Hydrology

Volumes 438–439, 17 May 2012, Pages 112-124
Journal of Hydrology

Origin of groundwater salinity and hydrogeochemical processes in the confined Quaternary aquifer of the Pearl River Delta, China

https://doi.org/10.1016/j.jhydrol.2012.03.008Get rights and content

Summary

The Pearl River Delta (PRD), located in the coastal area of South China, contains a large confined Quaternary aquifer. This aquifer has been identified to have high concentrations of total dissolved solids–ranging from 1 g/l in the inland area to 26.8 g/l near the southeast shoreline. This work aims to investigate the origin of groundwater salinity and the major hydrogeochemical processes controlling the groundwater evolution in the confined basal aquifer of the PRD. A total of 40 boreholes were drilled in the PRD during 2007–2009, and groundwater samples were collected for major ion (Ca2+,Mg2+,Na+,K+,NH4+,Cl-,SO42- and HCO3-) and isotope (δ18O, δ2H, 3H, δ13C and 14C) analyses. These analyses demonstrate that groundwater in the confined basal aquifer is recharged by local rainwater and seawater, and seawater is the major salinity source for groundwater. The contribution of seawater varies from less than 1% in the inland areas to 74% near the shoreline. SO42- reduction and methanogenesis occur in this reducing confined aquifer. The results of ion exchange show that Mg2+ exchanges mainly with the NH4+ ion, which is derived from the mineralization of sedimentary organic matter. Ion exchange between Na+ and Ca2+ and groundwater facies suggest that the brackish aquifer in the northwest inland area is undergoing freshening. 3H analysis suggests that infiltrated rainwater is younger than seawater. 14C analysis indicates that the seawater most likely infiltrated into the confined basal aquifer during the Holocene transgression period.

Highlights

► Groundwater from a Holocene–Pleistocene aquifer–aquitard system was studied. ► Salinity was caused by palaeo-seawater intrusion. ► Seawater can be kept in such an aquitard–aquifer system for a geologically long time. ► Mg2+ exchanges with NH4+ derived from a geological source.

Introduction

Groundwater salinization occurs in many coastal aquifers (Louvat et al., 1999, Edmunds and Milne, 2001, de Montety et al., 2008, Somay and Gemici, 2009). The origin of groundwater salinity in coastal aquifers has been studied by many researchers, and various sources of groundwater salinity have been identified, including wastewater (Ghabayen et al., 2006), fossil seawater (Tijani, 2004), and modern seawater induced by excessive groundwater pumping (Kim et al., 2003). Groundwater salinity in coastal aquifers has been studied in parts of Southeast Asia (Samsudin et al., 2008, Praveena et al., 2011), but the contribution of paleo-seawater intrusion to groundwater salinization in these areas is not clear.

Seawater intrusion is frequently observed in both confined and unconfined coastal aquifers (Groen et al., 2000, Jorgensen, 2002, Kim et al., 2003, Bouchaou et al., 2009, El Yaouti et al., 2009, Somay and Gemici, 2009), but few studies have investigated the hydrogeochemical processes in confined coastal aquifers that are intensively influenced by paleo-seawater intrusion. Understanding the processes and factors that control the evolution of brackish groundwater in subsurface system is of scientific and practical importance. Confined aquifers overlain by organic matter-rich aquitards, with groundwater having a long residence time, can experience complex hydrogeochemical processes that control the groundwater evolution.

The plain of the Pearl River Delta (PRD) has an area of 6932.5 km2 (excluding the area of Hong Kong) (Huang et al., 1982). This region has seen rapid population increase and economic growth particularly since the 1980s. The water supply of this region relies almost entirely on surface water due to abundant rainfall and the well-developed river network. However, rapid population and economic growth have led to the deterioration of surface water quality through pollution and declining river discharge (Cheung et al., 2003, Lu et al., 2009). Surface water alone can no longer meet the needs of the region, and people are looking to groundwater as an alternative supply. However, groundwater in many areas of the southern part of the PRD has been found to have total dissolved solids (TDS) of over 10 g/l (GHT, 1981). The recent increase in groundwater exploitation in the PRD has necessitated an evaluation of the origin of the salinity and an understanding of the chemical evolution of the groundwater resources in this region.

This research project was undertaken to determine the source(s) of salinity of brackish groundwater and to identify the hydrogeochemical processes related to groundwater evolution in the confined coastal aquifer of the PRD. To accomplish these goals, the spatial distribution of the brackish groundwater was mapped, and the origins of its salinity were investigated using hydrochemical and isotopic techniques. The major ion compositions and δ13C values of inorganic carbon in groundwater were used to determine the hydrogeochemical processes, and Pearson’s correlation analysis (Davis, 2002) was used to evaluate the ion exchange behaviour of the major cations. Radiocarbon dating was carried out to estimate the residence time of the brackish groundwater in the confined basal aquifer.

