| In this paper, Liao River estuary has been chosed as the study area. By testing grain-size, element content, 137Cs activity ratio of surface sediment from tidal flat and core sediment, using chemical mass balance method,system clustering method,rotation factor method,geoaccumulation index method,potential ecological risk index method and other research methods, the sedimental environment of tidal flat has been analyzed, the source of sediment has been explored, the elements'characteristics as distribution,occurrence,enrichment and migration on the tidal flat has been studied, the internal relations between elements of the tidal flat has been revealed, the pollution history of heavy metal of tidal flat in Liao River estuary has been discussed in the 50 years, and the potential ecological risk level of heavy metal in the tidal flat has been evaluated. Above all has provided a theoretical basis for ecological evirenmental protection of tidal flat.Seveal conclusions as follow has been maintained in this paper.(1)The surface sediments of tidal flat in Liao River estuary are mainly fine particulate matters(<63μm), The content of fine particulate matters has took up about 94.23% of the total sediments; the sediment grain size curve has shown a single peak, the source material is quite consistent in this region; the degree of particle size sorting is poor, the curve is extremely positively skewed, indicating that the grain size distribution peak in study area tends to towards the coarse side, the fine side has a low tail, the kurtosis of which is quite sharp. In the alongshore direction, the grain size characteristic of sediments from the tidal flat in Liao River estuary is that the clayey sedimental content in the estuary is higher than the seashore in the outer estuary ; in the offshore direction , the grain size has shown a zonal distribution tendency, which is that the content of silt and clay compenents reduced by turns, while the sand compenent has a increase tendence; the grain size of tidal flat sediment in the west coast is apparently coarser than east coast, which means that the tidal flat erosion effect has formed.(2)In the alongshore direction, the major elements in the study area has a randam distribution and has no obvious regularity. In the offshore direction, the characteristics of Al2O3,CaO,TFe2O3,MgO and other major elements are: the major element content in the high tidal flat is much higher, while it reduces by turns in the seawards direction; the sedimental distribution of SiO2 is just the opposite; SiO2,Al2O3,CaO and TFe2O3 has a good correlation, indicating that the sediments of Liao River estuary is mainly terrigenous deposits; K2O and Al2O3,CaO,TFe2O3 has shown a good positive correlation, which has to do with the potassium feldspar of the tidal flat sediments in Liao River estuary. The muddy sediments in Liao River estuary mainly origin from Upper Jurassic stratigraphy ( District II ) in the upstream of Xilamulun River, the contribution rate of this area is up to 76.08%, the second origin is the sediment of Quaternary in the downstream of Liao Rive, the contribution rate is 17.10%.(3)The trace element content characteristic of the surface sediment from the tidal flat in Liao River estuary has basically inherited the characteristic of the parent rocks and soil in the upstream. The heavy metal element of Pb has happened to enrich relatively, while V has occured to be leaching relatively. In the alongshore direction, the trace element distribution is determined by micro-topography,sediment transportation and tidal current; in the offshore direction, the trace element(except Ti and Ba ) of tidal flat has basically shown a zonal distribution, which is mainly controlled by the sedimental grain size, and the trace element content has increased with the increase of clay content. In Liao River estuary, of the surface sediment from tidal flat, the main features of element combination is Zn,Co,Mn,Cu,V,Ni,Pb,Zr,Rb,Ti and so on , which includes the heavy metal elements being siderophile and thiophile, and the element combination that has a relationgship with the fine grain clay in the sediments; the second combination is Sr,Ba and others, which has a relationship with potassium minerals; the third combination is Nb nd Y, which is related to the heavy minerals of weather resistance.(4)Using the time that has been derived from 137Cs activity ratio, we can figure out that the sedimental rate of tidal flat in Liao River estuary is: 2.4cm/a between 1954 and 1959, 4.8cm/a between 1959 and 1964, 0.55 cm/a between 1964 and 1986, 1.05 cm/a between 1986 and 2007. From 1954 to 2006. the average sedimental rate of tidal flat in Liao River estuary is 1.34 cm/a. In the core sediment of tidal flat in Liao River estuary, Cd,Mn,Pb,Al2O3,TFe2O3 and FeO have been devided into two combinations:①Combination of TFe2O3,Al2O3 and Mn, which is mainly natural source;②Combination of Cd,Pb and FeO, which is mainly artificial pollution source. The heavy metal of Cd had a most serious pollution during 1993; the pollution degree of Cd had been nearly moderate pollution between 1993 and 2000; in other periods, it had been mild pollution or non-pollution. The heavy metal of Pb and Mn had no pollution events during the 50-year sedimental history in the study area. (5)The potential ecological risk level of Cd, which is the heavy metal, has been serious pollution in the surface sediment of tidal flat in Liao River estuary; the heavy metal of As has been moderate pollution in Jinianbei of Liao River,Oil drilling paltform,Erjiegou,Delta and so on; other heavy metals like Mn,Zn,Cr,Cu,Pb have been mild pollution averagely, the potential ecological risk is quite low. In the general evaluation of potential ecological risk level, which has combined many heavy metal elements, we can find that: the potential ecological risk level of heavy metal in Oil drilling platform of Liao Rive,Delta Jinianbei r and Viewing platform is serious pollution and in Erjiegou and Sandaogou is moderate pollution.
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