土壤优先水流及污染物优先迁移的研究进展

土壤优先水流是非平衡管道流。土壤优先水流具有优先穿透、侧向入渗、穿透曲线不对称性和拖尾等特征。因其与土壤的作用较弱，并可携带溶质快速穿透土壤，造成土壤水分和养分利用率下降，同时导致地下水污染。向下移动的ＮＯ_３~－农药和其它溶质造成的地下水溶质浓度的变化远高于期望值。本文从土壤优先水流的产生机理出发，综述了土壤优先水流的特征、土壤优先水流的研究方法、土壤优先水流对环境的影响和土壤优先水流的研究进展，并对该领域研究的发展趋势进行了探讨。     

土壤中反应性溶质保持模拟模型概述

水溶性污染物和有机污染物在非饱和带的运动规律是当前土壤物理学研究的前沿领域．模拟反应性溶质的运移模型，必须包含溶质和土壤基质之间的保持和释放反应．本文简要综述土壤对溶质保持反应的平衡模型、动力学模型和多反应模型的主要特点．     

Assessing and simulating the major pathway and hydrogeochemical transport of arsenic in the Beitou–Guandu area,Taiwan

This study involved assessing and simulating the probable major pathways (surface and subsurface flow) and hydrogeochemical transport of arsenic (As) in the Beitou–Guandu area, Taiwan. A one-dimensional (1-D) generic, reactive, chemical transport model (PHREEQC) was adopted. The calibrated model showed that As transported to the downstream Guandu plain and Tan Shui river mouth accounted for 50.7 and approximately 100 % of the As in the subsurface flow pathway, respectively, suggesting that subsurface flow constituted a major As pathway. The highest As water concentration occurred near the Beitou geothermal valley because of the low pH and high redox potential in both the surface and subsurface pathways. However, As may be scavenged by aqueous Fe(II) in a reducing environment. The As concentrations in the downstream Guandu plain and Guandu wetland decreased as the simulated time increased, resulting in the adsorption of As on the surface of Fe oxydroxides and limiting the mobility of As in the surface flow pathway. The major retardation mechanism of As mobility in the subsurface flow pathway of the Guandu plain and Guandu wetland was governed by the adsorption reactions of iron-oxide and iron-sulfide minerals. The 1-D transport model was applied to predict the evolution of As in the subsurface flow pathway from 2013 to 2020. The results indicated that the As concentrations in all cells gradually increased. The geochemical redox reactions of As in the subsurface pathway subsequently led to the oxidization of As-bearing sulfides, causing As concentrations to rise substantially in the hillside area.

     

蚯蚓活动对重金属在土柱中淋溶行为的影响

采用土柱淋溶方法,以溴离子为示踪离子,研究了蚯蚓活动形成的土壤大孔隙对铜和锌离子在土柱中迁移行为的影响.运用平衡和非平衡对流-弥散模型对溴离子和铜、锌离子在不同土柱中的穿透曲线进行拟合.结果表明,蚯蚓活动形成的蚯蚓孔能够产生明显的优势流现象,这种优势流极大地加快了重金属离子在土柱中垂直向下迁移的速度.非平衡对流-弥散模型能很好地预测溴离子和铜、锌离子在蚯蚓孔土柱中的流出趋势,而平衡对流-弥散模型适合于预测溶质在对照（匀质）土柱中的流出趋势.     

人工湿地系统对含沼液畜禽废水净化效果试验研究

为了考察人工湿地处理含沼液畜禽废水的可行性,采用水平潜流人工湿地对含沼液畜禽废水进行处理实验。试验结果表明：在进水流量1.5 m3.d-1,水平潜流人工湿地系统对含沼液畜禽废水具有较好的处理效果。废水中COD、TP、TN和NH4＋-N浊度平均去除率分别为59.21%、53.80%、55.09%和55.57%.另外,通过对人工湿地沿程的污染物变化试验分析表明,人工湿地系统对污染物的降解是沿人工湿地水流方向逐渐降低的。     

