城市小型河流水动力弥散和潜流交换过程

为研究城市小型河流中污染物的物理迁移过程规律,分析基流条件下流动水体与暂态存储区之间的滞留交互作用,采用溴化锂(LiBr)作为保守性示踪剂进行野外现场示踪试验,结合一维溶质运移存储模型(One-dimensional Transport with Inflow and Storage model, OTIS)定量解析潜流交换特性,估算纵向弥散系数(D)、潜流交换面积(A_s)、主河道断面面积(A)和潜流交换系数(α).模型度量指标D_aI值和均方根误差值结果表征参数模拟结果可靠性高,拟合效果理想.由泵入点O至下游1 300 m设置的A、B、C、D 4处监测点的模拟结果表明,水文参数D、A_s、A和α均随水文条件而变,OB河段(0~600 m)潜流交换能力较弱,主要以对流弥散过程为主;BD河段(600~1 300 m)具有较强的暂态存储能力,对溶质的滞留时间长;BC(600~1 000 m)和CD(1 000~1 300 m)河段交换系数分别为(3.42×10-6±0.65×10-6)s-1和(2.87×10-6±0.81×10-6 )s-1;河段BC存在2.2×10-5m3/(s·m)的侧向补给流量.4个河段对比发现,城市河流渠道化、河床沉积物贫瘠等特征导致潜流交换能力弱化.     

Determination of the anisotropy of an upper streambed layer in east-central Nebraska, USA

Information on the anisotropy of streambed hydraulic conductivity (K) is a necessity for analyses of water exchange and solute transport in the hyporheic zone. An approach is proposed for the determination of K, developed from existing in-situ permeameter test methods. The approach is based on determination of vertical and horizontal hydraulic conductivity of streambed sediments on-site and eliminates the effects of vertical flow in the hyporheic zone and stream-stage fluctuation, which normally influence in situ permeameter tests. The approach was applied to seven study sites on four tributaries of the Platte River in east-central Nebraska, USA. On-site permeameter tests conducted on about 172 streambed cores for the determination of vertical hydraulic conductivity (K _v) and horizontal hydraulic conductivity (K _h) at the study sites indicate that the study sites have relatively small anisotropic ratios, ranging from 0.74 to 2.40. The ratios of K _h to K _v from individual locations within a study site show greater variation than the anisotropic ratios from the mean or median K at each of the study sites.     

胶体颗粒在潜流带中沉积过程及机制

潜流带是河流地表水和地下水交混区域,是河流中重要的物质能量交换和水生生物栖息的场所,而胶体颗粒在潜流带中沉积,会改变潜流带中的水动力结构和生态环境。本文利用室内循环水槽实验和多物理场耦合的数值模拟方法,旨在研究胶体颗粒在河流上覆水与潜流带中的迁移过程和胶体颗粒在潜流带中沉积分布特征及其对不同因素的响应规律。结果表明:河流上覆水中胶体会逐渐被河床截留且截留胶体集中于河床浅层;沙波水平方向截留量呈现出迎水面较高、背水面较低的趋势;胶体在潜流带沉积的主要机制是潜流交换、颗粒沉降与河床截留作用。本文能为胶体颗粒在潜流带中的生态环境作用研究提供科学依据,并为河流生态环境修复、河流健康管理提供理论支持。     

Reach-Scale Cation Exchange Controls on Major Ion Chemistry of an Antarctic Glacial Meltwater Stream

McMurdo dry valleys of Antarctica represent the largest of the ice-free areas on the Antarctic continent, containing glaciers, meltwater streams, and closed basin lakes. Previous geochemical studies of dry valley streams and lakes have addressed chemical weathering reactions of hyporheic substrate and geochemical evolution of dry valley surface waters. We examine cation transport and exchange reactions during a stream tracer experiment in a dry valley glacial meltwater stream. The injection solution was composed of dissolved Li⁺, Na⁺, K⁺, and Cl^-. Chloride behaved conservatively in this stream, but Li⁺, Na⁺, and K⁺ were reactive to varying degrees. Mass balance analysis indicates that relative to Cl^-, Li⁺ and K⁺ were taken up in downstream transport and Na⁺ was released. Simulations of conservative and reactive (first-order uptake or generation) solute transport were made with the OTIS (one-dimensional solute transport with inflow and storage) model. Among the four experimental reaches of Green Creek, solute transport simulations reveal that Li⁺ was removed from stream water in all four reaches, K⁺ was released in two reaches, taken up in one reach, and Na⁺ was released in all four reaches. Hyporheic sediments appear to be variable with uptake of Li⁺ in two reaches, uptake of K⁺ in one reach, release of K⁺ in two reaches, and uptake of Na⁺ in one reach. Mass balances of the conservative and reactive simulations show that from 1.05 to 2.19 moles of Li⁺ was adsorbed per reach, but less than 0.3 moles of K⁺ and less than 0.9 moles of Na⁺ were released per reach. This suggests that either (1) exchange of another ion which was not analyzed in this experiment or (2) that both ion exchange and sorption control inorganic solute transport. The elevated cation concentrations introduced during the experiment are typical of initial flows in each flow season, which flush accumulated dry salts from the streambed. We propose that the bed sediments (which compose the hyporheic zone) modulate the flushing of these salts during initial flows each season, due to ion exchange and sorption reactions.     

