摘要:

The effect of rate‐limited adsorption on transport of environmental contaminants is difficult to characterize at the field scale. This study investigated transport, during unsaturated water flow, of pulse inputs of bromide, simazine (2‐chloro‐4,6‐ bis(ethylamino)‐s‐triazine), and MS‐2 coliphage in a field lysimeter (0.8 m × 0.8 m square) containing undisturbed Tujunga loamy sand (mixed, thermic, Typic Xeropsamment). Sixty‐four fiberglass wick soil solution samplers collected drainage fractions from the exit surface (30 cm depth) following daily 2‐cm water inputs applied at 0.5 cm h−1. After 19.7 cm of cumulative drainage, the soil above 10 of the 64 locations was sampled to determine final depth distributions of simazine and virus. Most of the bromide was leached from the transport volume, while the sorbing pesticide and virus remained in the soil. Variance analysis indicated that local dispersion processes contributed more to the observed bromide spreading than did differences in local water velocities. A linear, first‐order, kinetic adsorption submodel was incorporated into a generalized linear transport model relating the bromide flux concentrations to the simazine and virus final resident concentrations. Least squares fitting showed that area‐averaged bromide transport could be described reasonably well by the two‐parameter convection‐dispersion model (CDM), although the mobile‐immobile water model provided a slightly better representation of effluent tailing. The CDM parameters fitted to the bromide data were then held constant while the two parameters of the adsorption submodel were varied to fit the pesticide soil concentrations at the end of the experiment at 10 days. A good fit was obtained for simazine, and the fitted value 0.54 d−1of the rate coefficient was in the range characterizing nonequilibrium adsorption. A batch adsorption/desorption experiment produced Freundlich isotherms describing nonlinear adsorption (exponentm= 0.85) and hysteresis in desorption. There was poor agreement between the retardation factor (R) estimated from a linearized batch distribution coefficientKdand theRfitted to lysimeter data. Virus concentrations fitted to the model yielded coefficients implying strong adsorption (R= 254) and rapid inactivation (inactivation rate coefficient of 1.64 d−1), whereas the laboratory sorption study implied that the virus should be very mobile in soil. The difference in field and laboratory sorption may be due to air‐water interfacial forces

ISSN:0043-1397    

DOI:10.1029/94WR02821

年代:1995

数据来源: WILEY

摘要:

This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one‐ and two‐stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate‐limited sorption and transport of the simazine added simultaneously with the Br. The calibrated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single‐stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratory‐measured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current

ISSN:0043-1397    

DOI:10.1029/94WR02822

年代:1995

数据来源: WILEY

摘要:

A field experiment was conducted to investigate preferential transport of herbicides and to explore processes that cause rapid movement. Bromide, chloride, and three herbicides (triasulfuron, atrazine, and terbuthylazine) with different mobility characteristics were applied to six 1.4 × 1.4 m field plots on a loamy and a sandy soil. At both sites, three of the plots were covered with a plastic roof 1 month before the beginning of the experiment to achieve different initial water contents between the plots. Two days before the beginning of the tracer experiment, crops were removed, and the soil surface was homogenized with a spade to a depth of 15–20 cm. One day after application of the chemicals the plots were irrigated with a sprinkling apparatus. The cumulative amounts of infiltration until the time of sampling were 30, 60, and 90 mm within 1, 2, and 3 days, respectively. A trench was excavated, and soil cores were taken horizontally from a 1 × 1 m profile in a regular 0.1 × 0.1 m grid. The loamy and the sandy soil showed completely different transport patterns. In the loamy soil the bulk mass of herbicides remained in the top layer; however, considerable amounts of herbicides were transported below the root zone. A few percent for triasulfuron and atrazine and<1% for terbuthylazine were detected below 0.5 m depth after 90‐mm cumulative infiltration. Traces of all herbicides were found down to 0.9 m. The depth distributions for anions and all herbicides were similar. These results show that the herbicides were only partly adsorbed by the soil matrix. A fraction of these chemicals was transported with or without minor adsorption along cracks or fissures. In the sandy soil, chemical movement was confined to the top 0.4 m, and the penetration depth of the herbicides was consistent with their mobility characteristics: triasulfuron showed greatest mobility, atrazine was moderately mobile, and terbuthylazine was the least mobile of all

ISSN:0043-1397    

DOI:10.1029/94WR02852

年代:1995

数据来源: WILEY

摘要:

A finite element flow and transport code, highly optimized for a Cray Y‐MP, is used to investigate the complex character of multispecies contaminant plumes that can develop in variable density flow systems. The reference data set for the simulation is based on a study of dense leachate plumes developed at Babylon, New York. A shallow unconfined aquifer receives leachates generated by annual recharge throughout the unlined landfill. The reference simulation generally reproduces the distribution of a density‐determining species in the leachate for both intermittent and continuous sources. A sensitivity analysis illustrates how the initial source concentrations for both a density‐determining species and a trace organic compound, the style of loading, and the reactive character of the organic compound can profoundly influence concentration distributions. For relatively large density contrasts between the leachate and the ambient groundwater, pockets of denser water sink to or near the bottom of the aquifer. A trace organic compound that is chromatographically separating from a dense plume due to sorption may follow different pathways as compared to the dense sp

ISSN:0043-1397    

DOI:10.1029/94WR02567

年代:1995

数据来源: WILEY

摘要:

Sorption isotherms are essential for describing transport of solutes in groundwater, and may be obtained in column experiments. In this paper a novel algorithm for the numerical computation of the sorption isotherm from the measured data is presented. The algorithm, which is quite different from classical minimization algorithms like the Marquardt process, uses the relative arrival times of dissolved components with a different sorption behavior. It has a large domain of attraction, and it is also rather inexpensive computationally. It is most effective as a preprocessor for a classical minimization process. The applicability of the algorithm is demonstrated on elution curves of K+, displaced by Ca++in a cation exchange reaction. The speed of the algorithm allows for the use of the bootstrap method for obtaining confidence domains for different sorption sites with different exchange constants.

