Dredging method effects on sediment resuspension and nutrient release across the sediment-water interface in Lake Taihu, China.

Environmental sediment dredging is one of the most common methods for the remediation of contaminated sediments in lakes; however, debate continues as to whether the effectiveness of dredging methods contributes to this phenomenon. To determine sediment resuspension and nutrient release following dredging with a variety of dredging methods, four dredging treatments at wind speeds of 0-5.2 m/s were simulated in this study, namely suction dredging (SD), grab dredging (GD), ideal dredging with no residual sediments (ID), and non-dredging (ND). Field sediments from suction and grab dredging areas (including post-dredged and non-dredged sediments) of Lake Taihu were used to assess the release abilities of soluble reactive phosphorus (SRP) and ammonia nitrogen (NH4+-N) from the sediment-water interface. The effects of residual sediments on nutrient concentrations in water were also evaluated. The results reveal that inhibition of resuspension of particulate matter and nutrients released through sediment dredging decreases with increasing levels of residual sediment. Total suspended particulate matter content in the mean water columns of ID, SD, and GD under wind-induced disturbance (1.7-5.2 m/s) decreased by 67.5%, 56.8%, and 44.3%, respectively; total nitrogen and total phosphorus in ID (SD) treatments were 19.8% (12.9%) and 24.5% (11.2%) lower than that in ND treatment. However, there were ~ 1.6 and 1.5 times higher SRP and NH4+-N in the GD treatment compared with the ND treatment at the end of the resuspension experiment (0 m/s). A significant increase in the SRP and NH4+-N release rates at the sediment-water interface was also observed in field sediments from a grab dredging area, indicating that GD may pose a short-term risk of nutrient release to the water body. Hence, dredging methods with less residual sediments both during and after dredging improves the dredging quality.

Optimum dredging time for inhibition and prevention of algae-induced black blooms in Lake Taihu, China.

Dredging, which is the removal of polluted surface sediments from a water body, is an effective means of preventing the formation of algae-induced black blooms. However, an inappropriate dredging time may contribute to rather than inhibit the formation of black blooms. To determine the optimum dredging time, four treatments were simulated with sediment samples collected from Lake Taihu: dredging in January 2014 (DW), April 2014 (DA), July 2014 (DS), and no dredging (UD). Results showed that typical characteristics associated with black blooms, such as high levels of nutrients (NH4 (+)-N and PO4 (3-)-P), Fe(2+), ∑S(2-) ([HS(-)] + [S(2-)]), and volatile organic sulfur compounds (VOSCs), including dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS), were more effectively suppressed in the water column by DW treatment than by UD treatment and the other two dredging treatments. The highest concentrations of NH4 (+)-N and PO4 (3-)-P in the UD water column were 4.09 and 4.03 times, respectively, those in the DW water column. DMS levels in the UD and DS water columns were significantly higher (p < 0.05) than those in the DW water column, but DMDS and DMTS levels were not significantly different between the treatments. After several months of dredging, surface sediments of the DW and DA treatments were well oxidized, and concentrations of Fe(2+) and ∑S(2-) were lower than those in UD and DS treatments because of material circulation between sediments and the water column. Water content, which is important for the transport of matter to the overlying water, was lower in the dredged sediments than in the undredged sediments. These factors can suppress the release of Fe(2+) and ∑S(2-) into the water column, thereby inhibiting the formation of black blooms. Black coloration occurred in the UD water column on the seventh day, 2 days later, and earlier, respectively, than the DW and DS water columns and almost on the same day as in the DA water column. This phenomenon is mainly attributed to the oxidation of the new sediment surface in the DW and DA water columns, suppressing the release of sulfur into the water column, because of a long incubation period. Thus, dredging in the winter can prevent the formation of black blooms, while dredging in summer may contribute to them.

[Environment effects of algae-caused black spots: impacts on Fe-Mn-S cycles in water-sediment interface].

The driving effects of algal cells settlement in the water-sediment interface on Fe, Mn, S biogeochemistry in laboratory through static cultivation device. Results showed that dissolved oxygen would be exhausted by algae cells in 50 min after the cyanobacteria cells settled to the sediment surface. Soon the water-sediment interface formed the severe anoxia and Fe-Mn oxides and sulfides were deoxidized quickly in the strong reducing environment. The Fe2+, Mn2+ content in interface increased to the summit at the 4th day and their concentrations were 4.40 mg/L and 2.35 mg/L, respectively. When it comes to the end of the experiment, the Fe2+ content had a little reduction and Mn2+ reduced quickly, their concentrations were 3.37 mg/L and 0.97 mg/L at the end of experiment. However, S2- concentration in interface reached the highest at the 2nd day and its content was 0.63 mg/L, and its concentration was only 0.12 mg/L at the end since it has been reduced. The ORP was--150 mV in the sediment surface and indicated that the sediment environment was a strong reducing environment. Phenomenon of algal cells induced black spots in water bodies was the main driving factors on Fe/Mn oxides and sulfides biogeochemistry cycle, and also the extreme anoxia environment would have great harm on the water body's ecology.

[Environment effects of algae-caused black spots: driving effects on the N, P changes in the water-sediment interface].

The impact and driving effect of deposited algal cells in the water-sediment interface on the N, P changes were studied through continuous extracted pore water with home-made static experiment. Results showed that dissolved oxygen in water-sediment interface was depleted in 50 min after algal cells settled. Soon the dead algal cells formed the anoxia and strong reducing environment and the dead cells had a severe anaerobic mineralization in the water-sediment interface, also the water bodies had a intense black and stink phenomenon. PO4(-3) -P, NH4(+) -N concentration in water-sediment interface increased from the 2nd day after added the algal cells to the sediment interface, and its concentration was 4.00 mg/L and 39.45 mg/L, respectively. Its concentration was the 10 fold and 241 fold higher than that the control experiments at the same time (PO4(-3) -P, NH4(+) -N concentration in control experiments was 0.42 mg/L and 0.16 mg/L, respectively). Anaerobic mineralization of dead cells in sediment surface drove the nutrients diffusing upward the overlying water, added the nutrients concentration in water bodies, and it also supplied the nutrient materials for the algal blooms happened again.