Heterogeneity of hydraulic conductivity and Darcian flux in the submerged streambed and adjacent exposed stream bank of the Beiluo River,northwest China

Recognizing the heterogeneity of hydraulic conductivity and hyporheic flow is critical for understanding contaminant transfer and biogeochemical and hydrological processes involving streams and aquifers. In this study, the heterogeneity of hydraulic conductivity and Darcian flux in a submerged streambed and its adjacent exposed stream banks were investigated in the Beiluo River, northwest China. In the submerged streambed, Darcian flux was estimated by measurement of vertical hydraulic conductivity (K _v) and vertical head gradient (VHG) using in-situ permeameter tests. On exposed stream banks, both horizontal hydraulic conductivity (K _h) and K _v were measured by on-site permeameter tests. In the submerged streambed, K _v values gradually decreased with depth and the higher values were concentrated in the center and close to the erosional bank. Compared to the exposed stream banks, the K _v values were higher in the streambed. From stream stage to the topmost layer of tested sediment, through increasing elevation, the K _h values increased on the erosional bank, while they decreased on the depositional bank. The values of VHG along the thalweg illustrate that downwelling flux occurred in the deepest area while upwelling flux appeared in the other areas, which might result from the change of streambed elevation. The higher value of the Darcian flux in the submerged streambed existed near the erosional bank.     

河流中潜流交换研究进展

河水和地下水交换——潜流交换对溶质和污染物的归宿起着重要作用。潜流交换机理主要包括泵吸交换和冲淤交换。泵吸交换是由于河床形态引起的水头梯度,这些水头梯度诱导了对流传输;冲淤交换发生是由于移动河床截留和释放孔隙水。潜流交换的主要影响因素包括:河道流量、河床水力传导性、河床形态、河道弯曲、河床不均匀和背景条件。还探讨了反应性溶质、胶体颗粒共存情况下潜流交换的规律。对潜流交换研究现存的主要问题及未来研究展望提出了看法。     

Heat tracing to examine seasonal groundwater flow beneath a low-gradient stream in rural central Illinois,USA

The thermal profile of a streambed is affected by a number of factors including: temperatures of stream water and groundwater, hydraulic conductivity, thermal conductivity, heat capacity of the streambed, and the geometry of hyporheic flow paths. Changes in these parameters over time cause changes in thermal profiles. In this study, temperature data were collected at depths of 30, 60, 90 and 150 cm at six streambed wells 5 m apart along the thalweg of Little Kickapoo Creek, in rural central Illinois, USA. This is a third-order low-gradient baseflow-fed stream. A positive temperature gradient with inflection at 90-cm depth was observed during the summer period. A negative temperature gradient with inflection at 30 cm was observed during the winter period, which suggests greater influence of stream-water temperatures in the substrate during the summer. Thermal models of the streambed were built using VS2DHI to simulate the thermal profiles observed in the field. Comparison of the parameters along with analysis of temperature envelopes and Peclet numbers suggested greater upwelling and stability in temperatures during the winter than during the summer. Upwelling was more pronounced in the downstream reach of the pool in the riffle and pool sequence.     

Stream water bypass through a meander neck, laterally extending the hyporheic zone

A meander lobe neck diverts stream water into a hyporheic flow path adjacent to a low gradient stream, Little Kickapoo Creek, Illinois, USA. Hyporheic processes have been well-documented in surface water–groundwater mixing zones underlying and directly adjacent to streams. Alluvial aquifers underlying meander necks provide a further extension of the hyporheic zone. Hydraulic head and temperature data, collected from a set of wells across a meander neck, show stream water moves through the meander neck. The hydraulic gradient across the meander neck (0.006) is greater than the stream gradient (0.003) between the same points, driving the bypass. Rapid subsurface response to elevated stream stage shows a hydraulic connection between the stream and the alluvial aquifer. Temperature data and a Peclet number (Pe) of 43.1 indicate that thermal transport is dominated by advection from the upstream side to the downstream side of the meander neck. The temperature observed within the alluvial aquifer correlates with seasonal temperature variation. Together, the pressure and temperature data indicate that water moves across the meander neck. The inflow of stream water through the meander neck suggests that the meander system may host biogeochemical hyporheic zone processes.     

