胶州湾地区水体中镭同位素分布特征及其影响因素

Distribution characteristics of radium isotopes and their influence factors in the water of Jiaozhou Bay area

2012年4-5月以胶州湾地区为研究地点,分析和研究了该地区地下水、河水、海水中224Ra和226Ra的分布特征及其影响因素。结果表明,232Th和238U浓度较高的花岗岩地区地下水和河水中224Ra和226Ra的活度均比其他地区高;受到海水混合的影响,沿岸地区地下水和河水由于水体盐度升高,224Ra和226Ra从固体颗粒的解吸量增加,从而导致活度较高;受河流输入(溶解态和悬浮颗粒解吸)以及海底地下水排泄(Submarine groundwater discharge,SGD)影响,胶州湾近岸海水224Ra活度相对较高,之后224Ra自身衰变以及外海水的混合稀释,使224Ra活度分布呈现随着离岸距离增加活度逐渐降低的特点;而226Ra活度在胶州湾内则保持一个较高水平,只出现个别226Ra低值分布区,主要原因是外海水与陆源淡水的226Ra活度较为接近且226Ra半衰期较长。

Background： The interaction between continental and oceanic process is much intense in coastal region. The terrestrial freshwater mixes with seawater here which leads to the chemical constituents in water undergo rapid change. Purpose： The aim is to analyze and study the distribution characteristics of 224Ra and 226Ra and their influence factors in the water of Jiaozhou Bay area. Methods： The water samples of 224Ra and 226Ra were collected from groundwater, river water and seawater around the Jiaozhou Bay from April to May, 2012. In the laboratory, 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. Results： The results show that the activities of 224Ra and 226Ra in groundwater and river water are much higher in the granitic area which has higher concentration of 232Th and 238U. Because of mixing with seawater, the salinity of groundwater and river water in coastal region rises which leads to the increasing amounts of 224Ra and 226Ra desorbed from the particles, resulting in higher activities of 224Ra and 226Ra in coastal region. Affected by the input of river （dissolved and desorbed from suspended particles） and submarine groundwater discharge （SGD）, the activities of 224Ra and 226Ra are relatively higher in nearshore seawater, then with the decay of 224Ra and mixing dilution effect of the offshore seawater, the activities of 224Ra gradually decrease from nearshore region to the open ocean. The activities of 226Ra in the seawater of Jiaozhou Bay remain high-level values, and there are only a few zones of low 226Ra activities. The main reasons are that the activities of 226Ra from terrestrial freshwater are close to those from offshore seawater and the half-life of 226Ra is very long. Conclusion： The activities of 224Ra and 226Ra in groundwater relate to the rock around Jiaozhou Bay and the water salinity. In addition to rock and water salinity, the activities of 224Ra and 226Ra in river water are also concerned with human activities and the process of groundwater recharge. The activities of Z24Ra and 226Ra in seawater depend on the sources and sinks of radium isotopes.