GEOPHYSICAL AND HYDROGEOLOGICAL INVESTIGATION REPORT
St Peters Catholic Church- Upendo in Kangawa, P.O. Box 230, Molo.
Through
The Catholic Diocese of Nakuru, through the Nakuru Defluoridation Co. Ltd P.O. Box 2943 - 20100 Nakuru
Plot LR No.; …………………… Kangawa-Upendo Village, Kangawa Sub-Location, Kuresoi North Location, Mau Summit Division, Molo Sub-County, Nakuru County.
Topo Sheet No.; 118/2
CONSULTANT:
Samuel Munyiri, BSc, MSc. (Hons) R. Geol. (Registered Hydrogeologist; Licence No. WD/WP/254) P.O. Box 798-10101, Karatina. Cell-phone: 0723 623 766 E-mail: [email protected]
Signed:______
Date:______
June, 2018
Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
EXECUTIVE SUMMARY
Introduction This report presents the findings of a geophysical and hydro-geological assessment carried out for St Peters Upendo Catholic Church-Kangawa on their 2 Acre parcel of land located in Kangawa Village, Kangawa Sub-Location, Kuresoi North Location and Mau Summit Division in Molo Sub- County of Nakuru County. The farm is located off the Molo-Londiani tarmac road section – at a distance of about 8km from Molo Town on the right hand side of this main road. The land can be accessed at the Kangawa Primary School junction and is about 1.8km west of this junction along a dry weather road. Water Demand and Use Kangawa-Upendo community is not served by any source of portable piped water, which would imply a near absence water supply on the client’s land. Apart from private boreholes and shallow wells in the area, there is no other feasible water supply. The community therefore relies on shallow wells, which have contaminated water due to percolation of fecal material from pit latrines. Residents with private borehole sell water to the community, making it unaffordable to the less fortunate. There is thus the need to provide a sufficient and reliable water supply to the farm to meet a domestic input estimated at 20.0m3/day. The Catholic Diocese of Nakuru, through the Nakuru Defluoridation Co. Ltd has partnered to sponsor a borehole facility and water reticulation system to the Kangawa-Upendo community. Geology Tertiary and Quaternary volcanics and sediments are the most dominant formations in the Mole area and basically in the entire Rift Valley region. The rift margins expose an older sequence of Trachytic and pyroclastics. Mau tuffs are exposed in the south and west of the Molo area. These materials comprise the majority of the Mau succession and are of unknown thickness. They represent a prolonged period of pyroclastic activity combined with a moist climate leading to fluviatile, mud-flow and lake bed deposits. Recent pyroclastics and sediments of the Mau Slope cover the Molo and Elburgon areas at surface, extending down onto the Rongai Plain to the east of the Molo area.
Hydrogeology This groundwater potential investigation carried out on this part of the Kangawa area identifies two potential water bearing zones. There is the possible water strike associated with the sediment aquifer, whereas the other aquifer is associated with the fractured systems of the volcanic lithology. The lowlands suffice, as the recipients of terminal waters from the Mau catchments-due to the low topographical setting and in relation to the dip of the volcanic rocks; that would tend to push their terminal flows into the fractured and faulted rift floor rocks and lake sediments. The bulk of the groundwater supplies will have to be derived from the sediments that lie between different strata of the volcanic rocks. Conclusion and Recommendations It is thus recommended that a borehole be drilled at the location of VES 2 with UTM coordinates 36M 0799758m E and 9976624m N and, degrees 00° 12’ 39.6” S & 35° 41’ 33” E, with an altitude of 2595m to a minimum depth of 150m to a maximum depth of 200 metres. It is a mandatory requirement to obtain the necessary pre-requisite permits from the Water Resources Authority (WRA) and Water Resource Users Association (WRUA) prior to the commencement of the drilling program. There are two boreholes that are within the 800m radial distance to the proposed borehole. To avoid aquifer interference, we recommend that the proposed borehole be drilled to a deeper depth of 150-200m. Shallow aquifers above 90m should be sealed off to avoid aquifer sharing and subsequent depletion.
May - 2018 II Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
TABLE OF CONTENTS
EXECUTIVE SUMMARY ...... II LIST OF ABBREVIATIONS AND GLOSSARY OF TERMS ...... 1 1. BACKGROUND INFORMATION ...... 3 1.1 Location ...... 3 1.2 Objectives of the Survey ...... 3 1.2 Climate, Drainage and Physiography ...... 4 1.3 Water Demand and existing water supplies...... 5 2.0 PROJECT APPROACH AND METHODOLOGY ...... 6 3.0 GEOLOGY ...... 7 3.1 Regional Geology of the area...... 7 3.2 Geological Succession ...... 7 3.6 Geological Structures ...... 8 4.0 HYDROGEOLOGY ...... 9 4.1 Surface Water Resources...... 9 4.2 Groundwater System...... 10 4.2.1 Rainfall, Percolation and Recharge ...... 10 4.2.2 Mau Escarpment Faults and Hydrogeology: ...... 11 4.2.3 Borehole Data ...... 11 4.4 Groundwater Storage, Recharge and Discharge ...... 12 4.5 Aquifer Parameters in the Study Area ...... 12 4.5.1 Transmissivity ...... 13 4.5.2 Hydraulic Conductivity ...... 13 4.5.3 Safe Yield...... 14 4.5.4 Specific Capacity and Specific yield/storage coefficient ...... 14 4.5.6 Groundwater Flux/recharge and Discharge Dynamics ...... 15 4.5.7 Recharge ...... 15 4.5.8 Discharge ...... 15 4.6 Groundwater Quality ...... 15 5.0 GEOPHYSICAL INVESTIGATION METHODS ...... 18 5.1 Resistivity Method ...... 18 5.2 Basic Principles ...... 18 5.3 Vertical Electrical Sounding (VES) ...... 19 5.4 Horizontal Resistivity Profiling...... 19 5.5 Field Work ...... 20 5.6 Resistivity Sounding (VES) Interpretation...... 20 6.0 CONCLUSIONS & RECOMMENDATIONS ...... 22 REFERENCES ...... 24
May - 2018 III Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
List of Figures
Figure 1: Climate data for the area ...... 4 Figure 2; Surface geology map of Molo and neighbouring Central Rift regions ...... 8 Figure 3; Surface water resources in the larger Mau catchment region ...... 10 Figure 4; Simulated surface runoff for the period 1980 to 1989...... 11 Figure 5; Map showing the distribution of Hydraulic conductivities in the area...... 13 Figure 6: An illustration on the principles of resistivity measurements ...... 19
List of Tables
Table 1; Existing Borehole Data...... 12 Table 2; Specific yield ranges ...... 14 Table 3; Ionic Concentration: WHO & Various Authorities...... 17 Table 4; VES- 1 & 2 interpretations ...... 21
List of Appendices
Appendix 1: Geophysical Data Curve ...... 25 Resistivity Pseudo-section ...... 27 Appendix 2: Drilling ...... 28 Appendix 3: Site sketch and Topo Sheet extract of the Project Site Location...... 31
May - 2018 IV Kangawa Area, Molo Sub-County LIST OF ABBREVIATIONS AND GLOSSARY OF TERMS
ABBREVIATIONS (All S.I Units unless indicated otherwise) agl above ground level amsl above mean sea level bgl below ground level E East EC electrical conductivity (mS/cm) h head hr hour K hydraulic conductivity (m/day) I litre m metre N North PWL pumped water level Q discharge sQ/s specific capacity (discharge – drawdown ratio; in m. cu/hr/m) Cu cubic Sq square S drawdown (m) S South Sec second SWL static water level T transmissivity (m.sq/day) VES Vertical Electrical Sounding W West WSL water struck level mS/cm micro-Siemens per centimetre: Unit for electrical conductivity 0C degrees Celsius: Unit for temperature Wm Ohm-m: Unit for apparent resistivity ra Apparent resistivity
Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
