Numerical Groundwater Flow and Nitrate Transport Modeling for the Prediction of Impacts of Land Use Changes on Water Quality in Akaki Catchment
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Date
2015-06
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Addis Ababa Universty
Abstract
Water is one of the most valuable resources that mankind needs for survival. One source of this precious resource is the groundwater, water beneath the surface. This resource is being contaminated by numerous organic and inorganic pollutants in the current study area. Thus, there is an urgent need to intensive investigations. Groundwater and contaminant transport modeling practices are one of the most powerful tools in groundwater studies. The present study has mainly performed a numerical advective-dispersive-reactive phenomenon of nitrate transport modeling to showing the effect of land use types and changes on groundwater quality by using a software called MODFLOW/MT3DMS. Primary and secondary data were collected from different sources. This modeling process has two independent but complementary modeling activities; the groundwater and contaminant transport process, the former being a precondition for the latter. Accordingly, the groundwater calibration process has produced a very cloth values between the observed and calculated values and statistically the ME, AME, and RMSE were found to be 7.6, 11.01 and 12.7 respectively. The groundwater modeling results shows that the maximum and minimum head values were 2729 and 1970 m.a.s.l. in the north and south ends respectively. Thus, groundwater flows north–south direction. The water balance of the model shows that a total volume of 665280 m3 water flows in the groundwater system in a day. Moreover, a very high groundwater-surfacewater interaction was observed. The model was very sensitive for hydraulic conductivity and recharge values.
The calibration process for the contaminant transport process was done in two fold; one calibration for time series data and calibration for 10 and 20 mg/l observed and calculated contour comparison. The former was done for 5 wells of 5 year time series data. Accordingly, the ME, AME, RMSE of the differences of these wells in the whole calibration time (5 years) was found to be 0.019, 0.6833 and 0.8760 respectively. The 10 and 20 mg/l contours produced for model simulated and observed nitrate concentration map has found to be visually similar; and the difference between areas covered with these contours of simulated and observed concentrations were very minimal. The general contaminant transport model result shows that the highly polluted areas were very populated built areas (from pit latrine and poor sewerage system) and these points were able to generate a huge mass of nitrate plume. There were 4 hotspot areas identified where nitrates migrates from. Nitrate migrates in all directions from these hotspots and advective transport mechanism were found to be the dominant one while dispersive and diffusive type migration were very significant. The temporal aspect of this model shows that more wells will be polluted as time increases even future nitrate loading is stopped. Spatially, Wells towards Gefersa were found to be very susceptible due to their proximity to the main plume center from the diffusive migration of nitrate. Wells towards Legedadi were the less vulnerable one due to their distance from the main plume center and relative position. But, these wells were endangered due to the new habilitation activities. Wells towards Akaki were found to be safe in the short term but very extremely vulnerable in the future. The scenario analysis part shows that about 9, 16 and 18 wells (out of 45 major wells) will exceed the WHO drinking water standard limit of nitrate concentration (50 mg/l) in the coming 25, 37 and 50 year if everything is kept constant. Further population increment with in the current land use will force other wells to exceed the drinking water standard. Distributing the upcoming population increment to the whole catchment will minimize the effect on the currently susceptible wells. There is less option of reducing the imminent impacts caused by the already contaminated water. In recommendation, it would be possible to minimize future nitrate loading by upgrading these infrastructures for already built area and setting a very tight structures to newly emerging villages.
Key Words: MODFLOWMT3DMS, Contaminant transport, Flow Modeling, Groundwater Contamination, Effect of Population, Addis Ababa
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Keywords
MODFLOWMT3DMS, Contaminant transport, Flow Modeling, Groundwater Contamination, Effect of Population, Addis Ababa