[Dissolved Ion Concentrations and Isotope Values in Agricultural Fertilizer Locally Applied in Henan Province]

Agricultural fertilizers (AFs) have provided vegetation with necessary nutrients, but unabsorbed constituents have been retarded in soil, potentially affecting the quality of adjacent surface water and groundwater. AFs element contents and stable isotope compositions have often been utilized to assess and calculate AFs pollution to nitrate and sulfate in surface water and groundwater; however, due to various AFs applied, the dissolved ion concentrations and isotope ratios are still unknown. This study collected commercial AF widely utilized in Henan province, China, to constrain their ion concentrations and isotope values. The dissolved ions (1 g AFs dissolved in 1 L ultrapure water), sulfate sulfur, and oxygen isotope values(δ³⁴S and δ¹⁸O) were analyzed, and total nitrogen (TN) contents coupled with nitrogen isotope values(δ¹⁵N) in solid AFs were determined to elucidate their elemental and isotopic compositions. These characteristics provided a scientific basis for further assessing their contributions to surface water and groundwater contaminations. The results indicated that pH values in the AFs solutions varied from 3.6 to 10.2, with a mean value of 6.7±1.5 (n=30, 1σ). Sulfate (SO₄^2-) and nitrate (NO₃^-) concentrations ranged from 4.38 mg·L^-1 to 827.29 mg·L^-1 and from 1.34 mg·L^-1 to 208.90 mg·L^-1, with median values of 192.80 mg·L^-1 and 13.51 mg·L^-1 and average values of (256.19±239.83) mg·L^-1 (n=30) and (37.07±53.21) mg·L^-1 (n=29), respectively. Dissolved sulfate δ³⁴S and δ¹⁸O values in AFs varied from -3.5‰ to 19.0‰ and from 6.7‰ to 18.5‰, with median values of 4.1‰ and 10.1‰ and mean values of (5.8±5.5)‰ (n=22, 1σ) and (10.7±2.7)‰ (n=22, 1σ), respectively. TN and δ¹⁵N values in AFs ranged from 0.5% to 38.9% and from -2.7‰ to 3.4‰, with median values of 13.3% and 0.0‰ and average values of (14.8±9.3)% (n=25) and 0.0±1.5‰ (n=24, 1σ), respectively. The lower averaged δ³⁴S values and positive averaged δ¹⁸O values potentially resulted from sulfuric acids added as raw materials, giving rise to a negative relationship between pH values and SO₄^2- concentrations (P<0.05). The δ¹⁵N values of AFs were close to that of air N₂, corresponding to the fact that NO₃^--N and NH₄⁺-N were synthesized via air N₂. Our results revealed the dissolved ion concentrations of SO₄^2-, NO₃^-, and NH₄⁺ and their δ³⁴S, δ¹⁸O, and δ¹⁵N values of typically applied AFs in Henan province, which provided the scientific basis for studying the AFs contributions to SO₄^2- and NO₃^- pollutions in surface water and groundwater surroundings.

Soil Column Experimental Study on the Effect of Soil Structure Disturbance on Water Chemistry

The changes in soil/rock structure caused by engineering disturbance or earthquakes could affect water chemistry by increasing the reaction surface, enhancing the oxidation condition, or exposing soluble rocks. However, the details of the mechanisms of the disturbance of soil/rock are little known. Based on the soil column experiment, this study analyzed the concentrations of sulfate (SO₄), sulfur, and oxygen isotopic composition of SO₄ (δ³⁴S-SO₄ and δ¹⁸O-SO₄) in effluent water. The water-rock interaction mechanisms in the disturbed soil and the contribution of this interaction to the SO₄ in groundwater were studied. The results suggest that the concentration of SO₄ in the first effluent water sample can reach up to 97 mg/L, much higher than that in natural groundwater (6.8 mg/L). The isotopic composition of SO₄ further suggested that SO₄ in the first effluent water sample was mainly derived from the dissolution of SO₄-containing evaporites. The proportion was estimated to be 93%. SO₄-containing evaporites accounted for 23% of the SO₄ content in all effluent water samples during the experiment. The disturbance of soil structure led to the exposure and dissolution of SO₄-containing evaporites, which were initially insoluble under natural conditions. This study is essential to the clarification of the water-rock interaction mechanisms following the changes in soil/rock structures.

[Seasonal Variation and Sources Identification of Dissolved Sulfate in a Typical Karst Subterranean Stream Basin Using Sulfur and Oxygen Isotopes]

Karst water, which provides approximately 25% of the world's drinking water, is especially vulnerable to anthropogenic pollutants. To determine the variations between high and low flow periods and the sources of dissolved sulfate (SO₄^2-) in small karst basins, SO₄^2- concentrations, stable sulfur and oxygen isotopes (δ³⁴S-SO₄ and δ¹⁸O-SO₄), and oxygen isotopes of water (δ¹⁸O-H₂O) were investigated in surface and groundwaters, during the high and low flow seasons, within the Babu subterranean stream basin. Analysis showed that: ① the water samples that were directly impacted by acid mine drainage exhibited high SO₄^2- concentrations (≥250 mg·L^-1) and significant seasonal variation, while the seasonal variation of non-AMD-impacted water with low SO₄^2- concentrations was not significant. ② During the high flow season, the mean δ³⁴S-SO₄ and δ¹⁸O-SO₄ values of surface water were -10.5‰ and 4.7‰, respectively, and -11.5‰ and 1.3‰ during the low flow period; the mean values of δ³⁴S-SO₄ and δ¹⁸O-SO₄ in groundwater samples were -2.9‰ and 7.1‰ during the high flow period, respectively, and -3.2‰ and 6.2‰ during the low flow period. Both surface and groundwater samples exhibited higher δ³⁴S-SO₄ and δ¹⁸O-SO₄ values during the high flow period than during the low flow period. ③ The values of δ³⁴S-SO₄ in the surface and groundwater samples were relatively stable, indicating that the sources of SO₄^2- at specific sampling sites were stable.④ The main sources of SO₄^2- in surface and groundwaters were rain, sulfide, and gypsum, accounting for 13%, 40%, and 47%, respectively, of SO₄^2- in samples taken from the basin outlet during the high flow season, and 18%, 39%, and 43%, respectively, in samples obtained during the low flow season.