Retention of sulfur in lake sediments

Rationale:

Although much has been learned in the past two decades about the inputs of sulfur to lake sediments, very little is known about the subsequent transformations and release from the sediments of the sulfur. Knowledge about release vs. retention of sulfur in lake sediments is important to understand the alkalinity balance of lakes, the carbon and nutrient cycles in lakes, the toxicity of trace metals in sediments, and the paleolimnological record of lake sediments. The ongoing work at MTU aims to quantify the release of sulfur from sediments of one well characterized lake, to develop a model for such release in all lakes, and to evaluate stable isotope ratios as a tool for assessing sulfur retention in lake sediments.

Study Site: Little Rock Lake

Little Rock Lake is one of hundreds of kettle-hole lakes in northern Wisconsin's Vilas County. Located in sandy outwash plains, Little Rock Lake is a seepage lake; it has no surface inflows or outflows. A small amount of groundwater flows into the southeast corner of the lake, and water seeps from the lake to the aquifer throughout other littoral areas.

Little Rock Lake has been intensively studied for the past 13 years. Beginning in 1984, the U.S.EPA funded an experimental acidification of this lake to study the effects of acid rain on lakes. The two equally-sized (9 ha each) basins of the lake were divided one from another with a Mylar curtain, and the north basin was acidified over the next 6 years. The pH was successively lowered from an initial value of 6.3; the pH was decreased by 0.5 units, held at that value for two years, and then another 0.5 units. This acidification regime was continued for 6 years, and since then the lake has been left to recover.

Sulfur in the lake water

As a result of the additions of sulfuric acid, concentrations of SO₄^2- increased relative to concentrations in the reference basin. The increased inventory of sulfate in the water column was much less that the total amount added to the lake; sequestration of sulfur in the sediments as well as outseepage accounted for approximately 50% of the added SO₄^2-. Following the cessation of acid inputs, concentrations of SO₄^2- in the lake have decreased. The rate of decrease is not, however, determined solely by the rate of flushing of the lake. Release of sulfur from the sediments is prolonging the time needed for full recovery.

Sulfur storage in and release from sediments

A comparison of sediment cores taken in 1985 and 1996 clearly reveals that sulfur is not stored permanently in the sediments. The core taken in 1985 showed an increase in sulfur toward the sediment surface. As a result of acidification, it was predicted that the sediment sulfur content would continue to increase (the solid points in the figure). However, the core taken in 1996 indicated that much less sulfur was present than predicted; either the anticipated fixation in the sediments never occurred, or the sulfur subsequently was released from the sediments. Analysis of archived cores taken in 1990 now is underway to determine which alternative is correct.

Comprehensive Model
Based on the 10 years of research on sulfur cycling in Little Rock Lake we are working to develop a mechanistic model for sulfur incorporation into and release from the sediments. Differences between the two basins of Little Rock Lake give insights into the effects of morphometry on S retention as well as the effects of increased sulfate concentrations and acidification. The fundamental process to be studied in this project, exchange of materials between lake water and sediments, is not unique to sulfur but is a critical component of the cycling of nitrogen, phosphorus, carbon, trace metals as well as xenobiotic compounds. The model developed in this project for sulfur may provide insights into models for these other substances of interest.

This project is being conducted by Amy Geisler, a Master's student in environmental engineering at MTU. Amy is shown at left with co-workers under typical working conditions.

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Last updated February 1997