Redai dili (Jul 2023)
Composition and Source Identification of Nitrogen in Groundwater in an Urbanized Basin
Abstract
The Maquekengshui basin in Pingshan District of Shenzhen was selected as a case study to reveal the composition characteristics and sources of nitrogen in groundwater, with water samples collected from domestic wells, bore holes with auger, and one foundation ditch in September of the wet season and December of the dry season of 2020. The major sources and transformation mechanisms of nitrogen were identified by combining multiple isotopes in nitrate (15N and 18O), ammonium, and water, and the contribution ratios of different nitrate sources were calculated using the Stable Isotope Analysis in R (SIAR) model. The results are as follows: (1) The main form of nitrogen in groundwater was NO3--N, followed by NH4+-N, whereas the NO2--N concentration was relatively low, and the average proportions of the three nitrogen contents were 96.8%, 2%, and 1.2%, respectively. The nitrate nitrogen concentration was significantly influenced by the type of land use. The nitrate nitrogen concentrations of the sampled waters near farmland were higher than those in construction land, however, there was no significant seasonal variation in the nitrogen concentration. (2) The δ15N-NO3- and δ18O-NO3- values of sampled waters ranged from -1.39‰ to 27.84‰ and from -0.27‰ to 40.05‰ in the wet season, while they ranged from 7.60‰ to 20.16‰ and from 2.25‰ to 25.39‰ in the dry season, respectively. The δ15N-NO3- values of the sampled waters also increased with increasing NO3- concentration. The δ15N-NO3- and δ18O-NO3- isotope signatures indicated that sewage and manure were the dominant sources of nitrogen in the groundwater, and nitrification was the main process of nitrogen transformation, whereas denitrification was weak. The 15N-NH4+ values of sampled waters ranged from 39.37‰ to 40.99‰ in the wet season and from 4.02‰ to 35.37‰ in the dry season, indicating that manure/septic effluents, swine effluents, and feedlot runoff were the main ammonia nitrogen sources. (3) The results from the SIAR model showed that the average contribution ratios of different nitrogen sources to groundwater nitrate during the wet and dry seasons were ranked in the same order with varied values. The average contribution rates of sewage and manure in the wet and dry seasons were 45% and 74%, respectively. Soil nitrogen in the wet and dry seasons was 22% and 11%, respectively; atmospheric nitrogen deposition in the wet and dry seasons was 17% and 16%, respectively; and fertilizer application in the wet and dry seasons was 8% and 7%, respectively. The average contribution ratios of different nitrogen sources for domestic wells and bore holes were ranked as sewage and manure (68%) > atmospheric nitrogen deposition (19%) > soil nitrogen (8%) > fertilizer (5%), and those for the foundation ditch were ranked as sewage and manure (82%) > atmospheric nitrogen deposition (15%) > soil nitrogen (2%) > fertilizer (1%). (4) This study revealed that the accumulated nitrogen legacies from intensive pig farms for more than ten years are a potential source of nitrogen pollution in regional groundwater flow, which subsequently affected nitrogen transformations. However, the deep groundwater of the foundation ditch located in the groundwater discharge area was affected more by the accumulated nitrogen pool. Nitrogen pollution from accumulated nitrogen pools is a hidden potential nitrogen source in groundwater.
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