济南泉水硝酸盐的升高机理及来源定量识别

The decline of water table changes the redox condition in the unsaturated zones, thereby leading to nitrification of the soil organic nitrogen and ammonia nitrogen and leaching to the saturated zones, which is an important cause for groundwater nitrate increasing. However, the importance and mechanisms of such processes have not been completely recognized. Thus, quantitatively identifying various sources of groundwater nitrates and their contribution rates is still challenging. Based on the analysis of regional groundwater flow systems, taking the four spring groups such as Baotu Spring and groundwater in Jinan spring catchment as an example, we will collect various waters (groundwater from wells and springs, rainfall, and surface water) and soils and soil water and air from the unsaturated zone at different depths in typical locations of groundwater flow system under different land use conditions and measure various physical and chemical indexes and isotopies of the samples. The leaching experiments of soil columns will be conducted in laboratory and the mechanisms of how the unsaturated zone control the nitrate inputs into saturated zone under water table decline will be revealed and the nitrate contribution rates of the unsaturated zone will be estimated in combining with the field data. Environmental tracers such as characteristic ions and stable isotopes, and δ18O-δ15N diagrams are comprehensively used to qualitatively distinguish the sources of groundwater nitrates in different seasons. The SIAR (stable isotope analysis in R) software based Bayesian algorithm is adopted to deal with the data of the stable isotopes of nitrogen and oxygen in nitrate, sulfur in sulfate, carbon, boron, and so on, to quantitatively estimate the contribution of the nitrate nitrogen from the different sources of precipitation, soils, fertilizers, sewages and so on. Finally, 1D vertical unsaturated models for nitrogen transport and transformation and 3D groundwater flow and nitrate transport models will be developed to estimate the inputs of groundwater nitrates derived from different zones of land use or disparate sources and to predict the nitrate trend in the spring water, consequently providing theoretical basis for the protection and utilization of spring water.

潜水位下降使包气带氧化还原条件变化、土壤中氮被硝化淋滤进入含水层，是地下水硝酸盐增加的重要原因，但人们对其重要性和机理认识不足；定量识别地下水硝酸盐来源及其贡献还存在很大挑战。本项目以济南趵突泉等四大泉群及泉域地下水为例，在区域水流系统分析的基础上，选择不同土地利用条件和水流系统典型部位，采集各类水样和包气带不同深度的土、土壤水及气样，测定其各种物理和化学指标及同位素。开展土柱淋滤实验，结合野外监测结果揭示潜水位下降对地下水硝酸盐输入升高的影响机理。综合运用特征离子和多种同位素及δ18O-δ15N图，定性判别不同季节地下水硝酸盐的来源。用SIAR软件处理氮、氧、碳、硼、硫等稳定同位素数据，计算降水、土壤、化肥、污水等来源硝酸盐氮贡献率。建立垂向一维包气带氮素迁移转化和三维地下水硝酸盐运移数值模型，估算不同土地利用区地下水硝酸盐的输入量，预测泉水硝酸盐变化趋势，为泉水保护和利用提供依据。

气候变化和人类活动引起潜水位波动使包气带氧化还原条件变化，必然影响潜水面和地表水-地下水界面氮素等组分的迁移转化过程和通量，但人们对其机理认识不足；定量识别地下水硝酸盐来源及其贡献存在很大挑战。本项目在济南泉域地下水流系统分析的基础上，选择不同土地利用条件和水流系统部位，调查采样，测定物理化学指标和同位素；开展土柱实验和数值模拟实验，探讨研究区地下水硝酸盐和硫酸盐的升高机理。结果表明济南泉域地下水总溶解性固体为202.9−977.3 mg/L，硝酸盐和硫酸盐浓度分别为2.0−139.9和6.3−255.6 mg/L。实验表明潜水位波动会改变包气带中硝酸盐进入饱水带的通量；地下水位下降时，包气带富氧区域增加，会增加动物粪便和氮肥硝化为硝态氮输入饱水带；水位上升时，包气带中硝酸盐被淋滤进入含水层；进而使地下水硝酸盐等有害组分升高。稳定同位素混合模型Simmr分析结果显示地下水硝酸盐主要来源于土壤氮（40.3%），其次为还原性氮肥（27.8%）及污水/粪肥（15.9%），硝态氮肥和大气沉降的贡献较少；地下水硫酸盐主要来源于土壤硫酸盐（37.3%）和污水（33.9%），其次是蒸发岩溶解（12.2%），硫化物氧化、洗涤剂、大气沉降和农业化肥的贡献比率较少。气候变化和人类活动加剧是济南泉水硝酸盐和硫酸盐升高的主要原因。基于溶质运移的光滑粒子流体动力学（SPH）方法，提出了修正SPH（C-SPH）、交互修正SPH（IC-SPH）和标准化核函数的解耦有限粒子法（DFPM-NK）三种改进的SPH新方法，构建三种方法相对应的溶质运移数值模型，新的数值模型更适用于非均质含水层。若不考虑计算成本，可用C-SPH和IC-SPH 方法提升小尺度地下水溶质运移的模拟精度。若同时考虑计算成本和精度，在保持模拟精度的条件下可用高效的DFPM-NK方法模拟较大尺度的地下水溶质运移问题。数值模拟结果表明颗粒有机碳促进反硝化作用的发生，降低地表水和地下水中硝酸盐浓度。非均质含水层中细粒结构有机碳含量丰富，能显著提高硝态氮的去除效率。项目成果可供济南泉域和类似地区水资源利用与保护参考。