This study concerns the impact of sea-level changes during a period of paleo-intrusion on coastal aquifer systems, with an emphasis on naturally-occurring high salinity groundwater. Although NH4+ is not generally regarded as a main component of either seawater or fresh groundwater, it exists in significant amounts in many coastal aquifers (Correll et al., 1999, Berg et al., 2001, Bratton et al., 2004, Santos et al., 2008). Naturally occurring NH4+, deriving from mineralization of sedimentary organic matter, has been identified as one of the major ions (concentrations up to 390 mg/l) in most of the confined basal aquifer of the PRD (Jiao et al., 2010). Unlike many other studies that only focus on the ion exchange of major cations of seawater (Ca2+, Mg2+, Na+, K+) in coastal aquifers (Martinez and Bocanegra, 2002, Appelo and Postma, 2005, Capaccioni et al., 2005), this investigation examines the ion exchange behaviour of naturally occurring NH4+ with the major cations of seawater. The findings should be useful for understanding the salinity processes of other coastal aquifers and other major estuaries in which sedimentary settings are largely influenced by transgressions, such as in the coastal aquifers of Malaysia (Samsudin et al., 2008) and the Netherlands (Post et al., 2003). Also, the major hydrogeochemical processes identified in the confined basal aquifer in this research can help to understand the groundwater evolution in other anoxic coastal aquifers, for example, the Gangetic plain in West Bengal and the Hanoi area in Vietnam (Berg et al., 2008, Chetia et al., 2011), which also contain abundant sedimentary organic matter in the sediments.

Section snippets

Geological and hydrogeological settings

Geologically, the Pearl River drainage basin (Fig. 1) was formed by the Tibetan Plateau uplifting during the Tertiary and Quaternary periods (Aitchison et al., 2007). The Pearl River Delta is largely covered with Quaternary sediments, and has an elevation ranging from 6 to 9 m above sea level in the north to 1–2 m near the coast (GHT, 1981). Bedrocks of Cambrian to Tertiary age crop out around and within the delta (Fig. 1).

During the Holocene period, the PRD experienced large-scale transgression.

Field and laboratory studies

A total of 40 boreholes were drilled in the PRD in the dry season (from December to March) during 2007–2009 (Fig. 1A). After drilling, steel pipes with diameter of 110 mm were emplaced to make temporary wells, and the open intervals were screened in the basal aquifer. Well development was carried out, and groundwater was sampled after the pumped water from the borehole became clean. Enough groundwater was collected from each site to fill two 125 ml, high density polyethylene bottles after

Distribution of groundwater salinity and spatial evolution of water facies in the confined Quaternary aquifer

Data from 113 boreholes (Fig. 1A) were used to generate a contour map of TDS (Fig. 2). Among them, 37 samples were collected during this study, the others came from other geological and hydrogeological surveys conducted by organizations of the Guangdong Geological Survey and the South China Sea Institute of Oceanology, Chinese Academy of Sciences. Fig. 1 shows all the 113 boreholes from which salinity data are obtained, and TDS data obtained during this study are presented in Table 1. For

Identification of groundwater origin by δ18O and δ2H isotope analyses

δ18O and δ2H values can provide information on precipitation, evaporated surface water and seawater sources, and thus can help to identify the origins of groundwater salinity. The δ2H and δ18O values of the investigated groundwater samples are shown in Table 1. The δ18O values of modern surface seawater in South China Sea are between −0.2‰ and 0.5‰ (Su, 2001), very close to 0‰ of the VSMOW. Data provided by the International Atomic Energy Agency (IAEA/WMO, 2005) show that the long-term weighted

Water–rock interactions

The consequences of a marine influence on groundwater chemistry in a reducing environment can be evaluated by comparing the chemical compositions of samples with the theoretical composition obtained from a conservative mixing between seawater and fresh groundwater (Appelo and Postma, 2005, Yamanaka and Kumagai, 2006, de Montety et al., 2008). For conservative mixing, the concentration of each ion can be calculated using the seawater fraction (fsea)mimix=fsea×mi,sea+(1-fsea)×mi,freshwhere mi,sea

Discussion of groundwater residence time

Radiocarbon dating is based on measuring the loss of the parent radionuclide (14C) in a sample. Groundwater age is calculated by the exponential loss of the parent according to its half-life (5730 years). The representative decay equation isat=a0×e-λtwhere a0 is the initial activity of the parent nuclide, and at is its activity after time t. The decay constant, λ is equal to ln 2/t1/2, and t1/2 is the decay half-life of radiocarbon (Clark and Fritz, 1997). The uncorrected groundwater ages are

Conclusions

Groundwater with high salinity exists widely in the confined Quaternary aquifer of the PRD, and the high salinity groundwater area (TDS > 1 g/l) extends about 75 km inland from the sea. Hydrochemical and stable isotopic investigations of the groundwater samples reveal that brackish groundwater mainly originates from the mixing of seawater and natural fresh groundwater. The calculated seawater percentages of the groundwater samples vary significantly from northwest to southeast of the PRD, with a

Acknowledgements

This study was supported financially by the General Research Fund of the Research Grants Council, the Hong Kong Special Administrative Region, China (HKU 702707P, HKU 703109P, and HKU 703010P), the China Geological Survey, the Guangdong Geological Survey and the “Sustainable Water Environment” Strategic Research Sub-Theme at HKU. We thank Haipeng Guo, Kouping Chen, Shubin Wang, Xingxing Kuang, Bingfa Zhi and Pu Yao for assistance with well drilling, sampling, testing in the field, and we also

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