摘要:

Large‐scale dispersion in a sandy unconfined aquifer in Denmark was studied by simulating subsurface transport of environmental tritium. Subsurface transport included transport in a moderately deep unsaturated zone and in a relatively long cross section of the aquifer. The tritium data from the site enabled a four‐step modeling analysis comprising (1) estimation of tritium content in the infiltration water, (2) transport in the unsaturated zone, (3) estimation of flux‐averaged tritium concentration in the recharge water, and (4) transport in the groundwater zone. The groundwater model simulations were sensitive to the longitudinal and transverse dispersivity parameters, αLand αr, as a set of parameters, but a model sensitivity analysis showed that it was not possible to identify a unique set of parameter values. A likely range of variation for the two parameters could be identified: (αL, αT); ∈ [(1 m, 0.005 m); (10 m, 0.0 m)] the two parameters being interdependent in that an increase in αLresults in a decrease in αTand vice versa. The reported dispersivities represent a scale of 1000 m, the approximate travel distance from the water table to the observation wells. If the estimated αLcan be regarded as being of intermediate reliability following earlier defined criteria, the range or the representative set of values then represent the largest scale of earlier reported values. Including our range of αLin the set of reported dispersivities suggests that αLdoes not increase indefin

ISSN:0043-1397    

DOI:10.1029/96WR02398

年代:1996

数据来源: WILEY

摘要:

We present a methodology for the analysis of chemical profiles from the unsaturated zone where ions from the sediment are extracted by two methods:centrifugation and water addition. The methodology enables one to (1) assess the ion amounts present in two different phases, pore water and minerals; (2) determine the depth in the unsaturated profile where the degree of saturation of each mineral is reached; (3) establish the principal direction of water flow; and (4) differentiate between upward transport of water either as vapor or as a saline solution. The methodology was applied to the unsaturated zone of the Dead Sea coastal area where the original salinity of interstitial water in these sediments was>300 g/L. Our analysis of the field data suggests that reduction of interstitial water salinity is the result of vertical upward transport of fresh water from a confined aquifer at a depth of 7 m. Flushing, up to the potentiometric surface (depth of 3.5 m), is due to the positive pressure head of the aquifer. Above the potentiometric surface, upward water transport is due to capillary forces, and flow is maintained by evaporation at the soil surface. Evaporation leads to an increase in the salinity of the rising interstitial solution and to the sequential deposition of salts such as NaCl and KMgCl3· 6H2O

ISSN:0043-1397    

DOI:10.1029/96WR02222

年代:1996

数据来源: WILEY

摘要:

Temporal and spatial variabilities of flow distribution significantly influence solute transport in soils. This laboratory study was designed to investigate the effects of temporal variation in flow velocity on pesticide transport in soils. Two pesticides, metribuzin (weakly adsorbed) and atrazine (moderately adsorbed), were chosen along with the following two soils: Cecil (<2 mm) and Sharkey (2–4 mm). Several tritium pulses were introduced into packed soil columns (15 or 30 cm in length) under different flow velocities to obtain velocity‐dependent dispersion coefficients (D). Subsequently, several atrazine and metribuzin pulses were introduced under conditions of constant and variable velocities. For each experiment, changes in flow velocity were stepwise using a piston flow pump and were carried out during pulse application and leaching. For constant and variable flow velocity experiments, approximately similar pulse volumes and average flow velocities were maintained. Values ofDversus pore water velocity (ν) from tritium breakthrough curves (BTCs) were well described using a linear equation for both soils. Identical BTCs for metribuzin were observed under conditions of constant or variable flow velocities in the Cecil soil column. However, metribuzin transport in the Sharkey soil was significantly influenced by velocity variations. Atrazine transport in the Sharkey soil was also significantly affected by variations in flow velocity. We further examined the error when an average rather than actual velocity distribution was used in BTC representation. The resulting experimental BTCs (concentration versus velocity‐averaged pore volume) exhibited early arrival and the appearance of multiple peaks. Moreover, predictions of such BTCs based on the convective‐dispersive equation were not successful. We concluded that actual water velocity distributions should be used in BTC representation, and, whenever possible, the use of an average velocity should be

