The role of small upstream reservoirs in trapping organic carbon, nutrients, and metals in the San Francisco Bay area

Laura Rademacher, University of the Pacific
Kristina Faul, Mills College

While the nature and sources of urban water quality impairments are well studied, questions remain about how watershed modifications resulting from urbanization and management practices impact biogeochemical cycling of these water quality impairments in small upstream reservoirs. Whether these upstream urban reservoirs act as "sources" or "sinks" of contaminants ultimately controls water quality of discharge into the "urban ocean". With more than 50% of the world's population living in cities and 75% of California's population living on the coast, there is a growing imperative to understand the role of urban watershed/reservoir systems in biogeochemical cycling of carbon, nutrients, and metals and how this role may change in response to factors such as climate change and continued urbanization.

The San Francisco Bay (SFB) area is one example of an increasingly urbanizing region with a long history of pollution pressures. Few studies investigate the role small urban watershed/reservoir systems, such as those in Oakland CA or the over 250 reservoirs in the SFB area, play in contributing to or mitigating inputs to the SFB system.  The proposed study address this need as well as focuses on how local authorities might best manage small ephemeral or perennial reservoir systems based on biogeochemical cycling in their specific system. 

2016 Research Update:

Five years of water samples from the three urban lakes have shown the impacts of the drought on both the incoming concentrations to the lakes and the lake stratification (e.g., warmer, less oxygen). Results suggest urban reservoirs are important controls on biogeochemical cycling in urban watersheds and downstream water quality. Reservoir mixing varies over the course of a year and leads to reducing conditions prevailing during warm summer months and oxidizing conditions dominating during winter months. The redox state of these reservoirs determines whether metals and nutrients are mobilized or retained by the lakes. Based on these samples, Lake Aliso is a source for arsenic, nickel and lead, and a sink for cadmium; Lake Anza is a source for lead, and is sink for arsenic, nickel, and cadmium; and Don Castro Lake is a sink for all metals. Preliminary carbon and nitrogen isotopic data suggests variations in source for organic matter to the lakes. 

Results suggest small, urban reservoirs are important controls on biogeochemical cycling in urban watersheds and downstream water quality. This research contributes to understanding how urban-impacted reservoirs cycle widespread pollutants, which can provide a scientific foundation for developing long-term, integrated management plans of these systems. Two peer-reviewed publications are in progress. 

2015 Research Update: 

Bimonthly water quality sampling (temp, dissolved oxygen, pH, conductivity, nutrients, metals, etc) of Lakes Aliso, Anza and Don Castro continue. Bimonthly depth profiles (every 1m) of the same Lakes for water quality parameters, metals and nutrients continue. Sediment cores were taken in all three locations and analysis is underway. Microbial analysis is also underway. 11 abstracts, presentations and magazine articles have come from the project so far.

Publications

• Impacts of agriculture irrigation recharge of groundwater in a basalt aquifer system: A multi-tracer approach 
• Impact of wildfire on source water contributions in Devil Creek, CA: Evidence from end-member mixing analysis 
• Application of sulfur isotopes for stratigraphic correlation, Isotopes in Environmental and Health Studies 
• A comparison of early Paleogene export productivity and organic carbon burial flux for Maud Rise, Weddell Sea and Kerguelen Plateau, South Indian Ocean, Paleoceanography