Evolution of the water chemistry of Lake Orta after liming

Since 1963 Lake Orta has been an emblematic case of industrial pollution by heavy metals and acidifying compounds (ammonium sulphate), to the extent that up to 1989 it was the largest acidified deep lake in the world. The low pH values of between 3.9 and 4.4 helped to keep high the levels of toxic compounds in solution, such as copper, aluminium, zinc and nickel. The liming performed in 1989-1990 brought the pH back to neutral values, determining the precipitation of the metals and the recovery of normal chemical conditions. The main results of researches conducted continuously on the lake water chemistry from 1988 to March 2000 are as follows. The whole water mass has been completely neutralised since the beginning of 1991; pH subsequently rose until in 1999-2000 it reached the values (6.7-6.9 units) of the years when the lake was in a natural condition. The alkaline reserve showed a continuous increase after the lake water was neutralised, until March 2000, when total alkalinity values levelled off at 0.19 meq l-1. The increase in pH has allowed a full recovery of nitrification processes; in fact, during the liming period the concentration of ammonium was drastically reduced, by over 80%; ammonium has been practically absent since the end of 1992, and it may be affirmed that the primary cause of the acidification of the lake has been completely removed. The nitrate content in the lake has almost halved compared with the mean concentrations measured before the liming; in March 2000 mean values of 2.0 mg N l-1 were measured, and it is likely that these values will fall further in the next few years, to below 1.5 mg N l-1. The concentrations of toxic metals have shown a progressive reduction, to the extent that in 1999 the content of copper and aluminium was close to zero in the whole water mass. The situation of Lake Orta has therefore improved enormously, and is now very similar to the original condition of the lake before it was polluted. However, this situation cannot be regarded as definitive, as a stable balance between the input and output of some important chemical species, such as bicarbonate and nitrogen compounds has not yet been achieved. Moreover, recent investigations have revealed that insufficiently treated industrial and urban waste is once more being dumped in the lake. In this situation there can be no doubts as to the necessity for continuing research on Lake Orta, not only to document the evolution of the chemical and biological characteristics of the lake, but also to verify the effectiveness of the treatment measures adopted and to determine the need for extending these measures to the whole watershed. In particular, the object of this research should be to quantify, control and reduce to an acceptable level the incoming loads of a) ammoniacal nitrogen, to eliminate the danger of the lake water becoming re-acidified; b) phosphorus, to prevent the onset of eutrophication processes; c) heavy metals, to minimise their toxic effects on the whole aquatic ecosystem.