Fate of Surfactants in Soil after Graywater Application for Landscape Irrigation

One innovative approach that is gaining popularity to water resources sustainability is household graywater reuse for residential landscape irrigation. However, limited scientific data is available on the fate of graywater chemical and microbiological constituents and for the safe operation of graywater irrigation systems. The objective of this research project is to elucidate information on the fate and occurrence of surfactants constituents and their long-term potential impacts on soil quality, as a result of its application for residential landscape irrigation. Samples were collected at four different households where graywater has been applied for irrigation for more than five years in Arizona, California, Colorado, and Texas. Soil cores were taken at depths of 0-15, 15- 30, and 30-100 cm separately in both an area irrigated with graywater as well as a control area with analogous soil and landscaping that irrigated with fresh water in each site for quantification of soil physical and chemical properties and surfactants. Soil samples were analyzed for pH, electrical conductivity, organic matter, total C, total N, extractable NH4-N, NO3-N, P, effective cation exchange capacity, sodium adsorption ratio, surfactants, and antimicrobials. Widely used surfactants including, linear alkyl benzene sulphonates, alcohol ethoxylates, and alcohol ether sulphates have been quantified by liquid chromatography-mass spectrometery in soil samples. In Colorado site, soil texture in both graywater and fresh water irrigated households is clay loam and soil composition is approximately the same. Compared to soil that received fresh water, 0-15 cm depth graywater-irrigated soil contained elevated amounts of organic matter and extractable P and Mn. There were no obvious differences in soil pH, EC, SAR, or extractable B although SAR values increased with soil depth in the graywater-treated area. Based on the soil data, surface soils in Texas site within the graywater-treated area had been amended at some point to alter soil texture, probably to improve aeration and drainage. The texture of the Texas soils is generally clay loams or clays, although the surface texture at the graywater area is a sandy clay loam. This soil also has a very high organic matter content (7.3%) compared to the fresh water-treated area (2.8%). The SAR value and concentrations of P and Mn of the 0-15 cm depth graywater-treated area are nearly twice that of the control area although the SAR value is still low overall. As a result these differences may affect the accumulation and moving of surfactants in the soil. In California sites, there were few striking differences in soil physical and chemical properties between the two treated areas. Concentration of NO3-N was relatively high in fresh waterirrigated soil (8.9 mg/L) compared to graywater-irrigated soil (4.4 mg/L). Sodium levels were highest among any of the household soils, at 6.9 ppm in the fresh water-treated soil and 7.8 ppm in the graywater treated soil. However, these concentrations are low overall, and the soil SAR values at the California household site are low as well. In all sampling sites, graywater irrigated soil samples has more surfactants concentration than fresh water irrigated soil samples. Based on measured values of different surfactant groups, paired t-tests have been conducted to evaluate probable accumulation of surfactants in surface soil samples. In order to evaluate the possibility of surfactants in the surface soil samples, a t-test has been done for all measured surfactants including LAS, AES and AE for all surface soil samples. Standard deviation of surfactant concentration in graywater and fresh water irrigated areas are 28.28 and 11.47 respectively which are relatively high. As a result, although average concentration of surfactant in surface soil samples in the graywater irrigated area is much higher than fresh water irrigated area, paired ttest results for sum of all measured surfactants also cannot support the surfactant accumulation hypothesis.