Effects Of Water Managements On Transport Of Escherichia Coli In Soil-plant System For Drip Irrigation Applying Secondary Sewage Effluent

Reuse of wastewater for agricultural irrigation is practiced both in water deficient areas and in water abundant regions(EPA,2004)to alleviate water crisis.As the increase of the reclaimed water irrigation,its healthy and environmental impacts cause wide concerns.The knowledge of the transport of typical indicator bacterial,Escherichia coli(E.coli),under varying soil and flow conditions is therefore essential for controlling pathogenic contamination to soil environment.In addition,it is quite necessary to investigate the effects of water management practices on transport of E.coli in soil and residuals in plants for drip irrigation while applying secondary domestic sewage effluent.Experiments were conducted to study the transport of a typical bacterium,Escherichia coli(E.coli),in a sandy and a sandy loam soil under different application rates and input concentrations.A 30° wedge-shaped plexiglass container was used to represent one twelfth of the complete cylinder in the experiments.The apparent cylindrical application rate was varied from 1.05 to 5.76 L/h and the input concentration of E.coli ranged from magnitude of 102 to 107 colony-forming unit CFU/mL.The applied effluent volume ranged from 4.8 to 12 L.The moving of water front during water application was recorded and soil samples were collected at different locations from the point source as the designed water volume was applied.A model of water and E.coli transport in unsaturated sandy and sandy loam soil was established when applying sewage effluent from a surface point source,and the model was implemented using the commercial software package HYDRUS.The model calibration and validation datasets of the performance indicators for water and E.coli distributions performed well in both soil types,indicating a good agreement between the simulation results and the experimental data.The simulated results confirmed the observations about water and E.coli transport under drip irrigation.The two-year experiments aimed at investigating the effects of water management practices on transport of E.coli in soil and residuals in plants for drip irrigation while applying secondary domestic sewage effluent.In 2014,the experiments were designed with three lateral depth of 0(SO),10(S1)and 20 cm(S2)below the soil surface along with three irrigation levels that were determined by pan coefficient of 0.6(I1),0.8(I2)and 1.0(13).In 2015,three irrigation intervals of 4 days(F1),8 days(F2)and 12 days(F3)along with three lateral depths similar to the 2014 experiments were used.Groundwater control experiments were applied for the treatments with pan coefficient of 0.8(I2)in the 2014 and the irrigation intervals of 8 days in the 2015.Soil samples were collected during the key growing period,and the growing of asparagus lettuce were recorded.The conclusions are as follows:(1)An extremely high concentration of E.coli was observed in the proximity of the point source,and the peak value increased with an increased input concentration.An increasing application rate of water suspended bacteria allowed a more rapid transport of bacteria,thus accelerating E.coli transport rate and resulting in a larger distributed volume of E.coli for both soil types.For the sandy soil,more than 70%of the E.coli that was detected within the entire wetted volume concentrated in the volume of 10 cm from the point source,and the concentration of E.coli decreased greatly as the distance from the point source increased.More than 98%of the E.coli was detected in a volume of 5 cm around the saturated wetted zone for the sandy loam soil.(2)The model calibration and validation datasets of the performance indicators for water and E.coli distributions performed well in both soil types,indicating a good agreement between the simulation results and the experimental data.The use of HYDRUS enabled to understand water and E.coli transport mechanism under drip irrigation and to strengthen the conclusions derived from the observations.(3)Surface drip irrigation more likely induced E.coli contamination on surface soil and the E.coli concentration demonstrated a decreasing trend after irrigation ceased.There is no obvious difference between the effluent and the comparing groundwater treatment 72 h after irrigation ceased.(4)In general,subsurface drip irrigation could avoid pathogen contamination when sewage effluent was applied.A more frequent irrigation and a higher containing level of E.coli increased short term E.coli contamination of soil as it increased contacting opportunities between effluent and soil.Drip irrigation with secondary sewage effluent did not lead to obvious accumulation of E.coli in soil.(5)On harvest,no E.coli was detected in the stems of asparagus lettuce and few counts of E.coli was detected on the leaves of the crop but a weak association between the irrigation management practices and E.coli contamination of leaves was found.Secondary treated sewage effluent irrigation increased yield of asparagus lettuce compared with groundwater irrigation.Subsurface drip irrigation could improve yield of asparagus lettuce,and irrigation frequency increase could improve yield and quality of asparagus lettuce.(6)In general,Compared with E.coli transport in saturated soil,E.coli limited in certain area in unsatureated soil,especially for sandy loam,E.coli were mainly stranded in the surface soil.The application rate prompts E.coli transport by increasing the water flux.The increasing effluent volume and initial water content also increase E.coli transport.Our study recommended that subsurface drip irrigation is a promising method to avoid E.coli contamination and increasing yield and quality of asparagus lettuce when applying sewage effluent.