Cd pollution in marine bay is one of the critical environmental issues, and therefore understanding the distribution characteristics of Cd is essential to environmental conservation. It has been found in previous studies that the distribution of pollutants in marine bay has feature of homogeneity, and this paper tried to analyze the mechanism of Cd's homogeneity based on investigation data in Jiaozhou Bay, eastern China. Results showed that the distributions of Cd contents in marine bay could be transformed between homogeneity and heterogeneity. In case of little or small Cd input to the bay, the distribution of Cr was homogeneity. While in case of large input, the distribution was heterogeneity. In generally, the heterogeneity was determined by Cd input, while the homogeneity was determined by tide and marine current.
Many marine bays have been polluted by various pollutants including Cr, and therefore understanding the migration rules and theories is essential to pollution control and environmental remediation. Jiaozhou Bay is a semi-closed bay located in Shandong Province, China. Based on the research on the distributions, trends, and migrations of Cr in Jiaozhou Bay, this paper provided five fundamental theories for migration rules of Cr, including 1) omogeneous theory for substance's content, 2) environmental dynamic theory for substance's content, 3) horizontal loss theory for substance's content, 4) migration trend theory for substance's content, and 5) vertical migration theory for substance's content. These fundamental theories were essential to environmental decision making and scientific research.
Based on the data from May, August, and October 1992, the Cd content in the Jiaozhou Bay waters varied from 0.11 to 1.53μg/L, up to the water quality standards of Class I and II seawater. This indicates that the water quality of the entire bay was slightly or not polluted by Cd during this period. In May, the variation of the Cd content in the bay ranged from 0.23 to1.53μg/L, more specifically, the Cd content reached a relatively high level of 1.00-1.53μg/L in the offshore waters near the estuary of Dagu River, Loushan River, Licun River and Haibo River and offshore waters of the north of the bay, leading to a mild contamination, while other waters in the bay were not polluted by the Cd content at all. In August, the variation of Cd content in the bay ranged from 0.11 to 1.11μg/L, more specifically, the Cd content hit the record high of 1.11μg/L in the offshore waters near the estuary of Dagu River and in the waters of the south of the bay mouth, making the waters polluted slightly, while other waters in the bay were not polluted by the Cd content at all. In October, the variation of the Cd content in the bay ranged from 0.12 to 1.11μg/L, more specifically, the Cd content reached a relatively high level of 1.04-1.11μg/L in the offshore waters near the estuary of Loushan River and Licun River, leading to mild contamination, while other waters in the bay were not polluted by the Cd content at all. There are three sources of the Cd content in the Jiaozhou Bay waters: rivers, main sea currents and surface runoff. In other words, these are three ways to transport the Cd content. In these ways, 0.96-1.53μg/L, 1.11μg/L and 1.10μg/L of Cd content are transported respectively. In this regard, the order from the largest to smallest of Cd content delivery is as follows: rivers, main sea currents and surface runoff. The Cd content transported by surface runoff is very close to that transported by main sea currents, indicating that the Cd content emitted by humans to land is consistent with that released to the oceans. Besides, the Cd content transported by the three sources is higher than or close to 1.00μg/L, suggesting that both lands and oceans are mildly polluted by the Cd content.
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