Aroca catchment’s hydrogeological characteristics are majorly controlled by deeply weathered crystalline rocks of northern Uganda. Groundwater derived from within the medium grained weathered clasts to the base of the unconsolidated regolith in addition to fissures in the underlying saprocks is widely used to supply water to most upcoming towns.
However, in addition to limited reliable data, little is known of the geometry and storage capacity of these aquifers. This study used magnetic resonance sounding (MRS) and complimentary electrical resistivity (VES), lithological logs and hydraulic tests to determine aquifer geometry and hydraulic properties of Aroca catchment. The main aquifer is largely a medium-grained sandy saprolite and exhibits leaky aquifer characteristics. It is supplemented by the saprolite-fissured boundary and the bulk of the fissured secondary aquifers. MRS and VES were useful in delineating the upper surface (up to a mean depth ~5 mbgl) of the aquifer geometry. The mean depth to the bottom of the saprolite and fractured layers was 33 and 70 mbgl respectively. The mean static water level across the catchment ranges from ~1.4 to 30 mbgl (with a median value of 13.7 mbgl). The mean T is 4.3 to 104 m2d-1, K is 7.0 md-1 and Sy is ~0.016 for this aquifer. MRS determined drainable groundwater storage of aquifers with a mean value of 380 mm, which is much lower than the value of 590 mm from similar geological formations of Benin. MRS water content and decay constant of the saturated aquifer had mean values of 6.4% and 156ms, respectively. Just like most geophysical methods, MRS suffers from resolution loss with depth and result into the fissured zone not being well resolved.
MRS complemented by VES and aquifer tests has provided a new insight in aquifer geometry characterization and groundwater storage in the deeply weathered crystalline rock aquifers of Aroca catchment and the region. The geometry and hydraulic properties of these weathered crystalline rock aquifers can further be constrained for better spatial representation by conducting a number of well distributed long duration aquifer tests and MRS measurements across the catchment and region. Hence, more site investigations is recommended to improve on the knowledge of storage capacities of the catchment/region aquifers for better future groundwater abstraction strategies, resource planning and management.
Table of Contents
1. INTRODUCTION
1.1 Background
1.2 PROBLEM STATEMENT
1.3 OBJECTIVES
1.3.1 Main Objectives
1.3.2 Specific Objectives
1.4 Justification and Significance of the study
1.5 Scope of work
1.6 Study Area
1.6.1 Location and Demography
1.6.2 Climate
1.6.3 Drainage and hydrology
1.6.4 Geology of the Aroca catchment
1.6.5 Hydrogeology
2. LITERATURE REVIEW
2.1 Geology and geological setting
2.1.1 Regional Geology
2.1.2 Evolution of weathered crystalline rock aquifers
2.2 Groundwater occurrance
2.2.1 Geometry of weathered crystalline rock aquifers
2.2.1.1 Lithological Logs
2.2.2 Geoelectrical resistivity
2.2.2.1 Principle of electrical resistivity method
2.2.2.2 Electrical profiling
2.2.2.3 Vertical Electrical Sounding (VES)
2.2.3 Magnetic Resonance Sounding (MRS)
2.2.3.1 Principle of MRS method
2.2.3.2 MRS measurements
2.2.3.3 MRS parameters and aquifer geometry
2.2.3.4 MRS flow related parameters
2.2.3.5 MRS storage related parameters
2.3. Hydraulic properties of crystalline rock aquifers
2.3.1 Lithological logs
2.3.2 Aquifer tests
2.3.2.1 Theoretical estimation of storage
2.3.2.2 Determination of aquifer hydraulic parameters
2.3.2.3 Derivative analysis
2.3.2.4 Hydrogeological conceptual models
3. MATERIALS AND METHODS
3.1 Materials
3.2. Methods
3.2.1 Archived/historical data collection
3.2.2 Field activities
3.2.2.1 Monitoring well construction
3.2.2.2 Aquifer tests
3.2.2.3 MRS measurements
3.2.3 Laboratory analyses
3.2.4 Data analyses
3.2.4.1 Litho-stratigraphic analysis
3.2.4.2 Use of geoelectrical resistivity (VES) and borehole lithological logs
3.2.4.3 Hydraulic parameters analysis
(a) Laboratory data analysis
(b) Aquifer test analysis
(c) MRS data analysis
3.2.5 Generation of a hydrogeological conceptual model
4. PRESENTATION AND DISCUSSION OF RESULTS
4.1 Aquifer characteristics
4.1.1 Borehole log analysis
4.1.2 Physio-chemical analysis
4.1.3 Spatial lithological distribution and basement spatial variation analysis
4.1.4 Delination of hydrogeologic units using geophysical technique
4.1.4.1 Geoelectrical resistivity (VES)
4.1.4.2 MRS aquifer geometry
4.1.4.3 Comparison of MRS, VES layer model and lithological logs
4.1.5 Hydraulic properties of aquifers
4.1.5.1 Estimates of hydraulic parameters from pedotransfer functions
4.1.5.2 Static water level and hydraulic head distribution
4.1.5.3 Aquifer tests analysis
4.1.5.4 Parameterization of MRS hydraulic parameters
(a) MRS flow hydraulic parameters
(b) MRS drainable groundwater storage
4.2 Conceptual hydrogeological model
4.3 DISCUSSION OF RESULTS
4.3.1 Aquifer Geometry
4.3.2 Hydraulic properties of aquifers in the region
4.3.3 Comparison of MRS hydraulic flow parameters
4.3.4 Conceptual cross-sectional model model
5. CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
5.1.1 Aquifer architecture
5.1.2 Aquifer hydraulic properties
5.1.3 Hydrogeological conceptual model
5.2 Recommendations
Objectives and Research Themes
The primary objective of this dissertation is to quantify groundwater storage and assess aquifer geometry within the deeply weathered crystalline rock aquifers of the Aroca catchment in Northern Uganda, utilizing a combination of geophysical methods and hydraulic testing to support sustainable groundwater development.