湖滨带复合型人工湿地氮磷的去除效果

湖滨带是连接湖泊水域生态系统与陆地生态系统的一个功能过渡区,是湖泊的最后一道保护屏障。在河流的入湖口建造湖滨湿地,可有效净化入湖径流中携带的部分有机污染物、营养盐等。以云南抚仙湖北岸的湖滨湿地—马料河复合人工湿地为研究对象,探讨了湿地不同功能区去除氮磷的效果。研究表明,沉淀池除氮效果最不明显,在该区内有机氮可能发生矿化作用而转变为氨氮。有植物系统的潜流和表流区除氮效果较为明显,潜流区对氨氮、硝氮和总氮的平均去除率分别达18.0%、19.7%和22.6%;表流区对三者的平均去除率分别达50.4%、35.9%和43.5%。沉淀池对磷有一定的截留作用,且在进水污染物质量浓度较高时表现明显,平均截留率为14.9%。潜流和表流区除磷效果不明显,可能是因为湿地运行了两年多,土壤吸附交换达到平衡,影响了表流区的除磷效果。潜流区除磷效果受降雨影响较大,雨季时,总磷的平均截留率为12.1%,主要是不溶性磷的吸附和沉积;雨季末期,湿地流量较小,水体流动性差,系统内处于厌氧状态,出现磷释放现象。     

Instantaneous transport of a passive scalar in a turbulent separated flow

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection.     

人工增氧型复合湿地污染物净化效果

针对传统人工湿地负荷低、硝化能力弱和易堵塞的缺点,结合曝气生物滤池与人工湿地强化理论的研究成果,自主开发了人工增氧型复合湿地工艺,即生物接触氧化预处理+微曝气垂直流湿地+水平潜流湿地。通过对滇池北岸城郊混合制污水处理的模型试验,分析该工艺流程各净化单元的污染物去除效果。结果显示,人工增氧型复合湿地对不同水力负荷和污染负荷都体现了较强的缓冲调节能力和较高的净化效果。对污染物的总去除率分别为SS 94.5%、CODCr71.3%、氨氮72.3%,在进水总氮10.0 mg.L-1条件下实现了41.9%的总氮去除率,出水总氮均值为5.8 mg.L-1。     

Using adsorption and sulphide precipitation as the principal removal mechanisms of arsenic from a constructed wetland – a critical review

This review has been undertaken to understand the role of various parameters such as redox potential, microbes, and organic matters on the fate and transport of arsenic in the constructed wetland. A conceptual diagram of arsenic fate and transport in the constructed wetland was developed. Role of various minerals which are produced due to microbial activities are described. The role of these minerals on arsenic adsorption is discussed. It was envisaged that iron sulphide would be the main adsorbent for arsenic in microbe-mediated adsorption process. Beside microbe-mediated arsenic adsorption, roles of various microbes, such as sulphate reducers and methane producers, on arsenic transformation are discussed. Role of various organic carbon sources and electron acceptors on the proliferation of the above mentioned microbes with respect to arsenic transformation was envisaged. Role of dissolved organic matters on arsenic transformation and transport was also discussed in details. To strengthen the review, laboratory studies and modelling of arsenic adsorption and transformation using the Visual Minteq were appended.     

Preferential leaching of nitrate,chloride and phosphate in an Australian clay soil

This paper investigates leaching of water and nutrients (NO^‐ ₃, Cl^‐, PO^3‐ ₄) from the unsaturated layer in an Australian soil using a multisegment percolation system (MPS). Large undisturbed soil cores were collected from a clay‐based, basaltic plain, agricultural soil at Grassmere, 300 km west of Melbourne, Australia. Significant heterogeneity (or preferential flow) of effluent moisture and solutes was detected (one‐way ANO VA, p < 0.001). Fifty percent of the applied nitrate and chloride leached from the soil core within three days after initial application. Hundred percent of the applied nitrate and chloride leached from the soil core within 8 days after application. These results indicate little incorporation into the soil matrix, and possible denitrification or mineralisation. In contrast, after 18 days, less than 1 % of the total applied phosphates leached from the soil, indicating strong adsorption. Our experiments indicate considerable heterogeneity in water flow patterns and solute leaching on a small spatial scale. Very rapid transport of nitrate and chloride through the soil was evident, in comparison phosphate leaching was negligible. These results have important implications for the management of nutrient schedules in agricultural soils, particularly those located in the Western District of Victoria, Australia.     