GLOSSARY OF TERMS Alluvium General term for detrital material deposited by flowing water. Aquifer A geological formation or structure, which stores and transmits water and which is able to supply water to wells, boreholes or springs. Colluvium General term for detrital material deposited by hill slope gravitational processes, with or without water as an agent. Usually of mixed texture. Confined aquifer A formation in which the groundwater is isolated from the atmosphere by impermeable geologic formations. Confined water is generally at greater pressure than atmospheric, and will therefore rise above the struck level in a borehole. Development In borehole engineering, this is the general term for procedures applied to repair the damage done to the formation during drilling. Often the borehole walls are partially clogged by an impermeable “wall cake”, consisting of fine debris crushed during drilling, and clays from the penetrated formations. Well development removes these clayey cakes, and increases the porosity and permeability of the materials around the intake portion of the well. As a result, a higher sustainable yield can be achieved. Fault A larger fracture surface along which appreciable displacement has taken place. Gradient The rate of change in total head per unit of distance, which causes flow in the direction of the lowest >head. Grit Coarse sandstone of angular grain Hydraulic head Energy contained in a water mass, produced by elevation, pressure or velocity. Hydrogeological Those factors that deal with subsurface waters and related geological aspects of surface waters. Infiltration Process of water entering the soil through the ground surface. Joint Fractures along which no significant displacement has taken place. Lava sheet Lava flow, in parts very thick, covering a large area. Percolation Process of water seeping through the unsaturated zone, generally from a surface source to the saturated zone. Permeability The capacity of a porous medium for transmitting fluid. Phenocrysts Large, conspicuous crystals in porphyritic rocks (i.e. rocks with visible mineral crystals in a generally fine groundmass). Phonolite Compact and fine textured volcanic rock, belonging to the trachyte-group (together with trachyte ss. and latite). Defined by a high portion of feldspar (40-90%) and feldspatoidic minerals (10-60%: analcite, nepheline, leucite, etc.), and very low to negligible quartz content (0-2%). Incorporated dark coloured minerals (0-40%) most commonly include hornblende, olivine, melanite and acmite. The structure is porphyritic with common phenocrysts of sanidine (orthoclase, or Potassium-feldspar) and nepheline. Piezometric level An imaginary water table, representing the total head in a confined aquifer: it is defined by the level to which water would rise in a well. Pyroclastic rocks Group of rocks consisting of volcanic dust, ashes, lapilli and coarse lumps of lava, explosively thrown up in molten condition, and deposited by gravity. Hardened masses of dust, ashes and lapilli are known as tuff, while coarse, consolidated pyroclastic debris is referred to as agglomerate. Porosity The portion of bulk volume in a rock or sediment that is occupied by openings, whether isolated or connected. Pumping test A test that is conducted to determine aquifer and/or well characteristics. Recharge General term applied to the passage of water from surface or subsurface sources (e.g. rivers, rainfall, lateral groundwater flow) to the aquifer zones. Static water level The level of water in a well that is not being affected by pumping (a.k.a. "rest water level") Transmissivity A measure for the capacity of an aquifer to conduct water through its saturated thickness (m2/day). Tuff Hardened volcanic ash. Unconfined Referring to an aquifer situation whereby the water table is exposed to the atmosphere through openings in the overlying materials (as opposed to >confined conditions). Yield Volume of water discharged from a well.
May - 2018 2 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
1. BACKGROUND INFORMATION
1.1 Location
The investigated site is situated at the edge of Eastern Mau Forest. It is located in Kangawa Village, Kangawa Sub-Location, Kuresoi North Location and Mau Summit Division in Molo Sub- County of Nakuru County. The farm is located off the Molo-Londiani tarmac road section – at a distance of about 8km NW of Molo Town on the right hand side of this main road. The land can be accessed through the Kangawa Primary School junction and is about 1.8km west of this junction along a dry weather road. The piece of land measures approximately 2 Acres. It is approximately 60 km from Nakuru town. The area covered in these investigations lays approximately UTM co-ordinates 36M 0799754m E and UTM 9976616m N at an average altitude of 2595m above sea level. The geology of the area is described in the geology of Mau report No. 96 (William, 1991) and the geological map Degree Sheet No. 42, SE quarter and Topographical sheet No. 118/2.
This report presents the results of hydrogeological and geophysical resistivity measurements executed at M/s St Peters Upendo Catholic Church-Kangawa property in Kangawa-Upendo area of Kangawa Sub-location. The results of the investigations are used to determine the geological and hydrogeological characteristics of the underneath strata and identify an optimum borehole site for the community’s domestic water requirements.
1.2 Objectives of the Survey
The Consultant was commissioned by the client to carry out a survey of the project site and subsequently present a hydro-geological report. The objectives of the survey were as outlined below:
i) Carry out a reconnaissance survey at the project site and generate a datum reference for the borehole site investigations; to conform to the WRMA requirements.
ii) Integrate hydrological and hydrogeological information with geophysical borehole data obtained from the field and previous work done to define the recharge/discharge boundaries for the project site.
iii) Undertake comprehensive assessment of the existing borehole facilities located in the neighboring areas with a view to quantify the groundwater potential in the area; and in addition use such data to define the operational aquifer parameters.
iv) Compute the hydraulic parameters of the aquifers in the general Kangawa area, the general Aquifer Transmissivities and the specific capacity of the operational aquifer data.
v) Conduct geophysical measurements and model the data to discern the underlying subsurface conditions and identify the optimum locations for drilling.
vi) Optimize the drilling depth, and in addition re-evaluate the likely aquifer performance in the proposed water supply borehole.
vii) Analyze the above available hydrogeological, geological, geophysical, hydrological data; and subsequently compile a comprehensive hydrogeological report for the project site.
To achieve these objectives, the consultant carried out a detailed review of the existing borehole database located in the immediate vicinity of the project site, for the purposes of statistical
May - 2018 3 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
evaluation of the borehole data, and also to define the general aquifer parameters and characteristics. The current study lays emphasis on the client’s specific water requirements and is geared towards development of a medium capacity borehole system with an estimated design flow of 7.0-10 m3/hr.
1.2 Climate, Drainage and Physiography
Molo's climate is classified as warm and temperate. The winters are rainier than the summers. The climate here is classified as Csb by the Köppen-Geiger system. The temperature here averages 14.1 °C. The average annual rainfall is 1131 mm (Fig. 1). The least amount of rainfall occurs in January. The average in this month is 35 mm. The greatest amount of precipitation occurs in April, with an average of 174 mm. The temperatures are highest on average in March, at around 15.1 °C. The lowest average temperatures in the year occur in July, when it is around 12.9 °C (Fig. 1).
Figure 1: Climate data for the area
The main river flowing down from the Mau Complex is the Molo River that has served citizens of the Rift Valley for hundreds of years. Over it’s approximately 100 km length that the river covers from the Mau Forest to Lake Baringo, this waterway is a primary source of livelihood amongst the communities it flows through. The constituencies that the Molo River serves along its coverage include: Kuresoi, Molo, Rongai, Mogotio, and Baringo Central.
The Physiography of the area is characterised by the higher lying Mau escarpments and valley patterns, grading into plateau like zones before reaching the Rift Valley floor, with the gradient occurring towards the east.
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1.3 Water Demand and existing water supplies.
Currently the Kangawa community does not have any piped water supply. However, the neighbours get their water from a commercial borehole located about 2 km NE of the project site. This is not a feasible water supply, especially for the proposed domestic use due to portability concerns.
The expected water demand has been approximated to be 10 m3/day, for both proposed residential house construction and domestic uses. A borehole with a capacity of 7-10 m3/hr would meet the existing demand, without any negative impacts on the aquifer. The Catholic Diocese of Nakuru, through the Nakuru Defluoridation Co. Ltd has partnered to facilitate a borehole facility and water reticulation system to the Kangawa-Upendo community. The community water development project will be developed in several phases. First, a productive well will be drilled and water pumped to the highest peak in the area. The water will be stored in a reservoir tank, which will be optimally designed by a qualified engineer. The reticulation main lines will then come from the main tank and supply target households in Kangawa-Upendo community.
