MODELING OF GROUNDWATER FLOW AND ITS SUSTAINABLE UTILIZATIONS FOR HANTEBET WATER SHED, TEKEZE BASIN, TIGRAY (SEHARTY SAMRE WOREDA)

Abstract

Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live, so it is very important to calculate the quantity and quality of this resource for better protection and management. The project was carried out in Hantebet basin, northern Ethiopia, southwest of Adeka ala village at about 60Km from Mekele town. The catchment is a closed basin covering an aerial extent of around 24.4 Km2. The main purpose of this project work is to make a quantitative estimation and sustainable use of groundwater resource for domestic, live stock and agricultural supply. To achieve this objective careful understanding of hydrology, hydrogeology and dynamics of groundwater flow in and around the study area was mandatory. The geology of the area was mapped at a scale of 1 :50,000 and it constitutes shale, limestone, siltstone and dolerite geological units. In the hydro geological investigation around 108 hand-dug wells were inventoried and their geologic and hydro geologic log was plotted in vertical section. Based on these loges the whole shallow wells are grouped in to nine major aquifer natures, and the pumping test data analysis was conducted manually on about 6- wells. The transmitivity and hydraulic conductivity of the formation was determined using Theis- Recovery methods. Accordingly the transmitivity and hydraulic conductivity potential of many of the aquifer was obtained in the range of 22.62 - 44.81 m2 /day and 8.38-26.39 m/day respectively. Aerial rainfall recharge was estimated using a step-by-step procedure of soil moisture balance in the unconfined zone and obtained 297.33mm/year. High abstraction rates from Hantebet aquifer during the previous years lead to a calibrated for steady state condition by matching simulated and observed hydraulic head data for the ·period 2006-2007. The root mean square error for simulated hydraulic heads for all wells was 36.58 meter. Simulated hydraulic heads were within ±15 meter of observed values for all observation wells. The results of the calibrated model showed that the horizontal hydraulic conductivity and transmitivity of the whole aquifer was 32m/d and 45m2 /day respectively. Using the calibrated model, water budget of the whole model domain was calculated with a percent discrepancy of 0.02. It includes the following inflow components to the ground water flow system: (1) recharge from Constant Head Boundary, with a value of 21, 109.490M3 (2) Groundwater recharge from precipitation, which is 392.175M3 /day.Total inflows were 21,501.66M3 /day The simulated groundwater outflow from the system includes Groundwater outflow by well pumpage with a value equal to 21,506.12m3 /day. The groundwater flow contour map and flow nets lines showed that there is a concentration flow towards southeast and this is an area of discharge to the surface streams Results on the physical and chemical water quality assessment of the area indicate that most of the water sample is magnesium chloride type. Based on the U.S. Environmental protection agency (1975) and WHO, the groundwater in the study area is suitable for irrigation and domestic purposes