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Water Supply

Main Issues

Newly fallen volcanic ash may result in short-term physical and chemical changes in water quality, increased wear on water-delivery and treatment systems (for example, pumping stations), and high demand for water during cleanup operations by residents of communities affected by ash fall. Historical eruptions generally have caused few water-quality problems, and hazardous chemical changes have been reported in only a few cases. The most common change in water quality results from the suspension of ash in open water-supply systems (uncovered reservoirs, lakes, streams, and water-catchment systems). Turbidity limits are easily exceeded by suspended ash. Water-quality monitoring programs can identify changes that may be hazardous and determine when warnings may need to be issued regarding potable water. Adverse affects on covered water supplies are minimal; for example wells with covered pump stations.

Precautions to keep ash out of water-supply equipment should be taken early to reduce or avoid damage to the equipment and other infrastructure, and increased water demand after ash fall should be anticipated.

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 ash web team for suggestions and contributions.


Water quality || turbidity || pH || chemistry ||

The addition of volcanic ash to water supplies can lead to a change in water quality. The most common ash-contamination problems result from a change in turbibity and acidity, but these usually last a few hours to a few days unless the ash fall occurs for prolonged periods of time. Hazardous changes in water chemistry are rare. Close to a volcano, however, water-soluble components that cling to particles of glass and crystals of the ash may lead to chemical changes in water supplies that render the water temporarily unsuitable for drinking.

Checking water quality of reservoir, New Zealand

Checking water quality at a water-supply dam for the city of Auckland, New Zealand. The reservoir lies on an active basaltic volcanic field.

Water-quality monitoring programs can identify and track water quality changes associated with ash fall that may be hazardous, and determine when public notices may need to be issued regarding potable water.

Relative vulnerability to volcanic ash contamination of water supplies (Johnston, 1997).

Type River/stream Reservoir Groundwater Roof-fed/water troughs
chemical low to medium low to medium low high
pH low to medium1 low to high1 low high
trubidity high medium to high low high
1 Depends on the ash thickness and water volume ratio


Background. Turbidity is a measure of water's ability to scatter and absorb light, and is caused by the presence of particulate matter, such as clay, silt, volcanic ash, colloidal particles, and microscopic organisms. Turbidity depends on a number of factors, such as the number, size, shape, and refractive index of particles in water. The greater the turbidity, the murkier the water.

Turbidity can have a significant effect on the microbiological quality of drinking water. Its presence can interfere with the detection of bacteria and viruses in drinking water; more importantly, turbid water has been shown to stimulate bacterial growth since nutrients are adsorbed on to particulate surfaces, thereby enabling the attached bacteria to grow more rapidly than those in free suspension (World Health Organization, 1996).

Effects of ash. Ash fall over areas supplied by open-water systems can increase turbidity significantly for short periods of time (days to a week) of time. For example, an ash fall of 3-6 mm in Anchorage, Alaska, during the 1953 eruption of Mount Spurr caused the turbidity of the public water supply to rise from 5 ppm to 290 ppm; it took six days to return to normal (Blong, 1984).

When ash fall causes water turbidity to increase, public officials typically advise consumers to boil water before drinking. The consumption of highly turbid water may constitute a health risk because excessive turbidity can protect pathogenic microorganisms from the effects of disinfectants and stimulate the growth of bacteria in distribution systems as noted above.

Volcanic ash suspended in water can clog and damage filters at intake structures and treatment plants and increase the wear on pumps used in water-delivery systems.


Background. Most precipitation has a pH level slightly below 7 (neutral); contact with carbon dioxide in the air makes it slightly acidic. "Acid Rain" is the term for precipitation with a pH of less than 5.6. Acidic water leads to corrosion of brass fixtures, copper plumbing, steel tanks, heating elements in hot-water heaters, and concrete.

Effects of ash.  Fresh volcanic ash typically lowers the pH of water. For example, an ash fall of 3-6 mm in Anchorage, Alaska, during the 1953 eruption of Mount Spurr caused the pH of the public water supply to fall to 4.5; within a few hours, the pH returned to 7.9. 


Potentially harmful substances in some volcanic ash are the water-soluble materials called leachates, mostly acids and salts, that cling to the particles of glass and crystals. These soluble coatings are derived from the interactions in an eruption column between ash particles and gas aerosols, which may be composed of sulphuric and hydrochloric acid droplets with absorbed halide salts. It is these components that make ash mildly corrosive and potentially conductive.