六价铬在具有渗透性反应墙的渗流槽中迁移实验研究

地下水中六价铬的污染修复是目前地下水污染修复研究的热点课题之一。以室内模拟为基础,进行了六价铬[Cr（Ⅵ）]随水流迁移的渗流槽实验,以及利用课题组研发的新型壳聚糖材料填充渗透性反应墙（PRB）,进行六价铬吸附试验研究。结果表明：在重金属迁移实验中,地下水的流速是影响Cr（Ⅵ）在含水层中迁移的主要因素,含水层介质的变化都会对Cr（Ⅵ）的迁移发生重要的影响。实验过程中,从进水口到出水口各个取样点依次出现Cr（Ⅵ）的质量浓度峰值,并且随着时间的延长,质量浓度峰值没有减少的趋势,证明了Cr（Ⅵ）在含水介质中基本上没有吸附。Cr（Ⅵ）的质量浓度峰值在含水介质中的持续时间不长,由于弥散作用造成了在相当长的时间内的较明显的拖尾现象。模拟渗透性反应墙的实验中,在同样的流速条件下,Cr（Ⅵ）的质量浓度峰值的出现时间相对先前的实验有明显滞后,并且质量浓度也有明显的减少,说明新型壳聚糖材料对Cr（Ⅵ）有较高的吸附作用,再达到质量浓度的峰值后,墙体后面的采样口中Cr（Ⅵ）的质量浓度减少的更为平缓,说明随着水流的流动,吸附在壳聚糖材料上的Cr（Ⅵ）有缓慢的析出。     

Treating wastewater under zero waste principle using wetland mesocosms

• Smart wetland was designed to treat wastewater according to zero waste principle. • The system included a dynamic roughing filter, Cyperus papyrus (L.) and zeolite. • It removed 98.8 and 99.8% of chemical and bacterial pollutants in 3 days. • The effluent reused to irrigate a landscape and the sludge recycled as fertilizer. • The plant biomass is a profitable resource for antibacterial and antioxidants. The present investigation demonstrates the synergistic action of using a sedimentation unit together with Cyperus papyrus (L.) wetland enriched with zeolite mineral in one-year round experiment for treating wastewater. The system was designed to support a horizontal surface flow pattern and showed satisfactory removal efficiencies for both physicochemical and bacteriological contaminants within 3 days of residence time. The removal efficiencies ranged between 76.3% and 98.8% for total suspended solids, turbidity, iron, biological oxygen demand, and ammonia. The bacterial indicators (total and fecal coliforms, as well as fecal streptococci) and the potential pathogens (Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa) showed removal efficiencies ranged between 96.9% and 99.8%. We expect the system to offer a smart management for every component according to zero waste principle. The treated effluent was reused to irrigate the landscape of pilot area, and the excess sludge was recycled as fertilizer and soil conditioner. The zeolite mineral did not require regeneration for almost 36 weeks of operation, and enhanced the density of shoots (14.11%) and the height of shoots (15.88%). The harvested plant biomass could be a profitable resource for potent antibacterial and antioxidant bioactive compounds. This could certainly offset part of the operation and maintenance costs and optimize the system implementation feasibility. Although the experiment was designed under local conditions, its results could provide insights to upgrade and optimize the performance of other analogous large-scale constructed wetlands.     

A non-negative and high-resolution finite volume method for the depth-integrated solute transport equation using an unstructured triangular mesh

This paper proposes a new high-resolution finite volume method for solving the two-dimensional (2D) solute transport equation using an unstructured mesh. A new simple r-factor algorithm is introduced into the Total Variation Diminishing flux limiter to achieve a more efficient yet accurate high-resolution scheme for solving the advection term. To avoid the physically-meaningless negative solutions resulted from using the Green–Gauss theorem, a nonlinear two-point flux approximation scheme is adopted to deal with the anisotropic diffusion term. The developed method can be readily coupled with a two-dimensional finite-volume-based flow models under unstructured triangular mesh. By integrating with the ELCIRC flow model, the proposed method was verified using three idealized benchmark cases (i.e., advection of a circle-shaped solute field, advection in a cyclogenesis flow field and transport of a initially square-shaped solute plume), and further applied to simulate the non-reactive solute transport process driven by irregular tides in the Deep Bay, eastern Pearl River Estuary of China. These cases are also simulated by models using other existing methods, including different r-factor for advection term and the Green–Gauss theorem for diffusion term. The comparison between the results from the new method and those from other existing methods demonstrated the new method could describe advection induced concentration shock and discontinuities, and anisotropic diffusion at high resolution without providing spurious oscillations and negative values.     

