We appreciate R. Brock's interest and comments on our paper but are unsure about the meaning of his comment 'although a nonuniform porosity can lead to significantly larger mound heights, this will not always be the case.' Certainly, we agree that the increase in mound height caused by a reduced specific yield beneath the recharge area is small when the reduced specific yield is nearly the same as the specific yield external to the recharge area. This is clearly shown in our Figure 4 [Ortiz et al., 1978]. Nearly uniform specific yields result when the recharge rate is small in comparison to the hydraulic conductivity as shown by (6). Calculations of mound height from Figure 4 will show that the ratio of the mound height with nonuniform specific yields to the mound height with uniform specific yield decreases as time increases, in agreement with Brock. The difference between the mound heights, calculated with and without reduced specific yield, increases, however. It is the relative error that decreases as was mentioned in our discussion of Figure 2. We are also aware that the effect on nonlinearity tends to offset the increase • Now with the Colorado-Wyoming Area, USDA, Science and Education Administration, Fort Collins, Colorado 80523.caused by a reduced specific yield. Brock provides a method for quantifying the effects of nonlinearity.It seems worth emphasizing that our procedures provide a means of relating the reduced specific yield to the recharge rate and, thereby, an improved way of relating mound growth to recharge rate. However, our equation (6) does not apply for the case where recharge has ceased and the mound is receding. The mound will not necessarily begin to recede at the moment recharge ceases, however, because drainage of the in-transit water will continue for some time after the flux at the ground surface becomes zero. The rate of drainage of in-transit water will decrease with time, and eventually, the mound will begin to recede. An appropriate expression for the effective specific yield during drainage of in-transit water and the subsequent recession of the mound has not been established. It seems reasonable that Brock's assertion of uniform specific yield during mound recession would be true in the later stages of recession, at least. We are currently investigating this problem. REFERENCE
Despite considerable scientific research over the past twenty-five years there still remains much speculation and uncertainty concerning the origin of the so-called tabular uranium deposits of the Colorado Plateau. One particular hypothesis suggests that the deposits resulted from geochemical reactions at the interface between a relatively stagnant solution and a dynamic, ore-carrying solution which permeated the host sandstones. The present study was designed to investigate some aspects of this hypothesis, notably the nature of fluid flow and the relations between the ore deposits and the host sandstones.Field studies involved an investigation of the character and origin of the host fluvial sandstones and their contained uranium deposits and an examination of the textural and porosity-permeability patterns in similar Holocene fluvial sand bodies. Data from the Holocene sand bodies was used to construct realistic porous media models. These models were then saturated with a humic acid solution andean aluminum potassium sulfate solution, causing a visible humic acid precipitate to form at the interface of the two fluids. Finally, an existing numerical model was modified and calibrated for use in predicting the shape and location of the interface between the two solutions.
Analytical solutions, accounting for the reduction in storage capacity caused by in‐transit water, are presented for the growth of groundwater mounds in an unconfined aquifer in response to uniform deep percolation from a rectangular and circular recharging area. The solution for the circular case was obtained by integrating the solution for the analogous case of heat flow in a composite cylindrical region with respect to time. In the rectangular case the solution was obtained by a product of two functions representing the solution for the instantaneous addition of a recharge slug over a long strip and integrating with respect to time. The aquifer is homogeneous, isotropic, infinite in areal extent, and resting on a horizontal impermeable base. Dimensionless curves are presented to afford a relatively simple means for the analysis of the growth of groundwater mounds.
The precipitation of humates at the interface between a humic acid solution and an aluminum potassium sulfate solution was studied in porous media models to gain some insight into the deposition of Colorado Plateau type uranium deposits. The effects of variable flow rates, porous‐media layering, mudstone lenses, fluid density differences, and geochemical reactions on the flow phenomenon and the resulting precipitate were evaluated. A numerical model was developed to predict the shape and location of the interface where precipitation occurs. The numerical model was verified by comparing its predictions with the results obtained from the porous media model. The association of precipitate bands with modeled sedimentary features and the similarities between these precipitate bands and actual deposits in the Colorado Plateau suggest that these model studies provide data useful both in understanding and predicting the relations between tabular ore deposits and ground‐water flow patterns.
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