Seasonal variations in 228Ra/ 226Ra ratio within coastal waters of the Sea of Japan: implications for water circulation patterns in coastal areas

Abstract

In this study, low-background γ-spectrometry was used to determine the 228Ra/ 226Ra ratio of 131 coastal water samples from various environments around Honshu Island, Japan (mainly around Noto Peninsula) at 1–3 month intervals from April 2003 until September 2005. Spatial variation in 228Ra/ 226Ra ratios was also assessed by analyzing 34 coastal water samples from five areas within the Sea of Japan during May and June 2004. The 228Ra/ 226Ra ratio of coastal water from all sites around Noto Peninsula shows seasonal variation, with minimum values during summer ( 228Ra/ 226Ra = 0.7) and maximum values during autumn–winter ( 228Ra/ 226Ra = 1.7–2). This seasonal variation is similar to that recorded for coastal water between Tsushima Strait and Noto Peninsula. The measured lateral variation in 228Ra/ 226Ra ratios within coastal water between Tsushima Strait and Noto Peninsula is only minor (0.5–0.7; May–June 2004). Coastal waters from two other sites (Pacific shore and Tsugaru Strait, north Honshu) show no clear seasonal variation in 228Ra/ 226Ra ratio. These measured variations in 228Ra/ 226Ra ratio, especially the temporal variations, have important implications for seasonal changes in patterns of coastal water circulation within the Sea of Japan.