ISSN:0043-1397    

DOI:10.1029/96WR02317

年代:1996

数据来源: WILEY

摘要:

This paper presents the conceptual and mathematical basis of a model for transport of nuclides in the geosphere between the near field of a radioactive waste repository and high‐conductivity fractures that are assumed to be closely connected to the biosphere. The geosphere is modeled as a network of fractures that are mapped onto a network of channels. The balance equations and boundary conditions on the scale of a single rectangular channel are given. The effects of one‐dimensional matrix diffusion in addition to advection, linear sorption in the fractures and the rock matrix, and radioactive decay are considered. A hierarchical linear response concept using Laplace transformation techniques is developed for solution of the balance equations (1) on the scale of a channel network, (2) in the individual channels, and (3) in the rock matrix adjacent to the channels. Examples with unlimited and limited matrix diffusion and for additional rock layers are discus

ISSN:0043-1397    

DOI:10.1029/96WR02508

年代:1996

数据来源: WILEY

摘要:

Percolation theory, commonly used to study quasi‐static immiscible displacement at the microscopic scale, is here extended to simulations of gravity‐stable drainage and imbibition in three‐dimensional porous media with spatially correlated macroscopic properties. The result of this extension is a macroscopic percolation on a regular lattice of sites, where lattice sites represent regions in a porous medium characterized by different macroscopic properties (e.g., absolute permeability, capillary pressure, and relative permeability curves). These properties are assigned to lattice sites by virtue of parametrizations in terms of local permeability. We present a general formulation of macroscopic percolation that accounts for gravitational effects, which can be important in large‐scale immiscible displacements with nonzero density difference. In such cases, we find that the local saturation distribution is markedly different from the distribution of saturation under conditions of negligible buoyancy. Displacements with nonzero density difference proceed with the formation of a transition zone of length inversely proportional to a macroscopic Bond number which characterizes the relative importance of capillary and buoyancy forces at the macroscopic scale. Several important features of percolation at the microscopic scale are also manifested at the macroscopic scale. These include the effects of lattice dimensionality and spatial correlation on the macroscopic percolation threshold and accessibility characteristics. In the absence of buoyancy forces, the large‐scale capillary pressure and relative permeability behavior of a heterogeneous system is dictated mainly by the structure of the permeability field and can be explained in terms of macroscopic accessibility. Spatial correlation of permeability is found to have pronounced effects on the large‐scale drainage relative permeabi

ISSN:0043-1397    

DOI:10.1029/95WR02216

年代:1996

数据来源: WILEY

摘要:

We consider solute transport in a porous medium for which we assume the retardation factorR(x), resulting from linear chemical adsorption, to be stochastically varying in space. For large times, the evolution of a solute plume developing from a pointlike, instantaneous solute injection is described by its effective velocity and dispersion. We calculate such quantities using perturbation theory and two different averaging procedures. The first and correct procedure calculates the central moments of the cloud for a given aquifer realization and averages over the ensemble afterward. The second method, which is mathematically less demanding, obtains large‐scale transport coefficients from the moments of the ensemble‐averaged concentration distribution. This last approach is often used to replace the correct procedure, tacitly assuming that the two averaging methods lead to the same effective quantities. We show that the results actually differ in one dimension, whereas the difference vanishes in higher dimensions. The effective retardation factor is found to be the ensemble average of the corresponding small‐scale quantity. The effective dispersion coefficient, on the other hand, differs from the retarded small‐scale dispersion coefficient. It is significantly enhanced by the inhomogeneous fluctuations of the disordered

ISSN:0043-1397    

DOI:10.1029/96WR01777

年代:1996

数据来源: WILEY

摘要:

