Tianming Huang

Sources of water pollution and evolution of water quality in the Wuwei basin of Shiyang river, Northwest China

Based on surveys and chemical analyses, we performed a case study of the surface water and groundwater quality in the Wuwei basin, in order to understand the sources of water pollution and the evolution of water quality in Shiyang river. Concentrations of major chemical elements in the surface water were related to the distance downstream from the source of the river, with surface water in the upstream reaches of good quality, but the river from Wuwei city to the Hongya reservoir was seriously polluted, with a synthetic pollution index of 25. Groundwater quality was generally good in the piedmont with dominant bicarbonate and calcium ions, but salinity was high and nitrate pollution occurs in the northern part of the basin. Mineralization of the groundwater has changed rapidly during the past 20 years. There are 23 wastewater outlets that discharge a total of 22.4 x 10(6)m(3)y(-1) into the river from Wuwei city, which, combined with a reduction of inflow water, were found to be the major causes of water pollution. Development of fisheries in the Hongya reservoir since 2000 has also contributed to the pollution. The consumption of water must be decreased until it reaches the sustainable level permitted by the available resources in the whole basin, and discharge of wastes must also be drastically reduced.

Processes affecting isotopes in precipitation of an arid region

The isotopic composition of precipitation has been measured in samples simultaneously collected during individual precipitation events at two neighbouring high-altitude stations (Houxia at 2100 m a.s.l. and Gaoshan at 3545 m a.s.l.) in the Tianshan Mts., northwest China. The observed changes of δ18O (δ2H) and deuterium excess with surface air temperature, altitude and season have been evaluated to derive information on the effects of subcloud evaporation and moisture recycling on the formation of precipitation and its isotopic composition under arid climatic conditions. Consulting the long-term monthly averages of ‘d’ excess and temperature of the nearest GNIP station Wulumuqi, a striking similarity was found with the results of the two high-altitude stations concerning the relation between ‘d’ excess and temperature. The ‘d’ excess-temperature plot of the Wulumuqi data shows an hysteresis effect which appears to signify seasonal changes in the interplay between subcloud evaporation and moisture recycling. Finally, for the first time a negative altitude gradient of the d excess has been found for all stations including two more GNIP stations in northwest China but far away from the study area. This ‘inverse altitude effect’ may manifest a decrease of the recycled fraction in air moisture with altitude.

Impacts of human activities on the occurrence of groundwater nitrate in an alluvial plain: A multiple isotopic tracers approach

Nitrate pollution is a severe problem in areas with intensive agricultural activities. This study focuses on nitrate occurrence and its constraints in a selected alluvial fan using chemical data combined with environmental isotopic tracers (18O, 3H, and 15N). Results show that groundwater nitrate in the study area is as high as 258.0 mg/L (hereafter NO3−) with an average of 86.8 mg/L against national drinking water limit of 45 mg/L and a regional baseline value of 14.4 mg/L. Outside of the riparian zone, nitrate occurrence is closely related to groundwater circulation and application of chemical fertilizer. High groundwater nitrate is found in the recharge area, where nitrate enters into groundwater through vertical infiltration, corresponding to high 3H and enriched 18O in the water. In the riparian zone, on the contrary, the fate of groundwater nitrate is strongly affected by groundwater level. Based on two sampling transects perpendicular to the riverbank, we found that the high level of nitrate corresponds to the deeper water table (25 m) near the urban center, where groundwater is heavily extracted. Groundwater nitrate is much lower (<12.4 mg/L) at localities with a shallow water table (5 m), which is likely caused by denitrification in the aquifer.

Integrated assessment on groundwater nitrate by unsaturated zone probing and aquifer sampling with environmental tracers

By employing chemical and isotopic tracers ((15)N and (18)O in NO(3)(-)), we investigated the main processes controlling nitrate distribution in the unsaturated zone and aquifer. Soil water was extracted from two soil cores drilled in a typical agricultural cropping area of the North China Plain (NCP), where groundwater was also sampled. The results indicate that evaporation and denitrification are the two major causes of the distribution of nitrate in soil water extracts in the unsaturated zone. Evaporation from unsaturated zone is evidenced by a positive correlation between chloride and nitrate, and denitrification by a strong linear relationship between [Formula: see text] and ln(NO(3)(-)/Cl). The latter is estimated to account for up to 50% of the nitrate loss from soil drainage. In the saturated zone, nitrate is reduced at varying extents (100 mg/L and 10 mg/L at two sites, respectively), largely by dilution of the aquifer water.

Groundwater recharge in an arid grassland as indicated by soil chloride profile and multiple tracers