不同剖面层次土壤磷素运移研究

以陕西杨凌地区塿土为研究对象,通过饱和稳定流土柱运移试验,测定磷酸盐(H_2PO_4~-)在不同层次土壤中的运移穿透曲线(BTC),应用CDE模型、CXTFIT软件程序对其实测穿透曲线(BTC)进行拟合求其运移参数,研究其运移规律。结果表明,磷酸盐在塿土中的运移受到土壤的强烈阻滞作用,随着CaCO_3含量增加,磷酸盐运移所受的阻滞作用增大。磷酸盐在塿土不同剖面层次中运移所受阻滞作用大小与CaCO_3含量呈极显著的正相关,r=0.9791;而与粘粒含量且成极显著的负相关、r=0.9318。磷酸盐在塿土不同土层运移所受阻滞作用大小顺序为:钙积层>堆垫表层>粘化层,其阻滞因子R值分别为:11.80～13.00,5.20～7.00,3.90～5.00。     

Decontamination efficiency and root structure change in the plant-intercropping model in vertical-flow constructed wetlands

Subtropical climatic conditions can contribute to the death of the aerial parts of constructed wetland plants in winter. This presents a barrier to the widespread application of constructed wetland and is an issue that urgently needs to be solved. Three contrasting experi- ments, the plant-intercropping model （A）, the warm- seasonal plant model （B）, and the non-plant model （C）, were studied in terms of their efficiency in removing pollutants, and the change in root structure of plants in the plant-intercropping model within the vertical-flow con- structed wetlands. The results indicate that model A was able to solve the aforementioned problem. Overall, average removal rates of three pollutants （CODcr, total nitrogen （TN） and total phosphorous （TP）） using model A were significantly higher than those obtained using models B and C （P 〈 0.01）. Moreover, no significant differences in removal rates of the three pollutants were detected between A and B during the higher temperature part of the year （P〉 0.05）. Conversely, removal rates of the three pollutants were found to be significantly higher using model A than those observed using model B during the lower temperature part of the year （P 〈 0.01）. Furthermore, the morphologies and internal structures of plant roots further demonstrate that numerous white roots, whose distribution in soil was generally shallow, extend further under model A. The roots of these aquatic plants have an aerenchyma structure composed of parenchyma cells, therefore, roots of the cold-seasonal plants with major growth advantages used in A were capable of creating a more favorable vertical-flow constructed wetlands media- microenvironment. In conclusion, the plant-intercropping model （A） is more suitable for use in the cold environment experienced by constructed wetland during winter.     

Numerical modeling and performance evaluation of passive convergence-permeable reactive barrier (PC-PRB)

● A 2D finite-element solute transport model, PRB-Trans, is developed. ● PC-PRB can significantly improve the remediation efficiency of PRB. ● PC-PRB can considerably reduce the required PRB dimensions and materials costs. ● The required PRB length decreases with the increase of pipe length, L _p. The passive convergence-permeable reactive barrier (PC-PRB) was proposed to address the limitations of traditional PRB configurations. To evaluate the hydraulic and pollutant removal performance of the PC-PRB system, we developed a simulation code named PRB-Trans. This code uses the two-dimensional (2D) finite element method to simulate groundwater flow and solute transport. Case studies demonstrate that PC-PRB technology is more efficient and cost-effective than continuous permeable reactive barrier (C-PRB) in treating the same contaminated plume. Implementation of PC-PRB technology results in a 33.3% and 72.7% reduction in PRB length (L_PRB) and height (H_PRB), respectively, while increasing 2D horizontal and 2D vertical pollutant treatment efficiencies of PRB by 87.8% and 266.8%, respectively. In addition, the PC-PRB technology has the ability to homogenize the pollutant concentration and pollutant flux through the PRB system, which can mitigate the problems arising from uneven distribution of pollutants in the C-PRB to some extent. The L_PRB required for PC-PRB decreases as the water pipe length (L_p) increases, while the H_PRB required initially decreases and then increases with increasing L_p. The effect of passive well height (H_w) on H_PRB is not as significant as that of L_p on H_PRB. Overall, PC-PRB presents a promising and advantageous PRB configuration in the effective treatment of various types of contaminated plumes.     

