Vulnerability of Aquifers to Pollution from Ash and Long-term Fire Retardants, after a Wildfire.
The ash produced by burnt biomass after a wildfire leads to the release of elements and trace elements that could end up in the groundwater, deteriorating groundwater quality. The usage of Long-term Fire Retardants by the Fire Services could contribute to groundwater pollution. The mechanism of the pollutants' release is described. The possibility the groundwater of the subjected area to be affected depends on aquifers' vulnerability. The latter is considered a multicomponent "function" of several parameters, also described.
Vulnerability of aquifers to pollution is a “function” of the type of the rock, the depth of the unsaturated zone, the climate of the area and the composition of pollutants. Burnt biomass affects the composition of the ash. Ash and Long-term Fire Retardants facilitate the accumulation of major elements and trace elements (the former), and of nitrogen, phosphorus, and sulfur compounds (the latter) 
The mechanism of pollutants' release and percolation
According to several laboratory-conducted studies that examine simulated combustions of biomass, ash is alkaline, rich in elements such as Ca, Mg, Na, K, Al, Fe and P, S and trace elements, such as Mn and Zn  . The elements prevailing in the ash are reported to be Ca, Mg, K, Si, P , Mn, Zn, V, Pb, Cr, and Cu . The ash is transported by runoff and wind to surface water bodies, while a part of it, after being steeped, penetrates the soil, filling up the water of aquifers. Hence, ash affects the soil profile and is possible to deteriorate the quality of water supplies. Since infiltrating water elutes cations to the soil, concentrations of nutrients and pH values of the soil-profile alter. Elevated pH values affect the availability of nutrients with potential consequences on vegetation. The values of pH increase and there is a release of cations from the ash, such as K, Na, Ca, and Mg. The ash produced at high temperatures of combustion is considered hydrophilic, facilitating infiltration. According to the literature, the ash produced by combustions with temperatures higher than 450 °C is characterized by high concentrations of metals . Metals are of major importance in earth science due to their high toxicity, their long life and their tendency to bioaccumulate in aquatic systems . The severity of wildfires affects both the type and the amount of eluted elements, while the Sodium Adsorption Ratio (SAR) appears to be higher for intense wildfires. Moreover, in areas with Mediterranean climate, as the new hydrological year begins, intense rainfalls succeed the period of wildfires. Rainfalls activate the transportation of ash to nearby sites by increasing the runoff .
The role of Long-term Fire Retardants
In literature several laboratory studies investigate the role of Long-term Fire Retardants. Long-term Fire Retardants are the same chemical compounds of nitrogen, phosphorus, and sulfur, also found in agricultural fertilizers  . Laboratory studies (i.e. treatments of biomass needles) have reported the leaching of ammonium ions and phosphates and the elution of Fe, Cu, Zn, and Mn into soil water, which could end up in the groundwater .
The multicomponent vulnerability “function”
The fact that aquifer vulnerability to percolation of ash-originated pollutants, is considered a function means that e.g. in cases the unsaturated zone is protected by impermeable rocks or/and the thickness of the unsaturated zone is prominent (granular aquifers), or/and the vertical component of hydraulic conductivity of the unsaturated zone is low, percolation may be obstructed . In addition, if the average annual precipitation is high enough (e.g. greater than 900 mm/y) the dissolution and transportation of pollutants through surface water and groundwater is facilitated  . Thus, the concentrations of infiltrating elements and trace elements in groundwater, if any, is possible to remain below the potable limits set by the European Council or the World Health Organization  . Moreover, as a rule, precipitation increases along with altitude. Coastal aquifers could be vulnerable to pollution (e.g. due to their smaller depths), while Karst aquifers, where limestones prevail, are prone to pollution  . (There are numerous methods in the literature for the assessment of aquifer vulnerability.)
All things considered, ash from burnt biomass after a severe wildfire, and the usage of Long-term Fire Retardants by Fire Services, lead to the release of pollutants that could end up in the groundwater. The possibility of hydro-chemical disturbances of groundwater quality, subsequent to a wildfire, depends on the vulnerability of the aquifers. Vulnerability of the aquifers, as explained above, is a “function” of several parameters regarding climate conditions, lithology, sensitivity of the aquifer, composition of the pollutants. Hence, each site subjected to a wildfire is recommended to be individually examined.
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