Temperature dependent sulfate transport in aquatic sediments

Sulfate, released to overlying waters from natural sources and human activity, has the potential to be reduced to sulfide within the anoxic environments of aquatic sediments and negatively impact the growth of aquatic vegetation. Wild Rice is of particular concern within Minnesota as it is both an economic and cultural resource within the state. This study was conducted to characterize the temperature dependence of sulfate transport, via diffusion, between overlying waters and sediment porewaters through the use of laboratory experimentation and mathematical analysis to study the transient response to changes in the overlying water concentration. Two riverine sediments with contrasting organic carbon content from the St. Louis River watershed in northern Minnesota were characterized for their bulk geochemistry and incubated under laboratory conditions to observe the temperature dependence of ion transport between overlying water and sediment porewaters. Two identical sets of laboratory microcosms, incubated under warm and cold conditions, were subjected to a sulfate loading phase in which the overlying water was spiked with sodium sulfate to induce a concentration gradient between the sediment porewaters and overlying water. At the end of the sulfate loading phase, the sulfate gradient was reversed by replacing the overlying water with fresh water, causing sulfate to diffuse out of the sediment, back into the overlying water. During the sulfate recovery phase, sodium bromide was spiked into the overlying water. Bromide, acting as an inert chemical tracer, provided a diffusion-only baseline with which to compare to reactive sulfate. The anion concentrations in the overlying waters were closely monitored to quantify changes in the concentration through the sulfate loading and recovery phases. Non-destructive porewater samples were collected using Rhizon® soil moisture samplers to measure concentrations of sulfate, bromide, ferrous iron, pH, and sulfide at discrete depths in the sediment during key times after changes in surface boundary conditions.Averaged results from both the high and low organic sediments showed sulfate transport occurred 49% faster out of the overlying waters into the sediments at 23°C when compared to 4.5°C. Estimated rates of sulfate reduction at 4.5°C were on average, 40% of those estimated at 23° C. After seven weeks of recovery from the sulfate loading, porewater sulfate concentrations in the warm microcosms had dropped back to ambient levels while slightly elevated sulfate levels were still noticed within the cold microcosm porewater. Even though more sulfate diffused into the warm sediments, the cold sediments retained the sulfate for a significantly longer period of time after the change in boundary layer conditions due to the retarded rates of diffusion and reaction. The longer the sediment is exposed to elevated sulfate levels a greater potential exists for the wild rice seed within the sediment to be exposed to sulfide.