Assessment of groundwater availability and its current and potential use and impacts in Tanzania

Abstract

The International Water Management Institute (IWMI) commissioned this study to assess groundwater availability and its current and potential use and impacts at the national scale for Tanzania as part of a wider study that extends across many sub-Saharan African countries. The study was a desktop study of existing geological, hydrogeological and hydrological data and reports that cover both biophysical and socio-economic aspects of groundwater. The report is based on a thorough review of white and grey literature from various government departments, NGOs, donor reviews and reports, student theses and consultant reports. The study came up with a number of key findings, conclusions and recommendations as highlighted below. The general geology of Tanzania comprises mainly the Precambrian (Archaean, Proterozoic) and Phanerozoic (Upper Palaeozoic, Mesozoic and Cenozoic) formations. The Archaean rocks are characterized by a granite-greenstone terrain. The Tanzanian Craton covers the central part of the territory up to south and east part of Lake Victoria. The occurrence of groundwater is largely influenced by geological conditions. Hydrogeologically about 75% of Tanzania is underlain by crystalline basement complex rocks of variable composition and ages, but predominantly Precambrian, which form the basement aquifers (for example the Pangani and Makutopora basins). Other aquifer types include karroo (found in Tanga), coastal sedimentary formation of limestone and sandstone (e.g. Dar es Salaam), and the alluvial sedimentary sequence, which mostly include clay, silt, sand and gravel, and volcanic materials (e.g. Kahe -Pangani basin). The groundwater potential of every type of aquifer differs significantly at the local scale as well as at the basin scale. The hydrogeology of Tanzania has not been thoroughly studied and owing to that, the quantification of the groundwater resources of the country has not yet been possible because of a lack of requisite data. In most cases, the only available information has been compiled from existing borehole log data. Groundwater development has concentrated mainly on shallow wells for domestic purposes over a wide part of the country (mainly rural areas). They are also commonly used in the peri-urban fringes where there is no distribution network and places with unreliable supply. Most boreholes are located in the internal drainage basin. The basin is characterized by semi-arid to arid conditions with rainfall less that 550 mm annually, making the dwellers dependent mostly on groundwater as the main source for water supply. The review has revealed that in areas where the static water level is less than 8 meters, shallow hand dug well fitted with hand pumps is feasible, which on average is about 40% of the Tanzania mainland area. There are limited extensive studies on recharge in Tanzania and owing to that the recharge rates are not known. However, based on very approximate basin-scale water balance calculations, the total ground water recharge on annual basis is estimated at 3,725 MCM (0.4 %). A general outlook on the various recharge estimates indicates that the values are greatly variable location- wise and are a function of the methods used. Low basin recharge rates implicate on groundwater development potential. Boreholes drilled for domestic water supplies indicate variable yields. Some boreholes in the Dodoma plain have exceptionally high yields of about 460m3hr-1. The average yield of boreholes (excluding Dar es Salaam and dry boreholes) is 11m3hr-1. The average static water level of productive boreholes is about 17 metres and the average total depth 62 metres. The cost for boreholes in Tanzania is about USD 6,000 for hand pumps and USD 12,000 for mechanised systems. These costs include the full facility, i.e. sitting, design, drilling, supervision, construction, and supply of equipment. The drilling cost contributes to only about 50% of the full facility cost. Groundwater has not been extensively used for irrigation largely due to the following reasons: • Detailed analysis on groundwater irrigation potential nation-wide has not been thoroughly explored. Most of the estimates are based on surface water information. • Tanzania still has enough areas that are potential for irrigation using surface water resources. Irrigation high potential area is estimated at 2.1 million ha in gross, as compared with 0.2 million ha currently irrigated. • There is scant information on the potential of aquifers and yields of individual boreholes. • Limited groundwater resources management plans. • The majority of people in the community have an inadequate understanding of groundwater resources and this has led to inappropriate development of groundwater. The national borehole database is maintained by the MoWI, Directorate of Water Resources in Dodoma. However, the data entry is not consistent; many boreholes have no data recorded and for others the data are incomplete and lack coordinates. This study recommends detailed groundwater studies be undertaken to assess the recharge and the sustainable groundwater yields, necessary to establish the groundwater potential for irrigation and for the other sectorial uses in Tanzania. Considering that groundwater in Tanzania is likely to be the key resource to improve the water supply coverage in many areas under the changing climate, the development of groundwater should be carefully managed to make full benefit of its potential, to protect its quality and to guard against over-exploitation of the aquifers. As a way of improving data management and information sharing, the existing database needs to be transformed into a Management Information Systems (MIS) that is integrated into a Geographic Information System (GIS). Key information like borehole location, groundwater quality, amounts of abstraction, and the hydrogeology should be maintained in the database.