May - 2018 5 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
2.0 PROJECT APPROACH AND METHODOLOGY
To achieve the set out project objectives, the Consultant adopted a methodology that would exhaustively address all issues regarding groundwater availability, depth and estimate the volume and water quality. Inferences were made to arrive at an optimum site that would ensure cost effectiveness and value for money. The consultant adopted a multi-discipline approach to arrive at the desired conclusion.
Reconnaissance studies were carried out to assess the local and regional factors affecting groundwater. Structural maps were analysed to optimize the borehole site. The site suitability was assessed using desktop studies, geophysical measurements, hydrogeological site assessment and data correlation. First, a few baseline conditions had to be set as boundary conditions in estimating and interpolating some of the aquifer parameters.
Once these baseline conditions were established, the effects of both abstraction to adjacent boreholes and the general impact on the regional and the localized effects on the aquifer system were evaluated.
The recommendations of the drilling procedure lay emphasis on the application of construction methodologies that allow for the attainment of the recommended depths whilst ensuring that aquifer efficiency was safeguarded. This also encompassed designs and sealing requirements of shallow seated aquifers to avoid possible interference with neighboring facilities located within the area.
The results of the project findings were consolidated in this survey report in total conformity to the WRMA requirements. The current study lay emphasis on the client’s specific water requirements and is geared towards development of a medium capacity borehole system with an estimated design flow of 7-10.0 m3/hr for domestic and small holder irrigation use.
Review of existing data and collection of additional data was encompassed in this study. To achieve the defined terms of reference, the Consultant carried out a detailed desktop study of the database inventory for boreholes located in the immediate vicinity of the project area; for the purposes of statistical evaluation of the down-hole borehole data. Satellite images were also employed to define the geological structures influencing groundwater flux. Digital Elevation Model from STRM was employed to identify structural trends. ASTER Infrared images were analysed to determine the surface recharge and discharge channels in the vicinity of the client’s farm. This data was also used to determine the orientation of the Vertical Electric Soundings (VES). Google maps (2016) were used to assess vegetation, administrative boundaries and land divisions in the vicinity and identify the rivers and local place names.
To define the general aquifer parameters and characteristics; it was necessary to review available test pumping data to define the values of transmissivity and specific capacities of the aquifers. Aquifer depth and thickness was also estimated from borehole data and VES soundings. Geological reports sourced from the Mines and Geology Department of the Ministry of Mining was used to assess the geology and general stratigraphy of Kangawa area. The results of this study have been consolidated in to an assessment report conforming to WRA standards; which is a mandatory requirement.
May - 2018 6 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
3.0 GEOLOGY
3.1 Regional Geology of the area.
The Kenyan rift began its development during the late Oligocene at approximately 30 Ma. Magma resurgence created weak points in the crust which later transformed to major volcanic landforms (Baker et al., 1971; Muchemi, 1992). The relatively shallow level of the lithosphere-asthenosphere boundary and Moho beneath the EARS is responsible for the high heat flow and geothermal gradient (200°C/km) within the rift (Wheildon et al., 1994 and Simiyu and Keller, 1997).
Quaternary volcanic activity was focused along the central axis of the rift and crustal thinning may have been responsible for formation of the key volcanic centres, including Eburru, Olkaria, Longonot and Suswa (Mohr, 1970; Mungania, 1999; Chorowicz, 2005). Explosive extrusions also created caldera volcanoes (Simiyu, 2010; Wood and Guth, 2015). Development of shallow magma chambers of intermediate to silicic composition formed the most important geothermal prospects. The volcanoes include Suswa, Longonot, Olkaria, Eburru, Menengai, Korosi, Paka, Silali, Emuruangogolak and Barrier (Clarke et al., 1990; Muchemi, 2000; Omenda, 2014).
3.2 Geological Succession The Mau escarpment is largely composed of the ignimbrite succession dominated by tuffs with only rare outcrops of agglomerates and lavas. The rifting has produced blocks down faulted to the east along the escarpment. The maximum exposed thickness is about 100m. The Rift Valley floor is largely covered with sediments that accumulated in the lakes during the Gamblian stage of the Pleistocene period. They contain a large proportion of their volcanic material and a few diatomaceous beds are known to occur. The rocks found on the Rift floor vary from unsaturated tephrites to highly acid rocks such as rhyolites and sodic rhyolites North to north west trending faults define the eastern and western Rift margins and most of this faulting has probably occurred prior to the development of volcanic centres on the rift floor. Mau tuffs are exposed in the south and west of the Molo area (Fig. 2). These materials comprise the majority of the Mau succession and are of unknown thickness. They represent a prolonged period of pyroclastic activity combined with a moist climate leading to fluviatile, mud-flow and lake bed deposits. Although often well-sorted, these beds are discontinuous; their mapping is further confused by possible subsequent faulting. The pyroclastics and sediments of the plain and Mau slope are locally composed of unconsolidated pumice. Elsewhere however, the Pleistocene to Recent succession is expanded with the inclusion of yellow sediments and white bleached tuffs at the base. Lateral variation from medium grained-pumice to fine-grained clays and sandy soils (produced by weathering of finer ash) occur within this formation. The pyroclastics and weakly consolidated sediments are particularly susceptible to severe erosion. Differential erosion of beds of slightly different compaction has produced steeply terraced walls and pillars, and weirdly sculptured forms.
Recent pyroclastics and sediments of the Mau Slope cover the Molo and Elburgon areas at surface, extending down onto the Rongai Plain to the east of the area (Fig. 2). This formation includes pumice, white bleached tuff and yellow sediments, which has decomposed to fine clay and sandy soils. These materials are at least partly lacustrine in origin.
The black ash of Rongai Plain has a fine-grained porous texture the friable texture and high susceptibility to erosion prevent it forming marked features. The most common exposures are in the floors of riverbeds where it invariably has a sooty waterlogged appearance. The base colour,
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most commonly black, varies locally to dark purple and purple grey with rusty partings. The composition of the ashes include grit-sized pellets of lava, obsidian, plate-like feldspar crystals and fragments of uncompressed pumice which is usually black and green in colour. Highly variable in proportion, but quite common are large glassy bombs within the composition. These bombs are themselves composed of pophyritic obsidian with prismatic feldspar phenocrysts. Underlying the black ashes in the north are the eutaxitic-welded tuffs of the north- east.
Figure 2; Surface geology map of Molo and neighbouring Central Rift regions. The red dot shows the location of the proposed borehole.
3.6 Geological Structures
The study area is part of the Gregory Rift Valley, which is a component of the greater rift system extending from Syria to the Zambezi. It is a multifaceted fault trough with a general north-south orientation (see Fig. 2), the entire system being broken up into oblique structures and other shorter fault troughs normal to the main trend, as demonstrated by Kavirondo Rift Valley.
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The major rift margin faults have surficial throws of more than 400 m that have generated major tilted blocks which act as catchment boundaries for the lake basins. For example, the blocks in the area around Menengai and Bahati are uplifted more than those towards the north or south. Thus, there is a northerly and southerly tilt on the ramps from Menengai-Bahati area. These ramps, assisted by the northerly and southerly tilts believed to be due to uplift and volcanism in the Central Rift Valley floor, serve to collect drainage from the marginal escarpments and channel it into the rift floor basins. The major detachment block in Central Kenya Rift Valley is the Mau detachment on the eastern margin of the area (Bosworth, 1987).
To the west is the Elgeyo detachment that borders the Bogoria-Baringo basins. These major ramp structures direct the major drainage into the widely separated rift floor basins of Turkana and Natron, leaving the Mau detachment lakes with little surface recharge. The Molo River, for example, drains parts of Mau escarpment into Lake Baringo, bypassing lakes Nakuru and Bogoria in the Central Rift Valley floor. The Kerio River, on the other hand, is assisted by ramps against the Elgeyo escarpment formed by Kamasia and Tugen hills, and flows north into Lake Turkana, instead of south into Lake Baringo or Lake Bogoria.
