Duke Ophori

Constraining permeabilities in a large-scale groundwater system through model calibration

Abstract The range of field estimates of permeabilities in rocks of the Atikokan Research Area (ARA), Canada, has been reduced by calibrating a groundwater flow model of the ARA. The flow model was calibrated to the recharge component of the water balance and to measured freshwater heads in boreholes. A good match between field and simulated recharge, and between measured and simulated heads was obtained with two different permeability-depth distributions that differed by up to two orders of magnitude in places. Solute transport simulation of the distributions of total dissolved solids (TDS), chlorides (Cl) and oxygen-18 ( 18 O) in groundwater in the area was performed to further refine the permeability estimates. Vertical average linear groundwater velocities from the calibrated flow model were used in one-dimensional solute transport models of the system to generate depth–concentration profiles of TDS, Cl and 18 O. Recharge-, midline- and discharge-area models of both the fracture zones and the rock mass were employed. The midline area is the narrow zone that separates the recharge (downward flow) area from the discharge (upward flow) area. Groundwater flow is essentially horizontal in the midline area. “Envelopes” formed by the simulated profiles that were created with the velocities of one simulation case more tightly enclosed the field data (TDS, Cl, 18 O) than those of another simulation case, indicating that velocities of the former simulation case better represent the hydrodynamics of the real system. The models demonstrate that the scatter of TDS, Cl and 18 O field data for the ARA is consistent with the groundwater flow model predictions and can be explained by the complexity arising from different hydraulic regimes (recharge, midline, discharge) and hydrogeologic environments (fracture zones, rock mass) of the regional flow system. Solute transport simulation of hydrochemical constituents can aid in refining and constraining permeabilities in large-scale groundwater systems.

The significance of viscosity in density-dependent flow of groundwater

Abstract Many modeling studies of variable-density groundwater flow have been performed in the last few decades. In most of these studies, fluid density is considered to vary with concentration, while the variation of viscosity with concentration is neglected. The consequences of this negligence is not completely known. The present study uses a numerical simulation approach to investigate the density-viscosity-concentration relationship during groundwater flow and solute transport through a density-stratified system. Fluid density is assumed to increase with depth from freshwater at the surface, through brackish and saline waters, to brines at the bottom half of the system. The system mimics field observations at the Atikokan Research Area (ARA) in northwestern Ontario, Canada. Hypothetical ‘unit basin’ models, consisting of recharge-, midline- and discharge-area regimes are employed. Simulations with the density-concentration equation of state and a constant (freshwater) viscosity in the density-stratified system causes groundwater to sink against the buoyancy forces of the system. More water is recharged into the system than necessitated by the buoyancy. The configurations and lengths of travel paths, and travel time of conservative contaminants are inaccurately predicted. Accounting for concentration in the viscosity equation causes groundwater floating in agreement with the expected buoyancy physics of the system. Overestimation of concentration-dependent density causes sinking, whereas, overestimation of viscosity results in overfloating and underestimation of groundwater recharge. Even in density-stratified fluids with salinity of seawater, recharge and through flow of water may be slightly overestimated if a concentration-dependent depsity is used along with a constant freshwater viscosity. The concentration dependence of both density and viscosity are to be analyzed carefully during groundwater flow and solute transport simulations in systems with considerable fluid density variations.

First approximations of soil moisture retention curves using the filter-paper method.

Relationships between gravimetric soil moisture content (w) and matric potential (ϕ), and between volumetric soil moisture content (θv) and pressure head (h) were approximated for the unsaturated zone on Long Island, New York. Soil samples were collected from two sites using a hand auger. The soil moisture content was determined using the filter-paper (wf) and gravimetric (w) methods, respectively. The wf was then used in an empirical equation to estimate ϕm. Each set of ϕm and w was combined with a straight-line empirical model to obtain a w(ϕm) relationship. Soil ϕm was converted to h, and w to the volumetric moisture content θv, in order to produce a θv(h) curve. Graphical and statistical comparison showed that the resulting θv(h) curves fell within one order of magnitude of similar curves generated by a more sophisticated non-linear model developed previously. The simplicity and low cost of the filter-paper approach described in this study recommends it for preliminary studies of hydraulic properties in the unsaturated zone. Copyright

Hydrogeological characterization of a tropical crystalline aquifer system

This research used a numerical groundwater flow model, calibrated under steady-state conditions to develop the groundwater flow system in the West Mamprusi District of Northern Ghana. It was aimed at conceptualizing the flow system to initiate a thorough hydrogeological study of the rocks in the area. Stable isotopes were used to relate groundwater recharge in the area to recent meteoric water that had been evaporated in transit down the surficial material. On this basis, direct vertical groundwater recharge from precipitation was applied in the numerical modeling. This study suggests that the prospects of commercial development of the aquifers are high as the estimated recharge ranges between 3.3% and 29% of the annual precipitation. Estimated horizontal hydraulic conductivity ranges between 3.2 and 48 m/d in the area. The variability in the horizontal hydraulic conductivity has led to the development of four prominent groundwater flowpaths in the area. However, a prominent NE–SW flow has been observed and is in consonance with the reported structural grain of the country. A groundwater flow divide noted in the southern part of the study area has been attributed to the structural heterogeneity rather than topographical complexities as the area is largely flat.