The extended Kalman filter (EKF) has long been recognized as a powerful, yet computationally intensive, methodology for stochastic parameter estimation. Three improvements to traditional algorithms are presented and applied to heterogeneous transmissivity estimation. First, the costly EKF covariance updates are replaced by more efficient approximations. Second, the zonation structure of the distributed parameter field being estimated is dynamically determined and refined using a partitional clustering algorithm. Third, a new method of merging first and second moments of random fields that have heterogeneous statistics is introduced. We apply this method, called random field union, as an alternative to conventional random field averaging for the systematic shrinking of covariance matrices as the dimensionality of the parameter space is reduced. The effects of these three improvements are examined. In applications to steady state groundwater flow test problems, we show that the first and second improvements reduce the computational time requirements dramatically, while the second and third can improve the accuracy and stability of the results. The resulting integrated method is successfully applied to a larger, more realistic calibration test case under steady and cyclostationary flow conditions (similar to regular seasonal fluctuations). When flow is steady, the method can be viewed as iterative; when flow is transient, the method is fully recursive.

ISSN:0043-1397    

DOI:10.1029/96WR02283

年代:1996

数据来源: WILEY

摘要:

During the past decade, significant spatial and temporal variability in the release of nitrate‐nitrogen (N) from catchments in a sugar maple forest in central Ontario was observed. To explain this variability, we tested the flushing hypothesis [Hornberger et al., 1994], where, when the soil saturation deficit is high, N accumulates in the upper layers of the soil and, as the soil saturation deficit decreases, the formation of a saturated subsurface layer flushes N from the upper layers of the soil into the stream. We used the Regional Hydro‐Ecological Simulation System to simulate water, carbon, and N dynamics. A N flushing index was modeled asS/S30, the ratio of the current day saturation deficit to the previous 30‐day average saturation deficit. A N source index was modeled as the ratio of N supply/demand. The relationship between the simulated N indices and the observed release of N indicated two mechanisms for the release of N from catchments: (1) a N flushing mechanism, where the N‐enriched upper layer of the soil is flushed, after a period of low demand for N by the forest (e.g., during spring snowmelt and autumn stormflow, the water table rising into previously unsaturated parts of a N‐enriched soil profile) or after a period of high demand for N by the forest (e.g., during summer droughts, the water table rising into previously saturated parts of a N‐impoverished soil profile following a period of enhanced rates of nitrification); and (2) a N draining mechanism, where spring snowmelt recharge of the groundwater translocates N from the upper layer of the soil into deeper hydrological flow pathways that are released slowly ov

ISSN:0043-1397    

DOI:10.1029/96WR02399

年代:1996

数据来源: WILEY

摘要:

Municipal water utilities commonly aim to set prices at average cost. Because of various omissions and owing to inflation, unadjusted accounting data understate the economic costs of fixed assets and thus generate inefficiently low prices and high consumption rates for the output of these enterprises. We investigate the nature and extent of undercosting and underpricing for a group of large urban water utilities in the United States. Economic costs appear to be significantly greater than the corresponding accounting measures for the cases studied.

ISSN:0043-1397    

DOI:10.1029/96WR02281

年代:1996

数据来源: WILEY

摘要:

Replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements. The bed shear velocityu*can be estimated within 3% using the depth‐averaged velocity in the vertically averaged logarithmic velocity profile. To be accurate, this method is limited to relatively simple flow geometries which may be expected to have the appropriate velocity structure. Estimates ofu*made using a single near‐bed velocity observation are less precise by a factor of 3 because of the larger uncertainty associated with a single observation. Accuracy of this method requires appropriate flow conditions only near the bed, so it may be applied in a wider range of flow conditions, including spatially variable flow. Estimates ofu*from the slope of the near‐bed velocity profile are the least precise and require the most restrictive flow conditions for accuracy but offer the advantage that they may be made without independent knowledge of the bed roug

ISSN:0043-1397    

DOI:10.1029/96WR02277

年代:1996

数据来源: WILEY