Vertical Flux of Arsenic from Bottom Sediments in a Shallow Urban Lake

Legacy arsenic in the bottom sediments of many south-central Puget Sound lakes serves as a long-term source of contamination to the aquatic system. This work uses high resolution measurements of turbulence and stratification together with oxygen and arsenic concentration taken over a one year period to assess the role of vertical turbulent flux in arsenic release from contaminated sediments and vertical transport in Lake Killarney, a shallow urban lake. Using both physical and biogeochemical parameters, we identify seasonal lake mixing regimes that have varying effects on the turbulent transfer of arsenic. Specifically, we used ADCP (acoustic Doppler current profiler) and ADV (acoustic Doppler velocimeter) measurements to evaluate the role of turbulent diffusion in vertical movement of arsenic, and temperature profiles and meteorological data to identify the external forces responsible for observed mixing patterns. We find that, in contrast to the generally assumed paradigm of a temperate dimictic lake, our study system has a variable-depth mixed layer that results in multiple mixing regimes with time scales of one day to months. During summer months, we observed diel patterns of midday stratification and nighttime convective overturning. On occasion, early morning deepening of the mixed layer reached the lake bed which results in a full overturning event occurring every day for periods of days to weeks. These observations suggest that near-bed turbulence generated by overturning could significantly affect diffusion of arsenic out of the sediments, a mechanism not previously considered. This work links the hydrodynamic characteristics of a small temperate lake with chemical concentration patterns, a connection that is often overlooked but needed to identify systems that are the most vulnerable to legacy arsenic contamination and to minimize the impacts of contamination on aquatic ecosystems and human populations.