Recycling Graywater for Landscape Irrigation: Impacts On Soil Quality.

Household graywater reuse for residential landscape irrigation is gaining popularity as communities seek innovative approaches to conserve and recycle water resources. However, application of graywater may pose certain risks to soil quality, including increased pathogens, changes in soil pH and salinity, and poor soil structure. The objective of this research project was to evaluate long-term effects of graywater irrigation on pathogen indicators, microbial communities, and physicochemical properties of urban soils. Soil samples (0-15, 15-30, and 30-100 cm depth increments) were collected from Arizona, California, Colorado and Texas residential landscapes receiving tap water (control) or graywater for a minimum of 5 years. Soils were analyzed for total coliforms and fecal indicators (E. coli, enterococci, and Clostridium perfringens), soil chemical properties including surfactants, microbial community structure and activity, and bulk density and water infiltration. The Texas household was the only household where total coliforms, E. coli and enterococci abundances were greater in graywater-treated than in control soil, but this was confounded by the presence of free-range chickens in the graywater-treated area. In general, there was no significant accumulation of nitrate or salts in soils receiving graywater, except in Colorado where SAR values increased with soil depth in the graywater-treated area. Concentration of surfactant linear alkyl benzene sulfonate (LAS) was greater in graywater-irrigated than in control soil, and increased with soil depth, at Texas and Colorado households. There were no negative impacts of graywater irrigation on water infiltration into soil at Texas and Colorado, but infiltration rate was reduced at the California household. Also in California, graywater-treated soil contained less microbial biomass and respiration activity, but greater dehydrogenase enzyme activity compared to control soil. Overall, graywater did not impact soil quality consistently across all household locations, which could be due to differences in graywater quality, application frequency and method, and inherent soil properties.