Accidental contamination during hydrocarbon exploitation and the rapid transfer of heavy-mineral fines through an overlying highly karstified aquifer (Paradiso Spring, SE Sicily)

The area around Ragusa in Sicily is well known for the exploration of petroleum deposits hosted in Mesozoic carbonate rocks. These reservoirs are overlain by less permeable rocks, whereas the surface geology is characterized by outcrops of Oligo-Miocene carbonate units hosting important aquifers. Some of the karst springs of the area are used as drinking water supplies, and therefore these vulnerable aquifers should be monitored and protected adequately.In the early afternoon (14:00) of 27 May until the late evening (19:30) of 28 May 2011, during the construction of an exploitation borehole (Tresauro 2), more than 1000m3 of drilling fluids were lost in an unknown karst void. Two days later, from 06:30 on 30 May, water flowing from Paradiso Spring, lying some 13.7kmSW of the borehole and 378m lower, normally used as a domestic water supply, was so intensely coloured that it was unfit for drinking.Bulk chemical analyses carried out on the water have shown a composition that is very similar to that of the drilling fluids lost at the Tresauro borehole, confirming a hydrological connection. Estimations indicate that the first signs of the drilling fluids took about 59h to flow from their injection point to the spring, corresponding to a mean velocity of ∼230m/h. That Paradiso Spring is recharged by a well-developed underground drainage system is also confirmed by the marked flow rate changes measured at the spring, ranging from a base flow of around 10–15l/s to flood peaks of 2–3 m3/s.Reflecting the source and nature of the initial contamination, the pollution lasted for just a few days, and the water returned to acceptable drinking-water standards relatively quickly. However, pollution related to heavy-mineral fines continues to be registered during flooding of the spring, when the aqueducts are normally shut down because of the high turbidity values. This pollution event offers an instructive example of how hydrocarbon exploitation in intensely karstified areas, where natural springs provide domestic water supplies, should be controlled effectively to prevent such disasters occurring. This pollution incident is also a useful example of how such “accidental” tracer tests can identify rapid karstic flowpaths over long distances.