摘要:

Tritium and He isotopes have been measured at a site where groundwater flow is nearly vertical for a travel time of 100 years and where recharge rates are spatially variable. Because the mid‐1960s3H peak (arising from aboveground testing of thermonuclear devices) is well‐defined, the vertical groundwater velocity is known with unusual accuracy at this site. Utilizing3H and its stable daughter3He to determine groundwater ages, we compute a recharge rate of 0.16 m/yr, which agrees to within about 5% of the value based on the depth of the3H peak (measured both in 1986 and 1991) and two‐dimensional modeling in an area of high recharge. Zero3H/3He age occurs at a depth that is approximately equal to the average depth of the annual low water table, even though the capillary fringe extends to land surface during most of the year at the study site. In an area of low recharge (0.05 m/yr) where the3H peak (and hence the vertical velocity) is also well‐defined, the3H/3He results could not be used to compute recharge because samples were not collected sufficiently far above the3H peak; however, modeling indicates that the3H/3He age gradient near the water table is an accurate measure of vertical velocities in the low‐recharge area. Because3H and3He have different diffusion coefficients, and because the amount of mechanical mixing is different in the area of high recharge than in the low‐recharge area, we have separated the dispersive effects of mechanical mixing from molecular diffusion. We estimate a longitudinal dispersivity of 0.07 m and effective diffusion coefficients for3H (3HHO) and3He of 2.4×10−5and 1.3×10−4m2/day, respectively. Although the3H/3He age gradient is an excellent indicator of vertical groundwater velocities above the mid‐1960s3H peak, dispersive mixing and diffusive loss of3He perturb the age gradient near a

ISSN:0043-1397    

DOI:10.1029/93WR00968

年代:1993

数据来源: WILEY

摘要:

An experimental approach to study physical processes affecting fluid behavior in three‐fluid porous media systems was designed and implemented. These experiments were designed to provide quantitative evidence of important pore‐scale displacement processes. A unique experimental approach using a two‐dimensional synthetic porous medium (micromodel) and digital image analysis (DIA) enabled both measurement of three‐fluid capillary pressure‐saturation relationships and analysis of fluid behavior at the pore level. Experiments were pressure controlled, and were designed to mimic measurements typically performed on laboratory cores. Only quasi‐static measurements were made, with a focus on capillary pressure‐saturation relations. Different orders of fluid infiltration with respect to wettability were studied. DIA made it possible to obtain quantitative information about the experiments, including fluid saturations, saturation changes between pressure steps, and movement of apparently isolated fluids through films. The results provide insights into important pore‐scale mechanisms and provide a basis for three‐fluid pore

ISSN:0043-1397    

DOI:10.1029/93WR00524

年代:1993

数据来源: WILEY

摘要:

Previous theoretical discussion of flow and transport in a single fracture has led to a difference of opinion as to whether the aperture estimated from a tracer test need always be less than (or greater than) the aperture estimated from a concurrent hydraulic test. Interpretation of previously published field results has, to our knowledge, not provided a defensible example where the ratio of these two aperture estimates has varied from less than 1.0 to greater than 1.0 in the same fracture using the same analysis techniques. In the present paper, field experiments involving concurrent hydraulic and tracer tests have been run in two different directions within an isolated fracture. Aperture estimates were derived from both the hydraulic and tracer tests. The ratio of the aperture estimated from the tracer test to the aperture estimated from the hydraulic test is shown to be greater than 1.0 in the first experiment and less than 1.0 in the second experiment. Despite noise in the hydraulic test of the first experiment, it is argued that our interpretation is reasonable. As such, this represents the first set of experimental results known to us in which a defensible interpretation leads to the conclusion that the aperture ratio varies from greater than 1.0 to less than 1.0 within the same fracture.

ISSN:0043-1397    

DOI:10.1029/93WR01073

年代:1993

数据来源: WILEY

摘要:

Porous media are considered to be composed of spatial distributions of particles, which individually follow Langmuir sorption isotherms (with their own distinct, randomly sampled sorption capacities and selectivity coefficients). Analyses of batch and column methods for determining the sorptive parameters of these porous media reveal that batch‐derived estimates of retardation based upon Langmuir and Freundlich isotherms systematically underestimate and overestimate that observed at low concentrations, respectively, when local equilibrium conditions are met in the columns. These results, derived by stochastically analyzing the sorptive behavior expected during batch and column experiments, follow from the failure of the homogeneous‐based Langmuir and Freundlich models to properly describe sorptive behavior in the ensembles of particles composing the media. As the variability in the underlying sorptive properties increases, the Langmuir isotherm ceases to describe well the behavior of the aggregate of (individually Langmuir) particles both under static (batch) and dynamic (column) conditions. In such composite porous media there tends to be an inherent uncertainty involved with characterizing sorptive behavior at lower concentrati

ISSN:0043-1397    

DOI:10.1029/93WR00967

年代:1993

数据来源: WILEY

摘要:

