Zhi-Qi Zhao

Natural and anthropogenic lead in soils and vegetables around Guiyang city, southwest China: A Pb isotopic approach

Soils, vegetables and rainwaters from three vegetable production bases in the Guiyang area, southwest China, were analyzed for Pb concentrations and isotope compositions to trace its sources in the vegetables and soils. Lead isotopic compositions were not distinguishable between yellow soils and calcareous soils, but distinguishable among sampling sites. The highest (207)Pb/(206)Pb and (208)Pb/(206)Pb ratios were found for rainwaters (0.8547-0.8593 and 2.098-2.109, respectively), and the lowest for soils (0.7173-0.8246 and 1.766-2.048, respectively). The (207)Pb/(206)Pb and (208)Pb/(206)Pb ratios increased in vegetables in the order of roots<stems<leaves<fruits. Plots of the (207)Pb/(206)Pb ratios versus the (208)Pb/(206)Pb ratios from all samples formed a straight line and supported a binary end-member mixing model for Pb in vegetables. Using deep soils and rainwaters as geogenic and anthropogenic end members in the mixing model, it was estimated that atmospheric Pb contributed 30-77% to total Pb for vegetable roots, 43-71% for stems, 72-85% for leaves, and 90% for capsicum fruits, whereas 10-70% of Pb in all vegetable parts was derived from soils. This research supports that heavy metal contamination in vegetables can result mainly from atmospheric deposition, and Pb isotope technique is useful for tracing the sources of Pb contamination in vegetables.

Tracing nitrate sources with dual isotopes and long term monitoring of nitrogen species in the Yellow River, China

A heavy load of nitrogenous compounds reflects nutrient loss and influences water quality in large rivers. Nitrogenous concentrations and dual isotopes of nitrate were measured to ascertain the spatial and temporal distributions of nitrate transformation in the Yellow River, the second-longest river in China. Assessment of the long-term record indicates that [NO3−–N] has increased by two-fold over the past three decades. Weekly observation of ammonium over a twelve-year period revealed high concentrations and suggests impairment of water quality, particularly since 2011. The estimated total dissolved nitrogen flux was 7.2 times higher in middle reaches than that at head waters. Anthropogenic nitrogen sources become more important in lower section of the upper reaches and middle reaches because of intensive agricultural activities and urban input. Nitrate in the lower reaches was mainly derived from transportation of upstream nitrate and point sources from cities. The spatial variation of ammonium and nitrate isotopes show that nitrification is a key process governing nitrogen transformation. Riverine biological processes could potentially be responsible for the shift of nitrate isotope signature. The first step to reducing nitrogen load and improving water quality will be containment and careful management of sources from urban input, sewage waste and irrigation runoff.

Boron isotopic geochemistry of karst groundwater in Guiyang City, China

Boron has two stable isotopes (l~ and liB) with relative abundances of about 20% and 80%, respectively. Boron isotopic ratios in natural materials show a huge range of variations, from -7096o to +60%0, when expressed with the classical 8laB notation. Most of these isotopic variations occur at the surface of the Earth. Hence, boron isotopic composition can be used as a sensitive tracer in geochemical study, for instance, to identify the different sources of contamination and factors controlling the salinity of groundwater. During the last decade, boron isotopes have been used to discriminate between the influences of seawater intrusion and anthropogenic discharge. But few of those researches can precisely identify the different sources of contamination. We measured the boron concentrations and boron isotopic ratios of groundwater samples collected in Guiyang City, as well as the major ions. The results indicate that the major ion composition of the groundwater in the investigated area is mainly controlled by the interactions between water and the dominant rock i.e. carbonates. All the water compositions are characterized by high concentrations of Ca 2+, Mg 2+, HCO3, SO42, and NOr, which are the dominant contaminants. Both dissolved boron concentrations and isotopic ratios show large variations among the ground waters, from 2 lag/L to 90 lag/L and from -6%o to +26%0, respectively. The boron concentrations and isotopic ratios indicate that the river across the studied city has been seriously contaminated by urban discharge. Boron concentrations of river water samples varied from 20 lag/L to 140 lag/L, with an average 811B value of +2.0%0. Using boron isotopic compositions and different geochemical indices allowed us to clearly identify and distinguish the two major sources of contamination, agricultural activity and urban wastewater. Both of the two sources are characterized by high boron concentrations but their boron isotopic compositions significantly differ. The 8nB values of urban effluents range from -6.0%0 to +5.0%o, clearly lower than the mean 8nB values of a typical fertilizer, about +15%o. For the studied city, some groundwater sites of the suburban area were contaminated by agricultural activities and most sites of the urban area were contaminated by sewerage waters. The two types of contamination present distinct evolution during a hydrologic cycle.

Mitigating ammonia emission from agriculture reduces PM2.5 pollution in the Hai River Basin in China

The Hai River Basin (HRB), one of the most populated areas in China, is experiencing high NH3 emissions, mostly from agricultural sources, and suffering from strongly enhanced PM2.5 concentrations in all urban areas. Further population growth and urbanization projected until 2030 may exacerbate this situation. Here, the NUFER (NUtrient flows in Food chains, Environment and Resources use) and GAINS (Greenhouse gas - Air pollution Interactions and Synergies) models have been coupled for the first time to understand possible changes of agricultural NH3 emission between 2012 and 2030 and their impacts on ambient PM2.5 concentrations, and to explore options to improve this situation. Results show that agricultural ammonia emissions in the HRB were 1179kt NH3 in 2012, 45% of which was from the hotspots at or near conurbation areas, including Beijing-Tianjin, Tangshan-Qinhuangdao, Shijiazhuang-Baoding, Dezhou, Handan-Liaocheng, and Xinxiang. Without intervention, agricultural ammonia emissions will further increase by 33% by 2030. The impacts of several scenarios were tested with respect to air pollution. Compared to the business-as-usual scenario, a scenario of improved technology and management combined with human diet optimization could greatly reduce emission (by 60%), and lead to 22-43% and 9-24% decrease of the secondary inorganic aerosols and PM2.5 concentrations, respectively, in the hotspots of NH3 emissions. Our results further confirmed that ammonia control is needed for air pollution abatement strategies (SO2, NOx and primary PM reduction) to be effective in terms of PM2.5.

The influence of climate and topography on chemical weathering of granitic regoliths in the monsoon region of China

Exploring the relationship between weathering and erosion is essential for understanding the evolution of landscapes and formation of soil under the influence of climate, tectonics, and topography. We measured the bulk chemistry of regoliths and calculated their weathering rates and intensity in three locations in China: Inner Mongolia in the mid-temperate semi-humid zone; Jiangxi Province, in the mid-subtropical humid zone; and Hainan Province, in the tropical humid zone. These profiles exhibited increased weathering with increasing temperature and precipitation. The low-gradient profile exhibited stronger weathering of saprolite than of soil, whereas the high-gradient profile showed a more constant weathering pattern. The regolith in the cold climate was the product of easily weatherable minerals, whereas weathering of K-feldspar and even secondary minerals occurred in hot and humid climates. The weathering of subtropical profiles was both supply- and kinetic-limited, controlled by weathering and erosion. The tropical profile experienced supply-limited weathering, indicating slow erosion and an intense weathering profile; the mid-temperate profile was not classifiable due to weak erosion and weathering. Long-term weathering fluxes of these profiles show that Si, Na, and K (or Mg) represent the bulk of the mass lost through weathering. This study underscores that weathering of granitic regolith is controlled by both climatic conditions and landscape.