Meta-analysis on penconazole sorption to soils – Modeling variability of K_OC and nonlinear isotherm parameters

A meta-analysis was conducted to examine the variability in log K_OC point-data (n = 612) and in the organic-carbon (OC) normalized Freundlich isotherm parameters (i.e., n_f, K_f,OC; n = 86) of penconazole, a polar systemic triazole fungicide, in soils and biosolids-amended soils. Log K_OC was not constant and varied from 2.15 to 4.24. Five existing log K_OC models were tested against experimental log K_d, and they produced biased prediction with moderate to low accuracy (RMSE = 0.60 to 1.14). n_f ranged from 0.38 to 1.02, with over 75% of n_f < 0.85, suggesting the sorption isotherms were mostly nonlinear. Point sorption data (i.e., S, C = sorbed and aqueous concentrations) were best modeled with a nonlinear function: S=10^3.26[ƒ_oc/(ƒ_oc+ƒclay)]^1.21C^0.81 (r² = 0.89, RMSE = 0.26, n = 557) where ƒ_OC/(ƒ_OC + ƒ_clay) represents the relative availability of OC sorption sites. Log K_f,OC also varied considerably (i.e., 2.61–5.69) and was found to linearly correlate with n_f: n_f = –0.194log K_f,OC + 1.517 (r² = 0.87). The observed sorption nonlinearity implies that assessment of subsurface mobility and ecotoxicological risk of soil penconazole is variable and site/soil-specific. The findings here challenge the conventional view of log K_OC constancy currently prevailing in existing risk assessment practices and in major predictive sorption models of organic chemicals in natural geosorbents. The extent to which log K_f,OC variability is a prevailing feature in other organic contaminants is unclear, and further investigations seem warranted.