Influence of artificially placed substrates on agro-drainage ditch water quality

Agricultural drainage ditch is a critical hydrological system in agro-ecosystems for transporting surface runoff to down-stream water systems and for removing drainage water pollutants. While pollutant removal is critical for the protection of water en-vironments, agro-drainage pollution continues to draw attention in several regions in China. In recent years, agro-drainage pollution in the Ningxia Yellow River Irrigation Region (NYRIR) heavily influences the quality of both local water systems and the Yellow River. The performance of such ditches regarding drainage water pollutant retention can be improved using suitable substrates. In this study, an experiment involving five treatments and a control was conducted in a NYRIR classical drainage ditch on Lingwu Farm of Ningxia Hui Autonomous Region. Soil, cinder, straw and sawdust were placed in the ditch as substrates and the capacities of sub-strates to remove pollutants from agro-drainage waters determined. The goal of the study was to augment understanding of substrate effect on water quality in drainage ditches and determining the practical effectiveness of the method of placing substrates in ditches on pollutant removal. The results showed that compared with other three materials, sawdust had a better total dissolved solids (TDS) and total nitrogen (TN) adsorption capacity 0.4 g(TDS)·kg^?1 and 0.3 g(TN)·kg^?1 , respectively. Soil, cinder and straw released TDS in the ranged of 5.3~50.8 g(TDS)·kg^?1 . Straw had the highest capacity for organic carbon release, which was 54.0 g·kg^?1 . However, the differences among total phosphorus (TP) adsorption capacity of the four substrates were statistically insignificant. The perform-ance of TP adsorption under the four substrates was related with the amount of the materials used and duration of the experiment. With the exception of ditches of straw and control treatments, examined water samples from experimental ditches suggested decreas-ing trends in TDS concentration. Meanwhile, chemical oxygen demand (COD), TN, TP, NO₃^?-N and NH₄⁺-N were effectively re-moved from drainage waters under sawdust and soil treatments. The experiment also revealed that the concentration of TDS, TN and TP (0.60~0.80 g·L^?1, 0.24~0.33 mg·L^?1 and 0.04~0.09 mg·L^?1 at the tail water section, respectively) decreased with the distance of water transport through the ditch. On the average, the results suggested that agro-drainage pollutants were intercepted in drainageditches. Proper substrates improved the ecological function of agro-drainage ditches.