Total suspended particle (TSP) samples were collected between 2005 and 2010. Weekly integrated samples were collected on acid washed quartz fiber filters (10"x8", Whatman®) using a Graseby Andersen TSP High Volume Sampler. Between November 2005 and May 2007, the sampler was located near the lake at the UC Davis Field Station (Hatchery) away from any local source of disturbance. After May 2007, the sampler was relocated about 300m south to reduce local impacts due to remodeling at the Hatchery. The TSP sampler was placed 3.2m above the ground and protected by trees from direct road dust inputs. TSP samples were collected at an airflow rate of 85m3 h−1. All filters were kept frozen until further analyses.
Water depth profile samples from Lake Tahoe were collected seven times at different seasons between the Spring of 2013 and Summer 2016. Van Dorn bottles (Wildco Beta Plus acrylic 2.2 L, with no metal parts that touch the sample) were used for water collection at depths of 50, 100, 150, 200, 250, 300, 350, 400, and 450m and a one-liter HDPE bottle attached to a 2.5m long plastic rod was used to collect surface water samples. Samples were collected at the Mid-lake Tahoe Profile (MLTP) station (39.09231° N; 120.00275° W). From each depth, one liter of water was dispensed into an acid-washed sample rinsed LDPE bottle for trace metals and Pb isotope analyses as described in Chien et al. (2017). Groundwater samples were obtained from two wells at the Lake Tahoe fire station and three wells at the Hatchery, and river water samples were collected from Third Creek, Trout Creek, Upper Truckee River, Ward Creek, Incline Creek, Blackwood Creek and General Creek. All water samples were filtered with acid washed 0.45 μm filters (SupaPore) before nutrient, trace metal and Pb isotope analyses. Samples for trace metal and Pb isotopes analyses were acidified to pH < 2 with concentrated double distilled nitric acid. MilliQ water blanks were also collected and analyzed similarly.
In order to separate Pb from the different sample matrices, 500 mL from each water sample and the digested TSP samples were dried down and re-dissolved in 100 μL of concentrated HBr (Optima grade, Fisher Scientific) three times. Pb was separated using AG1-X8 resin. Briefly, the matrix of the samples was eluted with 1 N HBr and the Pb fraction was eluted by 6 N double distilled HNO3, this eluent was then dried down and brought up with 2% HNO3 to ~2 ppb for analyses. Pb separation and trace metal pretreatments were done in the clean room in the Keck lab of UCSC. To determine Pb isotopic composition, analyses were carried out on a Thermo Element XR HR-ICP-MS in the Marine Analytical Laboratory at UC Santa Cruz following the method developed by (Zurbrick et al., 2013). 204Pb, 206Pb, 207Pb and 208Pb were analyzed and 200Hg and 202Hg were also monitored for isobaric interference correction on 204Pb. Pb isotopes in the samples were corrected by bracketing to NIST SRM-981 values; NIST SRM-981 was analyzed between every five samples. Typically, a 2 ppb Pb solution resulted in a signal of about 4x10^6 counts per second on 208Pb, external precision (2σ) for 206Pb/204Pb, 206Pb/207Pb and 208Pb/207Pb are 5.7‰, 3.7‰ and 2.2‰, respectively (based on 33 NIST SRM-981 analyses).
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