Jilai Liu

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.

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.

Novel fluorinated polyimides for microelectronics applications

A series of novel fluorinated polyimides were prepared, which could be easily dissolve in many organic solvents to give homogeneous and stable polyimide solution with solid content as high as 35-40 wt.%. The polyimide films obtained by casting the polymer solution followed by thermal imidization exhibited good thermal stability with high glass transition temperature and the temperature at 5% weight loss, and have outstanding mechanical properties. In addition, the fluorinated polyimide have low dielectric constants and dissipation factor, and very low moisture absorption.