Studies On The Nutrient Dynamics And The Heavy Metal Pollution In Lake Nanhu, Wuhan

The lake eutrophication is one of the most serious environment problems with social-economic development and pollutants discharge increment. Urban, shallow lakes are generally exposed to anthropogenic activities and particularly vulnerable to human interference that cause enhanced primary productivity and can rapidly be transferred into a state of highly eutrophication.Lake Nanhu is a shallow, polymictic lake located in Wuhan city, the middle reach of the Yangtze River. Its open water area has a surface area of 4.0 km2 and the water depth ranges from 1.85 m to 2.50 m. Many universities, hospitals, and residential areas located around the lake. In recent decades, with the intensification of human activities and the rapid development of economy, there were drastic changes in the lake environment, in which the lake eutrophication is most serious. The nitrogen and phosphorus dynamics in water column, sediment, interstitial water, rainfall, runoff, the phosphorus release from sediment and heavy metal pollution in sediment in Lake Nanhu were investigated from April,2005 to February,2006. The main results were as follows:1. The average of TN in water column was 12.66 mg/L. TN concentrations were increaseing during the sampling time. The dominant form of DIN in waters was NO3--N. The average concentrations of NO3--N, NH4+-N and N02--N were 1.076 mg/L,0.832 mg/L and 0.238 mg/L, respectively. The maximum of NH4+-N, and the minimum of NO3--N and NO2--N appeared all in August. And the averages of total phosphorus (TP) and orthophosphate were 0.240 mg/L and 0.033 mg/L, respectively. The maximum of TP and orthophosphate in water column occurred in November, and the lowest concentrations of TP and orthophosphate in February and August, respectively.2. The mean of TN in sediments was 3.3922 mg/g (DW). The seasonal variation of TN in sediments was the same as the TN in waters; and it decreased with the sediment depth in 0-25 cm layer. The average of NH4+-N in sediments was 0.0882 mg/g (DW) and the proportion to DIN ranged from 70% to 96%. The concentration of NH4+-N had the same trend as the TN in sediments in seasonal variation and vertical distribution. The concentration of NO3--N in sediments was very low. The average of NH4+-N in interstitial water was 11.59 mg/L and with a percentage of higher than 90% in DIN. The concentration of NH4+-N in interstitial water increased with sampling time; and vertically the maximum was in 5～15 cm layers. The averages of NO3--N and NO2--N in interstitial water were 0.212 mg/L and 0.025 mg/L, respectively; vertically, the maximum of NO3--N was in 5～10 cm layers and NO2--N decreases with the sediment depth. The nitrogen concentration was the highest and had the most significantly gradient at the siteⅣ. But it was the reverse at the site III. The results of correlation analysis showed that TN in sediments was an important source for the lake and the nitrogen diffuses mainly by NH4+-N. The average of TP in sediment was 1.005 mg/g (DW), and that of orthophosphate in interstitial water 0.209 mg/L. Seasonally, the concentration of TP in sediment increased from April to November, and then became stable; the concentration of orthophosphate in interstitial water reached the maximum in November and the minimum in February. Vertically, the concentration of TP in sediment decreased with sediment depth, and orthophosphate in interstitial water had the same pattern. Horizontally, the maximum concentration of phosphorus in sediment and interstitial water occurred at the siteⅣ. The dominant form of phosphorus fractionation in sediment was inorganic phosphorus (IP) and the proportions to the total extracted phosphorus ranged from 61.4% to 77.1%. The concentration of bio-available phosphorus (BA-P) in sediment was very high, but it decreased significantly with sediment depth. The high BA-P concentration in surface sediment suggested that there was a strong potential phosphorus release from sediments to the overlying water. According to the significant correlation between phosphorus fractionation in sediment and orthophosphate in interstitial water, the main source of TP released from sediments should be BA-P and ACa-P.3. As to the horizontal distributions of different forms of nitrogen and phosphorus in water, sediment and interstitial water, the maximum was at the site IV and the minimum was at the site III and V.4. The pH value and dissolved oxygen (DO) were the important factor affecting the release of phosphorus from these sediments. A simulated study was made for the influence of different pH value and DO on the concentration of orthophosphate in overlaying water. Under acid and alkaline conditions it favored the phosphorus release. The P release concentration was higher under acid than under alkaline condition; the maximal and the minimum release concentrations were in the pH values of 4.6 and 7.1, respectively in the experiment pH range. Both anaerobic and aeration condition could make P release from the sediments. But the release concentration and the release rate in anaerobic condition were about 10 times higher than the aeration condition.5. The average of the moisture concentration was 72.4% in sediment. There was a slight variation among months. The concentration decreased significantly with the sediment depth. The average of LOI was 10.20%. The maximum of LOI was in November. There were significant correlations among TN, TP and LOI. This meant that there were same sediment trend about N, P and organic matters. According to the present results, the dissolved nutrients in sediment and in interstitial water decreased with the sediment depth in the sediment cores in the sampling period. And the concentrations of the dissolved nutrients had a sharp gradient in 0-15 cm layer, then decreased slightly in 15～25 cm layer. This indicated that the regeneration depth was almost the same in these two cores, i.e., deeper to 25 cm. In the top 0-15 cm layer, the regeneration was actively, and then decreased with the sediment depth.6. The concentration of TN and TP in foothill vegetation runoff was higher than in rainfall and urban runoff. The main form of DIN was NH4+-N, then NO3--N. The NH4+-N concentration in rainfall was significant higher than in runoff. The concentration of soluble PO43--P was higher in rainfall than in either foothill vegetation runoff or urban runoff. During sampling period, the different forms of nitrogen and phosphorus in rainfall and runoff had a fluctuant variation (except NH4+-N in rainfall, which decreased in annual variation).7. The averages (mg/kg, DW) of Cd, Cu, Cr, Mn, Pb and Zn were 0.155,47.0,147.8, 1 026.5,55.7, and 132.1, respectively. Horizontally, the heavy metal concentrations at the siteⅣandⅠwere relatively higher. The heavy metal concentrations decreased with the sediment depth from surface to bottom and the concentrations in surface sediments were significantly higher than the deeper ones. The results suggested that there was a higher concentration of the heavy metals at the siteⅣandⅠ. According to Igeo values, the major heavy metal pollutions in Lake Nanhu were Cr and Cd and the highest concentrations (mg/kg, DW) of Cr and Cd were 231.9 and 0.348, respectively. But the pollution status of heavy metals in Lake Nanhu was generally light. The present results revealed that municipal sewage discharge and rainfall runoff had significant cause of heavy metal pollution in Lake Nanhu and it is necessary to control the pollutions from these "point-sources" input.