Integrating hydraulic conductivity with biogeochemical gradients and microbial activity along river–groundwater exchange zones in a subtropical stream

The pervious lateral bars (parafluvial zone) and beds (hyporheic zone), where stream water and groundwater exchange, are dynamic sites of hydrological and biological retention. The significance of these biogeochemical ‘hotspots’ to stream and groundwater metabolism is largely controlled by filtration capacity, defined as the extent to which subsurface flowpaths and matrix hydraulic conductivity modify water characteristics. Where hydraulic conductivity is high, gradients in biogeochemistry and microbial activity along subsurface flowpaths were hypothesized to be less marked than where hydraulic conductivity is low. This hypothesis was tested in two riffles and gravel bars in an Australian subtropical stream. At one site, gradients in chemical and microbial variables along flowpaths were associated with reduced hydraulic conductivity, longer water residence time and reduced filtration capacity compared with the second site where filtration capacity was greater and longitudinal biogeochemical trends were dampened. These results imply that factors affecting the sediment matrix in this subtropical stream can alter filtration capacity, interstitial microbial activity and biogeochemical gradients along subsurface flowpaths. This hydroecological approach also indicates potential for a simple field technique to estimate filtration capacity and predict the prevailing hyporheic gradients in microbial activity and biogeochemical processing efficiency, with significant implications for stream ecosystem function.     

Investigating spatial variability of vertical water fluxes through the streambed in distinctive stream morphologies using temperature and head data

Investigating the interaction of groundwater and surface water is key to understanding the hyporheic processes. The vertical water fluxes through a streambed were determined using Darcian flux calculations and vertical sediment temperature profiles to assess the pattern and magnitude of groundwater/surface-water interaction in Beiluo River, China. Field measurements were taken in January 2015 at three different stream morphologies including a meander bend, an anabranching channel and a straight stream channel. Despite the differences of flux direction and magnitude, flux directions based on vertical temperature profiles are in good agreement with results from Darcian flux calculations at the anabranching channel, and the Kruskal-Wallis tests show no significant differences between the estimated upward fluxes based on the two methods at each site. Also, the upward fluxes based on the two methods show similar spatial distributions on the streambed, indicating (1) that higher water fluxes at the meander bend occur from the center of the channel towards the erosional bank, (2) that water fluxes at the anabranching channel are higher near the erosional bank and in the center of the channel, and (3) that in the straight channel, higher water fluxes appear from the center of the channel towards the depositional bank. It is noted that higher fluxes generally occur at certain locations with higher streambed vertical hydraulic conductivity (K _v) or where a higher vertical hydraulic gradient is observed. Moreover, differences of grain size, induced by stream morphology and contrasting erosional and depositional conditions, have significant effects on streambed K _v and water fluxes.     

潜流带水文-生物地球化学:原理、方法及其生态意义

精确量化潜流带水文交换和生物地球化学反应一直是一个挑战,潜流带水文-生物地球化学研究的核心任务是将小尺度上的水文通量及生物地球化学反应动力学与更大尺度上它们对河流水质和生态的累积效应关联起来.基于潜流带水文-生物地球化学耦合原理,系统综述了渗流仪测量、测压管测量、示踪剂注射试验、温度示踪等潜流带水文学研究方法以及野外示踪试验、室内培养试验等生物地球化学研究方法,针对性地评述了潜流带水文-生物地球化学过程在更大尺度上的累积效应及其对河流生态系统的重要意义,并指出未来的研究将从潜流带研究技术方法的先进化、水文地貌理论与模型的深入化和潜流带生物地球化学过程的尺度化等方面持续地发展.      

床面形态驱动下潜流交换试验

为揭示河床形态特征引起的潜流交换规律,构建循环式水槽装置,通过NaCl示踪对比分析了4种河床地形驱动下的潜流交换规律,并基于扩散理论探讨了潜流交换与地表水水动力及床沙渗透特性之间的关系.结果表明,潜流交换可以发生在平坦河床地形,且交换速率随地形起伏度和雷诺数的增大而增大;在相似的地形条件下,地表流速是影响潜流交换速率的主导因素,地表水深对潜流交换速率影响较弱;此外,分析表明,有效扩散系数与河床特征粒径之间具有2次方幂率函数关系,且与河床渗透系数成正比;交换深度d正比于雷诺数Re的1/2次方.     

