2000 GSA Annual Meeting -- Reno, Nevada

Abstract 50379

CONCEPTUAL MODEL FOR DEVELOPMENT OF COLLAPSE SINKHOLES IN AREAS OF GROUNDWATER DISCHARGE, CENTRAL ITALY

Presented by Salvati, Roberto
Authors:
      Sasowsky, Ira D.,
      Capelli, Giuseppe.

Key words: karst, Italy, sinkholes, collapse, Latium-region

In Session 158     Hydrogeology III: Carbonate Ground-Water Flow Regimes (Posters) Wednesday, November 15, 2000 PM in Room: Hall C at 01:30 PM for .

Abstract: Sinkholes are erosional landforms that are characterized by internal (closed drainage) and a circular to sub-circular shape. They typically form in the upland zones of carbonate aquifers, where they serve as discrete recharge points to the aquifer. Two main types are known: solution and collapse. Collapse sinkholes cause significant societal impacts due to loss of buildings and infrastructure. Several areas within the Latium Region of central Italy have been experiencing collapse sinkholes for centuries. The collapses are sudden, and produce large water filled depressions. Occasionally, water in these depressions has a total head that is above the land surface, resulting in the creation of headwaters for new surface streams. The sinkholes are unusual, because they are formed in areas of regional groundwater discharge, rather than recharge. Most of the sinkholes occur on plains underlain by thick sequences of fluvial, lacustrine, or coastal sedimentary fill (some calcareous). The plains are within tectonic valleys adjacent to limestone uplands. We evaluated the historical occurrence of sinkholes in the Pontina and San Vittorino Plain localities. Based upon review of this data, along with regional geologic, chemical, and hydrologic information, we developed a conceptual model that accounts for the sinkhole development. Meteoric water recharges the aquifer in the adjacent limestone uplands, and flows towards regional discharge points in the valleys. The water quickly becomes nearly saturated with respect to calcite. Upon reaching the valley, the water is confined by clayey and calcitic horizons within the valley fill. Deep-source carbon dioxide ascends along basement faults to mix with the water, renewing aggression towards calcite. Dissolution of calcite in the valley fill allows localized breaching of the confining layer, and subsequent removal of mass from the valley fill. This leads to collapse of the land surface, as support is lost from below.

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