Bwalya Malama - Academia.edu (original) (raw)
Papers by Bwalya Malama
The Boise Hydrogeophysical Research Site (BHRS) is a field-scale test facility in an unconfined a... more 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 testing theories and models for groundwater flow and solute transport. The design of the BHRS supports a
Traditional methods of aquifer characterization, namely pumping and slug tests, provide the most ... more 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: they can be laborious, expensive, are intusive and yield spatially sparse data. Hydrogeophysical methods offer some promise to overcome some of these limitations. We discuss recently developed semi-analytical solutions for transient streaming potentials associated with pumping tests conducted in homogeneous confined and unconfined aquifers. Using these solutions, data obtained from field tests conducted (a) in a confined aquifer at a site located near Montalto Uffugo, in the region of Calabria in Southern Italy, and (b) in an unconfined aquifer at the Boise Hydrogeophysical Research Site (BHRS) in Idaho, US. Estimates of hydraulic parameters that compare well to those obtained by traditional methods were obtained. Our work indicates that transient streaming potential data, collected at land surface, may be used to provide preliminary estimates of hydraulic aquifer properties quickly and cheaply.
Agu Fall Meeting Abstracts, 2010
The characterization problem in hydrology - i.e., obtaining enough data to develop models that ma... more 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 the expected effects of remediation strategies. Secondly, at catchment and larger scales, there is a need for data sources that can be used to characterize extensive portions of an aquifer in order to accurately model regional flows and fluxes between the subsurface, surface, and biosphere. Geophysical methods, in many cases, provide a relatively inexpensive way to collect large amounts of data about subsurface variability. Likewise, since geophysical methods are not limited to the location of boreholes, they provide a promising method for collecting data over wide spatial extents. However, in order to convert geophysical information into hydrologic insight, it is necessary to determine what the relationship is between the geophysical parameters - which can be estimated with these methods - and the hydrologic parameters, which are of more pertinent interest. In many cases, it has been assumed that the relation between geophysical and hydrologic parameters is simple - either through petrophysical relations, or through relationships between geophysical and hydrologic boundaries - but a growing body of evidence suggests that these relationships are often more complex. In this talk, we will review some recent results showing non-unique petrophysical relationships observed in the field, and discuss the implications of this non-uniqueness for inversion and data integration. Given the success of Bayesian methods for inverting hydrologic data and recognizing uncertainty, we will discuss how new Bayesian methods may be used to both recognize non-uniqueness and to perform non-linear inversions which take into account these complications. In one proposed approach, we show how independent estimates of hydrologic and geophysical parameters can be used within a maximum likelihood optimization formulation to recognize different petrophysical relationships. By basing the problem in a probabilistic context, we can utilize statistical tools such as likelihood ratios and information theoretic criteria to both recognize discrete facies and to develop multiple petrophysical relationship models under uncertainty. For large-scale problems, where estimating boundaries between discrete facies is an important first-order control, we suggest a second strategy in which hydrologic and geophysical data are integrated during inversion using a boundary sensitivity concept. Through both field and modeling examples, we will discuss the benefits of these Bayesian problem formulations for data integration that are robust and at the same time recognize important sources of uncertainty.
A semi-analytical solution is presented for the problem of flow in a system comprising an unconfi... more 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-analytical since the exact solution is obtained in the double Laplace-Hankel transform space and is then inverted numerically. The solution presented here is more general than similar solutions obtained for confined aquifer flow as we do not adopt the assumption of unidirectional flow in the confined aquifer (typically assumed to be horizontal) and the aquitard (typically assumed to be vertical). Aquitard storage is not neglected.
A semi-analytical solution is presented for the problem of flow in a system comprising an unconfi... more 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-analytical since the exact solution is obtained in the double Laplace-Hankel transform space and is then inverted numerically. The solution presented here is more general than similar solutions obtained for confined aquifer flow as we do not adopt the assumption of unidirectional flow in the confined aquifer (typically assumed to be horizontal) and the aquitard (typically assumed to be vertical). Aquitard storage is not neglected.