4.0 HYDROGEOLOGY
The hydrogeology of an area depends on the nature of the parent rocks, structural geological features, degree of weathering, rate of evapotranspiration, the form and frequency of precipitation.
The upland areas of Mau and Bahati are often associated with high rainfall, which are the main sources of groundwater recharge for the lowland aquifers. Groundwater occurrence is greatly determined by the geological conditions and tectonics. Structural features such as faults often optimize storage, transmissivity and recharge significantly occurring in areas adjoining surface drainage system as seen in streams disappearing beneath land surface as typified by River Ngosur. Shallow groundwater table, low rainfall and moderately low values of recharge characterize the low lying areas.
Geological formations in the Molo area are restricted to the Cainozoic volcanic rocks (i.e. Tertiary-Quaternary) as well as sedimentary deposits, essentially of lacustrine origin, extending in time of deposition from the Tertiary era to the present day. The volcanic formations consist of pyroclastics and trachyte flows, together with intercalated tuffs and reworked tuffs holding abundance of water rounded fragments. Welded tuffs are widespread throughout the area depicting a grey glassy matrix choked with pyroclastic fragments and are trachytic in composition; their impervious nature is analogous to that of trachyte lava flows. The main aquifer is the lacustrine volcanic/sedimentary series usually occurring as fractured or reworked volcanics, or along the weathered contacts between lithological units.
4.1 Surface Water Resources.
The main surface water supply in the catchment is the Molo River, whose source is on the upper catchment in the Molo and Kuresoi areas (Fig. 3). The many streams that begin in the Mau Complex flow into the Molo River and are depended upon all the way down to Lake Baringo. An important aspect of the upper catchment is the Mau Complex, which is among the major water towers in the country with numerous rivers, other than Molo, emanating from it such as, Njoro River and Mara River (Fig. 3).
The Mau Complex plays an important hydrological function, as the underground water that the area brings is crucial in supplying many rivers that use this water to maintain their flow. It is important to focus on the upper catchment when viewing the Molo River as a whole, because of
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the effects that problems upstream have to the rest of downstream communities. Additionally, it is important for those downstream communities to understand the issues that are faced upstream, so that all communities involved can work together to conserve the environment.
Upper Molo River catchment has experienced rapid land use and land cover changes. Research studies show that forest was reduced by about 48% to agricultural land which increased from 27.4% to 41% and settlement from 14.6% to 21.5% in the period between 1986 and 2001 with significant change noted to have occurred in the period between 1995 and 2001 (Kirui, 2008).
Figure 3; Surface water resources in the larger Mau catchment region (BBC, 2018)
4.2 Groundwater System.
4.2.1 Rainfall, Percolation and Recharge
Assuming that suitable storage media exist below the ground, aquifer potential is also affected by the mechanisms of percolation of rainfall or river water down to the aquifer. If the infiltration capacity is low due to the presence of an aquiclude like clay, the recharge to the aquifer is low.
Percolation will depend on the soil structure, vegetation cover and the permeability of the rocks. Clayey formations naturally inhibit percolation. Aquifers may also be recharged laterally if the rock is permeable over a wide area.
From modeled data obtained from Njoro University, there was high surface runoff for 1982 which agrees well with the average increase in rainfall received in the catchment for that year
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(Kirui, 2008). In 1980 there was low surface runoff due to low rainfall as shown in the rainfall curve. In Figure 4 the surface runoff and rainfall followed the same trend. In 1988 there was high rainfall, but low surface runoff was simulated, this could be attributed to error in simulation. The model shows that runoff is directly proportional to rainfall. It is however not possible to accurately estimate percolation.
Figure 4; simulated surface runoff for the period 1980 to 1989 (Kirui, 2008).
4.2.2 Mau Escarpment Faults and Hydrogeology:
The Rift structure and the associated fault systems have a substantial effect on the groundwater flow system.
Faults may facilitate flow by providing channels of high permeability or may be barriers by offsetting zones of high permeability. On the Mau fault scarp, the block faults can be inferred to constitute hydraulic boundaries where horizontal flows predominate along the fault systems.
4.2.3 Borehole Data
The following is a summary of the hydrological data collected from the site through borehole drill logs and data base inventory. Source: Ministry of Water Resources Data Bank and from field observations.
They generally reflect the general ground water potential in the areas close to the Mau catchment fault scarp. The borehole data in the table below attests to the high prolific aquifer in this area. Note that most of the boreholes were drilled during the colonial period, using outdated and archaic methods. Their true productivity may therefore not be reflected by the displayed data, since the boreholes were poorly constructed. The aquifer development may have also been done poorly (Table 1). Comparing this data to more recent boreholes in the vicinity, the older production values are up to three times lower.
The borehole logs show water strikes of 66 – 74m (Table 1). The first aquifer occurs at the contact between pyroclastics and trachytes, while the second aquifer occurs in the fractured Molo Tuffs comprised of welded tuffs/ignimbrite succession and paleosols at a depth range between 100-130m.
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Table 1; Existing Borehole Data.
ID OWNER & COMPLETION TOTAL WSL WRL YIELD BEARING DATE DEPTH (m) (m) (m3/hr) (m) - Wanyoko; 0.4 km E 1977 60 - - - Mintos Co. Ltd; 0.6 Km - 87 - - - - E Maridas Farm; 3.3 Km July, 1951 182 - 37 0.9 351 SE Saitoti George; 2.9 Km March, 1987 87 74 15 - 737 SW DWD; 3 Km SW Jan, 1948 94 66 24 - 793
Boreholes in the area have been drilled to total downhole depth range of 60-182m bgl. The water table is relatively shallow around Kangawa area necessitating shallow drilling. Due to the step normal faulting along the rift shoulders, water levels become relatively deeper towards the escarpment, due to vertical and lateral displacement of aquifers. This requires that boreholes drilled along the rift flanks to be deeper to penetrate the deeper, displaced aquifers.
There are two boreholes that are in close range with the proposed borehole. The boreholes are within the 800m radial distance recommended as a buffer zone by the Water Resource Authority. To avoid aquifer interference, we recommend that the proposed borehole be drilled to a deeper depth of 150-200m. Shallow aquifers above 90m should be sealed off to avoid aquifer sharing and subsequent depletion. This is possible since the Wanyoko and Mintos boreholes have been drilled at nominal depths of 60 and 87m respectively. The proposed borehole will therefore abstract water from deeper aquifers from 130m. The deeper aquifers are underutilized and well recharged by the Mau catchment zone. They are therefore suitable for supplying domestic water to the community.
4.4 Groundwater Storage, Recharge and Discharge
From available geological and hydrogeological information, it is observed that the main water bearing rocks extend to the east and south towards the tuffs, trachytes and sediments. It is likely that an interconnected system exists of the fracture and sediment aquifers extending over a wide area. The aquifers are invariably semi-confined and it is therefore likely that a significant amount of the recharge takes place west of the area where the catchment is extensive.
4.5 Aquifer Parameters in the Study Area
This section briefly describes some of the most important aquifer characteristics of the aquifer rocks and attempts to give estimated quantitative measures of the different parameters. However, most data is lacking from drilled wells which would provide critical information. An attempt has been made to decipher the missing links.
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4.5.1 Transmissivity
The transmissivity is the product of the average hydraulic conductivity (or permeability) and the thickness of the aquifer. Consequently, it is the rate of flow under hydraulic gradient equal to unity through a cross-section of unit width over the whole thickness of the aquifer. It is designated by symbol KD or T. It has the dimension of length3/Time x Length or length²/Time and is, for example, expressed in m²/day.
For practical purposes, Transmissivity is calculated using the formula T=0.183 Q/S. However, this formula is applicable where well tested data is available in log scale. This log scale are unfortunately not available from Ministry of Water and Irrigation data base; where instead a summary sheet is used only to show only the pumped water and discharge; as part of the limited data that is provided. This has a limit to the use of the above formula.
