摘要:

A model is developed for predicting the location of major vegetation ecotones in eastern North America. The model is based upon the interaction of plant physiological characteristics, climate, and soil in order to be useful as an interactive boundary condition in atmospheric general circulation models. The model represents the relative competitive ability of different vegetation types by their annual net primary productivity as expressed through water use. At any given location, the vegetation type with the highest productivity is assumed to be dominant. Ecotones are located where competitive dominance shifts from one vegetation type to another. Productivity is computed as a function of annual evapotranspiration, which is a function of the length of the growing season, photosynthetic capacity, potential evapotranspiration, and soil moisture availability, among other things. When considering the coniferous boreal/deciduous forest ecotone, it is found that inherent physiological differences between conifers and deciduous trees lead to differences in productivity which are related primarily to temperature. The model predicts a reversal of deciduous/coniferous productive dominance at the latitude of the observed ecotone; that is, within the transition zone of mixed forest. The productivity model does not adequately explain the existence of the deciduous/southern pine ecotone. Soil chemistry apparently plays a role in the determination of this boundary.

ISSN:0043-1397    

DOI:10.1029/93WR02746

年代:1994

数据来源: WILEY

摘要:

This paper exposes the significant role played by the gas‐water interface in the fate and transport of colloids in porous media and also introduces a micromodel method to allow direct visualization of colloid behavior in pore networks. The gas‐water interface was created by trapping air in the pore space. Various types of latex and clay particles, as well as bacteria, were studied. The results suggest that the gas‐water interface sorbs not only hydrophobic but also hydrophilic particles. The degree of sorption is controlled by particle surface hydrophobicity, solution ionic strength, and particle charge sign. Sorption increases with increasing particle hydrophobicity and solution ionic strength, while positively charged particles have a very strong affinity for the gas‐water interface. The sorption on the gas‐water interface is essentially irreversible, in that the capillary free energy provides a large attractive force to hold particles on the gas‐water interface after its rupture. These findings reveal a mechanism of vadose zone transport: A static gas‐water interface behaves as a sorbent phase retarding the transport of particulate contaminants. The visualization method developed in this research is very useful for the investigation of particulate contaminant behavior and interface‐related transport, especially in the context of

ISSN:0043-1397    

DOI:10.1029/93WR02403

年代:1994

数据来源: WILEY

摘要:

For some potentially useful and emerging in situ bioremediation technologies it is important to control bacterial attachment to subsurface materials during the injection of microbial cell suspensions. In this study the attachment patterns ofMethylosinus trichosporiumOB3b were measured after horizontal injections into a two‐dimensional miniature aquifer simulator containing a wet homogeneous sand. In preliminary sand column assays, bacterial attachment to the sand was increased nearly 2 orders of magnitude compared to attachment in the presence of distilled water by raising the concentration of apH 7.0 sodium‐potassium phosphate buffer to 10 mM. The maximal concentration of attached cells was ∼4×108cells/g dry sand with both sand minicolumns and the wet sand aquifer simulator. For the latter this occurred on streamlines directly between the horizontal injector and withdrawal ports, where injection‐withdrawal velocities were the highest. The effects of a simulated groundwater cross flow during suspension injection on bacterial attachment to the aquifer simulator sands were also studied, and a peristaltic pumping method to counteract these groundwater flow effects resulted in a more localized pattern, i.e., without extensive downstream skewing of the bacterial attachment zone. Phenol red was utilized as a nonbinding, red‐colored tracer compound. It proved to be very convenient for quantitatively measuring the earlier breakthroughs of cells versus an inert tracer during the aquifer simulator and subsequent capillary tube cell injection experiments and also for visualizing the anticipated boundaries of cell attachment in the aquifer simulator. The effect of injection velocity on the observed bacterial attachment patterns in these experiments appears to be accounted for by colloid filtrat

ISSN:0043-1397    

DOI:10.1029/93WR02402

年代:1994

数据来源: WILEY

摘要:

