EXCESS RADON DISTRIBUTION IN WATER SAMPLES FROM SINKHOLES IN GROUNDWATER DISCHARGE AREAS (PONTINA PLAIN, CENTRAL ITALY)

Sinkholes in groundwater discharge areas are quite unusual. Most of the sinkholes occur on plains within tectonic valleys adjacent to limestone uplands and their occurrence has been so far reported in Central Italy only.

In this paper we report the preliminary results of a tentative Rn-222 budget, normally found at much higher concentrations in groundwater than surface waters, to assist in constraining the interaction between sinkholes and deep seated fluid circulation in the Vescovo Lake group in the Pontina Plain (Central Italy).

The Pontina Plain is located among the Ausoni and Lepini Mounts to the east and the current Tyrrhenian margin to the west. The overburden sedimentary succession developed from a marine depositional system to a transitional fluvial-coastal system early on, and to a fluvial-continental depositional system later (all of this Pliocene-Pleistocene). Within the overburden multiple confined and semi-confined aquifers are present. Below these, there is a regional confined aquifer that is recharged by circulation from the karst ridge. A hydrothermal circuit exists in the buried karst bedrock of the Plain, with main circulation along the fault network.

Water samples were collected in five sampling sites in the Green Lake (Vescovo Group) into top-to-bottom profiles during the month of June 2002 and Radon gas was extracted and counted using an electronic Radon detector (Durridge Co. manufactured RAD7, equipped with a solid-state ion-implanted, planar, silicon alpha detector).

Lake profiles generally showed an excess Rn-222 concentration in surface water. Excess Rn-222 depth profile is reversed in the eastern portion of the lake in proximity with the Lepini mountain border, indicating that groundwater enters Green Lake mostly through that region. Other physical and chemical data support the hypothesis that Rn-rich groundwater, less saline and dense than lake waters move upward along the eastern limit of the lake and form a fresher Rn-rich upper lens extending along the main axis of the lake, probably because of major evaporation processes. Locally in correspondence with sinkholes morphologies Radon values in surface waters are higher, probably because of gas emissions restricted to those particular areas of the lake bottom, where acid and corrosive fluids gave origin to the sinkhole collapse.