ISSN:0043-1397    

DOI:10.1029/94WR02383

年代:1995

数据来源: WILEY

摘要:

In the pump and treat approach to the problem of managing a contaminated aquifer, a key problem is to design an effective capture system that collects only the polluted groundwater without allowing any of it to escape. At present, it is customary to design a capture system using fully penetrating withdrawal wells. Very often, however, only part of the vertical thickness of the aquifer is contaminated, so the question may arise whether a more efficient capture system can be achieved using partially penetrating wells. Very little work has been done on the application of partially penetrating wells to this problem. A new semianalytic method that can be used in determining the geometry of the capture zone for steady state flow to a partially penetrating well that is screened from the top (or from the bottom) of a confined aquifer has been developed. By combining the velocity potentials for flow to the well with that for the regional flow field, a three‐dimensional velocity potential that can be used in determining the complete geometry of the capture surface has been developed. The results have shown that with a constant pumping rate the maximum horizontal extent of the capture surface at the top (or bottom) of the aquifer increases as the degree of penetration decreases. As one would expect, the maximum vertical extent increases as the depth of penetration increases. Thus, if one knows the actual location of the contaminant plume, an appropriate combination of the degree of penetration and pumping rate can be selected to create an effective capture zon

ISSN:0043-1397    

DOI:10.1029/94WR02707

年代:1995

数据来源: WILEY

摘要:

Simulation of flow through heterogeneous media often requires discretizing the flow domain into blocks and assigning an equivalent block conductivity value to each one of them. The process of defining block conductivities from point values is termed upscaling. A number of approaches to upscaling are available, most of which consider the uncertainty associated with any natural property, so that they cast the problem in a stochastic frame. Recently, Indelman and Dagan (1993a, b) provided a general stochastic methodology to upscaling in heterogeneous anisotropic formations by means of the dissipation energy function; unfortunately, they did not provide any “practical” method to compute block values from point ones. The objective of this work is twofold: First, we analyze different practical approaches to compute block conductivities and find that all of them provide very similar results in terms of actual computed values; second, we check that all approaches verify approximately a number of conditions stated by Indelman and Dagan (1993a). Specifically, we show analytically that for regular blocks, the methodologies of both Rubin and Gómez‐Hernández (1990) and Desbarats (1992) (which we call “practical” methodologies) satisfy the condition that the effective conductivity obtained from a field where the elementary conductivities are defined over a certain support (we call this the actual formation) is identical to that obtained from the same field with conductivities defined at a larger support (upscaled formation). The analysis is carried out by working with the logarithm of block conductivities and using a small‐perturbation expansion and thus is strictly valid for small variances. On the other hand, we show numerically that the two methodologies satisfy approximately an important condition stated in terms of the dissipation energy: that block‐averaged dissipation values computed are indeed very close to the true dissipation values in each block. The agreement is even better if we consider statistical moments instead of point values. As an important conclusion we should note that all practical methodologies considered in this work perform equally well and, more important, constitute a simple way to treat an otherwise very

ISSN:0043-1397    

DOI:10.1029/94WR02754

年代:1995

数据来源: WILEY

摘要:

Lumped models of convective solute transport in one‐dimensional porous media are derived from the mass balance and the law of mass action. These models predict cumulative leaching from cumulative solute application and cumulative irrigation or rainfall. The solutes can be subject to adsorption or exchange governed by a nonlinear isotherm. The most important feature of the models is that although the properties of the medium and the liquid phase are allowed to be nonuniform, application requires only knowledge of the integrated moisture storage and adsorption isotherm over the medium section under consideration. Sufficient conditions are derived for the validity of lumped models and convenient expressions for evaluation of the model output are given. The results will be applied in part 2 of this paper, forthcoming, which will deal with convective solute transport along fixed streamlines in three‐dimensional porous me

ISSN:0043-1397    

DOI:10.1029/94WR02527

年代:1995

数据来源: WILEY

摘要:

The water level of the Dead Sea, a terminal hypersaline lake (total dissolved solids, approximately 340 g/L) has decreased at an average rate of 0.5 m/yr since 1960 and by 0.8 m/yr between 1981 and 1989. The dramatic long‐term water level variation of the Dead Sea and the seasonal short‐term fluctuations are accompanied by parallel variations of groundwater levels in an adjacent aquifer. A general methodology based on a simplified yet reliable one‐dimensional flow model, together with continuous measurements of groundwater levels in observation wells, enables analysis of aquifer structural and hydraulic properties. Furthermore, this analysis enables prediction of future groundwater levels in unconfined and confined aquifers due to future changes in lake l

ISSN:0043-1397    

DOI:10.1029/94WR03154

年代:1995

数据来源: WILEY

摘要:

A three‐dimensional miscible transport model for seawater intrusion in a phreatic aquifer with a transition zone is presented. This model considers many important factors, such as the effect of variable density on fluid flow, the effect of precipitation infiltration and phreatic surface fluctuation on the process of seawater intrusion, the existence of great discharge pumping wells, etc. The difficulty in solving this problem can be tackled by the presented numerical method and iteration technique. This model is used to describe seawater intrusion in Huangheying, Longkou, People's Republic of China. The simulated values agree very well with the field data (e.g., the total mean values of the absolute error of Cl−concentration are 46.13 mg/L and 55.67 mg/L, respectively, and those of the water head are 0.24 m and 0.08 m, respective

ISSN:0043-1397    

DOI:10.1029/94WR02379

年代:1995

数据来源: WILEY