Groundwater Nitrate Contaminant Risk Mapping using the modified DRASTIC
Model: The Case of Bahir Dar City, Ethiopia.
Abstract
Nitrate contamination of groundwater often occurs in urban and
industrial areas due to point and non-point sources of anthropological
activities. Groundwater constitutes a significant portion of the water
supply system for Bahir Dar City in Ethiopia, though the level of
groundwater pollution is not known. This study was conducted to assess
the extent of the aquifer and groundwater pollution (nitrate) based on
contaminant vulnerability risk mapping using the GIS integrated modified
DRASTIC model. A field survey was conducted to collect samples from
boreholes for nitrate analysis and to modify the DRASTIC model. Compared
to the original intrinsic vulnerability assessment, land use as external
factors changed from moderate-high to high vulnerability class from 18
% to 88 %. The FR-APH modified model showed a good correlation (0.53)
compared to the other methods. Based on the FR-APH modified model, about
31% of the area was under moderate to high and high vulnerability
range, 39% was under moderate vulnerability range while 30% was under
low and moderate to low vulnerability range. The integrated
vulnerability map showed high risk in the central part of the City due
to the flat slope and shallow depth to groundwater. Besides, the
sensitivity analysis indicated that the contribution of aquifer media
and vadose zone to contaminant risk was found trivial. In general,
groundwater at Bahir Dar city was found vulnerable to nitrate
contamination and needs proper management.