Bwalya Malama

Verified

Bwalya verified their affiliation via an institutional email.

Learn more

Verified

Bwalya verified their affiliation via an institutional email.

Learn more

• PhD
• Professor (Full) at California Polytechnic State University

This study quantifies the transpiration of encroached lodgepole pine (Pinus contorta var. murryana (Grev. & Balf.) Engelm.) in a montane meadow using pre-restoration sap-flow measurements. Lodgepole pine transpiration and its response to environmental variables were examined in Rock Creek Meadow (RCM), Southern Cascade Range, CA, USA. Sap-flow data...

Analytical and semi-analytical models for stream depletion with transient stream stage drawdown induced by groundwater pumping are developed to address a deficiency in existing models, namely, the use of a fixed stream stage condition at the stream-aquifer interface. Field data are presented to demonstrate that stream stage drawdown does indeed occ...

Analytical and semi-analytical models for stream depletion with transient stream stage drawdown induced by groundwater pumping are developed to address a deficiency in existing models, namely, the use of a fixed stream stage condition at the stream–aquifer interface. Field data are presented to demonstrate that stream stage drawdown does indeed occ...

Compost application is commonly considered by winegrape producers to improve soil health while sequestering carbon (C) and mitigating climate change. However, inputs of available C and nitrogen (N) as nutrients can induce emissions of greenhouse gases (GHG) such as carbon dioxide (CO2) and nitrous oxide (N2O). A 2-year field experiment in a Mediter...

Background and Aims. No-till is considered a core practice of conservation and climate-smart agriculture. Nevertheless, recent evidence suggests that the benefits of this practice for climate change mitigation might be overestimated, particularly in the short term. Methods and Results. In a three-year field experiment, we investigated the environme...

Mathematical models for stream depletion with stream stage decline or drawdown are developed to overcome the deficiency in existing models that typically use the constant-head (Dirichlet) or general (Robin) boundary condition and source terms at the stream-aquifer interface. Existing approaches assume a fixed stream stage during pumping, implies th...

The results reported in this work are based in part on measurements of sap flow in a few select trees on a representative riparian forest plot coupled with a forest-wide randomized sampling of tree sapwood area in a watershed located along the Pacific coast in Santa Cruz County, California. These measurements were upscaled to estimate evapotranspir...

Of interest to the Underground Nuclear Explosion Signatures Experiment are patterns and timing of explosion‐generated noble gases that reach the land surface. The impact of potentially simultaneous flow of water and gas on noble gas transport in heterogeneous fractured rock is a current scientific knowledge gap. This article presents field and labo...

We report the results of field and laboratory investigations of stream-aquifer interactions in a watershed along the California coast to assess the impact of groundwater pumping for irrigation on stream flows. The methods used include subsurface sediment sampling using direct-push drilling, laboratory permeability and particle size analyses of sedi...

The continuum-scale electrokinetic porous-media flow and excess charge redistribution equations are uncoupled using eigenvalue decomposition. The uncoupling results in a pair of independent diffusion equations for “intermediate” potentials subject to modified material properties and boundary conditions. The fluid pressure and electrostatic potentia...

The continuum-scale electrokinetic porous-media flow and excess charge redistribution equations are uncoupled using eigenvalue decomposition. The uncoupling results in a pair of independent diffusion equations for "intermediate" potentials subject to modified material properties and boundary conditions. The fluid pressure and electrostatic potentia...

Slug tests are a widely used method for characterizing aquifers because they can be performed quickly, with minimal equipment and labor, and without the need to dispose of produced water. They involve rapid displacement of the water level in a well, followed by continuous measurement of the water level as it reequilibrates. Several mathematical mod...

A modified version of a published slug test model for unconfined aquifers is applied to cross-hole slug test data collected in field tests conducted at the Widen site in Switzerland. The model accounts for water-table effects using the linearised kinematic condition. The model also accounts for inertial effects in source and observation wells. The...

A multiporosity extension of classical double and triple porosity fractured rock flow models for slightly compressible fluids is presented. The multiporosity model is an adaptation of the multirate solute transport model of Haggerty and Gorelick [1995] to viscous flow in fractured rock reservoirs. It is a generalization of both pseudo-steady-state...

