In the midst of devastating fires across Australia, rainfall is desperately needed. However, following fires, contaminated runoff to waterways will present a new wave of challenges regarding risks to drinking water quality. I prepared this thread to highlight the key issues. 1/20 
When forests burn, they produce large masses of burnt wood and ash, which accumulate on the forest floor. A large rainfall event, when it comes, will wash much of the ash into waterways. In turn, drinking water reservoirs (“dams”) will be impacted by ash and sediment. 2/20
Loss of trees and other live plants in the catchment will leave soils highly exposed to erosion, which could also significantly add to large sediment loads to waterways. Other sources of contamination may include dead animals and burned homes, vehicles, power cables, etc. 3/20
Bushfire ash is largely composed of organic carbon, which will be biodegraded by bacteria, naturally present in waterways. As bacteria consume carbon, they also consume dissolved oxygen from water, leading to low oxygen concentrations or “deoxygenation”. 4/20
Deoxygenation of waterways can lead to a number of water quality outcomes. The most visible is a fish kill as many species cannot survive in low oxygen water. The water may also turn black (known as a “blackwater event”) due to chemical changes in the water. 5/20
Some naturally occurring minerals, such as iron and manganese, become soluble in low-oxygen water and these may be precipitated later when water is re-oxidised. And some substances, such as sulphate may be transformed to odorous gasses, such as hydrogen sulphide (H2S). 6/20
If minerals such as iron and manganese are dissolved, they don’t present public health risks. However, drinking water treatment can cause them to be re-oxidised, which can turn the water a yellow, orange or brown colour, causing concern to drinking water consumers. 7/20
Organic carbon, dissolved in water will react with chlorine, which is used in drinking water treatment. These reactions produce chemicals called disinfection byproducts, some of which pose health risks following long-term (e.g. most of a lifetime) exposure in drinking water. 8/20
Disinfection byproducts are included in Australian Drinking Water Guidelines and some guideline concentrations might be exceeded. Although efforts should be made to prevent this, we should remember that the guideline numbers are based on the assumption of long-term exposure. 9/20
In addition to organic carbon, ash contains high concentrations of nutrients, (nitrogen & phosphorus). Increased concentrations of nutrients can stimulate the growth of algae and cyanobacteria, as described in detail in this previous tweet thread. 10/20
As described in the previous tweet thread (above), growth of cyanobacteria can lead to numerous water quality issues, including taste & odour impacts and possibly the production of toxic chemicals, known as cyanotoxins. 11/20
Air-dropped fire retardants, used to help control bushfires, can also add to pollutants. Most are based on ammonium phosphate formulations, which add to the load of nutrients (nitrogen & phosphorous). Ammonia is also toxic to fish if it enters waterways. 12/20
A wide range of other contaminants may also be present in the untreated water. These include microorganisms (bacteria and protozoa) from dead animals, as well as metals and polyaromatic hydrocarbons from burned vehicles, homes and infrastructure. 13/20
Many water treatment plants include filtration. The filters will remove much of the suspended material (ash, algae, etc.), but doing so will clog the filters. This means the filters will need to spend more time “backwashing” and less time producing drinking water. 14/20
This increased rate of filter backwashing could slow down drinking water production such that it struggles to keep up with demand. This would require us to limit water use to prevent running out. A similar scenario was faced in Brisbane following major flooding in 2013. 15/20
Water managers have a number of ways to protect us from water quality impacts described in this thread. First, many large dams have a variable range of depths from which water can be drawn, potentially allowing the best quality water to be targeted and the worst avoided. 16/20
Drinking water treatment plants are quite variable in the technologies they use and their level of sophistication. So its difficult to generalise about sediment and chemical removal, but processes including coagulation and filtration are important and can be very effective. 17/20
Bacteria can be inactivated by chlorine disinfection, which is standard in NSW. But effectiveness depends on relatively clean water quality, -ether from the source or by effective pre-treatment processes. Plants which can’t achieve either may be at risk. 18/20
Other pathogens, such as cryptosporidium, are much less likely to be present. But if they are, they’re more difficult to remove and generally require a combination of excellent filtration performance and disinfection by ultraviolet radiation. Few plants have that capacity. 19/20
My intention is not to be alarmist, but to highlight risks that will require careful management. We’re up for some water management challenges and I think NSW authorities can meet them. But understanding the risks, attention to detail & broad cooperation will be necessary. 20/20
