OR/17/056 Factors for risk mapping
| Lapworth, D J, Stuart, M E, Pedley, S, Nkhuwa, D C W, and Tijani, M N. 2017. A review of urban groundwater use and water quality challenges in Sub-Saharan Africa. British Geological Survey Internal Report, OR/17/056. |
Preliminary assessment from literature review
From each of the chapters the key hazard factors have been extracted to inform the risk mapping process. These have been grouped into source-pathway-receptor categories (Table 10.1) with other local area factors such as population density. From the literature review a preliminary assessment of the importance of the various hazard factors can be made and these are shown as primary and secondary factors in the table.
Factors to be informed by local conditions
Whilst Table 10.1 sets out the key factors it does not always prioritise them or attempt to score them. The assessment can be informed by use of a risk assessment matrix (Figure 10.1). For a contaminant or a suite of contaminants, the various hazard factors are summed along the x-axis and a measure of the contamination along the y-axis. If the hazard factors selected are appropriate and/or correctly weighted the data will plot in the greyed areas of the diagram. Data which plot in the white areas can inform the weighting of the risk factors. In reality a scatter of data will still be obtained with some data points with excessive or unaccounted hazard factors.
The details of these will be provided from the project case study in Kabwe, Zambia and will underpin the risk mapping tool (Stuart et al., 2015) and this topic is covered in more detail in this accompanying report. There are a number of hazard details which do not appear to be covered in the review, e.g.:
- The critical separation between the source and the water supply in different settings.
- The role of borehole depth
- Relative importance of hazard factors
There may be other hazards that may be important which prove not to have been not covered by the summary in Table 10.1 at all which include:
- Socio-economic factors related to ownership, governance and legal frameworks;
- Other gaps in the knowledge base such as local understanding of recharge mechanisms.
| Hazards | Category | Primary factors | Secondary factors |
| Regional considerations | Population density Land use category Relief/slope Abstraction regime |
Managed aquifer recharge | |
| Aquifer vulnerability | Lateritic soils Shallow groundwater Surface flooding Karstic limestones River sediments |
Plinthosols and lateritic horizons Weathered basement conditions | |
| Contaminant sources | Municipal/household | Septic tanks Latrines Solid waste disposal Amenity pesticides |
Sewers Sewage effluent Storm water runoff Cemetery Abbatoir |
| Peri-urban agriculture | Livestock wastes Wastewater irrigation |
Pesticides Fertilisers Soil amendment | |
| Industrial | Process plant effluent–textiles, pharmaceuticals, detergents Industrial solid waste disposal Fuel stations/leaking storage tanks Chemical transport and pipelines |
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| Hospitals and treatment centres | Hospital waste discharge Medical waste disposal |
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| Mining | Tailings and stockpiles | ||
| Pathways | Aquifer vulnerability | Lateritic soils Shallow groundwater Karstic limestones River sediments |
Weathered basement Windblown sediments |
| Local/headworks pathways | Eroded or devegetated spring backfill Faulty masonry Gap between borehole riser and apron Damaged borehole apron Lack of dugwell headwall Lack of well cover Use of bucket and rope Gap between apron and well lining Damaged well apron Propensity to flood |
Lack of diversion ditch Lack of wastewater drain Lack of fence Animal access Uncontrolled use | |
| Receptors | Groundwater use Surface water |
Private unmonitored supply Public unmonitored supply |
Water treatment not installed No safety plan |