To ameliorate this short coming an estimate of the transmissivity is made using the Logan’s formula (Logan, 1964) i.e. T =1.22 Q/S. Based on the existing test pumping data, several boreholes in the area have a maximum discharge of approximately 80m3/day and a maximum drawdown of about 8m. T =1.2*80/3= 32m2/day. It is worthwhile to note that transmissivity values calculated using Logan’s formula may in most cases lead to an overestimation, but nonetheless it provides a fair indication of this aquifer characteristic.
4.5.2 Hydraulic Conductivity
Hydraulic conductivity is the measure of the ability of the soil to transmit water and depends upon both the properties of the soil and the fluid. Figure 5 below shows the spatial distribution of hydraulic conductivity over the study area. Each soil type under FAO classification has been assigned a hydraulic conductivity value. This was used in developing the hydraulic conductivity map (Kirui 2008).
Figure 5; Map showing the distribution of Hydraulic conductivities in the area.
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However, a simple estimation of hydraulic conductivity can be derived from the formula T=k d which can be rearranged to k=T/D, where k is the hydraulic conductivity, T is the transmissivity, and D is the aquifer thickness.
On the other hand groundwater flux can be estimated using the Darcy’s formula of Q=T.i. W, where T is the transmissivity of the borehole, I is the gradient and W the width. The slope, ¡ is calculated from the hydrostatic head difference between the Boreholes, i.e. ¡=h2-h1/L, where L is the distance between the two boreholes in metres.
Within hydro geological terms the actual aquifer yields are interchangeable with the aquifer transmissivity. High yields are indicative of high aquifer transmissivity and the vice versa.
4.5.3 Safe Yield
The aquifer in the investigated area is vast and thus, the annual average deep-percolation in this area has been estimated to be sufficient to yield the planned yearly groundwater abstraction of 7,300 m3/year from the aquifer system. Thus, even when there is no recharge of the aquifer from the precipitation, the proposed amount of water can be abstracted without changing the groundwater level dramatically. During dry periods, the natural lateral groundwater flow into the aquifer is estimated still to be higher than the proposed groundwater abstraction.
4.5.4 Specific Capacity and Specific yield/storage coefficient
This is a crude indication of the efficiency of the borehole as an engineered structure, and is calculated by dividing the discharge rate (as m3/day) by the total drawdown. High specific capacities generally indicate high transmissivities, low specific capacities the opposite. The specific capacity (yield-drawdown ratio) of the borehole is expected to be high and to decrease gradually at increasing abstraction rates. The water bearing layers are expected to be fractured volcanics and sediments which have medium to high hydraulic conductivity (K).
The storage coefficient and specific yield are both defined as the volume of water released or stored per unit surface area of the aquifer per unit change in the components of head normal to that surface. Both are designated by symbol S and are dimensionless (i.e. have no units).
The storage coefficient refers only to the confined parts of an aquifer and depends on the elasticity of the aquifer material and the fluid. It has an order of magnitude of 10-4 to 10-6
The specific yield refers to the unconfined parts of the aquifers. In practice, it may be considered equal to the effective porosity or drainable pore space because in unconfined aquifers, the effects of elasticity may be the order of 0.1 to 0.2 (Table 2).
Table 2; Specific yield ranges (after, Driscoll 1986) sediment Specific yield, % clay 1 - 10% sand 10 - 30% gravel 15 - 30% Sand and Gravel 15 - 25% Sandstone 5 - 15% Shale 0.5 - 5% Limestone 0.5 - 5%
The main aquifer of target in the study area consists of weathered volcanics and old land surface sediments marking the different emplacement episodes for the lava flows.
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4.5.6 Groundwater Flux/recharge and Discharge Dynamics
Groundwater flux can be estimated using the Darcy’s formula of Q=T.i.W, where T is the transmissivity of the borehole, i is the gradient and W the aquifer width. The gradient, i is calculated from the hydrostatic head difference between two Boreholes i.e. i =h2-h1/L, where L is the distance between the two boreholes in metres. However, due to lack of hydrostatic head data, these calculations could not be accurately calculated.
4.5.7 Recharge
The rate of recharge within Molo area aquifer cannot be accurately quantified, thus, the mechanism by which recharge occurs can only be postulated. The two possible recharge mechanisms are direct recharge at the surface and indirect recharge via faults or lateral water movement through the homogeneous aquifer beds.
Assuming that suitable storage media exist below the ground, the aquifer is affected by the downward percolation of rainfall or river water to the aquifer. If the infiltration is low due to the presence of an acquiclude like clay, the recharge to the aquifer is low.
Percolation will depend on the soil structure, vegetation cover and the state of erosion of the parent rock. Rocks weathering to clayey soils naturally inhibit percolation. Aquifers may also be recharged laterally if the rock is permeable over a wide area.
Some of the rainwater is also conducted underground by local faults though in the study area, no faults have been mapped on the surface. This mechanism is particularly important for recharge to the deep aquifers, which provide stable groundwater supplies.
The most important recharge area for the entire Molo aquifers is formed by the forested landscapes of the Mau forest. Here water percolates directly into the faults and cracks within the Pleistocene formation through which deeper and adjacent units are recharged over time.
Rainfall in Molo area is high ranging from 1,100 – 1,300mm/year. The amount of percolation and groundwater flow is known to be very significant e.g. the Molo River may yield 2,500m3/day during the heavy spell rainy season, which is quite a substantial flow.
4.5.8 Discharge
Discharge from aquifers is either through natural processes as groundwater base flow to streams and springs or artificial discharge through human activities.
A number of boreholes have been drilled in the Mau catchment area, rendering the artificial component comparatively higher than the natural outflow from the groundwater store.
The total effective discharge from the general Molo aquifer via either of the above means is not known. Increased consumer pressure and demand for groundwater will inevitably lead to impact of the aquifer systems but not in the very near future. The latter will take toll on the groundwater system as a result of increasing water demand for irrigational purposes as well as the enormous destruction of the catchment areas. Lake Bogoria basin is under threat of extinction and explains the impacts of the above factors.
4.6 Groundwater Quality
Intense weathering and/or hydrothermal discharges, coupled with the mainly alkaline petrology and volcanic and hydrothermal terrain, have produced regional water quality problems, the most
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important of which is an excess of fluoride (Edmunds, 1996). Many of the lakes of the Kenya Rift Valley system, especially the soda lakes, have extremely high fluoride concentrations: 1640 and 2800 mg L-1, respectively, in lakes Elmentaita and Nakuru (Nair et al., 1984). Similarly, high fluoride levels in groundwater (Nair et al., 1984), and the use of these waters as a potable source, has led to high prevalence of dental fluorosis (Manji & Kapila, 1986). In the lowlands, the water quality concerns revolve around both high fluoride and salinity concentrations, especially around lakes Baringo and Bogoria, as well as high temperatures of groundwater.
Fluoride in groundwater is contributed to by runoff, leaching and cation exchange in soils on the surface, and by similar processes acting in the soils and rocks. There is also some fluoride contribution attributed to interaction with deeper geothermal-related waters. The high level of nitrates in groundwater, likely as a consequence of fertilized agriculture, suggest that nitrates are introduced into the groundwater systems via infiltration through the soil horizon – overpumping may also result in a faster water infiltration rate, reducing the capacity of the soil’s natural filtering mechanisms to mitigate nitrate levels in groundwater. Some other inorganic constituents of agricultural additives include Cl, K, Ca, Mg, S, and a variety of minor elements (Böhlke, 2002).
Agricultural sources of some of these elements can dominate natural sources locally, and furthermore, agricultural effects on the recharge fluxes of various ions like NO3 - and H+ can cause changes in weathering rates and ion-exchange equilibria in the subsurface, thereby altering indirectly the concentrations of other constituents in groundwater (Böhlke, 2002). These indirect effects have important implications for geochemical water–rock interactions and can represent sources or sinks for a variety of problematic contaminants such as nutrients and toxic trace elements (Böhlke, 2002). However, anoxic conditions in groundwater aquifers may favour the reduction of nitrate to harmless nitrogen gas (Edmunds, 1996). Fluoride content is expected to be low at the catchment areas, where groundwater has not interacted so much with the weathered rocks. At higher elevations in the catchment, there is minimal interaction with deeper circulating geothermal brines which are enriched with salts. Areas close to the vicinity of the rift basins, along Lake Bogoria, have groundwater which exceeds the maximum WHO recommended fluoride level of 1.5ppm (Table 3). This is however expected with deeper volcanic geothermal aquifers.