The results of a tracer experiment conducted in a single fracture are interpreted using a semianalytical model which accounts for advective dispersion, matrix diffusion, mixing in the test zones, and for tortuosity. The experiment was conducted using a conservative tracer which was injected into a steady divergent flow field. Arrival of tracer was monitored in an array of 13 boreholes intersecting the fracture over a square area of approximately 30 m on a side. The arrival of tracer was detected in 11 of the 13 boreholes. Simulation of the transport process suggests that matrix diffusion may play a significant role in the migration of solutes. Contrary to previously published results, the relative influence of advective dispersion was found to be increasingly diminished with increasing scale. Fracture apertures determined from the results of the tracer experiment were found to be in general agreement with apertures calculated from independent hydraulic tests. The individual advective processes that contribute to advective dispersion remain unresolved on the basis of this experiment.

ISSN:0043-1397    

DOI:10.1029/93WR02401

年代:1994

数据来源: WILEY

摘要:

This paper discusses the hydrochemical evolution of groundwaters in the multilayer artesian aquifer of the Continental Intercalaire (the mainly Jurassic Agadez‐Dabla sandstones) aquifer of northern Niger. Isotopic and chemical changes in relation to the geological setting suggest that the increasing HCO3−alkalinity of the groundwaters is caused by interaction of deep CO2with the aquifer matrix. Alteration of feldspars by CO2with a δ13C value of −3.0‰ accounts for the observed alkalinity and isotopic and trends. Carbon 14 ages have been estimated for recent and Holocene groundwaters which have a distinct stable isotopic (2H and18O) signature compared with14C dead groundwaters from further west on the Irhazer Plain. Recharge temperatures, estimated by analysis of noble gas contents, show that the14C dead waters were recharged under conditions cooler than present averages in the region, possibly at the beginning of deglaciation (≈16 ka B.P.) or during the period 23–30 ka B.P. Trends in the hydrochemistry of U are related to the deep CO2alteration process. Radiogenic He contents increase along the flow direction and extremely high values occur in the proximity of U ores. Denitrification in the paleowaters of the Irhazer was assessed by measurements of nitrogen/

ISSN:0043-1397    

DOI:10.1029/93WR02226

年代:1994

数据来源: WILEY

摘要:

Variability in hydraulic properties and various mechanisms resulting in rate‐limited mass transfer influence field scale solute transport in aquifers. In this study analytical expressions are presented for the expected value and variance of the mass flux for a solute undergoing a nonequilibrium sorption reaction controlled by first‐order kinetics. The uncertainty in the mass flux, expressed through the variance, stems from finite solute injection volumes compared to the hydraulic conductivity integral scale. Two‐dimensional numerical simulations are performed under statistically identical conditions to the ones assumed in the derivation of the analytical results. Two cases of variability in the log hydraulic conductivity are considered (σY2= 0.1 and σY2= 1.0). The analytical model and the numerical simulations yield comparable predictions of the mass flux statistics for both variability cases. Comparing conservative and kinetically sorbing solute an apparent effect of sorption kinetics is to lower the discrepancy between the analytical model and the simulations due to increased spreading of the solute resulting from rate‐limiting mass

ISSN:0043-1397    

DOI:10.1029/93WR02654

年代:1994

数据来源: WILEY

摘要:

Finding the history of a groundwater contaminant plume from measurements of its current spatial distribution is an ill‐posed problem and, consequently, its solution is extremely sensitive to errors in the input data. In this paper, Tikhonov regularization (Tikhonov and Arsenin, 1977) is used in numerical experiments to recover the release history of a plume that has originated from a known, single site. The recovered release history is then used to reconstruct the plume evolution history. The method is found to be insensitive to round off errors, but its accuracy is affected by plume measurement errors, the extent to which the plume has dissipated, and, to a lesser degree, the accuracy of the transport parameter estimates. A regularization approach may be effective at finding a plume history when there are adequate dat

