Application of DGT/DIFS combined with BCR to assess the mobility and release risk of heavy metals in the sediments of Nansi Lake, China

The heavy metal contamination of the aquatic ecosystem is still prevalent even after reduction of the external anthropogenic inputs of the metals. The release of labile heavy metals from the sediments into the water is a potential risk, responsible for the contamination of the aquatic system. Herein, samples of sedimentary column cores were collected in Nansi Lake, and the distribution profiles of the labile and soluble metals (Cd, Cu, Ni, Pb, and Zn) were obtained by the diffusive gradient in thin films (DGT) and the high-resolution dialysis (HR-peeper) technique. Furthermore, the mobility, bioavailability and release risk of the heavy metals were assessed using the results of geochemical sequential extraction, DGT as well as the DGT-induced fluxes in sediments (DIFS) model. The results showed that the profile characteristics of the DGT-labile and soluble heavy metals showed irregular distribution in the sediment cores and Cd, Pb, Zn had an obvious positive correlation with Fe/Mn (p < 0.05). Ni, Cu, and Zn existed primarily in the residual fraction (accounting for 58-76%), while Cd and Pb existed in the reducible fraction (accounting for 50-67%). The Cd and Ni (0.027-0.185) had higher mobility coefficients compared with Pb, Cu, and Zn (0-0.011), and positive diffusive fluxes also proved that Cd and Ni were easy to be released from the sediments. In addition, the R values of five metals (0.18-0.85) ranged between R_diff to 0.95, indicating that all the metals had partially sustained case from the sediments solid phase. Based on the DIFS model, the five metals had weak mobility from the sediment to pore water, but the release risks in the Nansi Lake should also be of concern, especially for the highly mobile Cd and Ni in the Dushan Lake.

A study of synchronous measurement of liable phosphorous and iron based on ZrO-Chelex (DGT) in the sediment of the Chaiwopu Lake, Xinjiang, Northwest China

The water-sediment interface of lakes is an important and unique area of the water environment; the geochemical behavior of nutrients in this area has a significant impact on the quality of the water environment and ecosystems, especially in shallow lakes. However, most studies do not provide direct in situ evidence for this in shallow lakes in arid regions; in order to explore the coupling relationship between phosphorus (P) and iron (Fe) in a sediment profile, we conducted a high-resolution analysis of liable Fe and P in sediments taken from the Chaiwopu Lake using ZrO-Chelex thin film diffusion gradient technology (ZrO-Chelex DGT). The results show that (1) the vertical spatial distribution trend of the liable P and Fe in the sediments from each sampling site is essentially similar. The contents of the liable P and Fe ranged from 0.004-0.125 mg/L and 0.050-0.190 mg/L, respectively, and the synchronous distribution of the micro-interface concentration reflects the coupling relationship between them. (2) The correlation analysis of the liable P and Fe concentrations showed that there were significant linear correlations between them (P < 0.05, bilateral). (3) The diffusion fluxes of P and Fe were - 51.76~65.12 μg (m² d)^-1 and - 451.27~457.06 μg (m² d)^-1, respectively, and were shown to be negative at the sediment-water interface for most of the samples, which showed that P and Fe were released from the overlying water into the sediments. (4) This research showed that the diffusive fluxes at the different sites are quite different, which indicates that the phosphorus and iron pollution in the sediments of the Chaiwopu Lake is affected by exogenous inputs. There was no significant correlation between P release flux and pH, ORP, conductivity (EC), the TDS of the overlying water, or the pH, salinity (Ca²⁺, Mg²⁺), and nutrient (organic matter) content of the sediment. The release flux of Fe is affected by the pH of the sediment. The results of this study provide references for the research of elements in the water-sediment interface of shallow lakes in arid regions, as well as other areas.

Characterization of phosphorus accumulation and release using diffusive gradients in thin films (DGT) - linking the watershed to Taihu Lake, China

Taihu Lake is well known internationally due to its algae bloom, and phosphorus fertilization and accumulation in the watershed soil played an important role in the lake eutrophication. To explore the linkage between soil and sediment legacy P, the soils from four representative land uses in the upstream watershed and sediments from typical lake areas were sampled and analyzed for P species. Meanwhile, the DGT technology was used to characterize the labile P in the soil and sediment and its release dynamics. The results indicated that Taihu Lake was the major total phosphorus sink (TP = 481.7 ± 97 mg/kg) and wheat land the major reactive phosphorus stock (RP = 37.3 ± 9 mg/kg) in the watershed. The DGT-P dynamic with the deployment time (t) presented a downward exponential curve (f(t) = a × t^-b, b > 0) and the regression parameters implied the initial P stock (a) and release rate (b) of labile P. Although the result of the extract based method was statistically correlated to the DGT measured P (p < 0.05), the DGT result had advantages over the traditional method due to its in-situ measurement and kinetic characterization of available P. Since the regression parameters reflected combined factors that impact the interaction between solid and solution P, the soil or sediment properties such as pH and organic matters need to be further studied in details, although they presented a significant correlation with DGT-P in this work. Our work provides a new insight in the rapid assessment of the size and resupply capacity of the existing legacy P stocks.