Coping with Uncertainty

Collating in-depth information for risk assessment can be an extensively time-consuming exercise, and often information will not be readily available. Estimating nutrient loads from the catchment, understanding the ecological interactions within the waterbody and collecting information on socioeconomic aspects of water-use patterns may be particularly challenging. Therefore, it is necessary to consider the detail of information required for making decisions - possibly only preliminary ones, documenting uncertainties and information gaps along with the information. Risk assessment will then show which information gaps and uncertainties most urgently need to be closed, that is, those for which uncertainty precludes decision-making.

For example, reducing uncertainties in estimating a nutrient load may be critically important in a setting where controlling nutrient loading is the key measure to avoid cyanobacterial proliferation and thus cyanotoxin occurrence (e.g., an upstream reservoir in a fairly pristine catchment). In contrast, in a setting where nutrient loads and concentrations in the water- body are already excessively high and not readily amenable to local control (e.g., a downstream river reach with complex transnational nutrient sources), identifying key nutrient sources and taking action to reduce loads remain important in face of long-term benefits, but in the shorter term, other control measures may need to take priority.

Therefore, risk assessment is an iterative rather than a linear process: it should incorporate the WSP principle of “incremental improvement” as described above, with an emphasis on the importance of getting started and improving over time as information becomes available.

For the three case examples used in this chapter, Table 6.5 shows how the respective teams described the uncertainties of their risk assessments.

Documenting uncertainties and making them transparent, including information gaps to close, are important. This will inform decisions on which measures to take first - whether these should be interventions to reduce exposure or rather programmes to collect data and information before decisions on any investments into measures are to be made.

Cyanotoxin Risks In Relation To Other Public Health Risks From Exposure to Water

A key purpose of risk assessment in water safety planning is to determine priorities for maintaining, upgrading or implementing measures to control public health impacts from the hazards identified. For the overall target of protecting public health, it is important to assess the public health risk from cyanotoxins in relation to that from other hazards/hazardous events potentially occurring in the water. This is also useful because some of the events causing other hazards will also cause cyanobacteria - for example, sewage loads carry both nutrients that support blooms and pathogens. A comprehensive risk assessment would be developed in the context of developing a full WSP, but it is also valuable to contextualise the potential cyanotoxin risk even without developing a complete WSP. Generally, public health risks from pathogens in the water are likely to be of higher priority

Table 6.5 Three example settings: uncertainties arising during the assessments of the risk of cyanotoxins to occur in health-relevant concentrations

Examples of settings

Uncertainties of the risk assessment for each of the three settings introduced in Table 6. /

1: Slow-flowing large river serving as raw water source for drinking-water for a town of 500 000 inhabitants

Uncertainty due to the lack of cyanotoxin data, but the dominant cyanobacterial taxa almost certainly contain microcystins, and steps available in the drinking-water treatment system - while probably removing cells containing toxin - cannot remove dissolved toxins and may well lead to the lysis of some cells, thus releasing further toxins.Therefore.even without toxin data, exposure to microcystin concentrations in the range of a few pg/L is likely, particularly as occurrence is ongoing for periods of several months on end.

While uncertainty regarding the extent of private angling is high, it is clear that this lead to relevant exposure only for a small population, possibly for some people with low income frequently relying on fish from the river as a relevant source of protein.

2: Reservoir serving about 7000 people (three villages and a number of farms)

Uncertainty is considerable: although the visual reports suggest cyanobacteria to be the cause, it is unclear whether they indeed cause the discoloration. While catchment conditions do not indicate sufficiently high nutrient loads to support substantial biomass, this cannot be totally excluded, particularly in face of increasing tourism.

3: Farm dugout serving as water source for 20-50 people

Uncertainty regarding concentrations of phosphorus as well as cyanobacterial taxa and their concentrations is relevant, as the data from the three sampling occasions suggest them to be only slightly above thresholds for interventions to prevent human exposure, thus indicating that interventions to reduce phosphorus loads from erosion may be effective.

As finances for a more intensive monitoring programme are lacking, uncertainty will be addressed by intensified visual inspection for blooms.

