The past and prognosis of mining cessation impact on river sediment pollution

Abstract

Purpose: Remediation of mine sites is often aimed at reduction of river pollution after cessation of mining activity. However, the effects of these works overlap natural attenuation processes and their efficiency cannot be recognized well without detailed studies of the fluvial environment. The aim of the study is to predict changes of channel sediment pollution in rivers affected by mine sites after their rehabilitation, planned after mining cessation. Materials and methods: This study utilizes data on sediment pollution in three rivers polluted by lead-zinc mining, sampled over a time span of over 20 years in southern Poland. The former and the present pollution levels have been compared between the Biała Przemsza and Sztoła Rivers continuously influenced by mine waters and the Chechło River where mining has ceased. The observed changes in sediment pollution in the three catchments have been interpreted in terms of the impact of mining cessation on sediment pollution and the observed rate of the pollution changes in the past, reconstructed also from reservoir sediments. Results and discussion: In the drainage basins of the Chechło River, a mine has been closed and the rapid drop in metal concentrations in channel, and reservoir sediments was observed within a few years. In the other two drainage basins where lead-zinc mines still operate, the metal concentrations remained at the same level. Based on the similar catchment and pollution characteristics, a drop in metal concentration in these two rivers is expected soon after ore exhaustion. Nonetheless, considering the large contamination of soils and overbank sediments in the three drainage basins due to a long mining history, the drop will be followed by a very long period when their concentrations are raised above a permissible level. Conclusions: The study has shown that the natural fluvial attenuation processes can decrease heavy metal concentrations in channels cut in fine-grained alluvia within a few years after mine closure by one order of magnitude but the rate of the decrease depends on the frequency and magnitude of floods which will take place after mining cessation. Efforts made to mitigate pollution at mine sites may not be as efficient as fluvial processes to rapidly decrease channel sediment pollution below permissible levels.