High fluoride intake, especially by growing infants, may cause dental or skeletal flourosis. Over a short time span, the consumption of water with excess fluoride is not necessarily harmful to adults, especially if they rely on alternative sources for their main drinking water purposes. Again the level of 7.0 Ppm is not grossly excessive as to constrain the water usage.
In general and over most areas within the Molo volcanic system, the waters are suitable for domestic uses, with their concentrations falling within the WHO standards guidelines (table 3).
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Table 3; Ionic Concentration: WHO & Various Authorities
World Health Organization: European Community: 1983 1971 Int. EC Directive 1980 relating to the quality Guidelines Standard of water intended for human consumption
Substance or Guidelines Upper limit Guide level Max. Admissible Characteristic Value (GV) (HL), tentative (GL) Conc. (MAC) Inorganic Constituents of health significance: Antimony Sb 0.01 Arsenic As 0.05 0.05 0.05 Cadmium Cd 0.005 0.01 Chromium Cr 0.05 0.05 Cyanide CN 0.10 0.05 0.05 Fluoride F 1.5 1.7 1.5 Lead Pb 0.05 0.10 0.05 Mercury Hg 0.001 0.001 0.001 Nickel Ni 0.05 Nitrates 10(as N) 45 (as NO) 25(as (No) 50 (as NO) Selenium Se 0.01 0.01 ______Other Substances GV: Highest Max. GV MAC Desirable Permissible Level Level:
Aluminum Al 0.20 0.05 0.20 Ammonium NH 0.05 0.50 Barium Ba 0.10 Boron B 1.0 Calcium Ca 75 50 100 Chloride Cl 250 200 600 25 Copper CU 0.05 0.10 Hydrogen Sulphide H2S. ND ND Iron Fe 0.30 0.10 1.0 0.05 0.20 Magnesium Mg 0.10 30 150 30 50 Manganese Mn 0.10 0.05 0.50 0.02 0.05 Nitrite No 0.10 Potassium K 10 12 Silver Ag 0.01 Sodium Na 200 20 175 Sulphate Soq 400 200 400 25 250 Zinc Zn 5.0 15 0.10 Total Dissolved solids 1000 500 1500 1500 Total Hardness as CaCo3 500 100 500 Colour Hazen 15 5 50 1 20 Odour Inoffensive Unobjectionable 2 or 3 Ton Taste Inoffensive Unobjectionable 2 or 3 Ton Turbidity(JTU) 5 5 25 0.4 4 PH 6.5-8.5 7.0-8.5 6.5-9.2 6.5-8.5 9.5 (max) Temperature °C 12 25 EC us/cm 400 Notes ND-Not Detectable IO-Inoffensive GL-Guide Level UO-Unobjectionable
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5.0 GEOPHYSICAL INVESTIGATION METHODS
Investigations of the groundwater resources at Kangawa in Mau Summit Division included the use of geophysical techniques to probe the sub-surface. A variety of methods are available to assist in the assessment of geological sub-surface conditions. The main emphasis of the fieldwork undertaken was to determine the thicknesses and composition of the sub-surface formations and to identify water- bearing zones.
This information was principally obtained in the field using vertical electrical soundings (VES) with the SAS 1000 terrameter.
The VES probes the resistivity layering below the site of measurement. This method is described below.
5.1 Resistivity Method
Vertical electrical soundings (VES) were carried out to probe the condition of the sub-surface and to confirm the existence of deep groundwater. The VES investigates the resistivity layering below the site of measurement. This technique is described below.
5.2 Basic Principles
The electrical properties of rocks in the upper part of the earth's crust are dependent upon the lithology, porosity, the degree of pore space saturation and the salinity of the pore water. Saturated rocks have lower resistivities than unsaturated and dry rocks. The higher the porosity of the saturated rock, or the higher the salinity of the saturating fluids, the lower the resistivity. The presence of clays and conductive minerals also reduces the resistivity of the rock.
The resistivity of earth materials can be studied by measuring the electrical potential distribution pro- duced at the earth's surface by an electric current that is passed through the earth. The resistance R of a certain material is directly proportional to its length L and Cross-sectional area A, expressed as:
R = Rs * L/A (in Ohm)
Where Rs is known as the specific resistivity, characteristic of the material and independent of its shape or size.
With Ohm's Law,
R = dV/I (Ohm)
Where dV is the potential difference across the resistor and I is the electric current through the resistor. The specific resistivity may be determined by:
Rs = (A/L) * (dV/I) (in Ohm m)
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5.3 Vertical Electrical Sounding (VES)
When carrying out a resistivity sounding, current is led into the ground by means of two electrodes. With two other electrodes, situated near the centre of the array, the potential field generated by the current is measured (Figure 6).
From the observations of the current strength and the potential difference, and taking into account the electrode separations, the ground resistivity can be determined.
During a resistivity sounding, the separation between the electrodes is step-wise increased (in what is known as a Schlumberger Array), thus causing the flow of current to penetrate greater depths. When plotting the observed resistivity values against depth on double logarithmic paper, a resistivity graph is formed, which depicts the variation of resistivity with depth. This graph can be interpreted with the aid of a computer, and the actual resistivity layering of the subsoil is obtained. The depths and resistivity values provide the hydrogeologist with information on the geological layering and thus the occurrence of groundwater.
Figure 6: An illustration on the principles of resistivity measurements
5.4 Horizontal Resistivity Profiling
When carrying out the resistivity profiling, electric current is similarly led into the ground by means of two electrodes. With two other electrodes situated near to and equidistant from the centre of the array, the potential field generated by the current is measured along the profile line.
Two different configurations were employed for the present study. In the first one, the interval between each consecutive measuring point along the profile was 25 meters. Horizontal resistivity profiling using the wenner array was executed. The current electrode spacing applied was 150 meters, while the potential electrodes were spaced at 50 meters. The choice of the electrode spacing was made after carefully studying the geology and structure and having an estimation of the depth of the bedrock.
The results of the profile measurements reflect lateral changes in resistivity, corresponding to variations in lithology, salinity, thickness of formations, water content at a fixed depth along the profile. Resistivity profile data is normally interpreted qualitatively. The results are presented graphically in the respective sub-sections.
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5.5 Field Work
Fieldwork for this project was carried out on 31st May, 2018. The desk study and the compilation of the report were carried out from 3rd to 12th June, 2018.
The fieldwork involved a geological reconnaissance survey of the project site and the surrounding area and selecting the location for carrying out the Geophysical measurements (Vertical electrical sounding)
The geophysical procedure in a conventional survey methodology is used to delineate the sub- surface geological stratification and structure. The geophysical surveying method used was the combined Werner profiling and geo-electrical (resistivity) sounding. In this method, electric current injected into the ground by means of two electrodes reveal resistivity variations caused by geological and /or hydro-geological conditions of the sub-surface. A second pair of electrodes at the surface, which measure ground potentials, measures these patterns.
The data thus obtained can then be interpreted by use of relevant computer software to reveal sub-surface resistivity anomalies, and the associated hydro-stratigraphy.
One Vertical Electrical Sounding (VES) was carried out on the project site. The maximum electrode spacing used was AB/2 = 200m.
The interpretation of electrical resistivity field data is given on table 4 below, with the inferred geological layering inferred.
Three distinct groundwater manifestations had to be investigated: -
1: Infiltration waters are mainly collected by the down throw depressions but substantial groundwater storage is noted not to be within economic drill depths
2: Enhanced storage in fault scarp fracture systems.
3: Possibilities of the sediments between the lavas forming good aquifers within economic drill depth of boreholes.