Statistical and geostatistical features of streambed hydraulic conductivities in the Platte River, Nebraska

This paper presents streambed hydraulic conductivities of the Platte River from south-central to eastern Nebraska. The hydraulic conductivities were determined from river channels using permeameter tests. The vertical hydraulic conductivities (K _v ) from seven test sites along this river in south-central Nebraska belong to one statistical population. Its mean value is 40.2 m/d. However, the vertical hydraulic conductivities along four transects of the Ashland test site in eastern Nebraska have lower mean values, are statistically different from the K _v values in south-central Nebraska, and belong to two different populations with mean values of 20.7 and 9.1 m/d, respectively. Finer sediments carried from the Loup River and Elkhorn River watersheds to the eastern reach of the Platte River lowers the vertical hydraulic conductivity of the streambed. Correlation coefficients between water depth and K _v values along a test transect indicates a positive correlation – a larger K _v usually occurs in the part of channel with deeper water. Experimental variograms derived from the vertical hydraulic conductivities for several transects across the channels of the Platte River show periodicity of spatial correlation, which likely result from periodic variation of water depth across the channels. The sandy to gravelly streambed contains very local silt and clay layers; spatially continuous low-permeability streambed was not observed in the river channels. The horizontal hydraulic conductivities were larger than the vertical hydraulic conductivities for the same test locations.     

Heat tracing to determine spatial patterns of hyporheic exchange across a river transect

Significant spatial variability of water fluxes may exist at the water-sediment interface in river channels and has great influence on a variety of water issues. Understanding the complicated flow systems controlling the flux exchanges along an entire river is often limited due to averaging of parameters or the small number of discrete point measurements usually used. This study investigated the spatial pattern of the hyporheic flux exchange across a river transect in China, using the heat tracing approach. This was done with measurements of temperature at high spatial resolution during a 64-h monitoring period and using the data to identify the spatial pattern of the hyporheic exchange flux with the aid of a one-dimensional conduction-advection-dispersion model (VFLUX). The threshold of neutral exchange was considered as 126 L m^?2 d^?1 in this study and the heat tracing results showed that the change patterns of vertical hyporheic flux varied with buried depth along the river transect; however, the hyporheic flux was not simply controlled by the streambed hydraulic conductivity and water depth in the river transect. Also, lateral flow dominated the hyporheic process within the shallow high-permeability streambed, while the vertical flow was dominant in the deep low-permeability streambed. The spatial pattern of hyporheic exchange across the river transect was naturally controlled by the heterogeneity of the streambed and the bedform of the stream cross-section. Consequently, a two-dimensional conceptual illustration of the hyporheic process across the river transect is proposed, which could be applicable to river transects of similar conditions.     

河床地形影响潜流交换作用的数值分析

选取对潜流交换具有重要影响的河床地形作为主要研究内容,采用数值模拟(MODFLOW程序)的方法研究在河床横剖面地形不均匀的条件下,潜流交换量的空间分布以及地下水流场的演变机制。结果表明:在河床地形起伏不均的情况下,潜流交换量更易发生在河道的深水区域;地下水流向受河床地形影响较小;近河床界面处的地下水流速受地形起伏影响剧烈,深水区域的地下水流速远大于浅水区地下水流速;通过与现场试验结果对比分析,得出河床地形起伏是引起潜流带渗透系数非均质现象的重要原因之一。     

On-line fluorometry of multiple reactive and conservative tracers in streams

Performing tracer tests using artificial tracer compounds is a common practice to characterize natural streams regarding their (reactive) transport properties. Recently, the fluorescent compound resazurin was introduced as a reactive stream tracer to quantify hyporheic exchange and metabolic activity of streams. This tracer, together with its reaction product resorufin and a conservative tracer (in our case fluorescein), provides additional information about transport properties of the stream and its hyporheic zone and can therefore overcome restrictions that are commonly affiliated with the use of conservative tracers alone. However, all previously published studies using this tracer system were based on manual sampling of the water. This usually limits the number of measurements and thus the achievable temporal resolution, and potentially endangers data quality due to inadequate handling of samples. In this paper, a modified version of the GGUN-FL30 on-line fluorometer is presented in which the optics have been modified to allow measuring the concentrations of all three tracers simultaneously at intervals of 10 s. Experiments under controlled and natural conditions showed that the performance of the on-line fluorometer regarding tracer separation efficiency and practical detection limits is comparable to a high-performance laboratory spectrofluorometer. Furthermore, suggestions are given on how to correct tracer signal fluctuations caused by temporal changes in temperature and pH that might occur during a field tracer test.     

