Doctoral Degrees (Institute for Groundwater Studies (IGS))

Permanent URI for this collection

http://hdl.handle.net/11660/434

Browse

Browsing Doctoral Degrees (Institute for Groundwater Studies (IGS)) by Author "Gomo, Modreck"

Now showing 1 - 3 of 3
  • Thumbnail Image
    ItemOpen Access
    Characteristics of fluid electrical conductivity (FEC) profiles associated with a contaminant plume in porous and weathered basement aquifer systems
    (University of the Free State, 2023) Moleme, Malefa Florence; Gomo, Modreck
    The fluid electrical conductivity (FEC) profiling method has been commonly applied to aid in the compilation of site-specific conceptual models and understanding of the subsurface environment. Although research has recently been conducted to help improve the knowledge and understanding of the evolution of FEC profiles under natural and saline contaminated environments within the fractured-rock aquifer system, a research gap still exists for such studies in other aquifer types. The type of aquifer system plays a significant role in determining the migration patterns and behaviour of contaminants. Therefore, it is expected that the evolution of FEC profiles in different aquifer systems will vary, and this needs to be understood. It is against this background that this research aimed to investigate the behaviour of FEC profiles associated with a saline contaminant plume in typical unconfined porous and weathered basement aquifer systems, using laboratory-based aquifer models. This was done to improve the conceptual understanding of contaminant migration within these aquifer systems, which will essentially improve the interpretation of their FEC profiles. To achieve this, two physical models were developed in the laboratory to represent an unconfined porous aquifer system and a weathered basement aquifer system. The performance of the models was evaluated and tested, and subsequently used to investigate the progression of FEC profiles associated with a saline contaminant plume. The outcome of the laboratory tests was also verified in the field. This study also explored the effects that the distance of a source from a monitoring point would have on the shape of FEC profiles. Unlike previous studies which conducted the FEC profiling technique under induced groundwater flow, this study investigated the efficiency of a non-invasive approach of applying the method under natural gradient conditions. From the analysis of profiles obtained within the two simulated aquifer systems conceptual profiles were developed. Within the unconfined porous aquifer system, FEC profiles recorded from the borehole located closer to the source were notably different from the FEC profiles recorded from a borehole positioned further away from the source, thus it was evident that the distance of a monitoring point from the source influenced the orientation of the plume, and ultimately the resulting FEC profile. This brought to light the phenomenon of “plume orientation”. The orientation of the plume is usually disregarded in groundwater models and assessments, however this research showed that it is an important aspect which can be used to assist with FEC data interpretation and contaminated site characterisation studies. The orientation of the plume was strongly influenced by the magnitude of the forces acting upon it, primarily the gravitational and advection force. Closer to the source, the plume took on a vertical to sub-vertical orientation, whereas as the plume continued to migrate further away from the source it aligned with the flow lines of the system which resulted in a horizontal orientation. From the analysis of the weathered basement aquifer system two distinct signatures were identified and conceptualised: the low FEC profile and the elevated FEC profile. The low FEC profile not only represented a profile captured under natural conditions in the absence of contamination, but also represented a profile that would be observed when the majority of the contaminant has passed the borehole and the system was in the process of re-establishing initial conditions. It had three distinct zones: the weathered zone, transition zone, and the impermeable zone. The elevated FEC conceptual profile was associated with contaminated groundwater conditions within the weathered basement aquifer system. It had two distinct zones: the weathered zone and the impermeable zone.
  • Thumbnail Image
    ItemOpen Access
    Evaluation of the vulnerability of selected aquifer systems in the Eastern Dahomey basin, South Western Nigeria
    (University of the Free State, 2015-01) Oke, Saheed Adeyinka; Vermeulen, Danie; Gomo, Modreck
    This study aimed to evaluate the vulnerability of the shallow aquifer systems of the Dahomey Basin and formulate a simple vulnerability method with which data limited areas (which include the shallow unconfined aquifers in the Dahomey Basin) can be predicted. The Dahomey Basin is a transboundary aquifer which extends from Ghana to the western parts of Nigeria. The study covered the eastern section of the basin. The methodological approach involved a source–pathway–receptor vulnerability model. The Dahomey Basin was characterised through the geophysical, hydrological, litho-geochemical and hydrogeochemical approaches. The geology of the basin includes sedimentary rock types of sandstone, shale, limestone, alluvium conglomerate and the formations which are composed of sand, silt, clay, laterite and gravel. The geophysical study, which mainly aimed to estimate the depth-to-water table, identification of strata and vadose zone thickness, revealed topsoil, sandy clay, dry porous sandstone, conglomeratic sandstone, limestone and alluvium as the major lithological units in the basin. Geo-electrical curve types revealed an overlying multilayered rock. The vadose zone characterisation, which is the pathway through which contaminants infiltrate, aimed to determine the lithological properties which dictate the travel time of water. This was