Evaluation of simulated dredging to control internal phosphorus release from sediments: Focused on phosphorus transfer and resupply across the sediment-water interface.

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability.

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large, shallow lake

The disturbance of the water-sediment interface by wind-driven currents and waves plays a critical role in sediment resuspension and internal nutrient release in large, shallow lakes. This study analyzed the effects of the interactions between wind-induced currents an1d waves on the driving mechanism of sediment resuspension in Lake Taihu, the third largest freshwater lake in China, using acoustic and optic techniques to collect long-term, high-frequency, synchronous in situ measurements of wind, currents, waves, and suspended solid concentrations (SSCs). The results suggested that water turbidity started to increase at wind speeds of approximately 4 m/s and significantly increased when wind speeds exceeded 6 m/s. In most cases, wind-induced waves were the main energy source for changes in turbidity. Wave-generated shear stress contributed more than 95% to sediment resuspension and that only in weak wind conditions (<4 m/s) did the lake bottom shear stresses generated by currents and waves contributed equally. The relationship between SSC and bottom shear stress generated by wave was established by fitting the observed results. The processes of sediment dynamics were divided into four stages (A through D) according to three shear-stress thresholds. In stage A, SSC remained stable (about 45 mg/L) and τw was less than 0.02 N/m2. In stage B, the sediment bed was starting to be activated (SSC 45∼60 mg/L) and τw was in the range of 0.02∼0.07 N/m2. In stage C, a medium amount of sediment was suspended (SSC 60∼150 mg/L) and τw ranged from 0.07 to 0.3 N/m2. In stage D, large amount of sediment was suspended (SSC 150∼300 mg/L) and τw was larger than 0.3 N/m2. The findings of this paper reveal the driving mechanism of sediment resuspension, which may further help to evaluate internal nutrient release in large shallow Lake Taihu.

Effect of wave-current interactions on sediment resuspension in large shallow Lake Taihu, China.

Sediment dredging can permanently remove pollutants from an aquatic ecosystem, which is considered an effective approach to aquatic ecosystem restoration. In this work, a 2-year field simulation test was carried out to investigate the effect of dredging on nitrogen cycling across the sediment-water interface (SWI) in Lake Taihu, China. The results showed that simulated dredging applied to an area rich in total organic carbon (TOC) and total nitrogen (TN) slightly reduced the NH4(+)-N release from sediments while temporarily enhanced the NH4(+)-N release in an area with lower TOC and/or TN (in the first 180 days), although the application had a limited effect on the fluxes of NO2(-)-N and NO3(-)-N in both areas. Further analysis indicated that dredging induced decreases in nitrification, denitrification, and anaerobic ammonium oxidation (anammox) in sediments, notably by 76.9, 49.0, and 89.9%, respectively, in the TOC and/or TN-rich area. Therefore, dredging slowed down nitrogen cycling rates in sediments but did not increase N loading to overlying water. The main reason for the above phenomenon could be attributed to the removal of the surface sediments enriched with more TOC and/or TN (compared with the bottom sediments). Overall, to minimize internal N pollution, dredging may be more applicable to nutrient-rich sediments.

Effects of sediment dredging on nitrogen cycling in Lake Taihu, China: Insight from mass balance based on a 2-year field study

Eutrophication of lakes and reservoirs is a major water quality problem that poses significant environmental, economic and social threats around the world. Monitoring and managing lakes and reservoirs to prevent or limit eutrophication, therefore, has significant value. The literature has been reviewed to study ecological engineering and management methods that have been and can be applied to improve water quality. Ecological engineering has the potential to be utilized to improve the design and operation of lakes and reservoirs through monitoring and active management of biological, chemical and physical components. Phosphorus concentrations can be reduced by effective and sustainable management practices to improve water quality.

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Sustainable Management of Eutrophic Lakes and Reservoirs

Simulated research of internal loading and collecting and analyzing the samples from the lakes were carried out before and after dredging in polluted suburb lakes, Wuli Lake (Wuxi City) and Xuanwu Lake (Nanjing City). The research results showed that dredging can inhibit internal loadings in a certain degree in a short term. The discrepancy of dredging effect and technical level, namely dredging quality, by different dredging methods will result in a difference of control of lake internal loadings. The internal loadings' reversion will gradually appear along with the biogeochemical processes, including suspended particle precipitation, hydrodynamic disturbance and microbio-transformation. The reversion rate mainly depends on the dredging method and the change of interfacial processes on the newborn surface layer. The higher nutrient contents and organic matter in the sediment will enhance water-sediment interfacial processes and nutrients regeneration. It is very important to study the physicochemical and biological character of lacustrine sediments before dredging for determining the dredging methods and predicting their environmental effect.

ZHENGChaohai

Papers

Processes and mechanism of effects of sludge dredging on internal source release in lakes