Mitigation of agricultural nonpoint-source pesticide pollution in artificial wetland ecosystems

Contamination caused by pesticides in agriculture is a source of environmental poor water quality in some of the European Union countries. Without treatment or targeted mitigation, this pollution is diffused in the environment. Pesticides and some metabolites are of increasing concern because of their potential impacts on the environment, wildlife and human health. Within the context of the European Union (EU) water framework directive context to promote low pesticide-input farming and best management practices, the EU LIFE project ArtWET assessed the efficiency of ecological bioengineering methods using different artificial wetland (AW) prototypes throughout Europe. We optimized physical and biological processes to mitigate agricultural nonpoint-source pesticide pollution in artificial wetland ecosystems. Mitigation solutions were implemented at full-scale demonstration and experimental sites. We tested various bioremediation methods at seven experimental sites. These sites involved (1) experimental prototypes, such as vegetated ditches, a forest microcosm and 12 wetland mesocosms, and (2) demonstration prototypes: vegetated ditches, three detention ponds enhanced with technology of constructed wetlands, an outdoor bioreactor and a biomassbed. This set up provides a variety of hydrologic conditions, with some systems permanently flooded and others temporarily flooded. It also allowed to study the processes both in field and controlled conditions. In order to compare the efficiency of the wetlands, mass balances at the inlet and outlet of the artificial wetland will be used, taking into account the partition of the studied compound in water, sediments, plants, and suspended solids. The literature background necessary to harmonize the interdisciplinary work is reviewed here and the theoretical framework regarding pesticide removal mechanisms in artificial wetland is discussed. The development and the implementation of innovative approaches concerning various water quality sampling strategies for pesticide load estimates during flood, specific biological endpoints, innovative bioprocess applied to herbicide and copper mitigation to enhance the pesticide retention time within the artificial wetland, fate and transport using a 2D mixed hybrid finite element model are introduced. These future results will be useful to optimize hydraulic functioning, e.g., pesticide resident time, and biogeochemical conditions, e.g., dissipation, inside the artificial wetlands. Hydraulic retention times are generally too low to allow an optimized adsorption on sediment and organic materials accumulated in artificial wetlands. Absorption by plants is not either effective. The control of the hydraulic design and the use of adsorbing materials can be useful to increase the pesticides residence time and the contact between pesticides and biocatalyzers. Pesticide fluxes can be reduced by 50–80% when hydraulic pathways in artificial wetlands are optimized by increasing ten times the retention time, by recirculation of water, and by deceleration of the flow. Thus, using a bioremediation method should lead to an almost complete disappearance of pesticides pollution. To retain and treat the agricultural nonpoint-source po a major stake for a sustainable development.     

地下水中铵态氮的迁移转化过程

铵态氮进入地下水的主要途径是土壤淋失,通过室内土柱淋滤实验研究铵态氮在土壤中的迁移转化过程,测定不同时间和不同深度土壤中铵态氮及其转化物硝态氮和亚硝态氮的浓度变化,分析了影响铵态氮迁移转化的因素。实验表明：在土壤饱和、持续淋滤条件下,土柱中随采样深度的增加,铵态氮穿透时间延长,依次滞后;通过硝化能力分析,土柱上层发生了轻微的硝化反应,土柱底部发生了反硝化反应,导致硝态氮的浓度衰减。研究认为在铵态氮的迁移转化过程中,当入渗铵态氮浓度较低时,影响铵态氮迁移转化的显著因素是土壤对铵态氮的吸附;当入渗铵态氮浓度较大时,影响铵态氮迁移转化的显著因素是生物作用导致的铵态氮的硝化,以及土壤的渗透系数、弥散度等因素。     

Analytical solution of two-dimensional advection–dispersion equation with spatio-temporal coefficients for point sources in an infinite medium using Green’s function method

In the present study analytical solutions of a two-dimensional advection–dispersion equation (ADE) with spatially and temporally dependent longitudinal and lateral components of the dispersion coefficient and velocity are obtained using Green’s Function Method (GFM). These solutions describe solute transport in infinite horizontal groundwater flow, assimilating the spatio-temporal dependence of transport properties, dependence of dispersion coefficient on velocity, and the particulate heterogeneity of the aquifer. The solution is obtained in the general form of temporal dependence and the source term, from which solutions for instantaneous and continuous point sources are derived. The spatial dependence of groundwater velocity is considered non-homogeneous linear, whereas the dispersion coefficient is considered proportional to the square of spatial dependence of velocity. An asymptotically increasing temporal function is considered to illustrate the proposed solutions. The solutions are validated with the existing solutions derived from the proposed solutions in three special cases. The effect of spatially/temporally dependent heterogeneity on the solute transport is also demonstrated. To use the GFM, the ADE with spatio-temporally dependent coefficients is reduced to a dispersion equation with constant coefficients in terms of new position variables introduced through properly developed coordinate transformation equations. Also, a new time variable is introduced through a known transformation.