The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield or field-scale test facil... more 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 geophysical techniques; (b) examination of fundamental relationships and processes at multiple scales; (c) testing theories and models for groundwater flow and solute transport; and (d) educating and training of students in multidisciplinary subsurface science and engineering. The design of the wells and the wellfield support modular use and reoccupation of wells for a wide range of single-well, cross-hole, multiwell and multilevel hydrologic, geophysical, and combined hydrologic-geophysical experiments. Efforts to date by Boise State researchers and collaborators have been largely focused on: (a) establishing the 3D distributions of geologic, hydrologic, and geophysical parameters which can then be used as the basis for jointly inverting hard and soft data to return the 3D K distribution and (b) developing subsurface measurement and imaging methods including tomographic characterization and imaging methods. At this point the hydrostratigraphic framework of the BHRS is known to be a hierarchical multi-scale system which includes layers and lenses that are recognized with geologic, hydrologic, radar, seismic, and EM methods; details are now emerging which may allow 3D deterministic characterization of zones and/or material variations at the meter scale in the central wellfield. Also the site design and subsurface framework have supported a variety of testing configurations for joint hydrologic and geophysical experiments. Going forward we recognize the opportunity to increase the R&D returns from use of the BHRS with additional infrastructure (especially for monitoring the vadose zone and surface water-groundwater interactions), more collaborative activity, and greater access to site data. Our broader goal of becoming more available as a research asset for the scientific community also supports the long-term business plan of increasing funding opportunities to maintain and operate the site.
A theory for transport of a conservative tracer through a medium with spatially variable porosity... more 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 theory we have developed, porosity, n(x), and fluid flux, q(x, t) are treated as separate random fields with stationarily connected random fluctuations. Here, separate means that we do not lump the two random fields, n(x) and q(x, t), into a single random field, namely, the velocity field, v(x, t) = q(x, t)/n(x) at the outset before proceeding with ensemble averaging of the transport equation. This leads to ensemble moment equations that are significantly different from those typically obtained using the single random field v(x, t). In this preliminary work the equation for the first ensemble moment of tracer concentration is derived and used to predict solute transport for the case of a second-order stationary porosity field, and a steady-state flow field. Preliminary results indicate that close correspondence between the predicted mean and the observed behavior of the bromide tracer at the BHRS is achievable.
Riparian zones in semi-arid regions often exhibit high rates of evapotranspiration (ET) in spite ... more Riparian zones in semi-arid regions often exhibit high rates of evapotranspiration (ET) in spite of low soil moisture content due to vegetation that is able to withdraw water from shallow aquifers. This work better defines the relationship between ET and the saturated zone by comparing the observed water table drawdown to analytically modeled drawdown in fully penetrating wells of an unconfined aquifer in response to daily ET flux. ET at the Boise Hydrogeophysical Research Site or BHRS (a riparian zone in a temperate, semi arid environment) is calculated following the approach of Batra et al. (2006) but uses site (or near site) measurements for air temperature and net radiation while relying on Landsat 5 data for quantification of vegetation. The resulting ET calculations represent a data set consisting of high resolution (30m x 30m) ET flux values obtained from minimal site measurements. Water table levels in the shallow, unconfined aquifer were monitored over the summer and fall of 2009 to record the timing and magnitude of daily fluctuations at four separate wells at the site. ET derived from the radiation-driven model of Batra et al. (2006)
The Boise Hydrogeophysical Research Site (BHRS) is a field-scale test facility in an unconfined a... more 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 testing theories and models for groundwater flow and solute transport. The design of the BHRS supports a
We extend the models for slug tests developed by Hyder et al. (1994) and Butler and Zhan (2004) t... more 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 in wells in unconsolidated coarse fluvial aquifers, and others. The general semi-analytical solution allows for skin and formation storage as well as anisotropy in skin and formation hydraulic conductivity (K). The water table is treated as a fixed head boundary so the solution is applicable for wells screened below the water table. The model is validated by comparison with other models and by matching field data from unconfined fluvial aquifers at sites in Nebraska (MSEA) and Idaho (BHRS). We examine the effects of varying skin K and skin thickness to simulate the impact of a near-well disturbed zone that is lower (damage) or higher (filte...
The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield or field-scale test facil... more 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. The design of the wells and the wellfield provide for a wide range of single-well, cross- hole, multiwell and multilevel hydrologic, geophysical, and combined hydrologic-geophysical experiments. Recent efforts have been focused largely on: (a) establishing the 3D distributions of geologic, hydrologic, and geophysical parameters and (b) developing subsurface measurement and imaging methods including time- lapse tomographic imaging methods. Multiple lines of evidence from these efforts indicate that the hydrostratigraphic framework of the BHRS is a hierarchical system with at least three scales of sedimentary organization including layers and ...
