Many small communities obtain their drinking water from diverse sources (lakes, streams, springs and groundwater wells). Levels of treatment vary widely, from no treatment, to rudimentary systems with coarse screening or settling followed by disinfection (usually chlorination), to more sophisticated systems using a filtration step to reduce turbidity. Volcanic ashfall may have major effects on these systems. In general, the physical impacts of ash will be more important than any changes to the chemical composition of the water. However, there are exceptions including the island of Ambrym in Vanuatu where fluorine-rich gas and ash emissions from Marum and Benbow vents have led to elevated levels of fluoride in roof catchment drinking water tanks.
For systems with intakes in rivers or streams, common problems are that ash may clog intake structures, cause abrasion damage to pumps, and block pipes, settling ponds and open filter beds. The suspended ash may also overwhelm filtration systems, requiring an increased level of maintenance in cleaning filters. It may also be necessary to adjust the dose of disinfectant. It is essential to monitor chlorine residuals in the distribution system.
Open, shallow wells are used for household water supplies in some countries. If contaminated by volcanic ashfall they may become turbid. Options for treatment include adding a flocculating agent such as polyaluminium chloride (PAC) to reduce turbidity, then a disinfectant such as sodium hypochlorite (liquid bleach).
| Dosage of coagulant/litre1 | Dosage of disinfectant/litre | ||||
| Clarity of water | PAC (g) | Soda ash (g) | Alum (g) | Quicklime (g) | Chlorine2 (g) |
| Clear | - | - | - | - | 0.002 |
| Moderately clear | 0.05 | 0.05 | 0.15 | 0.05 | 0.002 |
| Cloudy | 0.1 | 0.1 | 0.3 | 0.1 | 0.002 |
For untreated water supplies drawn from surface waters, the major health risk is likely to be from pathogenic (disease-causing) micro-organisms. The risk from chemical contaminants is generally low from most ashfall events, although there are exceptions as noted above for countries such as Vanuatu.
Groundwater and spring-fed systems are generally very resilient to volcanic ashfalls, although pumping equipment may be vulnerable to airborne ash penetrating motors and to power outages. Any open channels will be vulnerable to becoming clogged with ash.