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Geology topics

Mary C. Hill

Publications and source records attributed to Mary C. Hill.

62 records · Page 4Linked to original sources
USGS1999

A new multistage groundwater transport inverse method: presentation, evaluation, and implications

More computationally efficient methods of using concentration data are needed to estimate groundwater flow and transport parameters. This work introduces and evaluates a three‐stage nonlinear‐regression‐based iterative procedure in which trial advective‐front locations link decoupled flow and transport models. Method accuracy and efficiency are evaluated by comparing results to those obtained when flow‐ and transport‐model parameters are estimated simultaneously. The new method is evaluated as conclusively as possible by using a simple test case that includes distinct flow and transport parameters, but does not include any approximations that are problem dependent. The test case is analytical; the only flow parameter is a constant velocity, and the transport parameters are longitudinal and transverse dispersivity. Any difficulties detected using the new method in this ideal situation are likely to be exacerbated in practical problems. Monte‐Carlo analysis of observation error ensures that no specific error realization obscures the results. Results indicate that, while this, and probably other, multistage methods do not always produce optimal parameter estimates, the computational advantage may make them useful in some circumstances, perhaps as a precursor to using a simultaneous method.

Water Resources Research↗
USGS1996

Presentation and evaluation of a new multi-stage parameter estimation method using advective transport observations

Observations of advective groundwater transport, generally inferred from concentration measurements, have been found to help in the estimation of groundwater flow parameters. Transverse dispersion, however, can make it difficult to determine the location of the groundwater advective front directly from concentration measurements. A three-stage iterative procedure is developed to estimate groundwater flow and transport parameters from head and concentration data, and is tested using an analytical model. The results are compared to those obtained when advective and dispersive transport parameters are estimated simultaneously. The iterative procedure can be effective at estimating advective and dispersive parameters that improve the fit to the concentration data, but the simultaneous procedure generally produces a better, optimal fit to the concentration data and, therefore, estimates more accurate and precise parameters values. This is accomplished in fewer iterations. It had been anticipated that this iterative procedure might be effective and efficient for using concentration data to estimate parameters of numerical models, but these preliminary results suggest that its utility is limited.

Book chapter↗
USGS1995

Three-dimensional inverse modelling using heads and concentrations at a Danish landfill

A three-dimensional inverse model using hydraulic heads and concentrations of non-reactive solutes is presented. Application to the Danish Grindsted Old Landfill is used to investigate how the concentration data affects estimates of horizontal and vertical hydraulic conductivity and longitudinal dispersivity as well as prediction uncertainty

Grindsted↗
USGS1990

Preconditioned Conjugate-Gradient 2 (PCG2), a computer program for solving ground-water flow equations

This report documents PCG2: a numerical code to be used with the U.S. Geological Survey modular three-dimensional, finite-difference, ground-water flow model. PCG2 uses the preconditioned conjugate-gradient method to solve the equations produced by the model for hydraulic head. Linear or nonlinear flow conditions may be simulated. PCG2 includes two reconditioning options: modified incomplete Cholesky preconditioning, which is efficient on scalar computers; and polynomial preconditioning, which requires less computer storage and, with modifications that depend on the computer used, is most efficient on vector computers. Convergence of the solver is determined using both head-change and residual criteria. Nonlinear problems are solved using Picard iterations. This documentation provides a description of the preconditioned conjugate gradient method and the two preconditioners, detailed instructions for linking PCG2 to the modular model, sample data inputs, a brief description of PCG2, and a FORTRAN listing.

Water-Resources Investigations Report↗
USGS1985

An investigation of hydraulic conductivity estimation in a ground-water flow study of Northern Long Valley, New Jersey

The purpose of this study was to develop a methodology to be used to investigate the aquifer characteristics and water supply potential of an aquifer system. In particular, the geohydrology of northern Long Valley, New Jersey, was investigated. Geohydrologic data were collected and analyzed to characterize the site. Analysis was accomplished by interpreting the available data and by using a numerical simulation of the watertable aquifer. Special attention was given to the estimation of hydraulic conductivity values and hydraulic conductivity structure which together define the hydraulic conductivity of the modeled aquifer. Hydraulic conductivity and all other aspects of the system were first estimated using the trial-and-error method of calibration. The estimation of hydraulic conductivity was improved using a least squares method to estimate hydraulic conductivity values and by improvements in the parameter structure. These efforts improved the calibration of the model far more than a preceding period of similar effort using the trial-and-error method of calibration. In addition, the proposed method provides statistical information on the reliability of estimated hydraulic conductivity values, calculated heads, and calculated flows. The methodology developed and applied in this work proved to be of substantial value in the evaluation of the aquifer considered.

Thesis↗