The fate of phosphorus in groundwater needs to be understood because phosphorus-rich groundwater is discharged into surface water bodies, which causes eutrophication, especially in urbanized areas. ...The present study investigated the spatial distributions and driving forces related to the groundwater phosphate levels in various aquifers in the Pearl River Delta (PRD), which has undergone three decades of urbanization, as well as the relationship between groundwater phosphate and arsenic was also discussed. The results showed that most of the high-phosphate (>1.53 mg/L) groundwater occurred in granular aquifers. The proportion of high-phosphate groundwater in granular aquifers was more than four times that in fissured aquifers, whereas high-phosphate groundwater was not observed in karst aquifers in the PRD. High-phosphate groundwater primarily occurred in urbanized areas in the PRD, and the proportion of high-phosphate groundwater had a significant positive correlation with the urbanization level. In granular aquifers, reductive environment and alkalization led to enrichment of the groundwater with phosphate. Anthropogenic sources such as wastewater from township-village enterprises (TVE) and animal wastes were the main sources of high-phosphate groundwater in urbanized areas, and the external input of phosphate enriched the groundwater arsenic levels in urbanized areas. By contrast, geogenic sources such as the release of phosphate from the reduction of Fe/Mn (hydr)oxides and the seawater intrusion accompanied by the release of phosphate from secondary minerals were mainly responsible for the occurrence of high-phosphate groundwater in peri-urban and non-urbanized areas, respectively. The high concentrations of both phosphate and arsenic in groundwater in fissured aquifers were mainly attributed to the infiltration of wastewater from TVEs. In contrast to the granular aquifers, the groundwater Eh and pH conditions were not conductive to the occurrence of high-phosphate groundwater in fissured aquifers.
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•Mostly high-phosphate groundwater occurs in granular aquifers in the PRD.•High-phosphate groundwater is positive correlation with the urbanization level.•Low Eh values and alkalization enhance groundwater phosphate in granular aquifers.•TVE wastewater and animal wastes are the main sources for high-phosphate groundwater.•External input of phosphate enhance groundwater arsenic enrichment in urbanized areas.
Township–village enterprises wastewater and animal wastes were the main sources of high-phosphate groundwater in study area. Low Eh and high pH enhanced groundwater phosphate in granular aquifers.
The moving of manufacturing industry from developed countries to Dongguan, China, promoted the semi-urbanization and rural industrialization in this area. It is urgent to acquire the impact of the ...enhanced anthropogenic pressure on the evolution of groundwater chemistry in this area. The objectives, in this study, were to understand the evolution of groundwater chemistry in Dongguan area based on the comparison of hydrochemical data variations and land use changes during the urbanization, to distinguish the impact of natural processes and anthropogenic activities on the groundwater chemistry by using principal components analysis (PCA) and hierarchical cluster analysis (HCA), and to discuss the origins of trace elements in groundwater. Eighteen physico-chemical parameters were investigated at 73 groundwater sites during July 2006. By analyzing the hydrochemical data, it shows that lateral flow from rivers and agricultural irrigation are the mechanisms controlling the groundwater chemistry in the river network area where the cation exchange of Na+ in sediments taken up by the exchanger Ca2+ occurs. Seawater intrusion is the mechanism controlling the groundwater chemistry in the coast area where the cation exchange of Ca2+ in sediments taken up by the exchanger Na+ occurs. The ion exchange reaction for fissured aquifer is weak in the study area. In addition, the comparison of hydrochemical data between in 2006 and in 1980 shows that anthropogenic activities such as excessive application of agricultural fertilizers, inappropriate emissions of domestic sewage and excessive emissions of SO2 are responsible for the occurrences of groundwater with NO3−, SO42− and Mg2+ types. Four principal components (PCs) were extracted from PCA, which explain 80.86% of the total parameters in water chemistry: PC1, the seawater intrusion and As contamination; PC2, the water–rock interaction, surface water recharge and acidic precipitation; PC3, heavy metal pollution from industry; and PC4, agricultural pollution and sewage intrusion. Four clusters were generated from HCA: cluster 1 is mainly influenced by the industrialization; cluster 2 is mainly affected by the water–rock interaction and the irrigation and lateral flow of river water; cluster 3 is mainly influenced by the seawater intrusion; and cluster 4 is mainly influenced by the sewage intrusion and agricultural pollution. The results show that both natural processes such as seawater intrusion, water–rock interaction and lateral flow of river water and anthropogenic activities such as industrialization, sewage intrusion and agricultural pollution are the two major factors for the evolution of groundwater chemistry in Dongguan area.