The most common leachates are Cl, SO4, Na, Ca, K, Mg, and F. Other elements reported but in lower concentrations include Mn, Zn, Ba, Se, Br, B, Al, Si, Cd, Pb, As, Cu and Fe. Most of these elements and compounds are naturally present in ground and surface water but become hazardous above threshold concentrations. Finer ash is able to carry more soluble ions than coarser ash because of its larger surface area; fine ash and smaller-sized ash travel greater distances from an erupting volcano and typically extend over very wide areas than larger ash particles.

Observations from historical eruptions show that concentrations of hazardous leachates in ash decrease with increasing distance from an erupting volcano, with few examples of serious chemical contamination of portable water supplies.

Fluorine: Excess fluorine is recognized as the most hazardous leachate in water supplies, but few historical eruptions are known to have resulted in fluorine poisoning in humans. The main concern of fluorine poisioning is for livestock, which graze on ash-contaminated grass and feed.

"Consumption of water with fluoride concentration of 2-10 ppm would not be expected to cause ill health if the contamination lasted only a few days, though it would be prudent for susceptible people, mainly those with chronic sickness, to use uncontaminated water. Acute exposure to higher concentrations can cause gastrointestinal illness. Consumption of water containing greater than 1 ppm fluoride over long periods could lead to dental mottling in children and, at higher concentrations, osteofluorosis." (Baxter and others, 1982, p. 271).

Hekla, Iceland: 14 May 1970; "Local groundwater is measuring high amounts of fluorine, which is toxic to sheep and horses. Fluorine concentration in creek water  has been measured at 10 mg/liter." (Smithsonian Institution Global Volcanism Program; see http://www.volcano.si.edu/world/volcano.cfm?vnum=1702-07=&volpage=var#cslp_7005).

Hekla, Iceland: 26 Feb 2000: "Ash from previous Hekla eruptions has often been the cause of fluorosis in grazing animals. However, during this time of the year most domestic animals are kept indoors, so fluorosis is not expected to become a problem. Freshly fallen ash was measured for soluble fluoride ions (F-). The result was 800-900 mg F/kg. Snow melted by the ash contained about 2,200 mg/l (ppm) of fluoride," (see Smithsonian Institution Global Volcanism Program; http://www.volcano.si.edu/world/volcano.cfm?vnum=1702-07=&volpage=var#bgvn_2502).

Mount St. Helens, U.S.: 18 May 1980: After the 1980 eruption of Mount St. Helens, the amounts of water-soluble materials were apparently not large enough to significantly affect well or surface water supplies. Many laboratories performed leaching tests on a variety of ash samples, and none of the tests indicated soluble chemical contaminants at concentrations great enough to exceed the Maximum Contaminant Level (MCL) for public water supplies. The tests simulated the effects of rain falling on the ash.

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Water demand

It may be necessary to control water demand in order to avoid water shortages for critical uses (especially for fire suppression, drinking water, and sanitation). A discrepancy between water demand and supply may be created by one or both of:

1. Water pressure and supply problems (water quality and supply system).
2. Increased demand, especially for ash cleanup.

The high demand for water that typically occurrs after an ash fall can lead to temporary water shortages, especially if there are problems with water quality and supply. It may be necessary to employ water-use restrictions or rationing after ash fall as people try to wash off cars, homes and buildings, and streets. For example, after the 1980 eruption of Mount St. Helens, some communities imposed an odds-even rationing system based on house numbers and dates and restricted water supply to the largest customers. One community imposed rationing only during the peak demand hours to ensure that line pressure was kept up in the event of a fire or some other high requirement situation. If uncontaminated ground water sources had not been available to most communities downwind of Mount St. Helens, the demand would have exceeded supply to a much greater extent (Blong, 1984). It may be wise to employ such restrictions early, even before any demand/supply discrepancy becomes apparent.

See also the Ruapehu (1995/96) and Mount Spur (1992) case studies below.

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Equipment damage

Volcanic ash suspended in water can clog and damage filters at intake structures and treatment plants, and ash can increase wear on pumps and other equipment used in water-delivery systems. Because volcanic ash consists of tiny pieces rock and volcanic glass, ash can infiltrate nearly every opening and abrade or scratch most surfaces, especially between moving parts of equipment. Ash particles easily clog air-filtration systems, which can lead to overheating and engine failure.

Damage to water-supply systems (pipes) from ash fall have generally been minor. Water reticulation systems have been easily damaged by lava flows, lahars (which can be caused by rainfall and subsequent erosion of ash into rivers), earthquakes, and ground deformation associated with magma intrusion and eruption. Such activity can easily clog water intakes and severe pipes.

