Determination of Surface and Groundwater Interaction of the Kilimanjaro Aquifer System Using Isotope Hydrology Techniques

Abstract

Sustainable management of the available water resources is the goal of any institution charged with the mandate of water resource management. However, several knowledge gaps exist that hinder the sustainable management of these resources. These knowledge gaps have greatly hampered the allocation and conservation of water resources. There is need to preserve and protect the available water resources, both surface and groundwater. The use of emerging technologies, and in particular isotope hydrology, have been practiced for quite some time to address various gaps in ground water hydrology, which cannot be solved by use of the conventional methods. Isotope hydrology techniques are also becoming more refined with time. This research attempted to use the emerging technologies and in particular environmental stable isotopes (18O and 2H) to compliment the conventional methods in addressing these gaps. The author used isotope hydrology techniques to compliment the conventional methods of hydrogeology, hydrology, and hydrochemistry investigations. Existing data on hydro geological surveys, discharging wells, exploration wells as well as water quality was also used in this research. Water sampling from rivers, lakes, shallow wells, boreholes and springs was done for hydro chemical and isotope data analysis including insitu measurements of physical parameters. Borehole water rest levels were also measured to assist in modeling groundwater flow direction. The research established that there is ground water- surface water interaction and also identified areas and altitude of recharge for the various water sources in the project area. The relationship between Lake Chala and the surrounding springs was also established, together with two types of mixing i.e. mixing of water types based on their chemical composition and mixing of water sources. The research also established the sources of salinity of water sources in the south of the project area and specifically Lake Jipe and Orkungu borehole. These sources are nature controlled and are due to evaporation and dissolution of gypsum as a result of the geology of the area. Ground water flow is from the highest point which is Loitokitok Town and varies in magnitude towards the other areas. Increase in flow magnitude was noted in the lower levels from 950 Masl, which could be an indication of regional groundwater flow. In conclusion, groundwater resources in the Kilimanjaro Aquifer System are recharged at different altitudes and locations. The waters of Mzima Springs and the springs from Mt. Kilimanjaro do not interact since they are recharged by different rainfalls regimes with different isotopic signatures and at different altitudes of recharge. The altitude of recharge for the waters from the Kilimanjaro ranges between 1550-2000Masl while that of the Chyulu, where Mzima is located is 1350-1450Masl. There is mixing of both water types and water sources. Relatively fresh waters are also mixing with mineralized waters from the Kilimanjaro. The observed groundwater interaction between boreholes and springs implies that over exploitation of groundwater resources from any of the boreholes will directly negatively affect these other sources. However, Mzima springs is only affected by mixing in terms of water types and not water sources. Therefore, interbasin transfer from this source will not adversely affect the other water sources and biodiversity along the waterways associated with it as long as the exploitation is done sustainably. Salinity in some water sources is as a result of evaporation and dissolution of gypsum as controlled by the geology of the area. Groundwater flow direction is from the highest point generally towards the easterly and south easterly directions as dictated by the Piezometric levels. There is also a high flow magnitude emerging at the lower altitudes and in particular between 750- 700 Masl towards the East and 950-850 Masl in the East and South East directions.