Following the radionuclide release event of February 14, 2014 at the Waste Isolation Pilot Plant (WIPP), actinide contamination has been found on the walls and floor in Panel 7 as a result of a release in Room 7 of Panel 7. It has been proposed to decontaminate Panel 7 at the WIPP by washing contaminated surfaces in the underground with fresh water...

A semi-analytical solution is presented for transient streaming potentials associated with flow to a pumping well in an unconfined aquifer, taking into account the effect of flow in the unsaturated zone above the water table. Flow in the unsaturated zone is modeled with a linearized form of Richards' equation using an exponential model for soil moi...

Predicting meaningful production decline in non-traditional shale oil and gas wells is a complex problem. We are developing a Bayesian framework to evaluate the use of naturally occurring noble gas tracers in fractured shale hydrocarbon wells. Using noble gas tracers along with traditional production analysis methods will increase the information c...

We present transient streaming potential data collected during falling-head permeameter tests performed on samples of two sands with different physical and chemical properties. The objective of the work is to estimate hydraulic conductivity (K) and the electrokinetic coupling coefficient (Cl ) of the sand samples. A semi-empirical model based on th...

Model applicability to core-scale solute transport is evaluated using breakthrough data from column experiments conducted with conservative tracers tritium (H-3) and sodium-22, and the retarding solute uranium-232. The three models considered are single-porosity, double-porosity with single-rate mobile-immobile mass-exchange, and the multirate mode...

This study presents a semi-analytical solution for the problem of leakage in an unconfined aquifer bounded below by an aquitard of finite or semi-infinite extent. The homogeneous anisotropic unconfined aquifer of infinite radial extent is pumped continuously at a constant rate from a well of infinitesimal radius. The aquitard is also homogeneous, a...

This report is a summary of the physical processes, primary governing equations, solution approaches, and historic testing related to brine migration in geologic salt. Although most information presented in this report is not new, we synthesize a large amount of material scattered across dozens of laboratory reports, journal papers, conference proc...

Key Points Analytical model developed for river leakage contribution to riparian zone ET River contributions to ET can be distinguished in relative drawdown at wells Improved relative contributions to ET from river and aquifer can be calculated

We develop a closed-form three-parameter model for unsaturated hydraulic conductivity associated with the Kosugi three-parameter lognormal moisture retention model. The model derivation uses a slight modification to Mualem's theory, which is nearly exact for non-clay soils. Kosugi's three-parameter lognormal moisture retention model uses physically...

Breakthrough data collected in a set of laboratory tracer experiments are used to constrain the multirate mass transport model STAMMT-L. The uniqueness and variance of the estimated parameters are explored using both Gauss-Marquardt-Levenberg (PEST) and Markov-Chain Monte Carlo (DREAM) algorithms. The efficacy and uniqueness of different multirate...

The streaming potential method has emerged as a promising technology for indirect acquisition of spatially dense measurements of the hydraulic system state response to pumping aquifers. The method relies on measurements of electric potentials generated by groundwater flow due to the existence of the electric double layer at the rock--water interfac...

Two models for slug tests conducted in unconfined aquifers are developed by (a) extending the uncon-fined KGS solution to oscillatory responses, yielding a model referred to herein as the unified model, and (b) replacing the constant head condition with the linearized kinematic condition at the water table. The models can be used to analyze the ful...

A recently developed unified model for partially-penetrating slug tests in unconfined aquifers (Malama et al., in press) provides a semi-analytical solution for aquifer response at the wellbore in the presence of inertial effects and wellbore skin, and is able to model the full range of responses from overdamped/monotonic to underdamped/oscillatory...

Research highlights ► The nonlinear kinematic condition at water table is linearized to include a diffusion term. ► This treatment implies diffuse propagation of water table during pumping tests, controlled by a linearization parameter, βDβD. ► βDβD can take values on [0, ∞], with βD=0βD=0 corresponding to Neuman (1972). ► βDβD modifies the slope o...

