A Preliminary Assessment of Wet Deposition and Episodic Transport of Total and Methyl Mercury from Low Order Blue Ridge Watersheds, S.E. U.S.A.

Abstract

Results from a preliminary sampling program designed to investigate total (THg) and methyl Hg (MeHg) deposition, cycling and transport at the Coweeta Hydrologic Laboratory western North Carolina are presented. Wet deposition samples were collected in June and July 1994 and throughfall, seep and streamwaters were intensively collected during and after a rainfall event in June 1994. All water samples were collected using ultra clean trace sampling protocol. Low elevation Watershed 18 streamwater THg concentrations peaked with discharge, increasing 6 fold to 9 ng L^-1. High elevation Watershed 27 which received less than one half the precipitation Watershed 18 received during the event, exhibited THg concentrations only 1.3 times over base flow conditions. Methyl Hg concentrations remained near detection limits (≤ 0.025 ng L^-1) in both streams. Dissolved MeHg concentrations were higher in shallow seep (0.097 ng L^-1), throughfall (0.135 ng L^-1) and precipitation (0.16 – 0.035 ng L^-1) than streamwaters. Initial estimates of annual THg and MeHg deposition and transport indicate >90% retention of Thg and a >80% retention or demethylation of wet deposition MeHg is occurring in these low order watersheds.