Effects of long-term greywater disposal on soil: A case study

Abstract

This study investigated the environmental health risks to soil and potential risks to groundwater associated with long term (8-18years) greywater disposal practices. Land application of greywater is likely to have environmental impacts, which may be positive or negative. Greywater can contain plant macronutrients that may benefit plant growth. Conversely, high levels of surfactants, oils, grease, sodium and potentially pathogenic organisms may negatively impact environmental and human health. In this study, land disposal of untreated greywater was practiced at five coastal domestic properties. At each property, soil samples were collected at two depths from areas used for greywater disposal and from control areas that were not exposed to greywater. Soils were analysed for chemical and biological responses to greywater exposure. Generally, greywater irrigated soils had higher pH, Olsen P, base saturation, and increased soil microbial activity (as measured by biomass carbon, basal respiration and dehydrogenase activity). A pH of >9 was recorded for some greywater treated soil samples. Escherichia coli (E. coli) were detected at up to 10(3)MPN/g in the greywater exposed surface soils at some sites. Terminal Restriction Fragment Length Polymorphism (TRFLP) analysis revealed that greywater affected the soil microbial community structure, which may have implications for soil health and fertility. Overall, this study shows that the long-term application of greywater at the investigated sites had a moderate impact on the soil environment. This may have been due to the sandy soils and high rainfall that would flush the soil. Increases in microbial biomass and dehydrogenase indicate that greywater application may be beneficial for plant growth. However, high levels of E. coli in some soils may be a risk to human health and sub-surface irrigation should be the recommended application method.

Keywords: Contaminated soil; Disposal; Greywater; Long term impacts; Soil chemistry; TRFLP.