Arsenic, drinking-water and health risk substitution in arsenic mitigation: A discussion paper
There are water-related health risks associated with all forms of water supply. In reducing one water-related health risk another may be substituted, sometimes of greater magnitude. In Bangladesh, a consequence of reducing the risk from microbial contamination of drinking water was the inadvertent substitution of a risk from arsenic.
In developing an emergency response to the arsenic crisis, the potential for risk substitution from other hazards must be considered. Water supply options should be selected within an overall risk management framework of Water Safety Plans. In selecting options, it is important that a consistent approach is adopted in evaluating all risks
The hazards that may substitute for arsenic include: microbial hazards (pathogens); toxins derived from cyanobacteria in surface water; and chemical contaminants from pollution. This report provides a qualitative risk comparison between arsenic and other hazards, but quantitative risk comparisons should be considered as a priority in the short term.
Risks and poverty
Risks from both arsenic and microbial hazards are strongly related to poverty and nutrition, and for microbial hazards there is a synergistic relationship between under-nutrition and repeated infection by microbial hazards.
Nature of health effects
Microbial hazards lead to acute health effects and attack rates commonly range from 20 to 70%. Effects range from self-limiting diarrhoea to mortality. Mortality is more common in particularly sensitive sub-groups (infant, children, immuno-compromised and pregnant women).
Arsenicosis is a chronic disease with a significant latency period for non-cancer and cancer effects. The proportion of a population exposed to elevated arsenic that will develop arsenicosis is uncertain, but may be significant.
Treatment of health effects
Medical treatment of infections by microbial hazards is generally well understood. In practice access may be limited to medical care, particularly among the poor.
Medical treatments for arsenicosis are not fully developed. There is indication that switching to arsenic-safe water and anti-oxidants may reverse symptoms in early stages.
Comparing the risks
Overall the risks posed by microbial hazards are greater than those posed by arsenic. This does not mean arsenic mitigation is not important but that emergency response measures must ensure that risks from microbial hazards do not increase.
For all options considered in the emergency programme, hygiene education will be essential to promote safe water handling.
In the short term it is unlikely that risks from cyanobacterial toxins will be greater than those posed by arsenic, but in the longer-term would need to be considered in defining appropriate water supply options.
Audits in emergency response
During implementation of the emergency response, third-party audit of construction quality is essential and should apply to all agencies undertaking construction.
Effective control of risks
Although designs can include effective control measures for microbial hazards, good operation and maintenance is essential to ongoing risk management, even within the short timeframe of an emergency response.
Community operators require proper training in O&M, including action-oriented monitoring and must have access to the appropriate tools. In addition to training of operators, O&M should be supported through development of an ongoing surveillance programme.
Pond sand filters
The performance of pond sand filters is often poor and there are concerns regarding both the ability to reduce risks from microbial hazards and cyanobacterial toxins. Consideration is being given to develop slow-sand filters as a more effective alternative to pond sand filters. The use of any technology for treating surface water must meet clear criteria regarding selection of ponds.
There is some evidence emerging of arsenic contamination of dug wells. Dug wells are also vulnerable to microbial hazards. Although these can be reduced through good design it is difficult to assure water safety in the monsoon and chlorination may be needed. It may be more appropriate to consider renovation of dug wells rather than construction of new wells.
Deep hand tubewells
Deep hand tubewells are an attractive option as microbial hazards are relatively easy to control. More recent data indicates that the deep aquifer is contaminated with arsenic in some areas. This needs urgent clarification.
The USGS study will provide further useful information to base decisions regarding the use of deep hand tubewells outside of areas that have been shown to be arsenic-safe.
The risks posed by rainwater harvesting are relatively easy to manage. Rainwater may not last the whole dry season and therefore promotion of rainwater will need to be combined with other solutions.
Arsenic removal technologies were not included in the short-list of emergency options as none had been formally verified through the ETV. The disadvantages noted for risk substitution for arsenic removal technologies are shared by other alternatives and do not appear adequate to disbar consideration in an emergency response. Community-level technologies would be more attractive than household options at this stage.
Household water treatment
Household treatment of water to remove microbial hazards could be considered as an option as there is evidence that these have a significant impact on diarrhoeal disease. Promotion of household treatment for microbial hazards could be considered in conjunction with other emergency interventions.
A programme of water quality surveillance should be developed to support the emergency response and the longer-term mitigation strategy. This can build on pilot activities in urban areas undertaken by DPHE and WHO and the protocol for surveillance in rural areas developed recently for DPHE. The surveillance programme should include testing for E.coli or thermotolerant coliforms and a rolling programme of repeat testing of tubewells for arsenic.