Short-term variability of ⁷Be atmospheric deposition and watershed response in a Pacific coastal stream, Monterey Bay, California, USA

Beryllium-7 is a powerful and commonly used tracer for environmental processes such as watershed sediment provenance, soil erosion, fluvial and nearshore sediment cycling, and atmospheric fallout. However, few studies have quantified temporal or spatial variability of ⁷Be accumulation from atmospheric fallout, and parameters that would better define the uses and limitations of this geochemical tracer. We investigated the abundance and variability of ⁷Be in atmospheric deposition in both rain events and dry periods, and in stream surface-water samples collected over a ten-month interval at sites near northern Monterey Bay (37°N, 122°W) on the central California coast, a region characterized by a rainy winters, dry summers, and small mountainous streams with flashy hydrology. The range of ⁷Be activity in rainwater samples from the main sampling site was 1.3–4.4 Bq L⁻¹, with a mean (±standard deviation) of 2.2 ± 0.9 Bq L⁻¹, and a volume-weighted average of 2.0 Bq L⁻¹. The range of wet atmospheric deposition was 18–188 Bq m⁻² per rain event, with a mean of 72 ± 53 Bq m⁻². Dry deposition fluxes of ⁷Be ranged from less than 0.01 up to 0.45 Bq m⁻² d⁻¹, with an estimated dry season deposition of 7 Bq m⁻² month⁻¹. Annualized ⁷Be atmospheric deposition was approximately 1900 Bq m⁻² yr⁻¹, with most deposition via rainwater (>95%) and little via dry deposition. Overall, these activities and deposition fluxes are similar to values found in other coastal locations with comparable latitude and Mediterranean-type climate. Particulate ⁷Be values in the surface water of the San Lorenzo River in Santa Cruz, California, ranged from