CHEMICAL WEATHERING OF BLACK SHALES AND THEIR ECOLOGICAL IMPACTS ON STREAMS

Recent studies of black shale weathering have shown that under natural weathering conditions, these rocks break down releasing trace metals that can impair the function and structure of the surrounding ecosystems such as streams, lakes as well as ground water. Based on the above premise, we carried out laboratory weathering experiments of black shales under near-neutral conditions (~pH 5) to study the processes that control metal release. Metal concentrations were measured by dynamic reaction cell inductively coupled plasma mass spectrometry (ICP-MS; Perkin Elmer DRCII) following US EPA 6020 (sediment) and 200.8 (water) methods. Additionally, we performed standard toxicity tests using surface water and sediments collected from streams draining black shale catchments to study the chronic toxicity effects of the released trace metals on aquatic organisms viz. waterflea, fathead minnow, and midge larvae. Our results showed that the concentrations of trace metals released during an 80-hour reaction period were exceptionally low (the content of most trace elements was < 1 ppm). This would be attributed to the short reaction time (i.e. not enough time for complete breakdown of minerals) However, we observed a pattern of metal release that was consistent with breakdown of carbonate minerals. In the streams draining black shale, the concentration of copper, cadmium, lead, and chromium in the sediments and surface water exceeded values promulgated in the US EPA Water Quality Criteria. On the other hand, growth, survival and reproduction impairments on the three test organisms were low despite the high metal concentrations in the test media (i.e. water and sediments). This latter observation may be due to high dissolved organic carbon (DOC), neutral pH, and high water hardness values (moderately hard to hard water) characteristic of the site water and sediments. We conclude that black shale chemical weathering under natural conditions leads to increased trace metal concentrations in water and sediments of the neighboring systems. We also suggest that studies on ecological impacts of black shale weathering need to incorporate chemical parameters such as pH, DOC, and water hardness.