Since 1979 the Dutch National Institute of Public Health and Environmental Protection (RIVM) has been developing the Dutch Groundwater Quality Monitoring Network (LMG). This network presently consists of about 350 monitoring sites. At each site, well screens are placed at two depths: 10 and 25 m below surface level. Samples are collected every year and are analyzed for all macrochemical parameters and some trace elements. Tritium contents were measured in the first sampling round. The geochemistry of Dutch groundwater is complex, due to the different sources (seawater, surface water and rainwater), complicated hydrogeology, and human impact on flow systems and pollution. Structuring or data analysis is required for the interpretation of the large number of hydrogeochemical data from such a monitoring network. An exploratory approach is to look within the data set for homogeneous groups, each with a typical (macro)chemistry. The selection criteria for the location of the monitoring sites of the LMG are mainly based on soil type and land use, and to some extent on the hydrogeological situation. However, a classification based on the two most reliable criteria, soil type and land use, does not result in chemically distinguishable homogeneous groups or water types. Fuzzycmeans clustering was successfully used to discern structure and natural groups in the LMG data for 1 year. A seven‐cluster model was adopted. The number of clusters was decided heuristically with the aid of nonlinear mapping, on the basis of the geographic distribution, the hydrogeochemical interpretability, and the unimodality of the distribution of the parameters per cluster. The consistency of the model is illustrated by the reproducibility of the clusters in different years. The clusters are related to geochemical processes, natural sources, and anthropogenic input and are designated as follows: (1) “seawater” in coastal areas, (2) “desalinization” in organic‐rich Holocene marine and peat deposits, (3) “surface water” for downward seeping river water or surface water near the main rivers, (4) “carbonate/reduction” in peat areas or old groundwater in seepage zones, (5) “carbonate” for carbonate‐dissolving “precipitation,” (6) acid “precipitation” water in sandy topographic highs, and (7) a “polluted” cluster characterized by agricultural contaminants. Although the influence of soil type and land use is noticeable in some of the clusters, the geochemical controls, which characterize the different clusters, appear to outweigh their anticipated influence on the hydrogeochemistry on the scale of the national groundwater network. Consequently, the homogeneous groups, obtained through the cluster analysis, present a better base for further statistical and hydrogeochemical evaluation than, e.g., a stratification of the

ISSN:0043-1397    

DOI:10.1029/93WR00970

年代:1993

数据来源: WILEY

摘要:

Cation exchange associated with displacement of aqueous solutions is a complex physicochemical process. In this study column experiments were combined with numerical modeling to determine the controlling chemical processes for natural systems at near‐neutralpH. The experimental results showed the importance of two processes in addition to exchange between free cations: exchange of monovalent metal‐bicarbonate complexes and proton buffering. The former primarily results from changes in CO2pressure, whereas the latter results from changes in aqueouspH. Neglecting these two processes had little effect when modeling the cation elution curves. However, thepH and alkalinity breakthrough curves were poorly fit, since nonreactive behavior of these variables was assumed. Inclusion of either specific sorption of Ca and Mg hydroxy complexes or transfer of Al hydroxy species to or from the exchanger sites resulted in a better fit. It is shown theoretically that the two additional processes have a major impact on calcite equilibria when considering calcareous syst

ISSN:0043-1397    

DOI:10.1029/93WR01067

年代:1993

数据来源: WILEY

摘要:

Previous flume studies of the velocity of saltating grains have almost all measured single grains traveling in otherwise clear water. However, such measurements disregard the likelihood that grain velocities are affected by the presence of other grains. In this laboratory study of overland flow we show that grain velocity is negatively related to sediment concentration. This negative relation is attributed to two mechanisms. The first mechanism is a decrease in water velocity as sediment concentration increases. Water velocity decreases because an increasing proportion of potential energy is dissipated in transporting sediment rather than being converted to kinetic energy. The second mechanism is an increase in the dispersion of the normal component of grain momentum as grain collisions increase with sediment concentration. The dispersive stress so created opposes bed load movement and causes grain velocity to decrease. Multiple regression analyses reveal relations in the experimental data that are consistent with these two mechanisms.

ISSN:0043-1397    

DOI:10.1029/93WR00771

年代:1993

数据来源: WILEY

摘要:

