12th October, 2009
GC/09/42
HYDROGEOLOGICAL SURVEY
REPORT
CLIENT: MTEZI CENTRAL PRIMARY
SCHOOL
P.O.BOX 47
KWALE
LOCATION: MTEZI
DISTRICT:
KWALE.
PROVINCE:
COAST
COMPILED BY: M. A.OCHARO, B.Sc, M.Sc,
REG . GEOLOGIST/ HYDROGEOLOGIST.
ABSRACT
This report describes the hydro geological and geological conditions prevailing at the
Mtezi Central Primary School area and its environs and covers an area of approximately
24 acres while the amount of water required by the client is approximated at about 5
cubic meters an hour for the purpose domestic especially for the chidrens’ cleaning and
maintaining the hygienic standards and to help the neighbors. The school is a full mixed
primary, which has a big problem with the water. The area has no source of water within
the vicinity and the climatic condition here is hostile during the dry seasons. The success
of this borehole will go a long way to alleviate the current critical water crisis.
The school is located S 04 04.994: E 039 30.985 GPS position at an altitude of 54 meters
above sea level. Currently the school is using roof catchment systems of harvesting rain
water and reserving in two large underground tanks, but at the current time the tanks are
empty due to the long dry season .
The climatic conditions of the area can be summarized as semi humid fairly to very hot in
some seasons especially December to mid- March with mean annual temperature ranging
between 24 to 30 degrees. The average annual evaporation is 2200mm whilst the mean
annual precipitation is approximated at 1100mm.
A comprehensive study of available data and the extrapolation of known geological
conditions of the area have enabled a fairly accurate picture of the groundwater
potentiality and the underlying rocks of the surveyed area to be concluded. Geophysical
Survey measurements have been carried out at the site, to investigate the geohydrological
and subsurface geological conditions.
In view of the above and the geophysical interpretation results it is observed and
concluded that there are chances of striking groundwater at both moderate and greater
depths.
Recommendations are therefore drawn to drill a borehole at the site known to the client to
maximum depth of 85 meters designated as GC/09/42 for the purpose of this reports’
records. This recommendation for borehole has been arrived due the amount of water
required at the center. Digging a hand dug well will limit the depth to a shallower depth
which will tap only surface and subsurface waters. The quality and quantity of the
borehole water can not be determined at this stage of the investigations until drilling
works is over and the necessary tests carried out.
The importance, correct, professional and comprehensive technique in this particular
field is of paramount importance and should be given the attention it deserves.
GEOLOGY.
The surveyed area is part of the Coastal belt whereby a series of three, more less parallel
zones or plains, each lightly dissected by denudation which rise in steps one above the
other towards the interior. Trending from East to West the zones are distinct.
1. The Coast plain, which is occupied by Pleistocene deposits.
2. The foot plateau, which is practically coincident with Jurassic outcrops.
3. Coast Range, formed essentially of the Upper Duruma Sandstone, owing its
eminence to resistant bands of grit that occur within it .
4. The Nyika, being the area covered by the Duruma Sandstonne series and gneisses
West of it.
The surveyed area falls under the Coast Range, formed essentially of the Upper
Duruma Sandstone owing to its eminence to resistant bands of grit that occur within
it.
HYDROGEOLOGY
a. Groundwater
In groundwater, the porosity of a rock is an important parameter in regard to its
potentiality as a water bearing zone. In sedimentary rocks the porosity differs with the
morphology and texture of the rock material. Sandstone which are compacted
possesses low to poor porosities while unconsolidated rock materials such as sands,
corals have high porosity which increases the recharge, therefore make good aquifers.
In the surveyed area, the stratigraphyical sequence is composed of Cretaceous age
formations as motioned above whilst, the permeability of these formations will
largely depend on the pore size of the cavity or pore space, the larger the pore the
higher the permeability and the small the size the low the permeability. The sand
formations normally make excellent aquifer material due to its morphology and the
higher rate of permeability and transmissivity rates that they possess.
b. Recharge
Recharge of any geological set-up is depended largely on the precipitation of the area.
