FLUVIAL METAL TRANSPORT NEAR SOURCES OF ACID MINE-DRAINAGE - RELATIONSHIPS OF SOLUBLE, SUSPENDED AND DEPOSITED METAL

The Afon Goch (Anglesey, UK) is a short (12 km source to estuary) stre am highly contaminated by acid mine drainage (AMD) throughout its leng th, due to past-mining at the head of the stream. Metal distribution i s strongly controlled by the pH, which increases downstream particular ly at confluences with two unpolluted tributaries. A pH increase cause s precipitation of metals, primarily Fe as hydroxide, thus altering th e transport of the metal load, potentially allowing storage of metal w ithin the river as deposited material. However, further work suggests that the controls on whether metal can behave non-conservatively, and therefore the controls on metal distribution, are more complicated tha n being purely pH dependent. This is because much of the Fe load, even at the low pH at the head of the stream, is not soluble Fe3+ but coll oidal Fe hydroxide. Consequently, coagulation is a requisite intermedi ate step between precipitation and potential for settling. It is possi ble that in reaches of the stream away from tributary confluences, the process of coagulation is the predominant influence on metal distribu tion. Furthermore, because much of the metal load in the water column is very fine, its deposition results in a sediment in which the metals can be intimately associated with a biofilm at the sediment/water int erface. Such associations change both deposition and erosion character istics of the sediment and have implications for subsequent diagenesis and mineral morphology.