Identifying Nitrogen Sources And Quantitfying The Contributions Of Nitrate Sources In Coastal Zone

Economic growth, rapid population increase and urban development lead to shortages of land and water resources in coastal regions. Groundwater reservoir and land reclaimation are effective method for increasing land and water resources. However, these human activities at the same time will disturb water environment, and produce pollutants exported into water, which will decrease water quality environment of the coastal region. Considering the environmental problems produced by land reclamation and groundwater reservoir, taken Laizhou City lied along the eastern coast of Laizhou Bay, China as examples, concentration analysis and stable isotope method are used to analyze nitrogen migration and transformation processes in water affected by these activities, and then evaluate their impact on the water, which will support the nonpoint pollution management and ocean resources development.(1) Four field investigations and samplings were conducted for analyzing nitrogen and phosphorus concentrations and the isotopes of nitrate-nitrogen and nitrate-oxygen in mariculture ponds, tital creeks and groundwater. The aim is to identify nitrate sources and nitrogen transformation processes and then evaluate the environmental impact of mariculture on surrounding water bodies. The results show that the phosphorus concentrations are significantly lower than nitrogen concentrations in mariculture ponds, tital creeks and groundwater in the study area. Mariculture may results in the increase in nitrogen concentration in the surrounding water and nitrate is the major part of the dissolved inorganic. The nitrogen concentrations increase from the mariculture zone to the hinterland. The isotopic compositions of nitrogen and oxygen in nitrate show that sewage and mariculture may be the main nitrate sources in groundwater, indicating that the nitrogen derived from mariculture ponds may be transported to the groundwater and than leads to the increase in nitrogen concentrations in groundwater.Nitrification and denitrification occur in groundwater.(2) Based on the structure of groundwater reservoir in coastal regions, three field investigations and samplings were conducted for analyzing nitrogen and phosphorus concentrations and the isotopes of nitrate-nitrogen and nitrate-oxygen in nitrate in the groundwater upstream and downstream. The aims are to identify nitrate sources and nitrogen transformation processes in groundwater upstream and evaluate the impact of the underground cutoff walls on nitrogen and phosphorus in groundwater. The results show that nitrate in groundwater upstream derive from fertilizers, manures and soils. The nitrate and total nitrogen concentrations in groundwater upstream are significantly higher than those concentrations in groundwater downstream, and both the seasonal and spatial difference of these concentrations in groundwater upstream are also lower than those in groundwater dowsnstream. These results indicate that the underground cutoff walls barrier the groundwater flow and then lead to the accumulation of nitrogen in groundwater. At the same time, the underground cutoff walls increase the groundwater residence time for creating a relative stable environment for nitrogen transformation processes. The relationships between the isotopic composition of nitrogen and oxygen in nitrate and nitrate concentrations conform to the isotopic compositon changes during nitrification and denitrification processes, indicating that these nitrogen transformation processes occur in groundwater upstream.(3) Base on the the impact of mariculture and groundwater reservoir on the nitrogen distribution in groundwater, three isotope mixing models are used to calculate the nitrate contributions of nitrate sources. The aims are to discuss the applicability of the three models in coastal regions and analyze the nitrate contributions of nitrate sources in the study area affected by human activities. The results show that the nitrate contributions from SIAR model have lowest standard deviations, indicating large uncertainty. The nitrate contributions from IsoError model have higer standard deviations which may be affected by the spatial variability of the isotopic compositions of nitrate-nitrogen and nitrate-oxygen in nitrate of nitrate sources and receiving water, and the higher spactial variability of which may leads to larger uncertainty.In the groundwater downstream, the nitrate contribution of mariculture is highest in September with a value of 45.06%, and lowest in Januray with a value of 21.57%. There is little difference of nitrate contributions of sewage between four seasons, and the value is about 40%. In groundwater upstream, nitrate contribution of manure is highest. But there is little difference of nitrate contributions of nitrate sources in three seasons due to the barrier of underground cutoff walls.