Study On Environmental Geochemistry Of The Sediments In Modaomen Estuary Of The Pearl River

Modaomen Estuary is a complex ecosystem influenced both by natural factors and human activities. It's the typical area that we do the research. The following major element ions(K+,Na+,Ca2+,Mg2+,Cl-,HCO3-,SO42-)and heavy metal(Cu,Pb,Zn,Cd,Cr,Ni,Hg,As)were chosen as the research objects. First, we analyse the dynamic environment,the physiognomy and the sediment character of Modaomen Estuary. Then analyse systemicly the content, distributing, ingection factors and the sources of the major elment ions and the heavy metals. Moreover, environment quality assessment and ecological risk assessment of the heavy metal in surface sediments were performed for the whole estuary. All this is to provide some scientific basis for the harmony development of the resource exploiture and the environment protection. On the bases of much data, the main conclusion can be drawn as follows.(1) Modaomen Estuary is a typical current-wave estuary at present. Both current and wave is strong. During a year, the maximum wave energy period and the maximum current runoff period is stagger. It has small tide disparity and low tide speed, and it belongs to weak tide estuary. During the flood season, the current is strong, and the estuary is controlled by the freshwater. During the low water season, the current weakens sharply, and it could reach over Dapaisha. The jet at the Modaomen outlet is turbulent during the flood season and buoyant during the dry season.(2) The former inner sea area is becoming two parts now after being fathered. One is the trunk current, the other is the distributary. The physiognomy character in the outlet is"one slot and one shoal", and outside the outlet it's character is"two slots and three shoals". Sandbar accretion is speeding up. The segments of the sediments along lognitudinal and transverse orientation distribution recognized as"coarser-finer-coarser"were differentiated, but the east sand grain is wider than the west. The distribution of sorting degree was recognized as"worse-better- worse"state.(3) The maximum of major elment anions and cations is commonly in the open sea except K+ and HCO3-. Their contents increase gradually from the riverbed to the sand bar and at last to the open sea. Concretely, the contents increase from the riverbed to the inner sand bar, and from the inner sand bar to the open sea, their contents decrease first and then increase. The contents of heavy metal have the highest value around the outlet and on the inner sand bar. Most heavy metal such as Cu, Pb, Zn, Cd, As, Ni, their contents decrease first and then increase.(4) The conclusion of clustering analysis of major elements is that they can be sorted to 4 genus: Mg2+ ,Cl-,Ca2+ and SO42- belong to one genus; HCO3-,K+ and Na+ each is one genus. Heavy metal can also be sorted to 4 genus: Cu, Cd, Ni, Zn and As belong to one genus; Pb, Cr and Hg each is one genus.(5) The relativity between pH value and the content of heavy metal is inapparent., and neither does relativity between Cl and heavy metal. It proves that pH value and Cl are not the main factors which affect the distribution of major elements and heavy metal.(6) Contents of major elements and heavy metal (except Pb) have positive relativity with content percent of clay in the sediments, and the relativity between wide grain and them is inapparent or negative, which proves clay has sorption to major elements and heavy metal, but not all the elements have the highest content in clay, the source of some elements are not from clay, but from industry and agriculture, life pollution or atmosphere sedimentation.(7) It can be drawn from the content and distribution character of heavy metal that the water quality of upriver Modaomen is good, and it's the drinking water source of Zhuhai and Aomen. To insure the water supply, Modaomen watercourse can only receive drainage underneath the Denglongshan break surface, so the main sources of heavy metal in Modaomen Estuary are from the weathering production of soil and rock, light industry and agriculture pollution, life pollution, atmosphere sedimentation and salt tide tracing upriver.(8) The conclusion of ecological risk assessment of the surface sediments in Modaomen Estuary is that there're middling-polluted areas around the inner sand bar and Jiaobeisha, and on both sides of the outlet. The most seriously polluted heavy metal are Cd and Hg. Secondly,pollution degree of As, Cu, Cr, Pb, Zn and Ni is light or zero-pollution. After all, most of the evaluate indexes of the assessment is low, and the pollution degree belongs to light or zero level and there're few overrun sites.(9) Modaomen Estuary can be divided into three sedimentary environment subareas. They are excellent area, good area and middling area, which can be denoted by A, B and C. The sedimental environment quality of A area is excellent, it mostly locates in the riverbed, the sand bar top and most part of the open sea. The sedimentary environment quality of B area is good, it lies in both sides of the outlet, the east side of the outer slope of the sand bar and the west side of the open sea. In C area, it's sedimental environment quality is middling. It is mostly around the sand bar.(10) Comparing with other area in China, the content level of heavy metal in Modaomen Estuary is low. It's sedimentary environment is better than other urbanized and industrialized estuary and coastal area, and it's pollution degree is lighter than theirs. While it's not very optimistic that the content amount level is equivalent to these area. Therefore, we must protect the sedimentary environment of Modaomen Estuary in order to make it develop continuably.