Distribution of Sediment Particle Size in South Dakota Lake Cores

By: Andrew Brachman, Advisor – Dr. Michael Penn

Abstract The purpose of this research project was to supplement research completed by South Dakota School of Mines and Technology by understanding mercury loading and its biogeochemical cycling in lakebeds across South Dakota. Mercury concentrations and sizes of sediment particles taken from sediment cores were investigated for relationships. No apparent relationship was found to exist between the level of mercury and size of particles. Questions arose indicting that further research might testify to relationships linking mercury levels and other chemicals such as sulfur.

Introduction: The state of South Dakota has many lakes with high mercury levels resulting in mercury

concentrations in the fish that exceed health standards for human consumption. Due to the rural location

of these lakes with no direct point sources of pollution, the source of the contamination is atmospheric

deposition. A primary source of atmospheric mercury is the burning of coal.

South Dakota School of Mines and Technology (SDSMT) received funding to study the sediment

that was deposited at the bottom of the lakes and reservoirs to develop an understanding of the mercury

loading and its biogeochemical cycling. To better understand mercury cycling, the concentrations of mercury

and other chemicals (phosphorus, iron, etc.) were measured in the sediment.

This research aims to supplement the original project by investigating relationship between the

concentration of mercury and the size of the sediment particles. It is predicted that high concentrations of

mercury will be associated with finer sized sediment particles. Smaller sediment particles have greater

surface area to volume ratio and thus a greater capacity for mercury adsorption.

Vertical sediment core samples from the bottom of lakes and reservoirs have been collected and

contain historic accumulations of sediment over many decades. One centimeter slices were analyzed for the

amount of mercury and associated chemicals (by SDSMT) and the particle size (this research). The sizes of

the particles were determined by a Malvern Laser Diffraction Instrument (Malvern) in the UWP Geology and

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Geography Program. The Malvern provides detailed information about the distribution of particle sizes in a

sediment sample allowing statistical analysis of the final data.

Methods:

Lake sediments were retrieved for testing with a Wilco KB corer (Figure 1). A vertical profile of

lake sediment is contained within each core (Figure 2). The core was then sliced into separate layers,

typically 1 cm in thickness ( Figure 3). Each layer represents a certain period of deposition in the lake

bed. Samples from the following lakes were analyzed for particle size distribution: Sinai, Island,

Sheridan, Lynn, West 81, Isabel, North Twin, Hurley, Wall, Pudwell, or Newell shown in Figure 4.

Figure 1: Sediment coring device.

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Figure 2: A retrieved sediment core.

Figure 3: Slicing a sediment core.

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Figure 4: Map of South Dakota depicting locations of lakes.

Various analyses were performed on the sediment slice samples, such as the water content of the

sediment, and metals and nutrient content. This research project was responsible for measuring the

particle sizes of the lake sediments.

The instrument that was used to measure the size of the particles was a Malvern Laser Diffraction

Instrument (Malvern). The Malvern uses laser diffraction to measure sediment particle diameters from 0.1

to 1000 micrometers (µm). Laser diffraction works on the principle that sediment particles that pass

through the laser will diffract the laser at different angles due to the size of the sediment particle. The

Malvern software, Mastersizer, uses the Fraunhofer Approximation and the Mie theory to calculate the

size of the sediment particles.

To use the Malvern, an aqueous solution of the sediment must first be prepared. To prepare a

solution, a dispersant, sodium hexametaphosphate (NaPO3)6, is added to approximately 100 mg of lake

sediment in 600 liters of distilled water. The dispersant is used to disperse aggregates into individual

particles for a period of 24 hours. Ultrasonic treatment is used to further disperse sediment aggregates for

five minutes prior to analysis. For each sample, particle size distribution is determined in triplicate.

Results:

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The particle size analysis produces a size distribution of particle sizes (diameter in µm) as percent

of sample sediment volume. Particle size distributions typically had patterns similar to that of Sinai Lake

(Figure 5).

Figure 5: Particle size vs. the percent volume from Sinai Lake.

Particle size variations with sediment depth within a profile are a function of changes in inputs

from runoff within the watershed over time, and perhaps due to changes in lake productivity. Diatoms are

a form of algae that produce silica frustules which are preserved in the sediment. Frustules from differing

diatom species vary greatly in size. Trends of the sediment particle sizes versus sediment depth varied

between lakes (Figure 6 through Figure 9). Without further research, explanations for these variations are

unknown.

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Figure 6: Relationship between particle size and depth from Sinai Lake.

Figure 7: Relationship between particle size and depth from Lynn Lake.

Figure 8: Relationship between particle size and depth from Island Lake.

Figure 9: Relationship between particle size and depth from Sheridan Lake.

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Sediment Hg concentration was plotted as a function of the median particle size, d50. The d10

and d90 were also determined, the diameters exceeded by 10% and 90% of the total particle volume

respectively. The values for the d90 and the d10 all followed the same relationship as the corresponding

values of the d50. The graph of d50 vs. Hg for Sheridan Lake is shown is in Figure 10. No relationship

was determined to exist between the particle size and the Hg concentration for the limited samples from

this lake. The same scenario was found to be true for Lynn Lake which is shown in Figure 11.