The recharge for this area is derived from local precipitation of about 1100mm
annually. The mechanism of the recharge into the aquifer is through vertical and
lateral infiltration processes.
c. Water Quality
As per the geoelectrical sounding carried out and the subsequence interpretation
results it is evident that the groundwater quality of this area is fair. It should be noted
here that the area falls within the coastal region where the factor of salinity is real
whereby the Gyben Herzberg relation(fresh water- salt water interface) is critical and
vital in groundwater investigations in such coastal areas. In this relation the salinity
increases with depth while the fresh water zone remains ‘thin’, while the interface
between the two grows ‘thinner’.
GEOPHYSICS
Tabulated below is the results of the one (1) geoelectrical sounding carried out at the
site to a maximum depth of 63 meters deep to determine the subsurface geology;
Vertical Electrical Sounding (V.E.S)
V.E.S .No
DEPTH(M)
GC/09/42
0-2.5
2.5 – 3
3–4
4–5
5–6
6–8
8 – 10
10 – 13
13 – 16
16 – 20
20 – 25
25 – 30
30 – 40
40 – 80
RESISTIVITY(ohm- GEOLOGICAL
m)
CONDITION.
440
Top weathered
formation.
390
weathered top layers
300
weathered material
200
Partially weathered
zone
90
weathered zone
85
Weathered
75
Moist weathered
zone
60
Aquifer zone
49
Aquifer zone
49
Aquifer zone
70
Moist weathered
zone.
110
Aquiferous zone
140
Saturated weathered
zone
150
Saturated weathered.
Zone
BOREHOLES
There are no documented and recorded boreholes within the vicinity of the surveyed
CONCLUSION AND RECOMMENDATIONS
In view of the foregoing it is concluded that the groundwater prospects at the
surveyed area are good and promising. It is therefore recommended that a borehole be
drilled to the maximum depth of 85 meters. The site is marked on the plot and shown
and known to the client. The quality and quantity of the water can not be determined
at this stage of the investigation, until drilling is completed and the tests carried out.
Drilling should be carried out according to the following procedures.
DRILLING TECHNIQUES
Drilling should be carried out with an appropriated drilling machine, either percussion
of rotary machine. However the latter is suitable due to its time frame of completion
of the job (Faster).Geological rock formation samples should be collected at every 2
meters interval. Struck water levels and static water levels should be monitored.
BOREHOLE/WELL DESIGN AND CONTRUCTION
The borehole/Well should be designed in such away to ensure that screens are place
to cover the optimum aquifer zones. The well should be cased and screened with
good quality standardizes P.V.C. casing and screens. The gravel pack is
recommended within the aquifer zones to avoid any silts blocking the screens, leading
to gradual siltation resulting to screen clogging. In well construction, the centralizes
of the screens and casings should be used at every 6 meters interval to insure
centrality within the borehole so it would be easier to install the gravel park all
around the screens. The remaining annular space should be backfilled with an inert
material and and the top 5 meters grouted with cement to ensure that no surface
water at well head can enter the well/borehole, so as to prevent surface pollution.
BOREHOLE / WELL DEVELOPMENT AND YIELD TEST
After the well design and construction the borehole should be developed well without
over pumping. Development aims at repairing or reconditioning of the aquifer which
may have been damaged during drilling. It also alters and improves the physical
characteristics of aquifer around the screens and removes fine particles. Thereafter a
long duration yield testing (Pump Testing) should be carried out to determine the
yield of the borehole (quality). The borehole water should undergo the chemical and
bacteriological analysis by a qualified Chemist, before it is put any use.
All the above works must be carried and conducted with the close supervision of a
qualified experienced hydro geologist who will certify the completion of the works as
per the required standard.
M.A.OCHARO.
REG . GEOLOGIST/ HYDROGEOLOGIST.
MTEZI CENTRALPRIMARY SCHOOL
VES No. GC/09/42
MTEZI AREA KWALE
Computer Interpretation Results
12th October, 2009