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Mitigation measures || source supplies || tanks and troughs ||

Stream, river, lake and reservoir supplies

Water supply intakes should be closed before turbidity and acidity levels become excessive; regular monitoring will determine when such levels are reached and indicate when the intakes can be opened again. High turbidity levels are usually manageable if water-treatment filters are cleaned or replaced frequently. Filters can become blocked, however, if turbidity levels become excessive. When turbidity is high, precautionary warnings to "boil water" might be issued to residents because the suspended ash may have decreased the effectiveness of any disinfection or flocculation process.

As the fine ash can remain in suspension for long periods (days to weeks) a coagulation-flocculating agent may need to be added. Alum is found to be the best agent (Hindin, 1981).

To reduce the physical damage to water supply systems, equipment and pumps should be covered when there is an impending ash fall, and the ash should be removed before normal operations resume.

Water tanks and farm water troughs

In addition to potential turbidity and acidity problems, bodies of water close to an erupting volcano with low volume-to-catchment-area ratios may be subject to chemical contamination by leachates, notably fluorine. When ash fall occurs, households with roof water supply should immediately disconnect down pipes connected to a water-supply tank. If ash collects on a roof and down pipes were not disconnected, it is recommnded that the tank water be tested before it is used for potable water. If turbidity remains high, even when tests show the water is non-toxic, the water should be boiled before use. If testing is not available it would be advisable to drain and flush the tank and refill with uncontaminated water.

Farm water troughs are highly vulnerable to contamination and would most probably need to be emptied and refilled after ash falls.

Water tank supplied by roof of small building

A covered water tank will be safe from direct ash fall contamination, and it may provide valuable water supply during periods ash fall if water use is carefully conserved. Water inflow should be closed before ash lands on the source (the house roof in this case), and the top must be regularly swept/shoveled free of ash to avoid collapse. The inflow should not be opened again until the chemical effect of the ash on water supply is determined safe, or the source is cleared of ash.

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Case studies || Ruapehu || Ambrym || Copahue || Mount St. Helens || Mount Spur || Irazú || Soufriere Hills || Hekla ||

Contamination of water supplies has been reported for several historical eruptions. The most common problems resulting from ash fall included unacceptable pH and turbidity levels and higher than usual demand for water. Post ash fall water supply is one of the most significant management issues to be addressed. Hazardous chemical changes in water supplies have been reported in only a few cases.

New Zealand - Ruapehu

The 1945 eruption of Ruapehu volcano produced deposits of several mm thick throughout the North Island of New Zealand, which caused problems for the water supply in the town of Taumarunui (Johnston, 1997). Ash suspended in the Wanganui River, from which the town draws its supply, blocked filters at the supply's intake structure. Pumping was reduced from 90,000 litres/hour to 31,500 litres/hour (Wanganui Herald 26/11/45). Turbidity problems were also experienced in the Whakapapa Village stream-fed water supply, located at the base of the mountain, forcing the Chateau, which was a hospital at the time, to close. Further problems were reported 10 years later when ash that was deposited in 1945 on Mt Ruapehu's snow fields was washed into streams that fed the Whakapapa Village water supply (Houghton and others, 1987).

The 1969 Ruapehu eruption deposited 6-7 mm of ash over an areas that contaminated the water supply of Iwikau Village (located at the Whakapapa Skifield) (Collins, 1978). Water is drawn from the roofs of most of the buildings in the village. The drain pipes were cut after the eruption began, but a number of storage tanks were found to be contaminated. Acidity of the tank water ranged from pH 6 to 4.4, and excessive fluoride concentrations (6.0 ppm) were measured in one tank. Also, Whakapapa Village received 1-6 mm of ash causing the pH of the stream fed water supply to drop to 5.3 and the turbidity to increase markedly. The Taumarunui Borough found for a short period after this eruption that the pH of their water supply dropped to 5.6 pH and turbidity was high.

During the 1995-1996 eruption of Ruapehu volcano ash fell over a wide area of the North Island. Contamination of water supplies was a common concern, and the public was advised to disconnect roof-fed water tanks as a precaution (Johnston and others, 2000). In some cases this proved to be very difficult to accomplish, especially newer enclosed systems. The Department of Conservation disconnected roof-fed water supplies at many of the huts in the Tongariro National Park. Many communities initiated special or enhanced monitoring of their water supplies due to the potential for ash contamination. In some cases extra supplies were stored.

