A Experimental Study On The Characteristic Of Nutrient Salts Release Of Sediments In Yuqiao Reservoir

Eutrophication is one of the environmental problems that draw our attention in the fields of water resources management. Eutrophication is mainly attributed to the excess nutrient substances in the aqueous systems in which N and P are the dominant factors. In most of the lakes (reservoirs), P is the restrictive factor of the natural algal growth. For many years, the main policy to treat eutrophic aqueous systems is to control the exotic import. However, few examples can be provided to prove the restore of the aquatic ecology through that way. And the reason is the phosphorus release of deposits in the aquatic system. Therefore, the release characteristic of the nutrient elements in the deposits becomes the emphasis of the scientific study of the lakes (reservoirs).In this experiment, Yuqiao reservoir in Tianjin City is the research object. Through laboratory simulation, the release characteristics of nutrient elements especially the phosphorous substances in deposits with respect to the environmental condition changes.The investigation indicates: the TP in the deposits ranges from 269.49 to 466.20μg/g, and the average concentration is 367.85μg/g. Fe-P consists 64.71% of the TP. The average pH value is 8.62. The bottom of the reservoir becomes seriously anaerobic or anoxic between May and September. Yuqiao reservoir satisfies the N, P release condition, the release of P of sediments intensifies the eutrophication of the reservoir.The experimental results indicate: when pH=6.5, the acidic dissolution of Ca-P is the main source of P in the water body. When DO<1 mg/L, the anaerobic condition makes Fe³⁺ reduced to soluble Fe²⁺, which leading to the phosphorus release to water body. As the concentration of PO₄^3- increases, the soluble P is absorbed by Fe²⁺ resulting into a state of chemical fixation, which can be proved through the fairly good correlation (coefficient 0.935). Under high pH value condition, the displacement of OH^- and PO₄^3- takes place leading to phosphorus release of the sediments. When DO<1 mg/L, the cooperation of the displacementand reduction happens, and the phosphorus release of the sediments speeds up greatly. The sediment Nitrogen release is affected by the DO concentration. When DO<2 mg/L, a short term release of TN takes place;when DO<1 mg/L, the denitrification strengthens to get a balance with the sediment release, as a result, the TN concentration stays at a certain level. The TN release has little to do with pH value. The organic release of sediments happens when DO<2 mg/L, and the high pH value neutralizes the organic acids from the pore spaces of the sediments, and the migration of the organic acids from the sediments to the water body speeds up. So it becomes easier for the organic release. Both TP and TN release are affected by the environmental temperature obviously. If the temperature rises by 5°C, the phosphorus release rate accelerates by 3.16 times, and the nitrogenous release by 1.15 times. Under neutral or acidic condition, the phosphorus release is mainly attributed to the dissolution of Ca-P and the reduction of Fe-P, and it takes effects from the water-sediment interface to nearly 8cm below. Under alkaline condition, TP release is mainly caused by the release of Fe-P, and it takes effects from the interface to 6cm below.