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•Human activities are responsible for the occurrences of NO3−, SO42− and Mg2+ types.•The anthropogenic and natural processes occurring are identified as four PCs.•Natural processes mainly control the evolution of groundwater chemistry.•Human activities are also the factors for the evolution of groundwater chemistry.
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•Severe NO3− and NH4+contamination occur in granular and fissured aquifers.•Proportions of high-NO3− and high-NH4+: urbanized areas ≫ non-urbanized areas.•High-NO3− mainly from the ...leakage of township-village enterprises wastewater.•Housing construction enhanced NO3− contamination in newly urbanized areas.•High-NH4+ in urbanized areas mainly from mineralization of organic N in strata.
High nitrogen loading in aquifers is an important issue because of its harmful effects on human health; its origins are often complicated, especially in urbanized areas. This study aims to investigate the distributions of nitrate, nitrite, and ammonium in various aquifers in the Pearl River Delta, and to identify their main sources using hydrochemical and socioeconomic data and principal component analysis. The results show that groundwater with high nitrate (>20 mg/L as N), high nitrite (>1 mg/L as N), and high ammonium (>0.5 mg/L as N) appeared in 7.0%, 4.3%, and 20.5% of granular aquifers, and in 8.3%, 0.8%, and 5.3% of fissured aquifers, respectively, but did not appear in karst aquifers. The proportions of high-nitrate and high-ammonium groundwater in urbanized areas were nearly or more than twice those in non-urbanized areas. High nitrate levels in both granular and fissured aquifers originated mainly from the wastewater leakage of township-village enterprises during the industrialization. Urbanization accompanied by leakage of domestic sewage was another main driving force for high nitrate levels in fissured aquifers. Housing construction aggravated groundwater nitrate contamination in newly urbanized areas. Nitrate contamination in granular aquifers was reduced via denitrification in urbanized areas developed in 1988–1998, whereas that in fissured aquifers was increased by nitrification in urbanized areas developed before 1998. High ammonium levels in granular aquifers in urbanized areas originated mainly from mineralization of organic nitrogen in overlying aquitards, whereas that in peri-urban areas and non-urbanized areas originated mainly from urbanization accompanied by leakage of domestic sewage and irrigation using ammonium-rich river water, respectively. By contrast, high ammonium levels in fissured aquifers are attributed mainly to mineralization of organic nitrogen in carbon-rich strata, which was enhanced by urbanization accompanied by a relatively anoxic environment. Our study implies that nitrate and ammonium contaminations in this area will worsen in the future owing to a lack of effective control and treatment in these rapidly urbanized areas. Naturally originating high-ammonium groundwater in this area can be used as a nitrogen resource, for example, as a nitrogen fertilizer in agricultural irrigation.
•Iodide-rich groundwater appears in 13.95% and 6.82% of granular and fissured aquifers.•Iodide-rich groundwater in fissured aquifers mostly appeared in urbanized areas.•Organic matter decomposition ...is a main origin for iodide-rich groundwater in the PRD.•Sewage leakage is a main source for iodide-rich groundwater in fissured aquifers.
