Seasonal Patterns of Mixing and Arsenic Distribution in a Shallow Urban Lake

Abstract

AbstractArsenic, a neurotoxin and carcinogen, is a legacy contaminant in the sediments of many urban lakes and poses health risks to aquatic ecosystems and lake users. Arsenic uptake into the aquatic food web is enhanced in shallow, polymictic lakes compared to deep, seasonally stratified lakes. We present the results of a 17‐month field study in Lake Killarney, a shallow, urban lake in Federal Way, Washington, USA, which examines the physical and biogeochemical mechanisms controlling arsenic mobilization and transport from sediment into lake waters, a prerequisite for arsenic uptake into the food web. In Lake Killarney, arsenic mobilization and transport into bottom waters occurred only when stratified conditions and elevated temperatures facilitated deoxygenation of bottom waters. Frequency of lake mixing varied seasonally and controlled the vertical distribution of arsenic in the water column. Convective mixing was the main contributor to elevated vertical turbulent intensity in the water column during periods of high arsenic mobilization, and thus to the upwards transport of arsenic from bottom waters. Maximum near‐surface arsenic occurred when the lakebed sediment temperature was elevated and the water column was overturning frequently. This work clarifies the mechanisms that contribute to vertical arsenic transport in shallow lakes and provides a basis for identifying contaminated systems with the physical and biogeochemical conditions that promote transport of arsenic into near‐surface water.