The Sedimentary Characteristics Of Phosphorus In The Sediment Of Chaohu Lake

In this paper, from sedimentology and element geochemistry view, we measured the phosphorus species, metal elements content, magnetic susceptibility, organic matter content and other indicators of surface sediments and a cylindrical core of the Chaohu Lake and its main inflow rivers, and analyzed relationship between sedimentary characteristics and spatial distribution of phosphorus and man-made pollution. Based on the 210Pb and 137Cs chronology, we studied relationship between accumulation characteristics and evolution of phosphorus in the Chaohu Lake sediments and human activities on the historical period. So we could understand scientifically of the Chaohu Lake eutrophication processes and human activities contribution, and provided references about pollution control and ecological restoration. The major results were:The HCl-P contents were more stable in core sediment, and the contents of NaOP-P, OP and TP increased gradually with significant phase variation. According to phosphorus content and its morphological composition and other geochemical indicators of changes, the 90cm-long cylindrical core was divided into five phases about the historical accumulation of phosphorus.The first stage (90-65cm, 1680 years ago): the contents of phosphorus species, magnetic susceptibility and metal element contents were more stable in sediments, and phosphorus contents were consistent with the metal element contents, showed that the main source of phosphorus was natural input in sediments.The second stage (65-54cm, 1680-1760 years): in this stage, the contents of TP increased in the sediments, particularly NaOH-P and OP contents; metal element contents and magnetic susceptibility also significantly increased, reflecting that the topsoil erosion had increased, and related to the rapid increase of population, deforestation and agricultural scale in this rapid.The third stage (54-25cm, 1760-1950 years): the contents of phosphorus species, magnetic susceptibility and metal element contents maintained at a relatively high level, the late (1860-1950 years) decreased, reflecting the impact of human activities (agriculture). The main driving factor was a sharp increase of phosphorus input flux that may lead to rise lake nutrient level in 1800 year.The fourth stage (25-16cm, 1950-1980 years): the contents of phosphorus and metal element increased gradually, reflecting the agricultural development after new china founding and topsoil erosion intensified; the enrichment factors of phosphorus increased, indicating the man-made pollution increased gradually.The third stage (16-0cm, 1950-up to the present): phosphorus contents reached the maximum value in the entire sedimentary core, particularly the increase of NaOH-P contents which was the most significant because the sediments accepted the domestic sewage and industrial waste water besides agricultural development.The distribution of total phosphorus (TP) contents in surface sediments were 221.9-1651.1mg/kg, and TP in the western region was higher than the eastern region, and TP in the north region was higher than the south region in Chaohu Lake. The mainly phosphorus forms were NaOH-P and OP, and their average contents were accounted for 40% and 30% of TP content, next was HCl-P that the average content was accounted for 19% of TP content, while the average content of the bio-availability of SRP was 6.4mg/kg, accounting for 1% of TP content. The spatial distribution of SRP and NaOH-P contents were consistent of TP content, the highest content was in the western region. In the main inflow rivers, the highest content of TP was 2858.9mg/kg in the Nanfeihe river with NaOH-P predominantly which was accounted for 68% of TP content, the remaining surface sediments in river estuaries of phosphorus contents were 256.3 -934.7mg/kg. The main inflow rivers in estuarine sediments were consistent with TP in water.The spatial distribution of phosphorus related to phosphorus and watershed geologic background and man-made pollution in the surface sediments of Chaohu Lake and its main rivers. The main reason lead to the higher phosphorus content of Nanfeihe river and western lake was that the Nanfeihe river accepted a large number of Hefei municipal sewage and industrial wastewater. Compared to 20th80s, the increase of phosphorus content was the most significant in northwest region of Chaohu Lake, and the man-made input phosphorus forms mainly was NaOH-P in sediments. The phosphorus stratigraphy were widely distributed within the northern Chaohu basin, rock-derived phosphorus input was also one of the main reasons for the high phosphorus content in northern lake. The enrichment factors (EF) for phosphorus forms could be quantified with the history sediments using background values and Al as the reference element. The EFs of TP in surface sediments were 1.0-7.5, the Nanfihe river estuary up to 9.4, and the EFs of NaOH-P, OP and HCl-P were 0.6-17.9, 0.7-2.0, 1.8-5.4. HCl-P hadn't significant change that it primarily reflected nature sources from river basin.