ISSN:0043-1397    

DOI:10.1029/93WR02656

年代:1994

数据来源: WILEY

摘要:

Constant flux infiltration experiments were performed at rates less than saturated hydraulic conductivity (Ksat) on uniform columns of soils and glass beads with low initial water contents. Tensiometers measured matric pressure (ψ) histories at various depths in the 0.05‐m diameter, 0.73‐m‐tall columns. One of two devices applied the steady fluxes: one was a ceramic plate applicator that allowed monitoring of pressures above the plate, and the other was a rain applicator that sprinkled water uniformly over the surface. Two columns were used. One allowed destructive sampling for water content in a way that prevented postrun changes in water content. The other allowed prerun replacement of pore air with gases of different solubilities. Contrary to predictions based on the Richards equation, transmission zone ψ values passed through maxima, then decreased continuously as wetting fronts moved down columns. Such nonideal behavior was observed in Aiken silt loam, Hanfoird sandy loam, Delhi sand, Oakley sand, and glass beads. At an infiltration rate equal to 70% ofKsat, values of ψ at 2, 5, and 8 cm depths in the glass beads column decreased an average of 52% from their respective maxima in about 3 hours. In the soil columns the nonmonotonic behavior was less pronounced but statistically significant at all rates examined (3–59% ofKsat). Matric pressure reversals were larger, and corresponding maxima higher, when a given rate was preceded by a series of step wise increasing rates. Tests of possible explanations provided no evidence to support hypotheses involving trapped‐air dynamics, convective air flow, particle rearrangement, or experimental artifacts. The results cast doubt on the ability of the Richards equation to predict the course of constant flux infiltration including the initiati

ISSN:0043-1397    

DOI:10.1029/93WR02476

年代:1994

数据来源: WILEY

摘要:

In this paper, a screening model for flow of a nonaqueous phase liquid (NAPL) and associated chemical transport in the vadose zone is developed. The model is based on kinematic approximation of the governing equations for both the NAPL and a partitionable chemical constituent. The resulting governing equation is a first‐order, quasi‐linear hyperbolic equation to which the generalized method of characteristics can be applied. This approach generally neglects the contribution to the NAPL flux from capillary pressure gradients. During infiltration under ponded conditions, or when the NAPL flux exceeds the maximum effective conductivity of the soil, the effect of capillary suction is included in the model through the usage of the Green‐Ampt model. All of the resulting model equations are in the form of ordinary differential equations which are solved numerically by a variable time step Runge‐Kutta technique. Results from a simple column experiment were used to evaluate the vadose zone flow model assumptions. Independently measured parameters allow simulation without calibration of the model results. The match of the model to the data suggests that the model captures the qualitative behavior of the experimental system and is capable of an acceptable degree of quantitative ag

ISSN:0043-1397    

DOI:10.1029/93WR02341

年代:1994

数据来源: WILEY

摘要:

Waechter and Philip (1985) obtained the asymptotic expansion of the mean infiltration rate for largesfrom a buried circular cylinder using a scattering analog. Heres(= αl/2) is defined as the ratio of the characteristic lengthlof the water supply surface (in fact, its radius) to the sorptive length 2α−1of the soil and a satisfies the relationshipK(ψ) =K(0)eαψ, whereKis the hydraulic conductivity, and ψ is the moisture potential. This exact solution cannot be used directly to obtain the separate contributions to the mean infiltration rate from the top and the bottom halves of the cylinder; our analysis is based on a new class of special functions derived from the modified Bessel equation with a forcing term. In this paper, we obtain the separate asymptotics for the two halves for large s to make a comparison with the results of the trench problem (Waechter and Mandal, 1993). The asymptotic expansions for top and bottom halves are (2/π)(0.69553s−2/3) and (2/π)(1+0.30066s−2/3), respectively, whereas for a semicircular trench, the mean infiltration rate is given by (2/π)(1

ISSN:0043-1397    

DOI:10.1029/93WR02468

年代:1994

数据来源: WILEY