Figure 10: Relationship between particle size and mercury concentration from Sheridan Lake.

Figure 11: Relationship between particle size and mercury concentration from Lynn Lake.

The relationship between particle size and Hg concentration for Island Lake is displayed in Figure 12.

Similar to the previous lakes, no relationship was found to exist between the particle size and the Hg

concentrations.

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Figure 12: Relationship between particle size and mercury concentration from Island Lake.

The expected trend of increasing Hg concentration with decreasing particle size was observed in Sinai

Lake, seen in Figure 13.

Figure 13: Relationship between particle size and mercury concentration from Sinai Lake.

A direct correlation was found to exist between the levels of concentrations of sulfur (S) and Hg.

This relationship was prevalent in Lynn, Sheridan, and Sinai Lake which can be seen in Figure 14 through

Figure 16. Island Lake showed irregular results as seen in Figure 17.

Figure 14: Relationship between S concentration and Hg concentration.

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Figure 15: Relationship between S concentration and Hg concentration.

Figure 16: Relationship between S concentration and Hg concentration.

Figure 17: Relationship between S concentration and Hg concentration.

An inverse relationship was observed between phosphorus (P) and Hg concentrations in three of

the lakes (Figure 18 through Figure 20). The level of Hg increased as the levels of P decreased. Contrary

to the other three lakes, Sheridan Lake demonstrated a direct relationship illustrated in Figure 21.

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Figure 18: Relationship between P concentration and Hg concentration.

Figure 19: Relationship between P concentration and Hg concentration.

Figure 20: Relationship between P concentration and hg concentration.

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Figure 21: Relationship between P concentration and Hg concentration.

Discussion:

After reviewing the results it was determined that no substantial relationship existed between the

Hg concentrations and the sediment particle sizes. Although no relationships were found in this study,

current research pertaining to this topic has indicated that such a relationship may exist. Two separate

projects testing the same hypothesis both came to the same conclusion that areas of finer, sized sediment

tend to exhibit high concentrations of Hg (Hunerlach, Alpers, Marvin-DiPasquale, Taylor, & De Wild,

2004), (Boszke, Kowalski, & Siepak, 2004). For this project’s results the concentration of Hg in the

sediment is most likely related to other factors other than particle size.

The levels of both S and P might have effects on the levels of Hg found in the sediment; along

with the time-variable historical loading of Hg to the lakes. Sulfur in particular may be a significant

factor pertaining to the levels of Hg, and areas in which Hg was found. Supporting research carried out in

South Florida attests to the relationship between S and Hg (Axelrad, et al., 2008). Due to the

biogeochemistry of Hg, S, and other chemicals, accumulations of methyl mercury (MeHg) can occur.

Higher levels of S can cause larger amounts of Hg to accumulate on the lake sediment forming MeHg.

“Preliminary research indicates that sulfate may promote phosphate and ammonium release from

Everglades sediments” (Axelrad, et al., 2008). It is acceptable then to recognize the inverse relationship

between S and P, which would affect the level of Hg.

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Conclusion:

After analyzing the results of this research project it is determined that no clear conclusions can

be drawn with respect to the relationship between sediment particle size and Hg concentration. The

hypothesis that high concentrations of mercury will be associated with finer sized sediment particles was not

supported, except in the case of Sinai Lake. Other factors influencing Hg concentrations appear to be more

significant than particle size.

Acknowledgements:

The author wishes to acknowledge the assistance of Dr. Michael Penn, research advisor, in this

research. The author is also grateful for the parallel research that was undertaken by the South Dakota

School of Mines and Technology. Finally, I am grateful for the generosity and knowledge of Dr. J. Elmo

Rawling 3rd for the opportunity to use the Malvern Laser Diffraction Instrument. This research was

funded by the program known as Pioneer Undergraduate Research Fellowship in coalition with the

University of Wisconsin-Platteville.

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References Axelrad, D.M., Lange, T., Gabriel, M., Atkeson, T. D., Pollman, C.D., Orem, W. H., et al.

(2008). Chapter 3B: Mercury and Sulfur Monitoring, Research and Environmental

Assessment in South Florida.

Boszke, L., Kowalski, A., & Siepack, J. (2004). Grain size partitioning of mercury in sediments

of the middle Odra River (Germany/Poland). Water, air and soil pollution, 159, 125-138.

Hunerlach, M. P., Alpers, C. N., Marvin-DiPasquale, M., Taylor, H. E., & De Wild, J. F. (2004).

Geochemistry of Mercury and other Trace Elements in Fluvial Tailings Upstream of

Daguerre Point Dam, Yuba River, California, August 2001. Scientific Investigations

Report, U.S. Geological Survey, Bureau of Reclamation and the California Department

of Fish and Game.