achieved by determining the hydraulic conductivity of the vadose lithology in the laboratory. Other important parameters such as grain size, porosity, shapes, textural classification and clay types were examined for their attenuation capacity. The hydrogeochemical investigation involving the collection and analysis of water samples from the hand-dug wells and shallow boreholes during the rainy and dry season was aimed at monitoring the groundwater quality of the basin. Ca-Mg-Cl water types and Na-K-Cl water types were delineated. Bacteriological examination of the shallow water reveals the presence of E.Coli, Heterotrophic bacteria and Salmonella/ Shigella. Precipitation which is a component of groundwater recharge ranged between 1 200–1 800 mm from the northern end to the southern end of the basin, respectively. Groundwater level were measured, monitored and average water level were delineated for the formations of the Dahomey Basin. The proposed RTt vulnerability method was applied to evaluate the groundwater vulnerability of the Dahomey Basin. The RTt method is an intrinsic physically based vulnerability method based on the concept of groundwater recharge from rainfall and travel time within the covering lithology over the aquifer. Travel time is the infiltration derived from multiplication of the slope and thickness of the vadose zone divided by fluid velocity. The fluid velocity is derived from the division of hydraulic conductivity by porosity. RTt method application results for the Dahomey Basin were presented on the RTt vulnerability map. The RTt vulnerability map was classified from very low vulnerability (12) to very high vulnerability (100). The RTt vulnerability results for the Dahomey Basin showed 18% of the areas classified as very high vulnerability, 7% of the areas classified as high vulnerability, 64% of the areas classified as moderate vulnerability and 10% of the areas classified as low vulnerability. The compared vulnerability maps of the RTt method and those of the DRASTIC, PI and AVI methods, showed similarities between the RTt method and the AVI and DRASTIC method, respectively. Areas classified as high vulnerability by these methods showed very shallow protective covers, high precipitation and porous aquifer materials, while areas classified as low vulnerability areas include thick protective cover, reduced rainfall, higher slope and higher depth-to-water. The RTt vulnerability map was validated with the hydrochemical tracer using chloride, DO and microbial loads as vulnerability indicators. This study has formulated an RTt method that can be used to predict the vulnerability of shallow unconfined aquifer systems, a key component in groundwater management. The major advantage of the RTt method is the use of less number of parameter to assess groundwater vulnerability. The method has been applied to investigate the regional aquifer of the Dahomey Basin and can be used to predict the aquifer vulnerability of similar basins across Africa with limited data.
  • Thumbnail Image
    ItemOpen Access
    A groundwater-surface water interaction study of an alluvial channel aquifer
    (University of the Free State, 2011-11) Gomo, Modreck; Van Tonder, G. J.
    The study describes the application complimentary geohydrologic tools to investigate the geohydrological properties of an alluvial channel aquifer and its interaction with the river surface water resources. Primary field investigations were designed to determine the geologic, hydraulic, hydrogeochemical and solute transport properties of the alluvial channel aquifer as an important component of the groundwater‐surface water (GW‐SW) interaction system. The secondary investigations were then aimed at assessing groundwater discharge and recharge mechanisms of the alluvial channel aquifer at a local scale (< 1000 m). A water balance model was developed for the groundwater‐surface system as a tertiary level of investigation. Geological characterisation results show the spatial variation in the physical properties of unconsolidated aquifer materials between boreholes and at different depth. The drawdown derivative diagnostic analysis shows that the alluvial channel aquifer system response during pumping can be described by the following major groundwater flow characteristics; Typical Theis response; transition period from initial Theis response to radial acting flow (RAF); radial acting flow in the gravel‐sand layer and river single impermeable boundary effects. Detailed studies of the hydrogeochemical processes in the alluvial aquifer system have shown that dissolution of silicate weathering, dolomite and calcite minerals, and ion exchanges are the dominant hydrogeochemical processes that controls groundwater quality. Quantitative and qualitative investigations indicate that the alluvial channel aquifer is being recharged through preferential infiltration recharge as facilitated by cavities and holes created by the burrowing animals and dense tree rooting system. Tracer tests under natural gradient were successfully conducted in an alluvial channel aquifer, thus providing some advice on how to conduct tracer breakthrough tests under natural gradients in a typical alluvial channel aquifer. The findings of the study also highlights the value of developing a water balance model as a preliminary requirement before detailed GW‐SW interaction investigations can be conducted. Based on the theoretical conceptualizations and field evidence it is suggested that studies be conducted to determine if alluvial channel aquifers can be further classified based on the nature of the hosting river channel. The classification would split the alluvial channel aquifer into alluvial cover and fractured‐bedrock, or a combination of the two. The applications of the PhD thesis findings are not only limited to the case study site, but have important implications for GW‐SW interaction studies, groundwater resource development and protection in areas where groundwater occurs in alluvial channel deposits.