Episodic acidification was examined in five mid‐Atlantic watersheds representing three physiographic provinces: Coastal Plain, Valley and Ridge, and Blue Ridge. Each of the watersheds receives a similar loading of atmospheric pollutants (SO42−and NO3−) and is underlain by different bedrock type. The purpose of this research was to quantify and compare the episodic variability in storm flow chemistry in Reedy Creek, Virginia (Coastal Plain), Mill Run and Shelter Run, Virginia (Valley and Ridge), and Fishing Creek Tributary and Hunting Creek, Maryland (Blue Ridge). Because episodic responses were similar from storm to storm in each of the watersheds, a representative storm from each watershed was discussed. Acidification, defined as the loss of acid‐neutralizing capacity (ANC), was observed in all streams except Mill Run. Mill Run chemistry showed little episodic variability. During storms in the other streams,pH decreased while SO42−, NO3−, and K+concentrations increased. Concentrations of Mg2+and Ca2+increased in Reedy Creek and Fishing Creek Tributary, but decreased in Shelter Run and Hunting Creek. Therefore the net effect of episodic changes on the acid‐base status differed among the streams. In general, greater losses of ANC were observed during storms at Shelter Run and Hunting Creek, watersheds underlain by reactive bedrock (carbonate, metabasalt); comparatively smaller losses in ANC were observed at Reedy Creek and Fishing Creek Tributary, watersheds underlain by quartzites and unconsolidated quartz sands and cobbles. Increased SO42−concentrations were most important during storms at Reedy Creek and Fishing Creek Tributary, but organic anions (inferred by anion deficit) were also a factor in causing the loss of ANC. Dilution of base cations was the most important factor in the loss of ANC at Shelter Run. Both increased sulfate and dilution of base flow were important in causing the episodic acidification at Hunting Creek. The role of SO42−in contributing to episodic acidification in these watersheds is similar to that documented in studies conducted in other regions of the United States, Scandinavia, Canada, and the United Kingdom. The importance of SO42−in mid‐Atlantic United States streams contrasts with northeastern United States streams, in which increased NO3‐derived from snowpack is more important in causing episodic acidification. Results support the importance of shallow subsurface processes in determining storm flow chemistry, regional climatic characteristics in determining the different sources of acidity during episodes, and the importance of bedrock geology in determining

ISSN:0043-1397    

DOI:10.1029/93WR01408

年代:1993

数据来源: WILEY

摘要:

The Lake Kinneret hydrologic budget includes two unknown terms: evaporation lossesQe, and inflow from hot saline springs which represent the groundwater component,Qs. This last term affects also the salt budget as well as the energy budget of the lake, which is, in turn, the basis for evaporation estimates. A new approach consisting of the simultaneous solution of the water, heat, and salt balance equations is presented to evaluateQeandQs. This approach gives a better estimation of evaporation, since it accounts for the net energy advected to the lake. Neglecting this term led to overestimation of evaporation during the dry winter of 1991 and to underestimation during the whole year of 1992, which was particularly rainy. The maximal monthly error occurred for both years in February: +55% for 1991, and −29% for 1992. The application of the approach to the period of 1988–1991 shows that the annual hydrograph of the saline springs system is characterized by two distinct periods of high discharge during each year. The first period, in winter, seems to be related to the structure and timing of the rainfall season. The second occurs generally at the beginning of the summer, after the rainfall season has ended. The overall trend for the period is declining peaks and annual discharges, which is consistent with the hydrologic impact of a 3‐year drought since 1989. The measured, as well as the estimated, components of the water balance equation are subject to uncertainty. The formulation of the balance equation in terms of a state‐space system and the application of the Kalman filter (KF) allows quantifying the effect of this uncertainty on the different components. Therefore for given noise statistics and a priori data, it is possible to determine the optimal linear estimates of the estimated components and to compare them to the values resulting from the simultaneous solution approach. The method was applied on monthly water budgets for a period of 60 months. The effect on evaporation varies within a range of ±5%. The estimate of runoff from the ungauged part of the direct watershed was generally increased by the KF, especially during the rainy winter of 1988. The effect on this estimate varies within a range of ±12%. The component that was affected the most is the estimated discharge from the saline springs system (±15%). The KF increases by 15% the winter peak of 1988 and decreases all the discharges estimates for the dry years, especially those of 1990. Therefore the stochastic approach strengthens the agreement between the trends in the estimated values and in the climatic conditions during the peri

ISSN:0043-1397    

DOI:10.1029/93WR01181

年代:1993

数据来源: WILEY

摘要:

A review of a calibration method developed earlier (Ibrahim and Liong, 1992) is presented. The method generates optimal values for single events. It entails randomizing the calibration parameters over bounds such that a system response under consideration is bounded. Within the bounds, which are narrow and generated automatically, explicit response surface representation of the response is obtained using experimental design techniques and regression analysis. The optimal values are obtained by searching on the response surface for a point at which the predicted response is equal to the measured response and the value of the joint probability density function at that point in a transformed space is the highest. The method is demonstrated on a catchment in Singapore. The issue of global optimal values is addressed by applying the method on wider bounds. The results indicate that the optimal values arising from the narrow set of bounds are, indeed, global. Improvements which are designed to achieve comparably accurate estimates but with less expense are introduced. A linear response surface model is used. Two approximations of the model are studied. The first is to fit the model using data points generated from simple Monte Carlo simulation; the second is to approximate the model by a Taylor series expansion. Very good results are obtained from both approximations. Two methods of obtaining a single estimate from the individual event's estimates of the parameters are presented. The simulated and measured hydrographs of four verification storms using these estimates compare quite well.

ISSN:0043-1397    

DOI:10.1029/93WR00971

年代:1993

数据来源: WILEY