Short-term Arsenic Cycling in a Shallow, Polymictic Lake
Samantha R. Fung1, Erin A. Hull2, James E. Gawel2, Alexander R. Horner-Devine1, Rebecca B. Neumann1
1 Department of Civil and Environmental Engineering, University of Washington, Seattle, WA, USA. 
2 Sciences and Mathematics Division, School of Interdisciplinary Arts and Sciences, University of Washington Tacoma, Tacoma, WA, USA. 
ABSTRACT
We observed repeated diel oscillations in arsenic (As) concentrations in the bottom waters of a shallow, temperate lake during a weeklong measurement period. Arsenic concentrations were highest during the morning or midday and lowest in the evening. In this work, we explore four mechanistic hypotheses to explain the diel As cycles based on the physical and biogeochemical processes that were investigated during the study. Despite pH being known to control As cycles in rivers, we determined that this mechanism was inconsistent with As dynamics observed in Lake Killarney. Instead, we found that iron and manganese concentrations oscillated simultaneously with As concentrations and, thus, concluded that redox conditions adjacent to the lakebed controlled the near-bed availability of these three elements. However, based on timescale analysis, we determined that biogeochemical processes at the sediment water interface alone could not have led to the daily oscillations in bottom water concentrations. Rather, turbulence from convective mixing was necessary to transport dissolved species from the lakebed into bottom waters. Notably, we saw that the timing and  intensity of peaks in convectively-driven turbulence were consistent with observed diel fluctuations in bottom water As. Our results indicate that physical mixing is key in controlling As transport and concentrations on diel timescales within shallow lakes. The daily cycling of redox-sensitive elements in shallow lakes and the potential physical controls on this phenomenon should be considered when designing sampling methods to assess the environmental health and water quality of contaminated sites.

DATA SET FILE DESCRIPTIONS:

1. Met_data_ShortTermAsCycling (.mat file)

Contains hourly meteorological data: in situ wind, in situ shortwave radiation (‘SW_Rad_insitu’), air temperature (‘AirTemp’), and vertical water temperature profiles (‘Thermistor_temp’) for the weeklong study period, August 29 – September 5, 2019. Thermistor depth (‘Thermistor_Z’) is depth below the water surface. Hourly datenum and datetime vectors are provided. 

2. As_Mn_Fe_DO_data_ShortTermAsCycling (.mat file)

Contains bottom water total arsenic (‘Bottom_As’), bottom water total iron (‘Bottom_Fe’), bottom water total manganese (‘Bottom_Mn’), and bottom and surface water dissolved oxygen (‘DO_Bottom’, ‘DO_Surface’) concentrations for the weeklong study period, August 29 – September 5, 2019. Timeseries of arsenic, iron, and manganese concentrations at a 6-hour period and are associated with the time vector, ‘Time_As_Fe_Mn’. Timeseries of dissolved oxygen concentrations are hourly and are associated with the time vectors, ‘Time_hourly’ and ‘Datetime_hourly’. 

3. Density_data_SI_ShortTermAsCycling (.mat file)

Contains four water density profiles (‘Density_data’) from September 1, 2019, as well as a time vector (‘Time_density’) with the times associated with the density profiles. 

4. Stratification_mixing_data_ShortTermAsCycling (.mat file)

Contains stratification and mixing data: hourly buoyancy frequency, or N_squared, profiles (‘Nsquared’) with associated depths (‘Nsquared_Z’). Hourly timeseries of averaged bottom and surface N_squared values (‘Nsquared_bottom’, ‘Nsquared_top’). Hourly vertical turbulent intensity values, both as hourly profiles (‘Vert_turbulent_intensity’) with depths associated (‘Vert_turbulent_intensity_Z’), and as an hourly depth-averaged value (‘Vert_turbulent_intensity_depth_avg’). Nsquared and vertical turbulent intensity data are hourly during the study week from August 29 – September 5 and timeseries are associated with the time vectors ‘Time_hourly’ and ‘Datetime_hourly’. Lastly, the file contains hourly profiles of horizontal water velocity magnitude (Horiz_Vel_Mag_Hourly_Aug6_Oct2_2019’) with associated depths (‘Horiz_Vel_Mag_Z_Aug6_Oct2_2019’) for a period spanning greater than the study week (August 6 – October 2, 2019). Time vectors associated with these data are included (‘Time_Hourly_Aug6_Oct2_2019’, ‘Datetime_Hourly_Aug6_Oct2_2019’). All depths are depth below water surface. See methods of paper for details on how these values were calculated.

DATASET UNITS:
- Depths [m]
- Thermistor [Degree C]
- DO [mg/L]
- SW radiation [W/m^2]
- Nsquared [s^-2]
- Vertical turbulent intensity [m^2/s^2]
- Air temperature [Degree C]
- Wind speed [m/s]
- Density [kg/m^2]
- Horizontal velocity magnitude [m/s]
- Total arsenic [-g/L]
- Total iron [[g/L]
- Total manganese [ g/L]