[Characteristics and Causes of High-manganese Groundwater in Pearl River Delta During Urbanization].

The high concentration of iron and manganese in groundwater is harmful to human health, and the sources of manganese in rapidly urbanization areas are complex. Based on more than 2500 sets of hydrochemical data in different historical periods, the spatial distribution characteristics, sources, and genesis of groundwater manganese in different aquifers and areas with different urbanization levels in the Pearl River Delta were studied by using mathematical statistics and principal component analysis. The results showed that the concentration of manganese in groundwater in the pore aquifer was obviously higher than that in the fissure and karst aquifer. The proportion of high-manganese groundwater in the pore aquifer was twice that in the fissure and karst aquifer. The proportion of high-manganese groundwater in urbanized and suburban areas was significantly higher than that in non-urbanized areas. On a regional scale, the decomposition of organic matter and the reductive dissolution of Fe-Mn (oxygen) hydroxide in sedimentary strata under reductive conditions may have been the main factors controlling the increase in manganese concentration in pore aquifers. High-manganese groundwater in fissured aquifers may have been affected by low-oxygen domestic sewage leakage accompanying urbanization and industrial wastewater leakage and infiltration accompanying industrialization. The pore high-manganese groundwater was controlled by reduction conditions, and the weakly acidic environment of fissure and karst high-manganese groundwater was the important influencing factor. In the past 10 years, the groundwater environment in the study area has been improving, and the increase in groundwater redox potential and pH was not conducive to the formation of high-manganese groundwater, which was also the main cause of the overall decrease in Mn2+ concentration in groundwater of different types of aquifers in the process of urbanization.

[Geochemical Characteristics and Driving Factors of High-Iodine Groundwater in Rapidly Urbanized Delta Areas: A Case Study of the Pearl River Delta].

The source of iodine in the groundwater of coastal urbanization areas is complex, and high-iodine groundwater is a potential threat to the safety of drinking water. Based on this, this study took the Pearl River Delta, which is developing rapidly in urbanization, as the research area. Additionally, the occurrence characteristics and driving factors of iodide in shallow groundwater of different aquifers and different urbanization levels in the Pearl River Delta were studied using mathematical statistics, principal component analysis, and other methods. The results showed that the concentration of iodide in the shallow groundwater was 2.34 mg·L-1 and undetected in the form of I-. Among 1567 groundwater samples in the study area, there were 120 groups of groundwater with high iodine content greater than 0.1 mg·L-1, accounting for 7.7%. Among them, 84 and 36 groups were detected in shallow porous and shallow fissure high-iodine groundwater, respectively, whereas no high-iodine groundwater was detected in the karst aquifer. The proportion of high-iodine groundwater was 8.0% in the shallow porous aquifer and 7.5% in the shallow fissure aquifer. Both the porous aquifer and the fissured aquifer with high iodine content were mainly distributed in the urbanized areas, the proportion of which was more than three times that of the non-urbanized areas. The chemical types of the high-iodine groundwater were mainly HCO3·Cl-Ca·Na and Cl-Na type water, which have the characteristics of high pH and low redox potential. The reduction and dissolution of iodine-containing Fe/Mn (oxygen) hydroxides and the decomposition of iodine-rich organics in sediments may be the main sources of high-iodine groundwater in the shallow porous aquifers of the Pearl River Delta Plain. The degradation and urbanization of organic matter in carbonate-rich rocks is accompanied by the leakage of reducing sewage, which may be the main source of high-iodine groundwater in shallow fissured aquifers. The neutral to weakly alkaline reduction environment with rich organic matter was the main cause of high-iodine groundwater in the Delta Plain area. Weathering, leaching, cation exchange, and sea-land interactions are the main hydrogeochemical processes in the evolution of high-iodine groundwater in the Pearl River Delta.

[Geochemical Characteristics and Driving Factors of NO3-Type Groundwater in the Rapidly Urbanizing Pearl River Delta].

