Decision support tool

Control measures

in drinking-water offtake

The determination of the type of raw water abstraction and abstraction sites is crucial. If surface water is directly abstracted it has be to ensured that the intake of cyanobacteria is avoided as far as possible and that the treatment in place is able to eliminate cyanotoxins. Water abstraction after a sediment passage (bank filtration [1] or artificial recharge [2]) does not necessarily require a subsequent treatment step as these techniques are able to remove cyanotoxins in the case of a suitable subsurface and retention times.

Water abstraction requires a careful planning and development of the respective systems (wells for bank filtration, off-take sites and depths in reservoirs). For drinking water safety it is crucial that the abstraction system in place retains or reduces cyanobacteria and/or the concentration of dissolved cyanotoxins as intended, and that this is continuously monitored according to operational parameters (e.g. pumping rates in wells or on-line fluorescence at the offtake).

For “drinking water abstraction”, expertise in hydrogeology and soil chemistry with respect to the removal of organic substances in the subsurface, as well as in hydrology and plankton-ecology (specifically for the occurrence of cyanobacteria), is required for surface water abstraction.

Examples of measures in drinking water abstractrion and their operational monitoring can be found here

Process step Examples of measures Examples of (operational) monitoring
Planning For bank filtration and artificial recharge:
Permits for drilling wells and for artificial recharge schemes based on a system of assessment which demonstrates that cyanotoxin break-through is unlikely; optimised choice of locations for production wells to ensure sufficiently large minimal travel times for cyanotoxin removal.
Review plans and applications for permits esp. with respect to professional analytical competence, i.e. the hydro(geo)logical data set.
Review retention times in the subsurface including tracer tests; grain size analysis of the sediment to review assumption during planning.
For surface water offtake:
Optimisation of the offtake site on the basis of cyanobacterial scum accumulation, e.g. sufficiently far from susceptible bays and leeward shorelines, or vertically sufficiently deep to remain beneath surface scums or above deep-layer (metalimnetic) accumulations.
Review records of cyanobacterial scum accumulation and of subsurface cyanobacterial maxima in relation to offtake site.
Design, construction and maintenance Contruction of wells according to best practice, avoiding filter bypass by developing preferential flow paths (which reduce filtering efficacy). Ensure that wells are constructed by trained experts; carry out maximum capacity pumping tests; TV-inspections and borehole geophysical examinations.
For surface water offtake:
Variable depth offtake to be able to react flexibly to high populations of cyanobacteria.
Monitoring of the integrity of structure and functioning of system that shifts offtake depths; control of the documentation of the choice of the depth and justification.
Operation For artificial recharge:
Avoidance of anoxic/anaerobic conditions by regular removal of clogging material.
Monitoring of dissolved organic carbon (DOC) in surface water and/or oxygen content in bank filtrates.
After removal of filter-clogging material in artificial recharge basins: meet required residence times by reducing pumping rates. Regular monitoring of pumping rates; inspection of records of filter-clogging layer removal, for documentation of ground water tables and pumping rates.
For depth variable offtake:
Control of the offtake depth in response to a selected indicator of cyanoabacterial biomass.
Monitoring of the indicator chosen for cyanoabacterial biomass at the offtake site and depth.

This list of examples is neither comprehensive nor should all measures listed be necessarily implemented. Rather, the nature of control measures and their monitoring should be demonstrated and provide the basis to develop your own control measures adequate for your setting.


Bank filtration is water abstraction from wells

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Artificial groundwater recharge is achieved by the infiltration

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