Iodine-rich groundwater is a cause for concern because it is harmful to human health, and determining the sources of groundwater iodine in coastal urbanized areas is complicated. This study aims to delineate the spatial distribution of groundwater iodide in various shallow and unconfined aquifers, as well as in areas with different urbanization levels in the Pearl River Delta (PRD), and to identify the origins of iodide-rich (>0.08 mg/L) groundwater in this region. Approximately 400 groundwater samples, 10 surface water samples, and 9 leachate samples were collected, and a total of 17 chemicals were analyzed. The result showed that iodide-rich groundwater was present in 13.95% of shallow granular aquifers and 6.82% of shallow fissured aquifers; however, it was not present in karst aquifers. In shallow granular aquifers, the proportions of iodide-rich groundwater in areas with different urbanization levels were similar. On the other hand, in shallow fissured aquifers, iodide-rich groundwater mostly appeared in urbanized areas, and its proportion was more than three times that of non-urbanized areas; however, it was not present in peri-urban areas. The reductive dissolution of iodine-loaded Fe/Mn (oxy)hydroxides and decomposition of iodine-rich organic matter in sediments were likely the main sources of iodide-rich groundwater in shallow granular aquifers. Urbanization accompanied by wastewater leakage was also a significant source for iodide-rich groundwater in granular aquifers of urbanized areas. In contrast, the degradation of organic matter in carbonate-rich rocks and urbanization accompanied by leakage of reductive sewage were most likely responsible for the occurrence of iodide-rich groundwater in shallow fissured aquifers. The leakage of landfill leachate was also a significant source for iodide-rich groundwater in the fissured aquifers of non-urbanized areas.
•Mean residence times of shallow groundwater were aged by tritium time-series data.•A negative correlation between groundwater age and ions was demonstrated.•Groundwater became more vulnerable to ...anthropogenic contaminants under intensive exploitation.
The deterioration of water quality in shallow groundwater systems over the past decades has attracted increased amounts of attention from hydrogeologists. Groundwater vulnerability and its changes are the factors that cause groundwater deterioration. Combining of the mean residence times (MRTs) with chemical characteristics of groundwater can be applied to understand groundwater vulnerability and its changes. The groundwater MRTs ranged from 6 to 180 years, with MRTs of 6 to 77 years in the shallow aquifer. Spatial distribution of groundwater MRTs suggested that young groundwater trended to extend. Relatively uniform MRTs and concentrations of ions were found above the main pumped well depth. High concentrations of ions (K+, Na+, Ca2+, Mg2+, HCO3−, Cl- and SO42-) were mainly distributed in the western piedmont plain areas with short turnover time but with low productivity, meaning that the aquifer is susceptible to human activities and hardly migrate out. Elevated concentrations of nitrate and others ions (such as K+, Ca2+, Cl- and SO42-) also were observed in moderately old groundwater (20 < MRTs < 80 years), indicating that some mixing with recent water within MRTs < 20 years likely occurs through vertical drainage from the upper to deeper in the single aquifer and that lateral flow occurs along flow paths towards the piezometric depressions caused by the pumping in urban areas. Slightly elevated HCO3− and K+ were found at east of the areas with long residence times suggested that water-salt equilibrium had also been disrupted. Therefore, the anthropogenic influence exceeded the natural attenuation in the current groundwater environment on the piedmont plain. And the Shijiazhuang groundwater system was identified to be tending more vulnerable under anthropogenic influence. Some measures must be taken to significantly reduce groundwater withdrawal and pollutant discharge.
Isotopes and hydrochemistry were used to characterize the recharge and flow of groundwater in the arid Santanghu basin in Northwest China. The results of isotopic measurement and hydrochemical facies ...indicate that the predominant recharge mechanism is via rivers and streams. Modern recharge only occurs in the piedmont area of Moqinwula Mountains by ephemeral rivers, and the main recharge source is precipitation in the mountain areas at an elevation of about 2,200 m above sea level. Two recharge mechanisms were identified by stable isotope methods: the ephemeral stream recharge in the piedmont with the occurrence of evaporation during recharge, and subsurface inflow recharge from the mountain regions. Diffuse recharge derived from precipitation in the basin can be ignored because of the scarcity of precipitation and intense evaporation. Groundwater in the Paleogene and Neogene confined aquifers could represent recharge during cooler climatic conditions, rather than latitude or altitude effects, signified by tritium-free samples and significant depletion of heavy stable isotopes throughout the study area. Groundwater age data suggest that central faults are controlling the flow paths of the regional groundwater flow system. Groundwater moves from the piedmont plain to the basin lowland area through lateral flow, and changes flow direction to the spring zone of Hanshuiquan Oasis because of the barrier of the central faults. A conceptual model was defined, for better understanding of the groundwater recharge and flow systems. The major findings of this study have significant implications for groundwater protection and management in arid basins.