5.6 Resistivity Sounding (VES) Interpretation
Interpreted results of the vertical electrical soundings are shown in Tables 4. While the resistivity curves are presented in the annexes.
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Table 4; VES- 1 & 2 interpretations
VES 1 Depth (m) Resistivity (Ω.m) Formation 0.0 – 0.5 200 Red loamy volcanic soils. 0.5 –1.00 100 Weathered volcanic rocks. 1.00 – 3.00 70 Mau Pyroclastics 3.00 – 8.00 20 Weathered pyroclastics 8.00 – 130.00 18 Pumice and pyroclastics >130.00 10 Trachytes (Main aquifer)
VES 2 Depth (m) Resistivity (Ω.m) Formation 0.0 – 0.5 250 Red loamy volcanic soils. 0.5 –1.50 120 Weathered volcanic rocks. 1.50 – 4.00 80 Mau Pyroclastics 4.00 – 15.00 20 Weathered pyroclastics 15.00 – 130.00 18 Pumice and pyroclastics >130.00 15 Trachytes (Main aquifer)
The VES electric trenching were carried out up to 200 metres. The VES were conducted using the Schlumberger electrode array. The sounding curves were interpreted using the GEWIN Nile Software and the smoothed curves obtained annexed in appendix 1. The sounding curve shows variations in downhole lithologies with the water bearing rocks exhibiting very low resistivity.
The top soil has a very high resistivity probably due to its unconsolidated nature and low moisture content. The clayey sub layer has a lower resistivity due to its wet nature and low grade alteration. A high resistive layer occurs beneath the clay from a depth of 0.6-1.5m bgl. This layer is composed of dry pyroclastic rocks, which are poor electricity conductors.
Pyroclastics are thick and occur from a depth of 1.5m to 130m bgl. They conduct groundwater around Kangawa area forming the minor aquifer from a depth of between 90-130m. They are poor water retainers due to their highly porous nature. Most of the fluid is stored at the contact between pyroclastics and trachytes at depths greater than 130m bgl.
From the correlation and comparison of the two pseudo-section (See Appendix 1) VES soundings carried out at the site, it has been definitely possible to recommend drilling of the borehole at VES 2, to a minimum depth of 150 meters and maximum depth of 200 meters. Analysis of existing data proves that the study area is suitable for drilling a borehole, which will not affect the neighbouring wells. Good quality water is also expected at this site.
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6.0 CONCLUSIONS & RECOMMENDATIONS
The regional groundwater aquifer system in the area is interconnected through a network of fissures and porous material, which partly comprises the sediment of the Lake sediments that underlie the volcanics. This system is mainly recharged in the high areas of the Rift Valley flanks where rainfall is high and water enters through permeable lava beds through which it sinks beneath the intermediate slopes of the ridge. The hydraulic gradient is high and favorable.
There are no known or established parameters of aquifer deterioration or depletion within borehole systems located in Molo area.
The hydraulic flow within this sequence of permeable sediments trends in the NW-SE, and with the higher permeability emanating from the porous media, the hydraulic gradient is expected to be high.
With this type of gradient, the shape of the cone of depression for a pumped borehole will be elongated along NW-SE in the direction of the recharge source.
In both situations, any boreholes that may be sited near the proposed boreholes can be pumped with maximum drawdown without any major impact on the aquifers system.
At medium ranked capacity of 7-10m3/hr, - balanced in pumped boreholes there can be no major depression of the respective cone of depression. The confining properties between the lava flows and the Tuffs will tend to confine the sediments aquifer and significantly reduce the possible static water levels (Tendency towards a shallow SWL implied).
Three distinct groundwater manifestations have been inferred from data analysis and structural geology of the area;
1: Infiltration waters are mainly collected by the down throw depressions but substantial groundwater storage is noted not to be within economic drill depths
2: Enhanced storage in fault scarp fracture systems.
3: Possibilities of the sediments between the lavas forming good aquifers within economic drill depth of boreholes.
Good water prospects exist at the project site with optimum conditions of transmissivity and storage being likely to be found within the sediments inter-bedded with the tuffs and Pyroclastics and sands. The likely occurrence of a lower compact confining boundary comprised of compact Ignimbrite/Molo tuffs will enhance the confining properties of the fractured contact aquifer at depth and thus down hole pumping heads are bound to be quite optimum. A borehole drilled at the surveyed prospects are likely to exhibit a yield of over 7.0m3/hr; that could meet the specific water requirements for the proposed community water project. The catchment factors and the recharge elements are quite optimum for the proposed groundwater development program.
From the forgoing conclusions, the study recommends drilling of 1 borehole at the location of VES 2 with UTM coordinates 36M 0799758m E and 9976624m N and, degrees 00° 12’ 39.6” S & 35° 41’ 33” E, with an altitude of 2595m to a minimum depth of 150m to a maximum depth of 200 metres. The proposed facility is expected to attain yields able to augment the community daily requirement of 20m3/day, for domestic purposes only. The borehole is to be drilled at a nominal diameter of 203mm drilling diameter with a nominal casing diameter of - 153mm.
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There are two boreholes that are within the 800m radial distance to the proposed borehole. To avoid aquifer interference, we recommend that the proposed borehole be drilled to a deeper depth of 150-200m. Shallow aquifers above 90m should be sealed off to avoid aquifer sharing and subsequent depletion.
To conform to specific Water Resources Authority construction requirements the following pertinent structures will constitute part of the proposed borehole works.
A monitoring tube is to be installed in the drilled intake to allow regular measurements of the water levels in the intake wells. This is a requirement for the final pumping equipment installation.
In case shallow aquifers are encountered it is recommended to seal these off within the upper 10 metres, in order to avoid any risk of chemical contamination. The potent risks of shallow aquifer contamination are practically possible and thus the need to installing a clay or cement seals in the annular space between the borehole wall and the metal casing to achieve the expected sealing tightness is recommended as part of the construction requirement.
Screens should only be installed at the deeper aquifers. We emphasize the use of plasma slotted pipes with enhanced density of slots to improve the lifespan of the bore.
The recommendations on well construction cannot be considered complete without the mention of the requirement to test pump the water supply bore to British standards BS 6316 (1992), which is an industry standard. This standard generates qualitative and quantitative discharge variations over time in response to the abstraction. At least 10hours of the step test at –2-hour interval followed by a CRT test for 30 hours is recommended. Recovery must be carried out to full Static Water Levels.
The drilling should ideally be carried out with a Rotary drilling plant rotary in order to attain the recommended maximum drill depth of 200m below ground level,
Great care should be taken over the completion and development of the well. This latter aspect is too often neglected in well construction in Kenya, which often leads to a reduction in maximum possible yield and a decrease in the hydraulic efficiency of the screens – which can mean higher pumping costs and shorter well life.
There is the need to define the Deepest Advisable Pumping Water Level (DAPWL) in the screens design during construction. The ultimate scenario without a properly defined DAPWL is the depreciation of the pumping water levels to the screened areas that leads to screen encrustations due to air exposure and water damp.
The important element for this drilling project is to ensure full penetration of the deep sections of the deeper fracture type aquifer to attain high and sustainable aquifer flows.
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REFERENCES
Baker, B.H., Williams, L.A.J., Miller, J.A., Fitch, F.J., 1971; Sequence and geochronology of the Kenya rift volcanics. Tectonophysics 11, 191–215.
Clarke, M. C. G., Woodhall, D. G., Allen, D. & Darling, G. (1990); Geological, volcanological and hydrogeological controls on the occurrence of geothermal activity in the area surrounding Lake Naivasha Kenya, 1–348. Ministry of Energy, Nairobi, Kenya and British Geological Survey, UK.
BBC, 2018; http://news.bbc.co.uk/2/hi/africa/8057316.stm, accessed on 11th June, 2018
Driscoll F.G., 1986; Groundwater and Wells, 2nd Ed. Johnson Division.
Ghosh, D P (1971); Inverse filter coefficients for the computation of apparent resistivity standard curves for a horizontally stratified earth. Geophysical Prospecting. v. 19, pp. 769-775.