&NA; Previous studies have shown that shallow groundwater in arid regions is often not in equilibrium with near‐surface boundary conditions due to human activities and climate change. This is especially the case where the unsaturated zone is thick and recharge rate is limited. Under this nonequilibrium condition, the unsaturated zone solute profile plays an important role in estimating recent diffuse recharge in arid environments. This paper combines evaluation of the thick unsaturated zone with the saturated zone to investigate the groundwater recharge of a grassland in the arid western Ordos Basin, NW China, using the soil chloride profiles and multiple tracers (2H, 18O, 13C, 14C, and water chemistry) of groundwater. Whereas conventional water balance and Darcy flux measurements usually involve large errors in recharge estimations for arid areas, chloride mass balance has been widely and generally successfully used. The results show that the present diffuse recharge beneath the grassland is 0.11–0.32 mm/year, based on the chloride mass balance of seven soil profiles. The chloride accumulation age is approximately 2,500 years at a depth of 13 m in the unsaturated zone. The average Cl content in soil moisture in the upper 13 m of the unsaturated zone ranges from 2,842 to 7,856 mg/L, whereas the shallow groundwater Cl content ranges from 95 to 351 mg/L. The corrected 14C age of shallow groundwater ranges from 4,327 to 29,708 years. Stable isotopes show that the shallow groundwater is unrelated to modern precipitation. The shallow groundwater was recharged during the cold and wet phases of the Late Pleistocene and Holocene humid phase based on palaeoclimate, and consequently, the groundwater resources are nonrenewable. Due to the limited recharge rate and thick unsaturated zone, the present shallow groundwater has not been in hydraulic equilibrium with near‐surface boundary conditions in the past 2,500 years.

How much information can soil solute profiles reveal about groundwater recharge

The unsaturated zone is an important link between groundwater and the land surface, and can provide storage capacity for both water and contaminants. Consequently, the solute profile of the unsaturated zone can provide much information on groundwater recharge using tracers. This study takes a rain-fed agricultural area (Luochuan loess plain) in the Chinese Loess Plateau as an example to demonstrate how much recharge information in the thick unsaturated zone can be obtained from the solute profile. The results show that the recharge rate is 36–67 mm yr–1 and infiltration rate is 0.19 to 0.30 m yr–1, as obtained using three methods, i.e., evaluating the Cl-peak, chloride mass balance, and nitrate levels. The amount of time needed for annual precipitation to reach the water table is 133–316 years, and the groundwater is tritium free. The nitrate introduced by human activities is mainly distributed in the upper unsaturated zone (above 6 m), and the groundwater nitrate content (3.7 to 9.6 mg l–1 as NO3) remains at baseline level. The storage capacity of the unsaturated zone is found to delay the entrance of pollutants into the aquifer.

Groundwater Baseline Quality in the Minqin Basin, NW China

Study on groundwater baseline quality is very important for groundwater management, as managers want to know the magnitude and reason for the elevated content (whether the elevated content is geological or anthropogenic in origin). As the baseline is defined as the range of concentration of a give element is derived entirely from natural sources under condition not perturbed by anthropogenic activity, it can be obtained from the palaeowaters. Based on the published data in the Minqin Basin, NW China, the groundwater baseline quality is determined. This will be one of the first groundwater baseline quality studies in China. Results show that the relatively high total mineral content in the basin is related to arid condition and low groundwater flow velocity and that the most groundwater quality has not been affected by human activities.

Geochemical processes during hydraulic fracturing: a water-rock interaction experiment and field test study

Various environmental problems related to flowback fluid have recently become a concern for hydraulic fracturing during shale gas production. Detailed studies on the geochemical reactions during hydraulic fracturing are performed for the Lower Silurian Longmaxi shale formation, Fuling block, Sichuan basin, SW China, through water-rock interaction experiment conducted under reservoir conditions, and comparisons with field test data are made. Results show that the total dissolved solids (TDS) of the flowback fluid increases and Ca2+, Na+, Cl– and SO42– make dominant contributions. The increases of Na+ and Cl– are assumed to be caused by halite dissolution and mixing with in situ formation water in the marine facies reservoir. The main geochemical reactions are inferred to be pyrite oxidation and the dissolution of calcite, dolomite and plagioclase, resulting in increases of major ions in the flowback fluid. Heavy metals, including Mn, Ni, Mo and Zn, exceed the drinking water criteria of China, and the potential threat from leakage should be a concern. Besides of CH4, as the specific pollutant to groundwater system, inorganic geochemical monitoring of shallow groundwater system should track TDS, major ions of Cl–, SO42–, Na+, and Ca2+, trace elements of Li, Ni, Pb, Cu, Zn, Mn, Mo and Rb, and stable isotopes, such as 18OH2O, 2HH2O and 13CDIC.

Combined conceptual model (V&P model) to correct groundwater radiocarbon age

An approach to groundwater radiocarbon correction (V&P model) based on characteristics of initial recharging water and C-13 mixing model is developed, which not only overcomes the input uncertainty of C-13 and C-14 from the unsaturated zone and open groundwater system, but also considers the probable dilution effects from carbonate dissolution in the closed groundwater system. The model is relatively simple and reasonable. The principles of choosing modern recharging groundwater are also referred. The model has been used to correct the radiocarbon groundwater age from the Minqin Basin, the North China Plain and has given the reasonable results.

Groundwater Recharge Environments and Hydrogeochemical Evolution in Beijing, China: Multi-Tracer Approach

The groundwater recharge environments and hydrogeochemical characteristics in Beijing were investigated using a combination of chemical indicators, stable isotopes, and radiocarbon dating. There are two distinct zones for those groundwaters: modern water (1000 a) are depleted, which implies that the palaeowaters were recharged in relatively cold climate. The TDS for all groundwater samples range from 312 to 914 mg/L. Groundwaters are supersaturated with respect to calcite and the δ13C value is stable (around -13‰), which suggests dissolution of calcite is limited. Dissolution of halite, sulfate and carbonate and exchange of cations are main processes of water chemistry. The results have important implications for groundwater management in Beijing, where a certain proportion of the unrenewable water has been mined.