河岸带蜿蜒性与植被密度对潜流驻留时间的复合效应

潜流驻留时间是反映潜流交换、溶质迁移转化、生物组成、生态过程的重要特征变量。应用双循环可控式河岸带模型,以NaCl为示踪剂,深入研究了河岸带蜿蜒性与植被密度对潜流驻留时间的复合效应;应用量纲分析和主成分分析法,探析了潜流驻留时间的关键影响因子及其作用程度和敏感性。结果表明:①河岸带植被密度对潜流驻留时间具有促进作用,但存在一定的阈值。②潜流驻留时间分布与河岸蜿蜒形态呈类镜像效应;潜流驻留时间在河岸带蜿蜒波不同位置差异明显,迎水面平均驻留时间约为背水面的1/2。③河岸带蜿蜒性与植被密度是影响潜流驻留时间的关键因子,总作用程度达91.07%。潜流驻留时间对蜿蜒性最敏感,对植被密度敏感性较弱,对复合因子的敏感性介于两者之间;迎水面内组合因子对潜流驻留时间具有共同促进效应,背水面内组合因子对潜流驻留时间具有抑制和促进双重效应。     

河岸带蜿蜒性与植被密度对潜流驻留时间的复合效应

潜流驻留时间是反映潜流交换、溶质迁移转化、生物组成、生态过程的重要特征变量。应用双循环可控式河岸带模型，以NaCl为示踪剂，深入研究了河岸带蜿蜒性与植被密度对潜流驻留时间的复合效应；应用量纲分析和主成分分析法，探析了潜流驻留时间的关键影响因子及其作用程度和敏感性。结果表明：①河岸带植被密度对潜流驻留时间具有促进作用，但存在一定的阈值。②潜流驻留时间分布与河岸蜿蜒形态呈类镜像效应；潜流驻留时间在河岸带蜿蜒波不同位置差异明显，迎水面平均驻留时间约为背水面的1/2。③河岸带蜿蜒性与植被密度是影响潜流驻留时间的关键因子，总作用程度达91.07%。潜流驻留时间对蜿蜒性最敏感，对植被密度敏感性较弱，对复合因子的敏感性介于两者之间；迎水面内组合因子对潜流驻留时间具有共同促进效应，背水面内组合因子对潜流驻留时间具有抑制和促进双重效应。     

A new method for mapping variability in vertical seepage flux in streambeds

A two-step approach was used to measure the flux across the water-sediment interface in river channels. A hollow tube was pressed into the streambed and an in situ sediment column of the streambed was created inside the tube. The hydraulic gradient between the two ends of the sediment column was measured. The vertical hydraulic conductivity of the sediment column was determined using a falling-head permeameter test in the river. Given the availability of the hydraulic gradient and vertical hydraulic conductivity of the streambed, Darcy’s law was used to calculate the specific discharge. This approach was applied to the Elkhorn River and one tributary in northeastern Nebraska, USA. The results suggest that the magnitude of the vertical flux varied greatly within a short distance. Furthermore, the flux can change direction from downward to upward between two locations only several meters apart. This spatial pattern of variation probably represents the inflow and outflow within the hyporheic zone, not the regional ambient flow systems. In this study, a thermal infrared camera was also used to detect the discharge locations of groundwater in the streambed. After the hydraulic gradient and the vertical hydraulic conductivity were estimated from the groundwater spring, the discharge rate was calculated.     

Measurement of streambed hydraulic conductivity and its anisotropy

 A method is described for the measurement of streambed hydraulic conductivity. Unlike permeameter methods, this method applies straight and l-shaped standpipes directly to streambeds for measurements of in-situ hydraulic conductivity in the vertical (K _v ) and horizontal (K _h ) directions, as well as in other oblique directions (K _s ). This method has advantages in determination of K _v values over grain-size analysis, permeameter tests, or slug test methods. Also unique to this method is that it provides K _s values of a streambed. The measured results can be used to construct a hydraulic conductivity ellipse and to evaluate the anisotropy of streambed sediments. Field examples from the Republican River, Nebraska, demonstrated the usefulness of this method in the determination of streambed hydraulic conductivity and anisotropy along or across a river channel. Results indicate that the K _h is about three to four times larger than K _v , whereas K _s values are larger than K _v but smaller than K _h . Received: 6 March 2000 · Accepted: 18 April 2000