The efficient solution of coupled hydrogeophysical problems both numerically and analytically is important to their use in parameter estimation. We present a general approach for decoupling the governing equations for groundwater flow and the associated electrokinetic problem. The approach can use either a symbolic or numerical eigenvector decompos...

Riparian zones in semi-arid regions often exhibit high rates of evapotranspiration (ET) in spite of low soil moisture content because riparian vegetation are adapted to withdraw water from shallow aquifers. This work better defines the relationship between ET and the saturated zone by comparing the observed and analytically modeled water table draw...

We present results of pneumatic slug tests conducted in an unconfined aquifer at various sites in the Butte--Silver Bow area in southwestern Montana. The results vary from monotonic decay to oscillatory, and seem to indicate that the water table does have a significant effect on slug test response. A model is presented where the linearized kinemati...

We present semi-analytical solutions for transient streaming potentials associated with pumping from unconfined and confined aquifer through a fully-penetrating line sink. Aquifer flow is assumed to occur without fluid leakage from the confining units. The solutions are fitted to field measurements of streaming potentials associated with an aquifer...

s u m m a r y We investigate the response of the saturated zone (unconfined aquifer) to evapotranspiration (ET) flux at ground surface. We neglect fluid flow and storage in the unsaturated zone and treat ET as a sinusoidal forcing function at the watertable. The linearized kinematic condition is imposed at the watertable. Ana-lytical solutions are...

The characterization problem in hydrology - i.e., obtaining enough data to develop models that make accurate, physically-based, and uncertainty-aware predictions - faces two key challenges. First, at the field scale, there is still a need for methods that can resolve fine-scale variability in order to accurately simulate contaminant transport and t...

The Boise Hydrogeophysical Research Site (BHRS) is a field-scale test facility in an unconfined aquifer with the goals of: developing cost-effective, non-invasive methods for quantitative characterization of heterogeneous aquifers using hydrologic and geophysical techniques; understanding fundamental relations and processes at multiple scales; and...

Traditional methods of aquifer characterization, namely pumping and slug tests, provide the most direct way of measuring system state variables (hydraulic head) and estimating hydraulic parameters (hydraulic conductivity, specific storage and specific yield) of aquifers. Despite this significant advantage, such methods have some serious limitations...

We extend the models for slug tests developed by Hyder et al. (1994) and Butler and Zhan (2004) to obtain a single general model for slug tests in unconfined aquifers in partially penetrating wells with a near-well disturbed zone (skin). The full range of responses, oscillatory to overdamped, is considered since both types of responses are common i...

We present a semi-analytical solution for the transient streaming potential response of an unconfined aquifer to continuous constant rate pumping. We assume that flow occurs without leakage from the unit below a transverse anisotropic aquifer and neglect flow in the unsaturated zone by treating the water-table as a moving material boundary. In the...

We present a nonlinear stochastic inverse algorithm that allows conditioning estimates of transient hydraulic heads, fluxes and their associated uncertainty on information about hydraulic conductivity (K) and hydraulic head (h) data collected in a randomly heterogeneous confined aquifer. Our algorithm is based on Laplace-transformed recursive finit...

We present a methodology for conditioning estimates of hydraulic heads and fluxes and their associated uncertainty on information about transmissivity, T , and hydraulic heads, h, collected within a randomly heterogeneous aquifer under transient conditions. Our approach is based on recursive finite-element approximations of exact nonlocal first and...

We consider the transient streaming potential response due to pumping from a confined aquifer through a fully penetrating line sink. Confined aquifer flow is assumed to occur without fluid leakage from the confining units. However, since confining units are typically clayey, and hence more electrically conductive than the aquifer, they are treated...

The importance of estimating spatially variable aquifer parameters such as transmissivity is widely recognized for studies in resource evaluation and contaminant transport. A useful approach for mapping such parameters is inverse modeling of data from series of pumping tests, that is, via hydraulic tomography. This inversion of field hydraulic tomo...

An analytical solution for flow in the vicinity of an observation well is developed. The observation is emplaced in a homogeneous aquifer of infinite radial extent that is pumped at a constant rate, and satisfies the Theis solution. We attempt to account for the effect of an observation well on the drawdown response in a neighborhood of finite radi...

