SEDIMENT FINGERPRINTING IN THE CLEAR LAKE, CALIFORNIA, WATERSHED: DETERMINING SOURCES OF NUTRIENTS AND SEDIMENT

Clear Lake, California, is a hyper-eutrophic lake with frequent harmful algal blooms driven primarily by nutrients nitrogen (N) and phosphorus (P). Understanding and controlling sources of nutrients to Clear Lake is a high priority for managers who intend to improve water quality, which will benefit the local economy and ecology. N and P are transported to Clear Lake from its tributaries in dissolved and particulate forms. Particulate nutrients may be deposited in lake sediment. Anoxic conditions in bottom waters can mobilize P associated with ferric iron; wind-driven mixing can disrupt vertical stratification and cause resuspension of sediment. A sediment fingerprinting study is underway with the goal of determining sources of particulate nutrients from specific tributaries and land uses. In addition to natural erosion, possible sources of fine-grained sediment (silt and clay) and nutrients include unpaved roads (including off-highway vehicles), agriculture, urban land uses, and wildfires, including a large fire in 2018 that affected much of the watershed. Nine of the largest tributaries to Clear Lake are being sampled for soil, streambed sediment, streamside (streambank or colluvial) sediment, and roadside ditches as source areas. Samples are distributed among the dominant land uses within each watershed. Integrator sites in each tributary watershed are being sampled multiple times to evaluate seasonal variation and to collect a sufficient number of samples for statistical power in matching characteristics to upstream sources. Samples of lake-bed sediment are also being collected to evaluate sources. Constituents that are being analyzed in all samples include 61 major and trace elements; grain-size distribution; forms of N (nitrate and ammonium/organic), P (leachable, calcium-associated, iron-associated, and residual), and carbon (C: organic, inorganic, and pyrogenic); and stable isotopes of C (δ¹³C), N (δ¹⁵N), and strontium (⁸⁷Sr/⁸⁶Sr). Stable isotopes of C and N are expected to vary with vegetation type and land use. The drainage basin includes a wide range of volcanic, sedimentary, and metamorphic rock types that are well suited to the use of strontium isotopes as a natural tracer. Results will be used to inform other watershed modeling efforts.