Water Resources Research, 2015
The Boise Hydrogeophysical Research Site (BHRS) is a field-scale test facility in an unconfined a... more 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 testing theories and models for groundwater flow and solute transport. The design of the BHRS supports a
Traditional methods of aquifer characterization, namely pumping and slug tests, provide the most ... more 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: they can be laborious, expensive, are intusive and yield spatially sparse data. Hydrogeophysical methods offer some promise to overcome some of these limitations. We discuss recently developed semi-analytical solutions for transient streaming potentials associated with pumping tests conducted in homogeneous confined and unconfined aquifers. Using these solutions, data obtained from field tests conducted (a) in a confined aquifer at a site located near Montalto Uffugo, in the region of Calabria in Southern Italy, and (b) in an unconfined aquifer at the Boise Hydrogeophysical Research Site (BHRS) in Idaho, US. Estimates of hydraulic parameters that compare well to those obtained by traditional methods were obtained. Our work indicates that transient streaming potential data, collected at land surface, may be used to provide preliminary estimates of hydraulic aquifer properties quickly and cheaply.
Agu Fall Meeting Abstracts, 2010
The characterization problem in hydrology - i.e., obtaining enough data to develop models that ma... more 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 the expected effects of remediation strategies. Secondly, at catchment and larger scales, there is a need for data sources that can be used to characterize extensive portions of an aquifer in order to accurately model regional flows and fluxes between the subsurface, surface, and biosphere. Geophysical methods, in many cases, provide a relatively inexpensive way to collect large amounts of data about subsurface variability. Likewise, since geophysical methods are not limited to the location of boreholes, they provide a promising method for collecting data over wide spatial extents. However, in order to convert geophysical information into hydrologic insight, it is necessary to determine what the relationship is between the geophysical parameters - which can be estimated with these methods - and the hydrologic parameters, which are of more pertinent interest. In many cases, it has been assumed that the relation between geophysical and hydrologic parameters is simple - either through petrophysical relations, or through relationships between geophysical and hydrologic boundaries - but a growing body of evidence suggests that these relationships are often more complex. In this talk, we will review some recent results showing non-unique petrophysical relationships observed in the field, and discuss the implications of this non-uniqueness for inversion and data integration. Given the success of Bayesian methods for inverting hydrologic data and recognizing uncertainty, we will discuss how new Bayesian methods may be used to both recognize non-uniqueness and to perform non-linear inversions which take into account these complications. In one proposed approach, we show how independent estimates of hydrologic and geophysical parameters can be used within a maximum likelihood optimization formulation to recognize different petrophysical relationships. By basing the problem in a probabilistic context, we can utilize statistical tools such as likelihood ratios and information theoretic criteria to both recognize discrete facies and to develop multiple petrophysical relationship models under uncertainty. For large-scale problems, where estimating boundaries between discrete facies is an important first-order control, we suggest a second strategy in which hydrologic and geophysical data are integrated during inversion using a boundary sensitivity concept. Through both field and modeling examples, we will discuss the benefits of these Bayesian problem formulations for data integration that are robust and at the same time recognize important sources of uncertainty.
A semi-analytical solution is presented for the problem of flow in a system comprising an unconfi... more 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-analytical since the exact solution is obtained in the double Laplace-Hankel transform space and is then inverted numerically. The solution presented here is more general than similar solutions obtained for confined aquifer flow as we do not adopt the assumption of unidirectional flow in the confined aquifer (typically assumed to be horizontal) and the aquitard (typically assumed to be vertical). Aquitard storage is not neglected.
A semi-analytical solution is presented for the problem of flow in a system comprising an unconfi... more 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-analytical since the exact solution is obtained in the double Laplace-Hankel transform space and is then inverted numerically. The solution presented here is more general than similar solutions obtained for confined aquifer flow as we do not adopt the assumption of unidirectional flow in the confined aquifer (typically assumed to be horizontal) and the aquitard (typically assumed to be vertical). Aquitard storage is not neglected.
