The effects of mixing on arsenic mobilization and transport in a shallow lake

Abstract

Arsenic, a heavy metal, is a neurotoxin and carcinogen that has been released into the environment at accelerated rates due to human activities. Arsenic has accumulated in the sediments of many lakes around the world and continues to have negative impacts on lake ecosystems and lake users. The negative effects of arsenic are magnified in shallow lakes, where spatial proximity between high arsenic concentrations and biota allow for the uptake of elevated levels of the arsenic into the aquatic food web. In this dissertation, I examine the basis of this phenomenon by examining how physical factors affect biogeochemical conditions and processes in a shallow, urban lake. In Chapter 2, I investigate lake mixing and arsenic distribution in Lake Killarney on a seasonal timescale during 2018 and 2019. We found that stratification and mixing exhibited seasonal patterns and greatly affected the fate of arsenic within the lake. Arsenic built up in lake bottom waters during the early and mid-summer when the water column remained stratified for week to multiweek periods. Warmer sediment temperatures during early summer, compared to the springtime, were shown to expediate the production of reducing conditions in the bottom water, a prerequisite for the buildup and maintenance of mobile arsenic in this region. Stratification was weaker and mixing occurred more frequently in the late summer. Frequent lake mixing during the late summer led to a more homogenous distribution of arsenic throughout the lake water column, with elevated surface water concentrations and diminished bottom water concentrations compared to early summer conditions. Thermal convection, rather than wind, was the main mixing force during the summertime in Lake Killarney and was responsible for the vertical transport of arsenic through the lake water column. This work provides a mechanistic understanding for why contaminated, polymictic lakes have high ecosystem and human health risks and underscores the need to prioritize these systems in management efforts. In Chapter 3, we present observations of repeated diel cycles in bottom water arsenic concentrations. Arsenic concentrations were highest in the early morning to midday and lowest in the evening. Bottom water manganese and iron concentrations exhibited diel signals that coincided with those in arsenic, demonstrating that the fluxes of these three metals were interconnected and likely caused by redox processes at the lakebed. Redox conditions were thus determined to control near-bed availability of arsenic. However, timescale analysis illustrated that redox processes occurring at the sediment water interface could not solely have led to the observed diel cycles in bottom water metals. Significantly, convective mixing was shown to occur nightly and create spikes in vertical turbulence that overlapped with periods when arsenic concentrations were increasing. Estimates of turbulent diffusivities were used to calculate an approximate transport time of arsenic from the lakebed to the bottom water and yielded results that could reasonably explain the observed diel arsenic cycles. Although short term oscillations in arsenic concentrations have been documented in rivers, there is a paucity of studies describing and investigating this phenomenon in lentic systems. Consequently, this study provides an important example of diel cycling in lakes and presents an explanation of the mechanisms behind these cycles. Our findings illustrate the need for lake sampling to include methods that integrate metal concentrations over multi-day periods in order to attain accurate representations of contaminant levels in lake water and the potential effects on biota and lake users. Finally, in Chapter 4, I expand on the findings in Chapter 2 by investigating the interannual variability in arsenic concentrations in the lake water and biota between the summers of 2018 and 2019. Mixing frequency varied between the two study years and was found to be a key control on aqueous and biological arsenic concentrations. Specifically, less frequent mixing caused a two-fold increase in arsenic concentrations in lake bottom water, phytoplankton, and zooplankton. Summertime meteorology did not differ significantly between the two years; although meteorology did have a prominent effect on surface thermal structure in both summers, surface stratification was transient and had minimal implications for the release of arsenic into lake bottom waters. Instead, bottom thermal structure, which was set by sediment temperature and the resulting sediment – water temperature difference, was the most important factor in lake mixing frequency and, thus, on the fate of arsenic diffusing upwards from sediment porewaters. Our findings demonstrate that lake mixing behavior and the fate of contaminants can change significantly from year-to-year, highlighting the necessity of understanding the ranges of these processes for effective lake management.