For example, at the Waiouru military base water was stockpiled in jerry cans, rubber bladders and fire engine tanks, and the stream supply was monitored regularly (New Zealand Herald, 27 October 1995). In October 1995 the Rotorua District Council constructed enclosures (costing $120,000) over the Rotorua city and Reporoa spring supplies to mitigate the potential of ash contamination (Johnston, 1997). This was to prove valuable in protecting the supply during the 17 June 1996 ash falls. However, Rotorua almost ran out of water after the ash fall when a resident washed ash into a power transformer. The transformer subsequently exploded, cutting electricity to the town's water pumps. Furthermore, the clean-up efforts of the city's residents almost drained the water supply; the district council imposed a hosing ban until the power supply was restored to the water supply headworks (Daily Post, 19 June 1996). On 17 June 1996 the water supply from the Raymond Dam in the Western Bay of Plenty was disconnected for four hours after the ash falls, while water analysis was carried out. The supply feeds Pukehina, Maketu, Pongakawa and Paengaroa and Te Puke. Some areas were without water for several hours (Bay of Plenty Times, 18 June 1996) but supplies were fully restored by the following day.

Vanuatu - Ambrym

Ambrym Island is a large basaltic shield volcano in Vanuatu. Fine ash falls from an eruption in 1953 led to ash contamination of tank water at Santo 130 km from the volcano (Blong, 1984). In a 1999 study Cronin and Sharp (2002) investigated water supplies from ash-affected areas from two volcanoes (Tanna and Ambrym); only samples from Ambrym were found to have excessive Fluorine levels.

Argentina - Copahue Volcano

Copahue Volcano is a large stratovolcano situated on the border between Argentina and Chile. During eruptions in July 2000, 30-50 mm of ash was deposited in and around Lake Caviahue. Water authorities in the area reported changes in water color from normal deep blue to greyish green. In the days/weeks following the eruption the pH of the lake dropped to 2.1. Ashfall reported 60 km from the volcano caused increased turbidity in streams and changes in pH from a typical pH of 7 to 2.5. Preliminary investigations by Argentina's Provincial Water Division revealed increased iron, fluorine and sulphate levels in water supplies. Ashfall related damage resulted in power outages to water treatment plants and cut off potable water supplies (see Global Volcanism Program, Smithsonian Institution, http://www.volcano.si.edu/world/volcano.cfm?vnum=1507-09=&volpage=var#bgvn_2506).

U.S.A. Washington - Mount St. Helens

The 1980 eruption of Mount St. Helens produced significant hydrologic and water quality effects in areas affected by ash fall. Ash contamination produced problems for a number of communities with excessive pH and turbidity levels reported in stream-derived water supplies (Blong 1984; Warrick and others, 1981). Samples of ash were found to contain 0.25% water soluble salts, mainly as sulphates and chlorides in the form of sodium salts (Moen and McLuca, 1980). In laboratory experiments distilled water became acidic when placed in contact with fresh ash but returned to normal pH within hours. No excessive chemical concentrations were found in community water supplies in the ash-affected areas.

Increased water demand was experienced in many ash affected communities (Blong 1984; Warrick and others, 1981). For example in Ellensburg, eastern Washington, demand during the first four days exceeded average demand 2.5 times (Warrick and others, 1981).

U.S.A. Alaska - Mount Spur

Anchorage is the largest city in Alaska, located 120 km east of Mount Spurr. The 1959 eruption of Mount Spur deposited 3-6 mm of ash on the city, causing short term pH and turbidity problems with the city's water supply (Wilcox, 1959). The pH level fell to 4.5 before returning to normal after a few hours. Turbidity rose from 5 ppm to 290 ppm and lasted six days before returning to normal.

The August 1992 eruption deposited about 3 mm of fine sand-sized volcanic ash on the city (Johnston, 1997b). The clean-up of ash resulted in excessive demands for water and caused major problems for the Anchorage Water and Wastewater Utility (AWWU) water production and distribution systems (AWWU pers comm.). The AWWU received a warning of the impending ash fall on the afternoon of 18 August. No action was taken that evening.

As one staff member described "we did not equate ash fall to high water demand... we were not prepared for what happened... had we known we would have moved to fill reservoirs sooner." By 10 am on 19 August (the day following the ash fall) a peak four hour demand of 230 million litres per day was recorded, about a 70 percent increase in normal demand (see graph below). Despite adequate production capacity, physical restrictions within the distribution system prevented the utility from moving sufficient water volumes to meet demand in parts of Anchorage.

Graph showing water production  rate in August 1992

The August 1992 water production rate for the city of Anchorage. (Source: unpublished Anchorage Water and Wastewater Utility (AWWU) data)

The high water demand caused widespread water-pressure and -supply problems throughout 19 August, with levels in several storage reservoirs dropping to dangerously low levels. Some reservoirs were isolated from the immediate distribution system to ensure adequate volumes for fire suppression if required. At least one reservoir was completely emptied. Had fires occurred in parts of the city no water would have been available.