Kirui Wesley K., 2008; Analysis of catchment hydrologic response under changing land use: the case of upper molo river catchment, kenya, MSc. Thesis, Egerton University, pp 85.
Ministry of Water & Irrigation borehole data bank
Muchemi G.G., 2000; Conceptual model of Olkaria geothermal field, KenGen, Kenya, KenGen internal report.
Odada, E. O. & Olago, D. O. (2005); Holocene climatic, hydrological and environmental oscillations in the tropics with special reference to Africa. In: Climate Change and Africa (ed. by P. S. Low), 3–22. Cambridge Univ. Press, Cambridge, UK.
Olago, D. O., Umer, M., Ringrose, S., Huntsman-Mapila, P., Sow, E. H. & Damnati, D. (2007); Palaeoclimate of Africa: an overview since the Last Glacial Maximum. In: Global Change Processes and Impacts in Africa: A Synthesis (ed. by L. Otter, D. O. Olago & I. Niang), 1–32. East African Educational Publishers, Nairobi, Kenya.
Omenda Peter, 2014; Geothermal Country Update Report for Kenya: 2014, Presented at Short Course IX on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Nov. 2-24, 2014.
Simiyu and Keller, 1997; An integrated analysis of the lithospheric structure across the east African plateau based on gravity analysis and recent seismic studies. Tectonophysics Vol. 278.
Williams L.A.J, 1991; Geology of the Mau area. Report no. 96, Mines and Geology Department, Government Press. Pp 55.
Whieldon et al. (1994); Heat flow in the Kenya Rift zone. In Crustal and Upper Mantle Structure of the Kenya Rift. Tectonophysics, 236, 131-149 pp.
May - 2018 24 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Appendix 1: Geophysical Data Curve
Depth [m] resist. [Ohm.m] 1.60 114.71 0 2.00 79.06 0 2.50 73.32 0 3.20 69.08 0 4.00 54.45 0 5.00 47.86 0 6.30 38.45 0 8.00 26.60 0 10.00 26.35 0 13.00 22.42 0 16.00 20.16 0 20.00 16.90 0 25.00 17.09 0 32.00 17.90 0 40.00 18.86 0 50.00 17.38 0 63.00 17.21 0 80.00 19.80 0 100.00 18.21 0 130.00 21.19 0 160.00 17.59 0 200.00 14.48 0 .00 .00 0
Depth [m] resist. [Ohm.m] .50 200 1.00 100 3.00 70 8.00 20 130.00 18 >130.00 10
May - 2018 25 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Depth [m] resist. [Ohm.m] 1.60 151.01 0 2.00 115.64 0 2.50 120.82 0 3.20 103.62 0 4.00 94.05 0 5.00 85.38 0 6.30 67.34 0 8.00 48.28 0 10.00 39.13 0 13.00 31.69 0 16.00 28.43 0 20.00 22.69 0 25.00 21.25 0 32.00 19.59 0 40.00 19.90 0 50.00 19.13 0 63.00 20.82 0 80.00 21.62 0 100.00 20.30 0 130.00 20.46 0 160.00 17.32 0 200.00 16.69 0
Depth [m] resist. [Ohm.m] .50 250 1.50 120 4.00 80 15.00 20 130.00 18 >130.00 15
May - 2018 26 Kangawa Area, Molo Sub-County Resistivity Pseudo-section – the blue zone shows the low resistivity areas, where high possibility of striking water exists. The left hand scale represents depth below ground level, while the right hand side has the resistivity colour scale and values.
Appendix 2: Drilling
Drilling A groundwater drilling permit should be obtained from the Water Apportionment Board of the Ministry of Water Resources.
Drilling Technique
Drilling should be carried out with an appropriate tool - either percussion or rotary machines will be suitable, though the latter are considerably faster. Geological rock samples should be collected at 2 meter intervals. Water struck levels, water rest levels and estimates of the yield of individual aquifers encountered should also be noted.
Well Design
The design of the well should ensure that screens are placed opposite the optimum aquifer zones. The final design should be made by an experienced hydrogeologist.
Casing and Screens
The well should be cased and screened with good quality material; considering the depth of the boreholes and the water demand, it is recommended that the borehole should be installed with 8" diameter uPVC casings and screens with slots of 1.5 mm and high percentage of open surface area. Alternatively, the borehole should be installed with 8” diameter epoxy or bitumen coated steel casings and Johnsons screens.
We strongly advise against the use of torch-cut steel well-casing as screen. In general, its use will reduce well efficiency (which leads to lower yield); increase pumping costs through greater drawdown; increase maintenance costs; and may even reduce the potential effective life of the well.
Gravel Pack
The use of a gravel pack is recommended within the aquifer zone, because the aquifer could contain sands or silts which are finer than the screen slot size. A 12" diameter borehole screened at 8" will leave an annular space of approximately 2", which should be sufficient. Should the slot size chosen be too large, the well will pump sand, thus damaging the pumping plant, and leading to gradual `siltation' of the well. The grain size of the gravel pack should be an average 2 - 5 mm.
Well Construction
Once the design has been agreed upon, construction can proceed. In installing screen and casing, centralizers at 6 meter intervals should be used to ensure centrality within the borehole. This is particularly important to insert the artificial gravel pack all around the screen. Gravel packed sections should be sealed off top and bottom with clay (2 m).
Well Development
Once screens, pack, seals and backfill have been installed, the well should be developed. Development aims at repairing the damage done to the aquifer during the course of drilling by removing clays and other additives from the borehole walls. Secondly, it alters the physical characteristics of the aquifer around the screen and removes fine particles.
We do not advocate the use of overpumping as a means of development since it only increases permeability in zones which are already permeable. Instead, we would recommend the use of air or water jetting, or airlifting, which physically agitates the gravel pack and adjacent aquifer material. This is an extremely efficient method of developing and cleaning wells.
Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Well development is an expensive element in the completion of a well, but is usually justified in longer well- life, greater efficiencies, lower operational and maintenance costs and a more constant yield.
Schematic design for borehole completion
Well cover Concrete slab
Sanitary casing
Groundlevel Groundlevel
Cement grout
Inert backfill
Plain casing
Bentonite seal
Screens 2-4 mm Gravel pack
Bottom cap
NB: Not to scale
May - 2018 29 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Well Testing
After development and preliminary tests, a long-duration well test should be carried out. Well tests have to be carried out on all newly-completed wells. A well test gives an indication of the success of the drilling, design and development, and it also yields information on aquifer parameters which is vital to hydrogeologists.
A well test consists of pumping a well from a measured start level (SWL) at a known or measured yield, and recording the rate and pattern by which the water level within the well changes. Once a dynamic water level (DWL) is reached, rate of inflow to the well equals the rate of pumping. Usually the rate of pumping is in- creased step-wise during the test each time equilibrium has been reached (Step Draw-Down Test). Towards the end of the test a water sample of 2 liters should be collected for chemical analysis and for microbiological testing.
The duration of the test should be 24 hours, with a further 24 hours for a recovery test or until the initial Static Water Level has been reached (during which the rate of recovery to SWL is recorded). The results of the test will enable a hydrogeologist to calculate the optimum pumping rate, the installation depth, and the drawdown for a given discharge rate. The pump should be installed at least 2 m above the screen, certainly not at the same depth as the screen.
May - 2018 30 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Appendix 3: Site sketch and Topo Sheet extract of the Project Site Location.
May - 2018 31 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Project Site
To Kinungi Kangawa area
To Londiani
To Nakuru Highway (Kibunja Road) To Molo Town
Molo Town
NB: Not drawn to scale.
May - 2018 32 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Google Map Extract
May - 2018 33 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
May - 2018 34 Kangawa Area, Molo Sub-County Hydrogeological and Geophysical Report St. Peters Upendo Catholic Church
Geological map of Mau Area – Degree Sheet No. 42SE
May - 2018 35 Kangawa Area, Molo Sub-County