Groundwater Management Action Plan - Stage 3. Characterising and prioritizing groundwater pollution threats - pollution risk assessments. Lessons learnt from Bishkek and Narayanganj.
For Groundwater Vulnerability Assessments:
• The overlay and index vulnerability mapping system was easy to devise in Bishkek because the geological setting was already well conceptualised, so existing maps could be adapted to draft the various overlays.
• In Narayanganj the information base was much weaker, and it was only the fact that shallow drilling by hand is so inexpensive in Bangladesh that the project could afford the luxury of supplementing its available data with additional field surveys. This made it possible to collect enough information on depth to water table and presence/thickness of the low permeability surface layer to construct maps. There will be numerous cities in a similar data-deficient situation to Narayanganj, and many will require significant resources to conduct a supplemental field survey programme to enable construction of an index and overlay-based vulnerability map.
Points added to Pollution Risk Assessment Methodology from applying
it to case-study cities:
• It is important to identify at the outset settings where land zoning will be ineffective, such as vulnerable karst limestone, and risk assessments will have little value.
• In practice, diffuse sources may be difficult to disentangle but it should still be possible to gain information on their role in the urban water balance. Links with the water supply/wastewater disposal utility makes assessment of both diffuse and point source loads easier because they should have information of the extent and type of wastewater arrangement in force.
• Point source information is difficult to obtain but commercial/industrial directories may be a useful source.
• Hot spot maps are a very useful tool for presenting information to the non-specialist. Care needs to be taken to avoid dividing the information presented into too many different classes giving a mosaic effect and confusing interpretation.
• Scheduling of production of the Planning Maps can be usefully linked to stakeholder mobilisation, giving an incentive to take ownership of the policies because they have been seen to be developed in discussion by all those interested.