Abstract
Acid rain with a relatively high concentration of ammonium and nitrate can accelerate rock weathering. However, its impact on groundwater nitrate is uncertain. This study evaluated the dual isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) from precipitation to groundwater in a rural mountainous area affected by acid rain. The average concentration for NH4+ is 1.25 mg/L and NO3- is 2.59 mg/L of acid rain. Groundwater NO3- concentrations ranged from <0.05 to 11.8 mg/L (baseline), and NH4+ concentrations ranged from 0.06 to 0.28 mg/L. The results show that groundwater δ18O-NO3- values (-4.7‰ to +4.2‰) were lower than the values of rainfall δ18O-NO3- (+24.9‰ to +67.3‰), suggesting that rainfall NO3- contributes little to groundwater NO3-. Groundwater δ15N-NO3- values (+0.1‰ to +7.5‰) were higher than the values of δ15N-NO3- derived from the nitrification of rainfall NH4+ (less than -4.7‰ in the study area), suggesting that nitrification of rainfall NH4+ also contributes little to groundwater NO3-. This implies that rainfall NO3- and NH4+ have been utilized. The dual isotopic composition of nitrate shows that baseline groundwater NO3- is derived mainly from nitrification of soil nitrogen. The denitrification process is limited in the groundwater system. This study shows that the rainfall NO3- and NH4+ contribute little to groundwater NO3-, improving the understanding of the nitrogen cycle in areas with a high concentration of NH4+ and NO3- in rainfall.
Highlights
Since the beginning of the twentieth century, the atmospheric concentration of acidic gases, such as SO2, NOx, and NH3 which are mainly a result of industrial activity and coal burning has increased steadily [1,2,3,4]
Shallow groundwater aquifers in the study area can be divided into four types (Figure 1) based on lithology and water abundance [31]: (I) fracturecave water occurring mainly in carbonate rock, (II) cave-fracture water generally occurring in shale with little limestone, (III) pore-fracture water only occurring in the Triassic sandstone with a relatively rich yield of 25–125 m3 per day per meter for a well, and (IV) fracture water occurring in sandstone and mudstone with a groundwater runoff modulus of less than 2 L s-1 km-2
Physical and chemical parameters such as electrical conductivity (EC), pH, oxidation-reduction potential (ORP), and temperature were measured in situ using a multiparameter device (Hach HQ40d)
Summary
Since the beginning of the twentieth century, the atmospheric concentration of acidic gases, such as SO2, NOx, and NH3 which are mainly a result of industrial activity and coal burning has increased steadily [1,2,3,4]. The mixing model of the dual isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) revealed that the nitrate from atmospheric deposition contributed 3% of the river nitrate in a river subbasin in Mecklenburg-Vorpommern (Germany) [27] This proportion was found to be 30% for direct NO3- input to spring water from rainfall in northeast Bavaria (Germany) without any microbial interaction [23]; here, the δ18O-NO3- values for spring water range from +11‰ to +33‰. This study aims to assess the potential input of nitrate from acid rain using chemical and isotopic data of rainfall and groundwater in a rural mountainous area in SW China. The results would have important implications for the groundwater nitrogen cycle in areas with a high concentration of rain ammonium and assessment of the baseline level of nitrate related to nitrate contamination due to anthropogenic activities
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