镭同位素示踪隆教湾的海底地下水排泄

福建省漳州市隆教湾海水中镭同位素的研究,目的是评价海底地下水排泄量。在2007年6月的航次中,垂直于岸线的9km剖面上布置15个站位,每个站位用潜水泵采集表层海水样60L于塑料桶中。水样运回实验室后,立即用装有锰纤维的PVC管以虹吸的方式富集水样中的镭同位素,水通过PVC管的流速小于300ml/min。224Ra活度用连续射气法测定,测完224Ra后密封7d以上,然后用直接射气法测定226Ra活度。224Ra和226Ra活度都呈现自岸向海逐渐降低的规律,表明扩散控制镭同位素的分布,由224Ra获得68.83km2d-1的扩散系数,同时226Ra形成-0.963dpm100l-1km-1的活度梯度。用扩散系数和活度梯度建立的226Ra的离岸通量为6.62×1011dpmkm-2d-1,这个通量一定是得到SGD输入的镭支持,从而获得隆教湾的海底地下水排泄量是3.03×109m3km-2d-1。该排泄量包括陆源地下淡水排泄量和再循环海水排泄量,绝大部分可能是再循环海水,有待进一步研究。

The Estimation of Submarine Inputs of Groundwater to a Coastal Bay Using Radium Isotopes

This paper reports the results of initial research on radium isotopes of surface ocean waters in the Longjiao Bay on the western flank of Taiwan Strait. The main objective of this research is to assess the submarine groundwater discharge (SGD). In order to assess the spatial distribution of 224Ra and 226Ra, the authors collected 15 surface ocean water samples (ca. -1 m in depth) during the low tide. The collection was performed within a period of 2 hours in the direction from onshore to 9 km offshore by using 60 L polypropylene buckets. In the laboratory, the water samples were immediately treated through gravity-fed PVC column (4.5 cm in diameter, 50 cm in length), filled with manganese oxide-impregnated acrylic fiber at a flow rate of 300 ml/min to retain radium. After that, the activities of 224Ra absorbed on the Mn-fiber were measured through the continuous emanation method. Finally, the Mn-fiber was sealed for more than 7 days, and the activities of 226Ra absorbed on the Mn-fiber were measured through the direct emanation method. 226Ra distribution yields an activity gradient of -0.963 dpm100 l-1km-1. The short-lived 224Ra distribution in this region yields an eddy diffusion coefficient of 68.83 km2d-1. The interpretation is that the eddy diffusion is the primary controlling agent for the shore perpendicular distribution of the long-lived 226Ra. In order to estimate the seepage rate of groundwater in the study area, the authors applied the approach developed by Moore. The short-lived radium isotopes, 224Ra, were used to establish the eddy diffusion coefficient for the near-shore study area. The product of the eddy diffusion coefficient and the offshore 226Ra activity gradient established the 226Ra flux of 6.62×1011dpmkm-2d-1. It is expected that this flux must be balanced by Ra input from the submarine groundwater discharge (SGD). On the basis of the flux of SGD within the shore, 226Ra budget was calculated as 3.03×109 m3km-2d-1. This magnitude of SGD includes terrestrially-derived fresh SGD and recirculated seawater SGD, However, further researches should be required to determine the terrestrially-derived fresh SGD on the one hand, and recirculated seawater SGD on the other hand.