In response to rapid economic development, nitrate pollution of groundwater is becoming a serious issue in many parts of China. Urbanization and industrialization are the main drivers of NO3-type groundwater expansion. Focusing on the Pearl River Delta, the occurrence and driving factors of shallow nitrate groundwater are discussed. Overall, groundwater nitrate concentrations are generally high in this region. Of 1538 groundwater samples, 5.7% had nitrate concentrations higher than the groundwater quality standard(88.6 mg·L-1) and 18.5% were classified as NO3-type waters, which are mainly distributed in the hilly and piedmont areas. Guangzhou, Dongguan, Foshan, Zhuhai and other areas show high total dissolved solid(TDS)-concentration NO3-type waters, which are affected by urbanization and industrialization. In comparison, low-TDS NO3-type waters are distributed in the hilly and valley areas. In the Xijiang and Dongjiang plains, the TDS concentrations on groundwater increased significantly due to inputs of industrial wastewater and saline seawater. The NO3- concentration in the groundwater in this area exceeded the class III water standard but did not change the hydrochemical type classification. However, industrialization has led to the frequent appearance of SO4-type water in this area. The NO3-type water occurs in acidic or weakly acidic environments, typically characterized by low TDS and total hardness concentrations, and high Cl-, SO42-, and K+ concentrations. The formation of NO3-type water is mainly affected by domestic sewage, industrial wastewater, agricultural nitrogen fertilizer, septic tank outflows, and landfill leachate leakage. Generally, the pollution loads of high-TDS NO3-type waters are higher than low-TDS NO3-type waters. The delineation of NO3-type waters, especially the low-TDS type, is helpful for identifying groundwaters posing greater risks for human activities, and those with low nitrate concentrations but potential pollution risk, which is of great significance in the prevention and control of groundwater pollution.

[Chemical Evolution of Groundwater in the Tacheng Basin of Xinjiang in the Process of Urbanization].

With the development of the local economy, the volume of groundwater production has increased continuously in the past decades in the Tacheng Basin of the Xinjiang Uygur Automous Region. Previous studies have not provided a clear pattern of the chemical composition evolution of groundwater and its driving force in this basin, which makes the future development and utilization of groundwater riskier. This study carried out systematic sampling and analysis of groundwater chemistry in this basin, and the chemical evolution of groundwater in the basin was analyzed by comparison with historical hydrochemical data. The results show that Ca2+ and Na+ are the main cations in the groundwater, HCO3-, SO42- are the main anions in the groundwater, and freshwater is widely distributed. The chemical types of groundwater changed from HCO3-Ca and HCO3·SO4-Ca·Mg in the source zone in front of the mountains to SO4·HCO3-Na·Ca type in the plain area. In comparison with the hydrochemical data of 1979, HCO3 and SO4·HCO3 type groundwater increased significantly. SO4 and Cl type groundwater with high total dissolved solids decreased significantly. However, the Cl- and SO42- concentration and total hardness in the groundwater around the cities and towns increased. Aquifer material and the change of flowing field are the two controlling factors of groundwater chemical change, but the leakage of waste water from city drainage channels also affects the groundwater chemistry drastically.

Transport of sucrose-modified nanoscale zero-valent iron in saturated porous media: role of media size, injection rate and input concentration.

The growing use of nanoscale zero-valent iron (NZVI) in the remediation of contaminated groundwater raises concerns regarding its transport in aquifers. Laboratory-scale sand-packed column experiments were conducted with bare and sucrose-modified NZVI (SM-NZVI) to improve our understanding of the transport of the nanoparticles in saturated porous media, as well as the role of media size, suspension injection rate and concentration on the nanoparticle behavior. As the main indicative parameters, the normalized effluent concentration was measured and the deposition rate coefficient (k) was calculated for different simulated conditions. Overall, compared to the high retention of bare NZVI in the saturated silica column, SM-NZVI suspension could travel through the coarse sand column easily. However, the transport of SM-NZVI particles was not very satisfactory in a smaller size granular matrix especially in fine silica sand. Furthermore, the value of k regularly decreased with the increasing injection rate of suspension but increased with suspension concentration, which could reflect the role of these factors in the SM-NZVI travel process. The calculation of k-value at the tests condition adequately described the experimental results from the point of deposition dynamics, which meant the assumption of first-order deposition kinetics for the transport of NZVI particles was reasonable and feasible.

Hydrochemistry indicating groundwater contamination and the potential fate of chlorohydrocarbons in combined polluted groundwater: a case study at a contamination site in North China.

Groundwater contamination characteristics and the potential fate of chlorohydrocarbons were investigated at a combined polluted groundwater site in North China. Groundwater chemistry and (2)D and (18)O isotope compositions indicated that high salination of groundwater was related with chemical pollution. The elevated salinity plume was consistent with the domain where typical chlorohydrocarbon contaminants occurred. The concentrations of heavy metals, oxidation-reduction potential, and pH in organic polluted areas significantly differed from those in peripheral (background) areas, indicating modified hydrochemistry possibly resulting from organic pollution. Under the presented redox conditions of groundwater, monochlorobenzene oxidation may have occurred when the trichlorohydrocarbons underwent reductive dechlorination. These findings suggested that inorganic hydrochemistry effectively indicated the occurrence of chemical contamination in groundwater and the potential fate of chlorohydrocarbons.