Review on effect of reclaimed water irrigation onsoil water movement in cropland

Abstract: Reclaimed water irrigation saves precious water resources and alleviates the shortage of agricultural water supply. Besides, the nutrients and microelements in reclaimed water promote crop growth and grain output. However, the organic and inorganic materials in reclaimed water change soil porosity, aggregate stability, soil-water interactions and clay particle dispersion, enhancing environment contamination risk. This research reviewed the effects and driving mechanism of suspended inorganic solids, macromolecular organic matter, oils, surfactants and salts contained in reclaimed water on the change of soil infiltration rates and hydraulic conductivity. When reclaimed water penetrates soil profile, suspended particles are filtered by soil pores and then accumulate in the upper few centimeters of soil profile. These effects form a seal on soil surface, increase soil bulk density, and decrease soil porosity, making soil infiltration rates and hydraulic conductivity decrease significantly. The impacts of macromolecular organic compounds on soil water movement are complicated. On one hand, macromolecular organic compounds, incorporated with those decomposed and degraded products, are beneficial to the formation of water stable aggregates; and on the other hand, they can plug the soil pores also, which depend on their properties and gradually expose and change with the development of soil biochemical activity. Oils in the reclaimed water are hydrophobic compounds with long-chain alkyls. When the long-chain alkyls coat mineral particles or aggregate surface, the soil is resistant to instantaneous wetting and form water repellency. The water repellent status of a soil depends greatly on soil water content: increasing when water content is low, and decreasing as the water content increases to a value beyond which the soil shows no repellency. Although water repellency increases the stability of soil aggregates, it decreases the soil infiltration rate and hydraulic conductivity, making the agriculture land hard to be irrigated and easily to generate preferential flow. These increase the contamination risk of groundwater and soil erosion under heavy rains. Surfactants in reclaimed water are mainly from laundry detergents, shampoo, bath lotions, and dishwashing detergents. Surfactants are usually organic compounds that are amphiphilic. Therefore, a surfactant contains both a water-insoluble (or oil-soluble) component and a water-soluble component. The surfactants affect the soil dispersity by changing the stability of soil aggregates and the stability of soil clay suspension: the more stable the soil aggregates are, the lower dispersity the soil presents; the more stable the soil clay suspension is, the higher dispersity the soil presents. The soil dispersion induced by the surfactants from the reclaimed water plugs soil pores, decreasing the soil infiltration rates and hydraulic conductivity. The salinity and the sodicity in reclaimed water are relatively high since secondary treatment does not remove salts from the water. The increase of soil salinity enhances the coagulation of soil particles, and the stability of soil aggregates and structure. These increase soil porosity and infiltrability. However, after the soil moisture being consumed by evaporation and transpiration, salts from the reclaimed water are left and accumulated in the irrigated soils, forming soil salinization when salinity reach a certain extent. High soil salinity increases osmotic potential of soil solution and physiological drought of crops, making yield decrease. Besides, excess salts destroy soil aggregates and block soil pores, decreasing soil infiltration rate and drainage capacity. The most urgent research directions of this field are the evolution of the soil structure and characteristics under reclaimed water irrigation and the driving mechanism behind it, and also the interaction between reclaimed water irrigated soils and unsaturated flow within them. This review is valuable for the design of reclaimed water irrigation projects, contaminant risk control and eco-environment protection.