Seasonal Distribution 224Radium Activities in the Venice Laggon as a proxy for groundwater discharge

The activities of 224Ra isotope were measured in 30 pre-determined locations within the Venice Lagoon, Italy, during different seasons. These surveys were performed in order to determine both seasonal variation and average annual activity within the lagoon. The first survey was completed in the surnrner of2005, the other two surveys were accomplished in the fall of 2006 and winter of 2007. End-member samples were also collected in the Adriatic Sea and a couple of rivers. The concentration of 224Ra in the pore water is often 2 orders of magnitude higher than its subsequent concentration in the open sea. We hence utilize the measured activities of radium to determine both the relative magnitude and location of the groundwater discharging into the Venice Lagoon. In addition point measurements of discharge using vented benthic chambers were taken in several locations during the sampling period in order to corroborate the radium measurements. Both integrated and point measurements display a large discharge of groundwater occurring in the northern lagoon as well as certain places in the southern area. The local discharge may vary with time though the general pattern does not display significant change between seasons. Though most of the discharge is likely to be recirculated salt water of varying origins, previous data show that this discharge is of a very large volume as compared to the fluvial discharge (as much as 20 fold greater). The zone where groundwater mixes with salt water has been likened to processes occurring in an estuary, and here many important chernic!ll reactions can take place. In addition, groundwater is often elevated in the concentration of contaminants so its discharge to the lagoon can be a major source of contamination.