The natural rehabilitation of an anthropogenically acidified tropical Lake: two decades of monitoring.

The rehabilitation of a pond after approximately 20 years of strong acidified conditions due to industrial and domestic waste deposition in its catchment basin is reviewed. We describe in this study the acidification process that occurred in a tropical pond in Northeast Brazil (Dunas Lake), the rehabilitation plan for the pond and the subsequent monitoring conducted over two decades. After the contamination assessment by the late 80s, a rehabilitation plan was carried out in the early 90s, in which the contaminated soil and water have been removed and reduced, respectively. No further attempt to neutralize the water or any remediation has been carried out. A toxicity monitoring plan based on toxicity assays with the fish Poecilia reticulata was employed to verify the natural rehabilitation of the pond. The data on toxicity, pH, conductivity, sulphate and dissolved iron recorded from 1994 to 2010 were also compiled and discussed. The collected data in 2003 and 2004 indicated changes in water quality and from them complementary management actions, namely improvement in the containment plant, were conducted in 2005. Results for toxicity assays and pH results indicated interannual changes in the water quality similar to rainy-dry periods. Moving average approach using pH data clearly showed the recovery process of Dunas Lake as well as the importance of the containment plan to reduce the contamination. Finally, a summary of the recent situation after two decades of rehabilitation is provided.