An update of Lake Orta chemical status 25 years after liming

The chemical monitoring of Lake Orta after the operation of liming (1989-1990), aimed at the neutralization of the lake acidity due to industrial pollution, has been performed regularly by the CNR ISE on a monthly (1990-2000) or seasonal basis (since 2001). All the main chemical variables (pH, conductivity, alkalinity, major ions, nutrients, trace metals) have been considered. A first evaluation of Lake Orta chemical status about 10 years after the liming was published in 2001, in a Special Issue of Journal of Limnology. In this paper we present a further update about the lake response to the restoration treatment, focusing on the data collected between 2001 and 2013. These data allowed an evaluation of the advancement of lake recovery as regards the acid-base status of lake water; they also provided an insight into emerging or potential critical issues, such as eutrophication or resuspension of heavy metals still present in the lake ecosystems. The most recent data available on Lake Orta provided evidence of an almost complete recovery from acidification: pH and alkalinity recovered to the pre-pollution levels and a mass budget applied to alkalinity data confirmed that present loads to the lake are in equilibrium with the output. The lake is presently oligotrophic, with mean annual values of total phosphorus steadily below 10 µg P L-1. This situation is due to the low nutrient loads from the main tributaries draining the lake catchment: those rivers showing quite high phosphorus and nitrogen concentrations in the 1980s and 1990s (e.g. Fiumetta, Lagna) showed evident signs of water quality improvement after 2000, thanks to the collection and treatment of domestic and industrial sewage. Data from lake monitoring put in evidence very low concentrations of heavy metals in the dissolved phase in recent years, due to the recovery of pH values to neutrality. However, metals can still be present in the lake ecosystem, in the sediment or in the particulate phase. The latter component was investigated in a study performed in 2013, with the aim to assess the possible role of particulate matter in the release of toxic metals to lake water. Sampling and analysis were performed also on Lake Maggiore and Lake Mergozzo, to assess the situation of Lake Orta with respect to these lakes. Results were also compared with previous data available on trace metal concentrations in Lake Orta sediments and, for Cu and Fe, in the particulate matter.