| As point source pollution is controlled effectively, agricultural non-point source pollution has been one of the primary factors to restrict agricultural sustainable development and threaten ecological environment security. Agricultural non-point source pollution control can be divided into "source control" and "sink control" according to contaminants migration processes."Sink control" focuses on the outflow contaminant removal, which is one of the important measures to protect surface water quality. As surface runoff is a main delivery pathway for contaminants, constructing vegetative filter strip between pollution source area and receiving water has been considered to be an effective way to intercept runoff contaminants and has been widely researched and used in USA and Europe countries.There were two kinds of runoff yielding patterns in this study. One was simulated runoff and grass filter strips consisting of North China region soil and plant of alfalfa and tall fescue were constructed on simulated runoff systems (6m of self-designed soil bin device). The other one was simulated rainfall and grass filter strips consisting of North China region soil and plant of tall fescue, white clover and bermuda were constructed on simulated rainfall plot (length of12m). Taking typical cropland runoff pollutants (suspended solids, nitrogen, phosphorus and organic pesticide) as target compounds, contaminant migration patterns and intercept mechanisms in surface runoff and subsurface runoff were analyzed on the bases of compound existing status and absorptivity. Moreover, mitigation mechanism of organic pesticide was discussed further. The study mentioned above would provide important theoretical references to research on ecological function and regulation mechanism of grass filter strip. The main points of this study are shown as follows:(1) Grass filer strips were effective to intercept runoff contaminants. The removal efficiencies of particulate contaminants including SS(suspended solid)、PP(particulate phosphorus) and CODMn were higher than those of dissolved contaminants including NO3--N、NH4+-N and TDP. The removal efficiencies of NH4+-N and TDP with stronger absorptivity were higher than NO3--N with weaker absorptivity. The removal efficiency of TP or TN increased as the proportion of particulate component increased.(2) SS and PP were mainly trapped in the first part of filter strip, and their outflow concentrations decreased exponentially with runoff extension，whereas NO3--N, NH4+-N and TDP outflow concentrations decreased linearly.(3)The effects of vegetation type, pollutant concentration, flow velocity and slope on runoff pollutant removal efficiency were studied by comparing contrast system, alfalfa filter strip and tall fescue filter strip. Results indicated that the influence of vegetation type, pollutant concentration, flow velocity and slope on pollutant removal efficiency varied with the status and nature of pollutants. The trapping of particulate pollutant SS, PP and CODMn was affected by the above mentioned four parameters obviously. The particulate pollutant removal efficiencies in the alfalfa filter strip and tall fescue strip were significantly superior to the control system, and decreased with the increasing of influent concentration, influent flow rate and slope. The removal efficiencies of dissolved pollutant NO3--N、NH4+-N and TDP (total dissolved phosphorus) were affected by vegetation type and influent concentration significantly. The dissolved contaminant removal efficiency in the tall fescue filter strip was better than alfalfa filter strip as well as contrast system. Nitrate removal rate reached highest in moderate concentration. The removal efficiencies of NH4+-N and TDP increased with the increase of their concentrations. Nitrate removal rate decreased remarkably with increasing influent flow rate. The removal rate of NH4+-N decreased significantly with increasing slope.(4)Moderately sorbed herbicide atrazine (Koc=100L Kg-1) and strongly sorbed fungicide chlorothalonil (Koc=1380L Kg-1) were studied as target organic pesticide contaminants. Simulated runoff experiment showed:atrazine and chlorothalonil outflow concentrations decreased as runoff extended, and neither exponential equation nor linear equation could fit the outflow concentrations versus runoff extension distances very well. The pesticide removal efficiency was calculated by inflow and outflow concentrations, and pesticide contaminant trapping features could be drawn that the removal efficiency of strongly sorbed fungicide chlorothalonil was higher than moderately sorbed herbicide atrazine, and plant can promote pesticide interception. Comparing the sorbed pesticide concentrations in sediment gained from the bottom of collecting containers of inflow and outflow (2m), sorbed atrazine concentrations did not show significant differences between inflow and outflow, whereas sorbed chlorothalonil concentrations of outflow were much higher than those of inflow. The implication was that organic pesticide transported with finer sediment，and sorbed pesticide concentrations increased as fine silt or clay fraction increased. Taking sorbed contaminate load reduction to evaluate sorbed pesticide trapping, grass filter strip was effective to intercept organic pesticide contaminates, and plant condition and soil organic matter content did not influence the removal significantly.(5) Atrazine and chlorothalonil outflow concentrations increased successively in4simulated rainfall experiments, and the outflow concentration of contrast system changed most significantly. Taking the reduction of grass filter strip outflow concentration compared to that of contrast system as evaluation index, atrazine removal efficiencies were above60%, whereas the removal efficiencies decreased as experiment times increased. Chlorothalonil removal showed different features from atrazine, and its trapping efficiencies increased firstly and decreased later. There was no significant difference of chlorothalonil outflow concentrations between grass filter strip and contrast system in the first and the fourth experiments, whereas its outflow concentrations of grass filter strips were much lower than those of contrast system in the second and the third experiments, with the removal efficiency of85.43%～87.69%and22.36%～37.21%.respectively.(6)The results of seepage experiment are as follows:contaminant concentrations of subsurface outflow were lower than those of correspongding surface outflow, and the trapping effectiveness by0.35m soil layer infiltration in grass filter strip were higher than that in contrast system. Contaminant removal efficiency was calculated based on the reduction of subsurface outflow concentration compared to surface outflow concentration, and the removal efficiency of SS was higher than those of dissolved contaminants NO3--N, NH4+-N and TDP. The removal efficiencies of TDP and NH4+-N with stronger absorptivity were higher than that of NO3--N with weaker absorptivity. TP or TN trapping effectiveness is a comprehensive reflection of particulate and dissolved fractions retention, and their removal efficiencies increased with the increase of particulate fraction proportion. Dissolved nitrogen, especially NO3--N, posed the higher risk of groundwater pollution than sediment and phosphorus. Electrical conductivity in subsurface outflow was lower than surface outflow, and the electrical conductivity reduction verified that dissolved contaminants removal by soil percolation was mainly attributed to ion adsorption. The pH values in subsurface outflow changed insignificantly compared with surface inflow, which meant the concentrations of H+and OH" were not changed by0.35m Soil layer infiltration. As for organic pesticide, plant condition and soil property did not influence pesticide removal efficiency significantly, whereas trapping efficiency of chlorothalonil with strong absorptivity (Koc>1000L Kg-1) were higher than atrazine with moderate absorptivity (100L Kg-1<Koc<1000L Kg-1). It can be drawn from seepage experiment that atrazine poses the higher risk of groundwater pollution than chlorothalonil by subsurface transport. |
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