Hydraulic conductivity in soils irrigated with wastewaters of differing strengths: Field and laboratory studies

Abstract

Application of wastewater can decrease the hydraulic conductivity of soils, and so reduce future loading rates. A possible mechanism for this decrease is blockage of soil pores by microbial growth and extracellular carbohydrate production. Our objective was to determine whether wastewater characteristics that increased microbial biomass and carbohydrate also decreased soil hydraulic conductivity. In field trials, irrigation of secondary-treated wastewater for 7 years onto a sandy soil increased carbohydrate, but had no effect on microbial C or unsaturated hydraulic conductivity, relative to non-irrigated soil. Irrigation of tertiary-treated wastewater for 5 years onto an allophanic soil had no significant effect on these soil characteristics. Effects on soil properties of wastewater with differing carbon : nitrogen (C : N) ratio were examined in the laboratory on repacked cores of the allophanic soil over 14 weeks. Unsaturated hydraulic conductivity decreased significantly in cores irrigated with the high C: N wastewater. These cores also exhibited an increase in soil carbohydrate content, microbial biomass, and respiration relative to cores irrigated with water. This study has demonstrated that the land treatment of industrial wastewaters of high C: N ratio and high biochemical oxygen demand could be problematic, even on freely draining soils.