Discussion on the way of determining environmental dredging depth based on nitrogen and phosphorus adsorption/desorption method: taking the demonstration section of Fenhe River in Taiyuan as an example

Effective dredging depth is the key index to determine dredging cost and treatment effect. There is no standard stipulation for nutrient content in sediment in the current standards and specifications in China, which is very unfavorable to the evaluation and determination of effective dredging depth of polluted sediment in rivers and lakes with excessive nutrient salt. Taking the demonstration section of dredging of Fenhe River in Taiyuan as an example, the sediments with the depth of 0.3-0.4, 0.9-1.0, 1.4-1.6 and 1.9-2.0 m were taken as representative core samples. The simulated water of ${\mathrm{NH}}_4^{+}$ -N and ${\mathrm{PO}}_4^{3-}$ with Grade Ⅰ to worse than Grade Ⅴ and the overlying water of core samples were designed respectively to carried out nitrogen and phosphorus adsorption/desorption experiments, and the nitrogen and phosphorus adsorption/desorption characteristics of sediment at different depths were also analyzed. The results showed that ${\mathrm{NH}}_4^{+}$ -N in the sediment within 1.6 m of the core sample was released to the water body, and the sediment at a depth of 1.9 m had certain adsorption to ${\mathrm{NH}}_4^{+}$ -N in the water body. ${\mathrm{PO}}_4^{3-}$ in the sediment within 1.6 m had a risk of release to water body, and the sediment at a depth of 1.9 m had certain adsorption to ${\mathrm{PO}}_4^{3-}$ in water body. The concentrations of TN and ${\mathrm{NH}}_4^{+}$ -N in the sediment at a depth of 1.3 m showed an obvious inflection point, and the concentration of TP in the sediment at a depth of 1.6 m showed an obvious inflection point. Comprehensively considering the influence of the sediment on overlaying water body and the vertical variation of sediment nitrogen and phosphorus concentrations, the effective dredging depth of Fenhe River demonstration section in Taiyuan was determined to be 1.6 m.