The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield or field-scale test facil... more 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 geophysical techniques; (b) examination of fundamental relationships and processes at multiple scales; (c) testing theories and models for groundwater flow and solute transport; and (d) educating and training of students in multidisciplinary subsurface science and engineering. The design of the wells and the wellfield support modular use and reoccupation of wells for a wide range of single-well, cross-hole, multiwell and multilevel hydrologic, geophysical, and combined hydrologic-geophysical experiments. Efforts to date by Boise State researchers and collaborators have been largely focused on: (a) establishing the 3D distributions of geologic, hydrologic, and geophysical parameters which can then be used as the basis for jointly inverting hard and soft data to return the 3D K distribution and (b) developing subsurface measurement and imaging methods including tomographic characterization and imaging methods. At this point the hydrostratigraphic framework of the BHRS is known to be a hierarchical multi-scale system which includes layers and lenses that are recognized with geologic, hydrologic, radar, seismic, and EM methods; details are now emerging which may allow 3D deterministic characterization of zones and/or material variations at the meter scale in the central wellfield. Also the site design and subsurface framework have supported a variety of testing configurations for joint hydrologic and geophysical experiments. Going forward we recognize the opportunity to increase the R&D returns from use of the BHRS with additional infrastructure (especially for monitoring the vadose zone and surface water-groundwater interactions), more collaborative activity, and greater access to site data. Our broader goal of becoming more available as a research asset for the scientific community also supports the long-term business plan of increasing funding opportunities to maintain and operate the site.
A theory for transport of a conservative tracer through a medium with spatially variable porosity... more 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 theory we have developed, porosity, n(x), and fluid flux, q(x, t) are treated as separate random fields with stationarily connected random fluctuations. Here, separate means that we do not lump the two random fields, n(x) and q(x, t), into a single random field, namely, the velocity field, v(x, t) = q(x, t)/n(x) at the outset before proceeding with ensemble averaging of the transport equation. This leads to ensemble moment equations that are significantly different from those typically obtained using the single random field v(x, t). In this preliminary work the equation for the first ensemble moment of tracer concentration is derived and used to predict solute transport for the case of a second-order stationary porosity field, and a steady-state flow field. Preliminary results indicate that close correspondence between the predicted mean and the observed behavior of the bromide tracer at the BHRS is achievable.
Riparian zones in semi-arid regions often exhibit high rates of evapotranspiration (ET) in spite ... more Riparian zones in semi-arid regions often exhibit high rates of evapotranspiration (ET) in spite of low soil moisture content due to vegetation that is able to withdraw water from shallow aquifers. This work better defines the relationship between ET and the saturated zone by comparing the observed water table drawdown to analytically modeled drawdown in fully penetrating wells of an unconfined aquifer in response to daily ET flux. ET at the Boise Hydrogeophysical Research Site or BHRS (a riparian zone in a temperate, semi arid environment) is calculated following the approach of Batra et al. (2006) but uses site (or near site) measurements for air temperature and net radiation while relying on Landsat 5 data for quantification of vegetation. The resulting ET calculations represent a data set consisting of high resolution (30m x 30m) ET flux values obtained from minimal site measurements. Water table levels in the shallow, unconfined aquifer were monitored over the summer and fall of 2009 to record the timing and magnitude of daily fluctuations at four separate wells at the site. ET derived from the radiation-driven model of Batra et al. (2006)
The Boise Hydrogeophysical Research Site (BHRS) is a field-scale test facility in an unconfined a... more 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 testing theories and models for groundwater flow and solute transport. The design of the BHRS supports a
We extend the models for slug tests developed by Hyder et al. (1994) and Butler and Zhan (2004) t... more 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 in wells in unconsolidated coarse fluvial aquifers, and others. The general semi-analytical solution allows for skin and formation storage as well as anisotropy in skin and formation hydraulic conductivity (K). The water table is treated as a fixed head boundary so the solution is applicable for wells screened below the water table. The model is validated by comparison with other models and by matching field data from unconfined fluvial aquifers at sites in Nebraska (MSEA) and Idaho (BHRS). We examine the effects of varying skin K and skin thickness to simulate the impact of a near-well disturbed zone that is lower (damage) or higher (filte...
The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield or field-scale test facil... more 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. The design of the wells and the wellfield provide for a wide range of single-well, cross- hole, multiwell and multilevel hydrologic, geophysical, and combined hydrologic-geophysical experiments. Recent efforts have been focused largely on: (a) establishing the 3D distributions of geologic, hydrologic, and geophysical parameters and (b) developing subsurface measurement and imaging methods including time- lapse tomographic imaging methods. Multiple lines of evidence from these efforts indicate that the hydrostratigraphic framework of the BHRS is a hierarchical system with at least three scales of sedimentary organization including layers and ...
Water Resources Research, 2015