Yiman Li

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.

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.

A preliminary study of background concentrations of elements in plants of the Xizang Plateau

A total of 29 major and trace elements have been determined in 43 plant samples collected from the Xizang (Tibet) Plateau using INAA at a SLOWPOKE mini-reactor in order to study elemental background concentration values in Xizang plants. In this paper are reported elemental concentrations of different parts of the plants compared to average values of plants worldwide. The preliminary results suggest that nutritional, plant elements in this region are quite low indicative of a state of malnutrition, while the contents of Al, Ti, Cr and Ba are comparatively high. These findings indicate that the concentrations of most elements in the plants are mainly affected by local soil geochemical characteristics. Some variation in elemental distribution within the plants was also evident: in general, higher concentrations were found in stems compared to roots, and, differences for nutritional elements among the plant species were much greater than found for other elements.