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Sediment source analysis at Malakoff Diggins State Historic Park, Nevada County, CA
Malakoff Diggins State Historic Park (MDSHP), a historic hydraulic mine, is a known source of fine‐grained sediment and contaminant metals to the Humbug Creek Watershed. The purpose of this study was to provide land managers insight into erosion and episodic transport of fine‐grained sediment in MDSHP. Erosion of the pit walls deliver sediment to Hiller Tunnel, which discharges to Humbug Creek, an impaired body of water for chromium (total), copper, iron, mercury, sedimentation/siltation, and zinc. In‐situ sediment samples were collected during 2015 from six vertical transects along the cliff walls of the mine pit. Surface water samples were collected in 2014 and 2016 from drainage networks below the in‐situ sampling sites during runoff conditions. In‐situ and suspended sediment samples were analyzed for grain‐size distribution, by X‐ray diffraction (XRD) for quantitative mineralogy, by inductively coupled plasma‐mass spectrometry (ICP‐MS) for trace elements, and by inductively coupled plasma‐optical emission spectroscopy (ICP‐OES) for major elements. X‐ray diffraction data indicated that the fine suspended sediment in Hiller Tunnel is composed predominantly of kaolinite and halloysite, with minor smectite, illite, and muscovite. Kaolinite is abundant in many of the in‐situ samples but relatively few stratigraphic units have abundant halloysite. In‐situ sediment samples from within the pit were found to increase in halloysite concentration, from stratigraphic bottom to top. SedUnMix was used to explore the potential contribution of sediment from different sites in the mine pit to the Humbug Creek discharge and may be used to inform management efforts in MDSHP. Finegrained sediment, which is discharged out of Hiller Tunnel, comes from all areas in the pit. Contributions from runoff above the pit rim were minimal, based on low suspended sediment concentrations contained in storm runoff. Elevated concentrations of heavy metals were higher in storm runoff samples collected in the west of the pit than in the east. Total mercury concentrations ranged from 29 to 2290 ng/L in 2014 and 21.1 to 6700 ng/L in 2016.