Masters Thesis

The contribution of inflow tributaries to surface water, metal concentration, and total suspended sediment load at Hiller Tunnel, Malakoff Diggins

This thesis reports on the Malakoff Diggins Rim Trail tributary streams and their contribution to Hiller Tunnel, the main surface water outlet of the Malakoff Diggins mine pit. The study utilized field measurements, hydrologic box modeling, and the ArcGIS software. Malakoff Diggins State Historic Park was one of California's largest hydraulic gold mines. Since the California Gold Rush, discharge containing metals and sediment has continued to be released from the mine, contaminating the Humbug Creek Watershed. Research on the contribution of inflow Rim Trail streams that are located along the north rim of the mine pit, is an important step towards mitigating discharge as a part of mine site remediation. Study objectives included a) locate Rim Trail tributaries and quantify surface water discharge from these inputs and at the Hiller Tunnel output; b) determine Rim Trail discharge metal concentrations for mercury, copper, nickel, and zinc; c) determine total suspended sediment (TSS) loads from the Rim Trail tributaries and Hiller Tunnel, and d) estimate water budgets for the Rim Trail subwatersheds and Hiller Tunnel during storm events. Quantification of surface water discharge and assessment of water quality were conducted during five storm events in Water Years 2014 and 2015. Storm runoff was sampled at the Rim Trail tributary streams and Hiller Tunnel and analyzed for TSS and metal concentrations (notably total and dissolved mercury, copper, nickel, and zinc). LiDAR (Light Detection and Ranging) datasets and ArcGIS software (Spatial Analyst Tools: Hydrology) were used to delineate the Rim Trail subwatersheds and the Hiller Tunnel watershed. A hydrologic box model was used to quantify fluxes and volumes of water within the Rim Trail subwatersheds and the Hiller Tunnel watershed. The cumulative instantaneous discharge from seven Rim Trail tributaries ranged from 0.4 to 3.8 ft3/s (± 10%), while the Hiller Tunnel instantaneous discharge ranged from 2.5 ft3/s (± 10%) to 47 ft3/s (± 25%). The Rim Trail tributaries contributed approximately 6.5-26% towards Hiller Tunnel outflow discharge. Surface water entering the Malakoff Diggins Pit from the Rim Trail tributaries met the regulatory water quality standards for mercury (50 ng/L), copper (9.0 μg/L), nickel (52.0 μg/L), and zinc (120.0 μg/L). However, during a storm event on 2/9/2014, measured concentrations of mercury (500 ng/L), copper (136 μg/L), nickel (109 μg/L), and zinc (158 μg/L) in the surface water exiting the pit (Hiller Tunnel outlet) exceeded the water quality standards for these metals. The Rim Trail tributary TSS load was 508 mg/s, which contributed less than 1% towards the Hiller Tunnel TSS load of 3.4 kg/s on 2/9/2014. This indicates that surface water originating from the north rim of the pit primarily picks up metals and sediment after it enters the pit and transports sediment within the Malakoff Diggins Pit downstream towards Hiller Tunnel.

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