The use of isotope tracers as a tool for assessing aquifer responses to intensive exploitation is demonstrated and used to attain a better understanding of the sustainability of intensively exploited ...aquifers in the North China Plain. Eleven well sites were selected that have long-term (years 1985–2014) analysis data of isotopic tracers. The stable isotopes δ
18
O and δ
2
H and hydrochemistry were used to understand the hydrodynamic responses of the aquifer system, including unconfined and confined aquifers, to groundwater abstraction. The time series data of
14
C activity were also used to assess groundwater age, thereby contributing to an understanding of groundwater sustainability and aquifer depletion. Enrichment of the heavy oxygen isotope (
18
O) and elevated concentrations of chloride, sulfate, and nitrate were found in groundwater abstracted from the unconfined aquifer, which suggests that intensive exploitation might induce the potential for aquifer contamination. The time series data of
14
C activity showed an increase of groundwater age with exploitation of the confined parts of the aquifer system, which indicates that a larger fraction of old water has been exploited over time, and that the groundwater from the deep aquifer has been mined. The current water demand exceeds the sustainable production capabilities of the aquifer system in the North China Plain. Some measures must be taken to ensure major cuts in groundwater withdrawals from the aquifers after a long period of depletion.
Clay aquitards are semipermeable membranes that allow groundwater flow while retarding solute migration has been researched extensively but also subjected to much debate. At present, there is no ...evidence of whether the physical and chemical properties of clay soil and the isotopic composition of pore water affect the semipermeable membrane effect. In this study, we collected clay samples from drilling cores (30–90 m) in the Hengshui area located in the North China Plain (NCP), then extracted pore water using a high‐pressure squeezing device. Vertical hydrochemical and isotopic profile variation trends for the pore water were revealed using hydrochemical (Cl−, Na+, Ca2+, K+, Mg2+, and SO42−) and stable isotopic measurements of H, O and Cl. The results showed that the hydrochemical clay interlayer pore water of the saline aquifer is Cl/SO4‐Na/Mg type and the average total dissolved solids (TDS) are 10.17 g/L. However, the hydrochemical clay aquitard pore water is of the Cl/SO4‐Na/Ca type with an average TDS of 1.90 g/L. The hydrochemical clay interlayer pore water of aquifer II is of Cl‐Na/Ca type with an average TDS of 1.10 g/L. Our results showed that the water quality of the aquifer II is not affected by the upper part of the saline aquifer, thus the clay aquitard acts as a significant barrier to salt movement. A polarization layer concentrated in ions was formed between the upper part of the saline aquifer and the clay aquitard. The concentration polarization layer increases the salt‐inhibition effect. H, O and Cl isotopic composition results showed significant fractionation. The pore water of aquifer II lacked heavy isotopes (2H, 18O, 37Cl), but had significant heavy isotope enrichment in the concentrated polarized layer (the δ2H value was −76‰, the δ18O value was −8.4‰, and the δ37Cl value was 1.59‰). Hyperfiltration thus played a significant role in isotope fractionation.
Porewater hydrochemical and isotope analysis collected from the North China Plain show that clay aquitard has the characteristics of semipermeable membrane and ultrafiltration, which affects the fractionation of hydrogen, oxygen and chlorine isotopes and also leads to the change of hydrochemistry.