Opening of the Anchorage International Airport was delayed for several hours due to shortages of water to clean the runways. Stranded passengers were unable to use the toilets due to the lack of water.

Coasta Rica - Irazú

The 1963 eruption of Irazú volcano caused water supply problems in the capital with fine ash clogging filters at the river-feed water works (Blong, 1984). The main water pumping station was reported to have been out of action in mid-June 1964 with water being trucked in to the city.

Montserrat - Soufriere Hills

The eruptions of Soufriere Hills during 1997 produced chemical contamination of rainwater and surface water. Water sampling in January 1997 indicated highly acidic water with high concentrations of sulphates, chloride and fluorides. Similar results were recorded until June 1997 although all fell within World Health Organization recommended levels for all measured components (see Smithsonian Institution Global Volcanism Program; http://www.volcano.si.edu/world/volcano.cfm?vnum=1600-05=&volpage=var#bgvn_2207).

Iceland - Hekla

After tephra falls during the 1947 eruption of Hekla, fluorine contents of stream waters in areas that received between 1 to 10 cm of ash were found to be between 1.0 and 4.5 ppm for a short duration (Thorarinsson, 1979), elevated from the normal background level of 0.2 ppm. In the 1970 eruption of Hekla, excess fluorine levels (up to 8 ppm) were recorded in the Galtalaekur River. Levels returned to normal with a few hours (Oskarsson, 1980). Although fluorine contamination of water supplies was short lived and caused only a minor environmental disturbances, fluoride contamination of pastures did cause major stock losses (Blong, 1984; Thorarinsson, 1979).

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Baxter, P.L., Bernstein, R.S., Falk, H., French, J. and Ing, R., 1982, Medical aspects of volcanic disasters: An outline of the hazards and emergency response measures: Disasters, v. 6, n. 4, p. 268-276.

Cronin, S.J., and Sharp, D.S., 2002, Environmental impacts on health from continuous volcanic activity at Yasur (Tanna) and Ambrym, Vanuatu: International Journal of Environmental Health Research 12, p. 109-123.

Hindin, E., 1981, Rendering ash contaminated water potable, in Keller, S.A.C. (eds), Mount St. Helens One Year Later: Eastern Washington University Press, p. 237.

Hoverd, J., Johnston, D., Stewart, C., Thordarsson, T. and Cronin, S., (in prep), Impacts of volcanic ash on water supplies in Auckland: Science Report, Institute of Geological and Nuclear Sciences, Wellington.

Johnston, D., Dolan, L., Becker, J., Alloway, B., Weinstein, P., 2001, Volcanic ash review ? Part 1: Impacts on lifelines services and collection/disposal issues: Auckland Regional Council Technical Publication, No. 144, 50 p.

Johnston, D.M., 1997, Physical and social impacts of past and future volcanic eruptions in New Zealand, Unpublished Ph.D. thesis, University of Canterbury, Christchurch, 288 p.

Moen, W.S. and McLucas, G.B., 1980, Mount St. Helens ash: Properties and possible uses: Report of Investigation 24, Washington Department of Natural Resources, Division of Geology and Earth Resources, 60p.

Mount St Helens Technical Information Network Bulletins: U.S. Federal Emergency Management Agency, Federal Coordinating Office.

Oskarsson, N., 1980, The interaction between volcanic gases and tephra: Fluorine adhering to tephra of the 1970 Hekla eruption: Journal of Volcanology and Geothermal Research, v. 8, p. 251-266.

Thorarinsson, S., 1979, On the damage caused by volcanic eruptions with special reference to tephra and gases, in Sheets, P.D. and Grayson, D.K. (eds.), Volcanic activity and human ecology: Academic Press, p 125-156.

Warrick, R.A., Anderson, J., Downing, T., Lyons, J., Ressler, J., Warrick, M., Warrick, T., 1981, Four communities under ash - after Mount St. Helens: Program on Technology, Environment and Man, Mongraph 34, Institute of Behavioral Science, University of Colorado, 143 p.

Weniger, B.G., Gedrose, M.B., Lippy, E.C., Juranek, D.D., 1983, An outbreak of waterborne giardiasis associated with heavey water runoff due to warm water and volcanic ashfall: American Journal of Public Health v. 73, p. 868-872.

Wilcox, R.E., 1959. Some effects of recent volcanic ash with special reference to Alaska: U.S. Geological Survey Bulletin 1028 N, Washington D.C., U.S. Government Printing Office, p. 409-476.

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