基于溴离子和氢氧同位素示踪的潜水蒸发评价研究

Phreatic water evaporation represents the water transfer from groundwater to soil water, as well as the process of soil water movement upwards to surface. However, the current research on phreatic water evaporation haven not taken fully account of the water movement upwards in vadose zone. It will limit our understanding of process and mechanism of phreatic water evaporation, and go against with the accuracy of evaluation. The purpose of this project is to analyze the process and mechanism of phreatic water evaporation from the aspect of soil water movement, by using the bromide and hydrogen and oxygen isotope as tracer. The migration of bromide under the conditions of different evaporation intensity and time, different water table depth and different lithology and structure of vadose zone will be studied to indicate the movement of liquid water in vadose zone through laboratory soil-column experiments. At the same time, the characteristic of hydrogen and oxygen isotope of shallow soil water will been measured to seek the interface of water-vapor, which can represent the process of water transfer from liquid to vapor at the shallow parts of vadose zone. Based on the results of laboratory soil-column experiments, several methods, such as lysimeter, temperature tracer and numerical simulation using variably saturated flow model, together with the bromide and hydrogen and oxygen isotope will be used to evaluate the phreatic water evaporation at different water depth. The results of different methods will be compared to confirm the accuracy of chemical tracers. The lowland plain of Hanjiang river will be chosen as the typical research area, fields test will be carry out to analyze and summarize the applicability and localization of chemical tracers. Based on the achievements we will get above, the detail evaluation scheme of phreatic water evaporation using bromide and hydrogen and oxygen isotope will be propose. The results of this project will be of great significance in further understanding the process and mechanism of phreatic water evaporation, accurate evaluation of the quantity of phreatic water evaporation, and it will enrich the methods system of phreatic water evaporation evaluation.

潜水蒸发是四水转换的重要环节，表征了地下水与土壤水之间的水分交换及包气带水分传输过程。但目前潜水蒸发研究一般未充分考虑水分在包气带中的向上传输过程，局限了对潜水蒸发过程和机理的认识，不利于潜水蒸发量的准确评价。本项目将人工溴离子和氢氧同位素作为示踪剂，基于室内土柱试验分析蒸发强度及时间、水位埋深、包气带岩性结构等因素影响下的溴离子运移规律，以指示蒸发条件下包气带液态水分的向上运移；通过分析土柱表层土壤水氢氧同位素特征确定持续蒸发条件下的水气交界带特征及其决定因素，指示水分由液态到气态的转换过程；利用地中渗透仪实验、变饱和数值模拟、温度示踪等方法对比印证示踪剂评价潜水蒸发量的准确性；选取汉江下游低平原区作为研究典型区，探求应用示踪剂评价潜水蒸发的适用性及局限性，提出详细的评价方法及方案。本项目对于深入认识潜水蒸发过程及机理，准确评价潜水蒸发量，丰富潜水蒸发评价体系具有重要理论和现实意义。

为探讨利用溴离子和氢氧同位素作为示踪剂评价潜水蒸发的可行性及适用条件，本项目采用室内土柱试验、野外示踪试验及变饱和数值模拟等方法，对蒸发条件下包气带中溴离子运移规律及表层土壤水氢氧同位素分布特征进行了分析，进而探讨了包气带岩性及厚度、蒸发条件等天然因素以及投放浓度、投放方式、布孔方式等人为因素对潜水蒸发评价的影响，并利用蒸渗仪数据揭示了入渗对利用溴离子示踪法评价潜水蒸发的不利影响。研究表明：持续蒸发作用使溴离子随包气带水分由投放深度向地表运移，溴离子运移规律随包气带特征变化差异显著，表明其作为示踪剂响应潜水蒸发强度变化的灵敏度高；投放深度、投放浓度对溴离子运移规律影响不显著；试验条件下，蒸发作用使表层土壤水氢氧同位素发生分馏，0~15cm深度范围内，δD、δ18O值随深度增加明显减小，指示土壤水发生相变，表明气液交界带位于表层15cm范围内；包气带岩性颗粒细、水位埋深浅时，溴离子在包气带中运移快，浓度峰值运移明显，潜水蒸发速率计算误差相对较小；天然非稳定蒸发条件下，潜水蒸发速率评价结果能够响应大气蒸发能力的变化；示踪剖面平均含水率随示踪剂投放深度增加而增加，导致潜水蒸发速率评价结果增大而偏离实际值；布孔方式及投放浓度仅影响示踪剂取样范围及样品浓度，不会使评价结果出现显著误差，4孔正方形布孔对应示踪剂分布范围较大（比3孔高19.2%）且中心点示踪剂浓度峰值最高，具有更好的示踪效果；入渗补给和潜水蒸发对溴离子运移的影响相反但相对独立，没有耦合效应；溴离子示踪法可适用于不同条件下的潜水蒸发速率评价，指示水分传输带过程，氢氧同位素分布特征可用于划分水分传输带、气液交界带和水汽扩散带，明确溴离子示踪区间上界面，指示潜水蒸发全过程；联合运用溴离子和氢氧同位素作为示踪剂开展潜水蒸发评价研究，可充分利用并结合两者优势，具有较高的可行性及适用性。本项目研究成果可为开展野外潜水蒸发评价研究提供一定的理论和技术支撑。