Water contamination in the Anzasca valley (NW Italy): the long-term effects of historical Au-mine activities

Gold mining activities can have long-lasting impacts on the environment. These impacts include soil and water contamination due to the generation of acidic drainage (AMD) and the release of potentially toxic elements (PTEs) including metals and metalloids as well as other chemical residues derived from ore processing.

In the Anzasca Valley (NW Italy), the Pestarena and Crocette gold mines were exploited from the Middle Ages until their final closure in 1961. The study area is an alpine valley where paragneiss, mycascists and orthogneiss outcrop. The metamorphic rocks at the valley floor are covered by alluvial deposits that host a phreatic aquifer connected to surface water bodies.

The gold mineralisation is associated with sulphides (pyrite and arsenopyrite) that were initially processed by mercury amalgamation, followed by cyanidation. Waste from ore processing was abandoned in the large area near the processing plants and deposited in waste dumps. The mobilization of PTEs due to the leaching of mining waste and the drainage of mine waters has led to significant soil and water contamination.

Previous studies of soils in the area indicate acidic conditions, with pH values ranging from 3.8 to 6.2, and concentrations of PTEs exceeding Italian legislative threshold, including antimony (up to 40 mg/kg), lead (up to 2360 mg/kg), mercury (up to 470 mg/kg) and in particular arsenic (up to 25800 mg/kg). Furthermore, surface water (SW) found arsenic concentrations peaking at 280 µg/l. However, until now, no studies have evaluated the quality of groundwater (GW) in these areas.

The current study aims to assess the level of water contamination in Pestarena and Crocette. Analyses were carried out during three water sampling campaigns in May, July and September 2024, to highlight possible chemical variations over time in GW and SW. In Crocette, 11 samples were collected, including 2 GW samples and 9 SW samples. At Pestarena, 18 samples were collected, equally divided between GW and SW. pH varied slightly but remained neutral or sub-acid, while electrical conductivity (EC) and total dissolved solids (TDS) increased during the summer months, with particularly high levels observed in GW near the waste dumps in Pestarena. Arsenic levels exceeded the Italian limit (70 µg/l) in 83% of the GW and SW samples. While other metals remained at low concentrations in SW, elevated levels were found in GW at Pestarena downstream of the mining waste, including aluminium (up to 7266 µg/l), iron (up to 1785 µg/l), lead (up to 25 µg/l), manganese (up to 276 µg/l) and nickel (up to 86 µg/l). Cyanide and mercury analyses are currently underway.

These preliminary results confirm that GW also is affected by past mining activities and emphasise the need for long-term monitoring to assess contamination levels and future remediation activities. Further studies are needed to fully understand how factors such as precipitation, snowmelt and soil characteristics influence these parameters, as well as the mobility and concentration of PTEs throughout the seasons.