Attenuation Capacity Of The Vadose Zone In Water Reuse For Managed Aquiferartificial Recharge

Groundwater input to the city of Addis Ababa has been increasing significantly especially with in the last few years. Increasing pressure is being imposed on the aquifers in order to meet the increasing demand by the city for more water. Major input from the groundwater as a source for Addis Ababa comes from the study area of the research, Akaki Well Field, currently pumping water in the order of 40,000m3/d. Other distributed abstractions are also undergoing in the city by AAWSA itself, different private/public and governmental institutions, all pumping water from a groundwater system that prevails in the same River catchment.rnIncreased but sustainable abstraction rate is practically proved to be possible on various projects by having additional planned recharges through a method of Artificial Recharge (currently called “Managed Aquifer Recharge”). However, before its implementation various factors and parameters need to be assessed on the supposed site. Among these, few are addressed in this laboratory scale study.rnThe main objective of this thesis paper is to study the change in water quality resulting from the attenuation/filtration effects of the vadose zone (made of soil layers) in the study area for assumed future implementations of Managed Aquifer Recharge. For this, water quality analysis for 17 different parameters was done along with soil test for ESP (Exchangeable Sodium Percentage), EC and CEC (Cation Exchange Capacity) values.rnTwo water sources, treated wastewater discharge from Kaliti sewage treatment plant and Akaki River were used as an input for the column experiments. Water quality test for 17 parameters for both the inflow and outflow of water samples showed the change that occurred as passing through the Black Cotton soil in the area. Relatively intact, 50cm long soil samples were collected in 4 different columns from the well field and operated for seven days to study the water quality change in the laboratory scale. Simulation/flow-through of water in these soil columns showed that the input waters used changed in important ways for most parameters in different percentages, signifying the capacity ofrn- viii -rnthe vadose zone in contaminant attenuation from recharging waters. Actual results showed that concentrations of input waters for TDS, EC, Hardness (CaCO3), NH3, NO2-, PO4-2, K+, Ca+2, Mg+2, BOD, COD and Total Coliform decreased as water flow through the soil columns while it increased from slight to huge percentages for pH, NO3-, SO4-2, Na+ and DO parameters.rnComparison between water quality results of the outflow water and existing groundwater in the area showed analogous values. Estimation in the field scale occurrence was also done and results suggested that increased percentages of attenuation would occur in natural field-scale vadose zone.rnAt the end, it was concluded that the vadose zone (Black Cotton Soils) attenuation capacity in changing the recharging water qualities was highly important and a planned safe reuse of different water bodies through a method of Managed Aquifer Recharge would prove to be highly beneficial. However, before making such useful method practical more detailed researches should be undertaken in varying spatial, temporal, case-by-case laboratory and field-scale levels.rnKeywords:rnVadose Zone; Soil Layers (Strata); Attenuation Capacity; Artificial Recharge/Managed Aquifer Recharge; Treated Wastewater/Surface Water; Water Reuse

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