Tracing the impact of geothermal plants in the Monte Amiata area, Tuscany, Italy: evidence from Hg contents in stream sediments and tree barks

The Monte Amiata area hosts the 3rd largest mercury (Hg) district worldwide, and an important geothermal field actively exploited for energy production. Mining activities ceased in the 1980s, but left an impressive legacy of environmental impact; on the other hand, the geothermal fluids contain several potentially toxic substances (gaseous Hg, hydrogen sulphide, ammonia....), that may be released into the environment during exploitation. For this reason, since 2002 abatement measures have been implemented at geothermal plants; emissions are constantly monitored by the regional environmental agency (ARPAT). It is obviously important to discriminate the contributions from different sources for impact minimization and optimal resource and land management. Ongoing studies of Hg dispersion from the Monte Amiata mining district in the environment, mostly in the catchment of the Paglia River, give also indications on the contribution from geothermal power plants. A recent survey conducted in the catchment of small creeks draining the eastern flank of Monte Amiata showed that most stream sediments are highly anomalous (>1 mg/kg) for Hg contents, but the highest values (up to 1,900 mg/kg) occur in creeks directly draining abandoned mines and/or smelting plants; creeks closest to present day geothermal plants show definitely lower (<10 mg/kg) Hg contents in stream sediments. The use of tree (Pinus nigra) barks as biomonitors of airborne Hg clearly indicates that the highest (up to 28.8 mg/kg) Hg contents in barks are all within 2 km from the largest former mining and smelting site; tree barks closest to geothermal plants show lower (<1 mg/kg) contents. An ad hoc monitoring program involving more trees at increasing distances from geothermal plants could reinforce this conclusion, and give more quantitative constraints on the long-term Hg contribution from geothermal plants.