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