Uncertainty also exists with regard to the efficacy of the point of use filters and whether filtered water is reliably used for food preparation or not.

due to the potential for severe acute illness, even death, and also because even a small number of people infected through exposure to water containing pathogens can communicate the infection to a potentially exponentially increasing number of others.

For the three case examples used in this chapter, Table 6.6 shows how the respective teams related the health risks due to cyanotoxins to other health risks in the respective setting.

Table 6.6 Three example settings: health risk assessments due to cyanotoxins in relation to risks from other hazards

Example 1: Slow-flowing large river serving as raw water source for a town of S00 000 inhabitants

Severity of public health impact

Minor impact

Moderate impact

Major impact

Often

Contamination of drinking-water with musty taste and odour (of unclear origin) possibly due to ineffective water treatment

Exposure to cyanotoxins in drinking-water (chiefly microcystins) due to insufficiently effective water treatment

Microbial contamination of the drinking- water due to failure of filtration in water treatment

Likelihood of occurrence

Occasional

Microbial contamination (i.e., Legionella) in household installations due to inappropriate management of internal plumbing systems

Rarely

Exposure to cyanotoxins due to recreational contact (bathing)3 Exposure to cyanotoxins due to the consumption of contaminated foods3

Chemical contamination of source water due to spills of hazardous chemicals

Drowning, injuries due to illegal swimming in boating channels *

Dark grey=high risk; Medium grey=medium risk; Light grey=low risk

Rationale: high risk from cyanotoxins (see Table 6.4); high risk of pathogen breakthrough, particularly of Cryptosporidium (which are resistant to disinfection). Legionella are known to have caused numerous cases of serious pneumonia and two deaths in two hotels and one senior citizens’ residence. Public concern is high for spills of hazardous chemicals but actual risks are low, due to the lack of industry in nearer catchment (if they occur, concentrations would be low). Slight risk from musty taste and odour with increased risk if people use other less safe water sources.Two known incidents of teenagers severely injured by boats when swimming in spite of warning notices.

(Continued)

Example 2: Reservoir serving about 7000 people - three villages and a number of forms

Severity of public health impact

Minor impact

Moderate impact

Major impact

Likelihood of occurrence

Often

Sunburn due to excessive exposure to sun as a result of underestimating the impact at 1000 m altitude2

Occasional

Exposure to cyanotoxins (including potentially neurotoxic ones) due to recreational contact (bathing)2

Exposure to microcystins (chiefly from P. rubescens) in drinking-water due to ineffective water treatment, probably at low concentrations based on likely low cell densities (< 10 Pg/L)

Cranial and spinal injury due to unsafe diving2

Rarely

Microbial contamination of drinking-water due to the presence of inadequately treated human effluent from the bathing area

Dark grey = high risk; Medium grey=medium risk; Light grey=low risk

Rationale: risks from cyanotoxins provisionally moderate for drinking-water and low for recreation (seeTable 6.4). Diving injuries reported more than once a year; sunburn frequently. Pathogens from sewage seeping through rock fissures and from people using the beach are not likely to reach the waterworks (inactivation by long travel times in the reservoir).

(Continued)

Example 3: Farm dugout serving as water source for 20-50 people

Severity of public health impact

Minor impact

Moderate impact

Major impact

Likelihood of occurrence

Often

Microbial contamination of farm

reservoir used for

drinking-water due to

defecation and run-off from livestock

Occasional

Exposure to cyanotoxins due to the consumption of contaminated foods (i.e„ cyanotoxins on irrigated vegetables and fruit)3

Possible exposure to cyanotoxins in drinking-water due to uncertain reliability of household-level treatment Exposure (including young children) to cyanotoxins due to recreational contact (bathing)3

Rarely

Dark grey=high risk; Medium grey=medium risk; Light grey=low risk

Rationale: moderate risks exists from exposure to cyanotoxins through recreational contact, low for foods (see Table 6.4). Cryptosporidium likely, due to cattle uphill of the dugout, which may cause severe illness, so risk is high.

Note: The above tables include a number of risks (denoted bya) that are not typically considered under a conventional WSP for drinking-water, but would be assessed in a risk assessment/ management plan for recreational water safety or for food under the similar principles of HACCP.

 
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