ABSTRACT
Excessive use of fertilizers presents a significant threat to groundwater safety. To mitigate nitrate leaching and ensure the sustainable utilization of groundwater resources, it is crucial to quantify the spatial heterogeneity of nitrogen leaching and its drivers. Therefore, accurate modeling of deep nitrate leaching at large regional scales is necessary. In this study, we have created a computational framework to analyze the transport of unsaturated zone water and nitrate at a regional scale. The framework is based on a process-oriented, watershed-scale computational model that segments the study area into a grid system, with each grid modeled using Richards-based advection-diffusion equations for water and solutes. The research model estimated nitrate nitrogen leaching, accumulation, and denitrification in the vadose zone of agricultural fields in the Baiyangdian watershed, which is a typical agricultural region with complex land use and soil deposition conditions in the North China Plain. The results showed that there were significant spatial differences in nitrate N leaching, denitrification and accumulation with values of 0-388 kg/ha/year, 30-177 kg/ha/year and 75-4778 kg/ha. Groundwater recharge in the wheat/maize, vegetable, and cotton area exhibited a negative correlation with nitrate N accumulation while showing a positive correlation with nitrate N leaching. Nitrate nitrogen distribution indicated spatial heterogeneity, attributable mainly to the heterogeneity in soil texture, structure, and land use. With nitrate nitrogen leaching and denitrification levels reaching 327-388 kg/ha/year and 133-175 kg/ha/year, respectively, vegetable fields pose a direct threat to groundwater. Meanwhile, wheat/maize fields showed the greatest nitrate nitrogen accumulation, ranging from 624 to 4778 kg/ha. This excessive buildup of nitrate in these fields presents a potential hazard to groundwater quality. Soil texture in the root zone had a greater influence on the amount of nitrate leaching and denitrification than soil texture below the root zone. Deeper soil texture (>2 m) was found to mainly control total nitrate accumulation in the vadose zone. To assess nitrate leaching, denitrification, and accumulation at a regional scale within the deep vadose zone, a process-oriented model was developed, considering the intricate associations among land usage, soil texture, and biochemical reactions.
ABSTRACT
Concentrated rainfall and water transfer projects result in slight and dramatic increases in flow volume over short periods of time, causing nitrogen recontamination in the water-receiving areas of nitrogen-rich rivers. This study coupled hydrodynamic and biochemical reaction models to construct a model for quantifying diffusive transport and transformation fluxes of nitrogen across the water-sediment interface and analysed possible changes in the relative abundance of microbial functional genes using high-throughput sequencing techniques. In this study, the processes of ammonium (NH4+-N) and nitrate (NO3--N) nitrogen release and sedimentation with resuspended particles, as well as mineralisation, nitrification, and denitrification processes were investigated at the water-sediment interface in the Fu River during slight and dramatic increases in flow volume caused by concentrated rainfall and water diversion projects. Specifically, a slight flow volume rise increased the release of NH4+-N from the sediment, inhibited sedimentation of NO3--N, decreased the mineralisation rate, increased the nitrification rate, and had little effect on the denitrification process, ultimately increasing the nitrogen load to the river water. A dramatic increase in flow volume simultaneously increased NH4+-N and NO3--N exchange fluxes, inhibited the mineralisation process, promoted nitrification-denitrification processes, and increased inorganic nitrogen consumption in the river. This study provides a solution for the re-pollution of rivers that occurs during the implementation of reservoir management and water diversion projects. Furthermore, these results indicate a potential global nitrogen sink that may have been overlooked.
Subject(s)
Rivers , Water Pollutants, Chemical , Denitrification , Nitrates/analysis , Nitrification , Nitrogen/analysis , Water/analysis , Water Pollutants, Chemical/analysisABSTRACT
The variations of hydrogen and oxygen isotopes in rainfall are critical for understanding the sources of rainfall and the influence of local evaporation. Satellite precipitation products with high time resolution (for instance 1 h) could be helpful for testifying the accuracy of water sources, as it can clearly illustrate the route of cloud movement. In this study, we analyzed the composition of hydrogen and oxygen isotopes in different rainfall events in three stations from 2015 to 2018 along the transection of 38° N latitude from Taihang Mountains to the coastal region in North China, Taihang Mountain Station (mountainous area), Luancheng Station (pre-mountain plain) and Nanpi Station (coastal low plain). By selecting typical rainfall events, water vapor sources and its influence rainfall on hydrogen and oxygen isotopes were analyzed with hourly available CMORPH satellite precipitation products. Results showed that the hydrogen and oxygen isotopes of precipitation were cha-racterized by enrichment in the rainy season and depletion in the dry season. The hydrogen and oxygen isotopes in the rainy season showed a tendency of depletion with the increases of precipitation. The slope and intercept of the fitted relationship of hydrogen and oxygen isotopes in the piedmont region of the mountains were the lowest, indicating that precipitation in the piedmont plain was significantly affected by secondary evaporation fractionation. The effect of evaporation resulted in the largest variations of isotope ratio in the dry year. In the mountainous station, due to the heavy rainfall, large isotopic variation was found in rich precipitation year. Based on the route analysis of sate-llite precipitation products, dominant water vapor in the region was inland and northwest-oriented water vapor, while water vapor in the rainy season was from southwest and from the Pacific Ocean. There was a significant difference in the hydrogen and oxygen isotopes of precipitation in the mountainous and plain stations in 2016, owing to water vapor sources and effects of rainfall for the mountainous and evaporation for plain. The results from HYSPLIT model showed that during the rainstorm on 19th July in 2016, water vapor at the mountainous station was mainly from the southwest, while that in the coastal plain was a mixture of southwest and southeast sources. Overall, our results showed that spatial and temporal variations of hydrogen and oxygen isotopes were controlled by both water sources and evaporation processes along the transection of 38° north latitude in North China.
Subject(s)
Hydrogen , Steam , China , Environmental Monitoring , Hydrogen/analysis , Oxygen Isotopes/analysis , Pacific Ocean , Rain , SeasonsABSTRACT
Nitrate in groundwater is increasing in hilly areas of the world due to diverse land-uses and intensified anthropogenic activity. However, the key factors that control nitrate in groundwater as it leaches through the thin vadose zone are still poorly understood. In this study, the behavior of nitrate in the vadose zone during a normal year (2015) and a wet year (2016) were investigated in a cultivated farmland (FL) under wheat-maize double cropping and a field of natural vegetation (NV) in a headwater region of the Taihang Mountain. Water chemistry and N balances were quantified to estimate the accumulation and leaching of nitrate. Transport and fate of nitrate in the vadose zone were identified using stable isotopes of nitrate (δ18O-NO3-, δ15N-NO3-). Accumulation of NO3- was mainly in the shallow layer (0-50 cm) under both NV and FL in 2015. After large rain events (20-50 mm/day) during the rainy season in 2015, the leaching rates of NO3- at NV and FL sites were 86.04 and 9.61 kg/hm2, respectively. The accumulation of NO3- decreased 30% and 7% at NV and FL, respectively. However, the leaching rates of NO3- were 63.8 and 22.63 kg/hm2, and the accumulation of NO3- decreased 84% and 43% at NV and FL after extreme precipitation in 2016, respectively. Additionally, nitrate isotopes indicated that the different sources of nitrate in soil water were due to the land-use. However, nitrate isotopes showed that increased nitrate concentrations in groundwater at two sites in 2016 were due to the extreme precipitation. Accumulated high nitrate in surface soil in a normal year and leaching after extreme precipitation lead to increased nitrate concentration in groundwater, which poses a major threat in the future. The results are critical for informing land-use and water management when mitigating groundwater contamination in hilly area.
