Weathering of spent mushroom substrate: Leachate chemistry, solute transport and environmental impacts

Field weathering is a popular method to treat piles of spent mushroom substrate (SMS) prior to its reuse. During this field weathering process, leachate is generated and percolates into the underlying soils. Field and laboratory studies were conducted to investigate the chemistry of SMS leachate, its transport behavior in soil profiles, and impacts on the underlying soils and groundwater quality. Two SMS piles were deposited (90 cm and 150 cm in height) in an open field over a Typic Hapludult and weathered for 24 months. Leachate and groundwater samples were collected biweekly and analyzed for inorganic and organic constituents. The underlying soils were sampled upon removal of the SMS piles and analyzed for water soluble and exchangeable components. The SMS field weathering process was simulated in the laboratory by leaching a 150 cm high SMS column with water for 6 months, and leachate was analyzed for chemical compositions. Leachate was applied to undisturbed and homogenized soil columns (10 cm long by 7.6 cm i.d.) and effluents were analyzed for chemical changes. SMS leachate contains high concentrations of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and inorganic salts. Inorganic chemistry of the leachate was dominated by K+, Ca2+, Cl − and SO₄2−. SMS field weathering significantly increased water-soluble components and altered exchangeable compositions of the underlying soils. The top 90 cm of soil retained 20–89% of the leachate solutes, and leachate solutes had migrated in the soil profiles at least 200 cm depth from the surface in 2 years. With a water table of 760 cm from the surface, no significant changes of groundwater quality influenced by SMS weathering was observed in the experimental period. Transport of SMS leachate in undisturbed soil columns was initially through preferential flow followed by matrix flow. Leachate DOM and K+ were retained in the soil columns while other solutes were excluded. Weathering of SMS in piles of 90 cm depth or greater significantly influences underlying soil chemistry and may adversely impact groundwater quality. It is recommended that weathering of SMS be conducted on compacted surfaces and the leachate be collected and disposed of properly.