REPORT: Is Residential Wood Ash Safe for Use to Help Solve the Calcium Decline Problem in Muskoka?

The overarching goal of the HATSEO project is to evaluate the use of residential wood ash – a readily available waste product – to help solve the Ca decline problem in Muskoka forests and lakes. In Chapter 1 of this report, the metal concentrations of 11 regulated trace metals in 14 local sources of residential wood ash are reported. The concentrations of the trace metals are then compared with target thresholds of the content of regulated materials (CM) for non-aqueous non-agricultural source materials (NASM) set by Ontario Regulations 267/03 of the Nutrient Management Act (Government of Ontario 2002). This analysis revealed that the metal concentration in the vast majority of the wood ash samples fell well below the CM1 level with two exceptions. Levels of the essential metals, copper and zinc, slightly exceeded CM1 (the level set for unrestricted use), but were well below CM2 levels. Wood ash with metal concentration above CM2 guidelines are treated as hazardous waste that must be landfilled. None of our ash samples had any metal levels that approached or exceeded CM2 guidelines. The amount of carbonates in the dry ash are also examined in Chapter 1. This component provides information on the amount of carbonate that is present in wood ash samples, particularly on the mineral form of the Ca, an important feature for explaining its aqueous solubility.

Chapter 2 of this report addresses the water solubility of wood ash samples over 30 days. This experiment revealed that wood ash types varied in readily extractable nutrients, with ash from wood pellet combustion exhibiting a higher percentage of Ca compared to hardwood and hardwood-softwood mix of ash. The results indicated that following land application, wood ash would likely slowly release nutrients (e.g., Ca, potassium, and phosphorus) over time to the environment.

Chapter 3 of this report addresses the short-term aquatic toxicity and longer term sediment toxicity of wood ash on Daphnia, the common water flea that is the “white rat” of aquatic toxicology. These experiments revealed some toxicity of wood ash elutriate concentrates on Daphnia in the short-term. This was not a surprise given that the Concentrate was the product of a 10% ash solution, i.e., 1 part ash in 9 parts of water. The short-term toxicity was not attributable to metals or high pH, as we first suspected, but to high levels of soluble potassium, a nutrient, in our concentrates. As this potassium is rapidly diluted in nature, we consider this observation of toxicity in our concentrates to be a simple product of our use of high sediment to water ratios in our undiluted bioassays. We do not believe it is of much environmental relevance.

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