- Characterization of aquifer geometry using magnetic resonance sounding (MRS), electrical resistivity (VES), and lithological logs.
- Determination of hydraulic properties (transmissivity and hydraulic conductivity) through aquifer tests and MRS parameterization.
- Estimation of drainable groundwater storage and water content using MRS techniques.
- Development of a hydrogeological conceptual model to understand the subsurface architecture and groundwater occurrence.
Excerpts from the Book
1.6.5 Hydrogeology
The hydrogeological characteristics of the catchment are majorly controlled by the deeply weathered crystalline rocks of northern Uganda. Beneath the soil zone, the regolith is often clay-rich with a correspondingly low hydraulic conductivity (K ~0.003 to 0.3 md-1; Taylor and Howard, 1998; this study). The first water strike ranges from about 15 to >40 mbgl whereas the main water strike ranges from about 25 mbgl to >40 mbgl suggesting mean thickness of the regolith to range from 15 to >40m (DWRM, 2012).
At the base of the regolith, the clay content is significantly reduced and the hydraulic conductivity increases (K = 0.5 – 5 md-1, Tindimugaya, 2008; 0.3 – 10 md-1, this study). Fractures present in the upper 20 mbgl to 40 mbgl of the crystalline rocks also provide an additional groundwater source of importance, particularly where the weathered zone is thin. The static water level is generally less than 15 mbgl in most parts of the catchment (DWRM, 2012) which is suggestive of leaky to confined aquifers in the catchment. Most wells in the catchment draw groundwater from aquifers within medium-grained, weathered clasts to the base of the unconsolidated regolith in addition to the fissures in the underlying saprock. Recharge annual rate of the Aroca catchment has been estimated to vary between 100 and 200 mm (Taylor and Howard, 1996, 2000).
Summary of Chapters
1. INTRODUCTION: Outlines the significance of groundwater in Uganda, stating the problem of limited reliable data regarding aquifer geometry and storage in the Aroca catchment.
2. LITERATURE REVIEW: Explores the regional geological setting and existing methodologies for characterizing weathered crystalline rock aquifers using geophysical and hydraulic tests.
3. MATERIALS AND METHODS: Describes the field activities, including the construction of a monitoring well, aquifer testing, and MRS/VES geophysical measurement protocols.
4. PRESENTATION AND DISCUSSION OF RESULTS: Analyzes the gathered borehole logs, geophysical parameters, and hydraulic test data to define aquifer characteristics and build a hydrogeological model.
5. CONCLUSIONS AND RECOMMENDATIONS: Summarizes the findings on aquifer architecture, hydraulic properties, and provides guidance for future groundwater abstraction and management strategies.
Keywords
Groundwater, Magnetic Resonance Sounding (MRS), Transmissivity, Storage, Geometry, Aroca catchment, crystalline rock aquifers, hydraulic conductivity, hydrogeological model, regolith, saprolite, vertical electrical sounding (VES), borehole logs, aquifer tests, Northern Uganda
Frequently Asked Questions
What is the core focus of this research?
The research focuses on characterizing the geometry, hydraulic properties, and groundwater storage capacity of deeply weathered crystalline rock aquifers in the Aroca catchment, Northern Uganda.
Which primary methodologies are employed in this study?
The study integrates non-invasive Magnetic Resonance Sounding (MRS) with Electrical Resistivity (VES), analysis of borehole lithological logs, and long-duration aquifer pumping tests.
What is the main objective of the dissertation?
The main objective is to provide a reliable quantification of groundwater storage and a comprehensive assessment of aquifer geometry to aid sustainable safe water provision.
What are the critical thematic areas covered?
The topics include geology and geological settings, groundwater occurrence, aquifer hydraulic properties, and the generation of a conceptual hydrogeological model for the region.
What does the main body address?
The main body presents field data collection techniques, analysis of subsurface hydrogeological materials, and the comparative evaluation of various aquifer models.
What defining keywords characterize this work?
Key terms include Magnetic Resonance Sounding (MRS), Transmissivity, Groundwater storage, crystalline rock aquifers, and hydrogeological conceptual models.
Does the MRS method provide accurate aquifer details at greater depths?
The study notes that MRS, like most geophysical methods, suffers from resolution loss with increasing depth, making the fissured zone beneath the regolith more challenging to resolve clearly.
How does the aquifer system in the Aroca catchment behave?
The aquifer system is characterized as mostly unconfined to leaky, utilizing water from weathered clasts to the base of the regolith and fissures within the underlying saprock.
- Quote paper
- Joseph Okullo (Author), 2016, Estimating Groundwater Storage in Weathered Crystalline Rock Aquifers of Aroca Catchment, Northern Uganda using Geophysical Methods and Aquifer Tests, Munich, GRIN Verlag, https://www.grin.com/document/1488069
