222Rn activity in a karst coastal aquifer under active seawater intrusion

226Ra and 222Rn are considered by oceanographers as potential tracers of coastal and submarine groundwater discharge (SGWD). The use as tracers is based on the occurrence that 226Ra and 222Rn concentrations in coastal discharge are a few orders of magnitude greater than in coastal seawater. The paper is aimed at studying these isotopes in/and in the aquifers, in order to fili the gap of knowledge about their behaviour before they reach the coastal discharge. A part of the Salento karstic coastal aquifer (Southern Italy) has being selected as representative for the presence of many coastal and submarine springs and the occurrence of seawater intrusion , which represents the main process indicated as relevant in the production of high isotope activities. The significance of the presence and behaviour in coastal zone of 222Rn is examined. Base knowledge has been established by means of laborato~ experiments. The terra rossa, final product of limestone dissolution, resulted the main source of 26Ra in the aquifer. In the common conditions found in freshwater environments, where Ra is blocked in "terra rossa", 222Rn comes from this source. In dlsnamic conditions (physical !aboratory mode!) 222Rn activity demonstrates to be more influenced by 2 6Ra activi~ of terra rossa and its specific surface than flow velocity. Moreover the interpretation of the role of 2 2Rn has been carried out on the light of the mechanisms that regulate the presence in groundwaters of 226Ra in relation to seawater intrusion (desorption in high ionic strength). Lab experiments allow to state the extent of 226Ra desorption from terra rossa at different water salt content. The interpretation of the Rn contents related to numerous coastal springs and groundwaters belonging to the karst coastal aquifers of Puglia region and to the selected area lead to put forward some hypotheses about the factors, which should regulate the activity of 222Rn in the concerned aquifers.