Investigation of hydrochemical anomalies of the Roe Valley catchment, Northern Ireland

Anomalies in stream water concentrations for several metals (Al, Cr, Cu, Ni, Pb, Zn) over the Roe Valley, Co. Londonderry, were detected in 1996. The catchment has a variety of geological formations, with base-rich and base-poor lithologies and peat dominated headwaters. A selection of stream water sites was re-sampled (2009) to compare with the older survey sampling (1996) and to try to elucidate the hydrochemical processes. Additionally, some of the sites had concentrations close to, or exceeding those, of the Water Framework Directive (WFD) Environmental Quality Standards (EQS). Since 1996, chromium, nickel and zinc concentrations have changed only moderately. Copper concentrations have decreased significantly between 1996 (median 2.7 μg/L) and 2009 (median 1.3 μg/L). The trace element concentrations can be related to local natural sources (such as mineralisation in the Sperrin Mountains), and solubility controls related to pH and dissolved organic carbon. The change in copper concentrations is not completely understood, but may relate to subtle variations in solubility controls, or a decline in the sources of copper to the catchment