This saying is especially true when it comes to natural disasters, such as volcanic eruptions and wildfires, which are two of the three main sources of toxic ash. The third source is coal ash, for which we certainly cannot blame Mother Nature. But whether toxic ash results from natural or man-made disasters, its effects are crushing. Toxic ash not only destroys buildings and forests, it is the number one cause of the poisoning of our water sources.
Toxic Ash—Wild Fires
During October, November, and December of 2017, California’s wine country, the Napa Valley, suffered some of its worst fires in recent times. Forty two people died, more than 8,400 houses and buildings were completely destroyed, over 100,000 people were displaced from their homes, and over 160,000 acres were burned. But the scary part is that it will take as long as four years before we know if the greatest potential damage has been done—the poisoning of the area’s water sources. 
Ash, the residue left by forest fires, contains numerous harmful chemicals such as carbon, calcium, and magnesium. Even worse, when fires also consume buildings and homes, the resulting ash can contain lead, mercury, iron, copper, nickel and other poisonous substances.
Thunderstorms, which you would think would be the answer to your prayers in a wildfire, are in fact a nightmare when it comes to spreading the poisonous chemicals. The thunder clouds absorb the dust and toxic ash and spread them to land areas that are near water sources or to the water sources themselves. The chemicals remain in the soil for years, slowly leaking into the water. 
Toxic Ash—Coal Ash
In December of 2008 525 million gallons of wet coal ash flooded the Tenessee River when a retaining wall crumbled at the nearby power plant in Harriman. The sludge from the toxic coal ash destroyed 12 homes, killed a large number of fish, and the drinking water, nine years later, still contains elevated levels of lead and thallium. Authorities claim that standard water purification methods will filter out the poisons – and let’s hope they’re right. 
In February of 2014 there was a similar accident in Eden, North Carolina when 39,000 tons of carbon ash spilled out of the Duke Energy River Steam Station from a corroded pipe about 80 miles upstream from the Kerr Reservoir. While the EPA, local enviromental agencies, as well as the US Army Corp of Engineers were all out the following day to clean up, it took five months to clear away the ash and debris. Again, the drinking water sources will feel the effects of the spill for many years to come.
The Duke Station was ultimately fined $102,000,000 for negligence. Plant officials were aware of leaks and requested $20,000 in cameras to check the four main pipes — including the one that burst. The company’s head office turned down their request. 
There are almost 1,000 sites where coal ash is disposed of in the United States, with over 500 of these sites storing wet ash. Chemicals in the ash include: arsenic, lead, mercury, cadmium, chromium and selenium. The known effects of these leaked chemicals include various cancers, kidney disease, respiratory problems, and other illnesses. 
Toxic Ash—Volcanic Ash
While 9% of the Earth’s population lives in areas where there are active volcanoes, there is much less danger to the water sources than with ash from wildfires or carbon ash. Learning the lessons from Pompeii, virtually no residential or industrial areas are in the path of flowing lava.
Airborne volcanic ash contains large amounts of fluoride that can be dangerous if ingested, as well as causing difficulty in breathing. But potential danger to drinking water can be avoided with proper water purification methods. 
While there is nothing that can be done against volcanoes, damage from wildfires can be cut to a minimum with proper preparation and firefighting equipment
There have been studies that show that it is much safer to dispose of carbon ash by non-combustible means that would prevent a lot of the toxic sludge that exists today.  Together with proper maintenance of power plants and their retaining walls, carbon ash disasters can and should be eliminated completely.
Extreme water testing has to be done in all areas where there has been any kind of toxic ash incident and should continue for years — literally.
On the brighter side, volcanic ash and non-urban fires actually cause a rebirth in the affected areas. The nutrients in the ash enrich the soil, allowing flowers and trees to flourish once again.
 Adam Rogers, After the Napa Fires, a Disaster-in-Waiting: Toxic Ash, October 29, 2017
 Aregai Tecle and Daniel Neary, Water Quality Impacts of Forest Fires, July 14, 2015
 Scientific American EarthTalk, The Lasting Damage of the Tennessee Coal Ash Spill, March 2009 EPA, Duke Energy Coal Ash Spill in Eden, NC History and Response Time
 Gerry Broome, AP, Duke Energy Fined $102 Million Dollars in Coal Ash Spill
 Physicians for Social Responsibility, Coal Ash Toxics: Damaging to Human Health
 Carol Stewart, Can Volcanic Ash Poison Water Supplies?, May 2009
 IPEN, Toxic Ash Poisons Our Food Chain, April 20, 2017