Water Supply

• Water Demand
• Large Treatment System
• Small Treatment System
• Mitigation
• Roof Catchment
• Case Studies

Fresh water supply and wastewater collection and treatment become vulnerable during a volcanic ashfall, which can:

• cause changes to water quality in raw water sources.
• create high water demand during the cleanup phase, which can in turn lead to water shortages.
• cause operational problems for water treatment plants.

An increased water demand is extremely common following an ashfall, as the cleanup phase begins, which should be anticipated and planned for.

Further research is underway for ash-related mitigation measures for water supply and changes in water chemistry resulting from volcanic ash following historical volcanic eruptions. Please contact the Volcanic Ash Web Team for suggestions and contributions.

Turbidity affects water quality and water treatment.

Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates. The more suspended material in the water, the higher the turbidity. Volcanic ashfalls can increase water turbidity if the finer particles remain suspended in the water. In general, the major water quality affect of ashfall on raw water sources is increased turbidity rather than changes in chemical composition.

Increases in turbidity can cause problems for water treatment plants, although the extent to which this occurs is strongly dependent on the design of the treatment system. Resilient design features may help to mitigate against damage. Suspended particles act to 'shelter' pathogenic (disease-causing) micro-organisms, which reduces the efficiency and effectiveness of disinfection treatments at water treatment facilities. A vitally important step in drinking-water treatment is turbidity reduction prior to disinfection. Steps that remove turbidity throughout the water treatment process include settling wells, initial coagulation/flocculation treatment, and filtration. Filtration options commonly include slow sand filter beds or pressure sand filters.

Impacts of suspended solids on water treatment plants are discussed in the wastewater treatment plants section.

Ash can cause chemical changes that affect water quality.

Changes in water composition for a particular water body depend on the depth of ashfall and its 'cargo' of water-soluble elements; the area of the catchment and volume available for dilution; and the pre-existing composition of the water body. Release of readily soluble elements from freshly fallen ash may lead to concentration increases in surface waters.

Typical effects of ashfall on the quality of surface water bodies

Turbidity	Ash suspended in water will increase turbidity in lakes, reservoirs, rivers and stream. Very fine ash will settle slowly and residual turbidity may remain in standing water bodies. In streams, ash may continue to be mobilised by rainfall events, and lahars may be a hazard in some regions.
Acidity (pH)	Fresh ashfall commonly has an acidic surface coating. This may cause a slight depression of pH (not usually below pH 6.5) in low-alkalinity surface waters.
Potentially toxic elements	Fresh ash has a surface coating of soluble salts that are rapidly released on contact with water. The most abundant soluble elements are typically Ca, Na, K, Mg, Al, Cl, S and F. Compositional changes depend on the depth of ashfall and its 'cargo' of water-soluble elements; the area of the catchment and volume available for dilution; and the pre-existing composition of the water body. In rivers and streams, there will be a short-lived pulse of dissolved constituents In lakes and reservoirs, the volume is usually large enough that changes in composition are not discernible The constituents most likely to be elevated above background levels in natural waters are Fe, Al and Mn, because these are normally present at very low levels. Thus water is likely to become unpalatable due to discolouration or a metallic taste before it becomes a health hazard.

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