4.3 Control Measures in Drinking-water Offtake and their Operational Monitoring

Defining these begins with basic decisions on offtake systems to use (i.e. whether to abstract surface water directly or to use infiltration through river banks or through artificial groundwater recharge systems). It includes adequate planning and design of the physical structures for offtake (e.g. bank filtration wells or abstraction towers in reservoirs) and operational controls such as meeting restrictions for pumping rates to avoid break-through of cells or toxins.

The monitoring and surveillance of such control measures is crucial to ensure that they are in place and effective. This does not primarily imply cyanotoxin monitoring, but rather checking whether controls are operating as intended, i.e. operational monitoring as well as surveillance over plans, design and maintenance of structures.

When developing your Water Safety Plan (WSP) your WSP-team will assess the control measures already in place. If they are found to be insufficient, it will suggest upgrading or select new ones. The list of examples suggested below is not comprehensive, but merely intended for demonstrating the nature of control measures and their monitoring, and to trigger your own development of control measures adequate for your setting. This requires expertise in hydrology and – if infiltration is used – in hydrogeology.

For each control measure chosen, your Water Safety Plan should document the reasons for its choice and the targets it should achieve as well as how you validate that it is adequate for achieving the targets you set. Furthermore a management plan should be developed which defines how their performance is monitored and which corrective action should be taken if monitoring indicates poor performance or if incidents occur.

Note: this is not a comprehensive catalogue of examples, but merely intends to trigger your own setting-specific concept of control measures !

Process Step	Examples of control measures for catchment management	Operational monitoring, surveillance and verification
Planning	For infiltration, require permits for drilling wells and for siting artificial recharge schemes that are based on a system assessment which demonstrates that cyanotoxin break-through is unlikely	Review plans and applications for permits in relation to cyanotoxin occurrence and hydro(geo)logical conditions, potentially also in relation to subsequent water treatment
	For infiltration, optimise choice of locations for production wells to ensure sufficiently large minimal travel times for cyanotoxin removal	Review plans and applications for permits in relation to hydrogeological information
	For surface water offtake, plan site on the basis of observations of scum accumulation, e.g. sufficiently far outside of susceptible bays or vertically sufficiently deep to stay underneath surface scums or above deep-layer (metalimnetic) accumulations	Review records of scum accumulation in relation to offtake site
	… ?	… ?
Design, construction and maintenance	Ensure that wells are constructed according to best practice, avoiding filter bypass through the development of preferential flow paths	Ensure that wells are constructed by trained experts, carry out maximum capacity pumping test, TV-inspection and borehole geophysical examination
	For infiltration, ascertain that minimum residence times are achieved	Validate with tracer investigations
	Modify well locations / filter depth, if material proves coarser than expected	Analyse grain size of aquifer material prior to well lining to validate assumptions
	For direct surface water use, construct offtake with options for varying depth in response to an adequate indicator of cyanobacterial accumulation (e.g. turbidity or fluorescence)	Monitor integrity of structure and functioning of system that shifts offtake depths, validate adequacy of scheme inducing these shifts
	… ?	… ?
Operation	For artificial recharge, avoid anoxic / anaerobic conditions by regular removal of clogging layer	Monitor DOC in surface water and oxygen content in bank filtrate; validate that removal scheme is adequate to prevent anoxic conditions
	After clogging layer removal in artificial recharge basins: meet required residence times by reducing pumping rates	Monitor pumping rates regularly and occasionally validate that they are adequate to prevent break-through; for surveillance inspect records of clogging layer removal for documentation of ground water tables and pumping rates
	For variable surface water offtake, switch depth in response to a selected indicator of cyanoabacterial accumulation	Monitor the indicator chosen for cyanoabacterial accumulation at the offtake site; for surveillance check records of monitoring and of response in offtake site selection
	… ?	… ?

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