Ground water flow associated with pumping and injection tests generates self-potential signals that can be measured at the ground surface and used to estimate the pattern of ground water flow at depth. We propose an inversion of the self-potential signals that accounts for the heterogeneous nature of the aquifer and a relationship between the elect...

Using an integrated geophysical and hydrological modeling approach along with a field test, we investigate the use of ground penetrating radar (GPR) to monitor the drawdown response of a heterogeneous, unconfined aquifer to pumping. The pumping test was conducted at the Boise Hydrogeophysical Research Site (BHRS) where we have access to an unconfin...

The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield or field-scale test facility developed in a shallow, coarse, fluvial aquifer with the objectives of supporting: (a) development of cost- effective, non- or minimally-invasive quantitative characterization and imaging methods in heterogeneous aquifers using hydrologic and geophy...

A semi-analytical solution is presented for the problem of flow in a system consisting of unconfined and confined aquifers, separated by an aquitard. The unconfined aquifer is pumped continuously at a constant rate from a well of infinitesimal radius that partially penetrates its saturated thickness. The solution is termed semi-analytical because t...

The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield or field-scale test facility developed in a shallow, coarse, fluvial unconfined aquifer with the objectives of developing cost-effective, non-invasive methods for quantitative characterization and imaging in heterogeneous aquifers using hydrologic and geophysical techniques. Th...

We thank Bohling and Butler (this issue) for their kind words regarding our paper (Cardiff et al. 2009) and for the opportunity to clarify some statements made in the introduction of our paper. In our paper, we first discuss data analysis techniques that can be used to analyze hydraulic data in a tomo-graphic manner, including the use of Bohling's...

A series of combined hydrogeophysical tests were conducted during a two-week period in June 2007 in the unconfined coarse fluvial aquifer at the Boise Hydrogeophysical Research Site (BHRS). The tests included three component experiments -- hydraulic tomography (HT), self potential measurements (SP), and electrical resistivity tomography (ERT) -- wh...

Semi-analytical solutions for the problem of leakage in an unconfined aquifer bounded below by an aquitard of finite or semi-infinite extent are presented. The homogeneous anisotropic unconfined aquifer of infinite radial extent is pumped continuously at a constant rate from a well of infinitesimal radius. The aquitard is also homogeneous, anisotro...

A semi-analytical solution is presented for the problem of flow in a system comprising an unconfined aquifer and a confined aquifer that are separated by an aquitard. The unconfined aquifer is pumped continuously at a constant rate from a well of infinitessimal radius that partially penetrates its saturated thickness. The solution is termed semi-an...

A theory for transport of a conservative tracer through a medium with spatially variable porosity is presented. The work is motivated by observed anomalous behavior of a conservative tracer (bromide) at the Boise Hydrogeophysical Research Site (BHRS) which cannot fully be accounted for by spatial variability of hydraulic conductivity alone. In the...

To simulate brittle rocks, a mixture of glastone, sand and water was used as a model material. Thin galvanized sheets of thickness 0.254mm were used to create joints in blocks made out of the model material. To investigate the failure modes and strength, both the intact material blocks as well as jointed model material blocks of size 35.6×17.8×2.5c...

Nonlocal stochastic moment equations have been used successfully to analyze transient flow in randomly heterogeneous media conditional on measured values of medium properties. We present a geostatistical inverse algorithm that makes it possible to further condition such analyses on measured values of state variables, notably hydraulic head and flux...

A new semi-empirical model that can be used to predict fracture deformation behavior under normal compressive loading is presented. The development of a simple exponential model is presented first after which a modified and more general exponential model, with an additional degree of freedom in the model parameters, is obtained. The simple and the...

Laboratory experiments and numerical simulations, using Particle Flow Code (PFC3D ), were performed to study the behavior of jointed blocks of model material under uniaxial loading. The effect of joint geometry parameters on the uniaxial compressive strength of jointed blocks was investigated and this paper presents the results of the experiments a...