Characterisation of bushfire residuals in source water and removal by coagulation

[Graphical abstract present]

A bushfire is a spontaneous vegetation fire that can fundamentally affect lives, property, the environment, and even the global climate. Ash from fire carries hazardous pollutants like metal oxides/hydroxides, minerals, black carbons, and by-products of partial combustion, such as hydrocarbons and colloidal charcoal. Bushfire gases and residues can heavily pollute surface and groundwater resources. 

This paper focuses on the impact of bushfire residue on water quality and explores methods to remediate impacted water supplies. Soils burned in controlled furnace conditions between 150 °C, and 600 °C were characterised, suspended in water, and changes in water quality was measured following leaching from the burned residues. Results indicate that once the soil is burned at temperatures above 300 °C, there is little evidence of leached organic matter. At temperatures below 300 °C, the water discolouration was evident after 24 h leaching, and much higher quantities of leached organic matter were measured. Higher burning temperatures resulted in more alkaline residues. 

Leachate and charred sample characterisation data shows that the charcoal is highly porous and mainly consists of- amorphous material. The ash is a heterogeneous concoction of smaller particles and comprises significant mineral content. The results also indicate that the primary pollutant among the brushfire residuals is ash which increases pH, alkalinity, turbidity, and UV₂₅₄. Coagulation experiments reveal that dual coagulation systems with metal salts- organic polyelectrolyte reduced the turbidity by 84 %, and dissolved organic carbon (DOC) reduced by 68 % of water containing ash residues. However, some other treatments are needed to reduce the alkalinity.