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A Chemical Analysis of Sediment Sources in the Le Sueur River, Southern Minnesota

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A Chemical Analysis of Sediment Sources in the Le Sueur River, Southern Minnesota A chemical analysis of sediment sources in the Le Sueur River, southern Minnesota By John Leaf A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Arts (Geology) at GUSTAVUS ADOLPHUS COLLEGE 2009 A chemical analysis of sediment sources in the Le Sueur River, southern Minnesota By John Leaf Under the supervision of Professor Laura Triplett Abstract Recently, it has been found that the Minnesota River is contributing a disproportionate amount of sediment to the Mississippi River. Much of this unnatural influx comes from the Le Sueur River watershed in Blue Earth County near Mankato, MN. Using geo- chemical techniques we attempted to determine the source of Le Sueur River sediment. By using rare earth and trace elements as “fingerprints”, we analyzed topsoil, bluff, and ravine sediments of the river valley. We found that the geochemical composition of heavy minerals can be used to discriminate between bluff and ravine sediments. However, due to a shortage of suspended sediment sample we could not measure the heavy mineral fraction of suspended sediments and thus could not use our findings to determine the sediment source proportions. Acknowledgments In making geology an enjoyable experience the last four years, I first thank all of my fellow Gustavus geology majors. Whether scrambling to finish structural geology labs or visiting a local paleo-channel on a field trip, they always managed to make it fun. I also thank Alan Gishlick and Jim Welsh. Both helped build a broad base to my geology knowledge throughout my time at Gustavus. Next, I thank Russell Shapiro, whose Unatural Disasters course and teaching style inspired me to become a geology major. In conjunction with this, I thank my family for all the support they have given me and in passing on to me their love for the earth. Who knew walks looking for agates on the beach, or playing in the creek at our cabin would fit perfectly into my major. Lastly, I thank Laura Triplett. She passed on her passion for geomorphology to me and has served as a wonderful mentor the last two years. I couldn’t have done this without her. Contents Introduction………………………………………………………………………….Pg. 1 Geologic Setting……………………………………………………………………..Pg. 5 Site Description……………………………………………………………………....Pg. 8 Methods……………………………………………………………………………...Pg. 11 Results……………………………………………………………………………….Pg. 14 Discussion……………………………………………………………………………Pg. 16 Conclusion…………………………………………………………………………...Pg. 18 Figures and Tables 1. Minnesota and Le Sueur River watersheds: Pg. 1 2. Glacial Lobes of the last glaciation in the Upper Midwest: Pg. 5 3. Glacial Lake Agassiz and Glacier River Warren: Pg. 7 4. Topographic Map of Le Sueur River from St. Clair to the confluence with the Blue Earth River: Pg. 9 5. Le Sueur River bluff near Monk’s Road: Pg. 10 6. Le Sueur River bluff near Wildwood County Park: Pg. 10 7. Ravine picture courtesy of Stephanie Day: Pg. 10 8. Ravine picture courtesy of Stephanie Day: Pg. 10 9. Table One: “p” values of elements compared in bluff and ravine samples: Pg. 15 10. Table Two: Trace element concentrations in bluffs and ravines: Pg. 15 11. Table Three: Major element concentrations in bluffs and ravines: Pg. 15 INTRODUCTION: In recent years, the Minnesota River and its watershed (fig. 1) have been implicated in the pollution of Lake Pepin, a natural widening in the river downstream of the confluence of the Minnesota River with the Mississippi River. According to Environmental Protection Agency regulations, Lake Pepin does not meet the standard for turbidity, a measure of suspended sediment in water, largely due to unnatural fluxes of sediment and nutrients entering the lake from tributaries upstream (Engstrom et al, 2009). However, the precise source of this sediment on the landscape is unknown. To identify the most important source of sediments entering the lake, Kelly (et al., 2000) attempted to geo-chemically fingerprint the major Figure 1a: Minnesota Figure 1b: Le Sueur River watershed River watershed tributaries of Lake Pepin. By analyzing 42 elements in suspended sediment samples from the Upper Mississippi, St. Croix and Minnesota rivers, Kelly was able to statistically distinguish sediment between the three. Using a sediment core from Lake Pepin, the group then used a chemical mass balance 1 model to determine the proportions each river contributed to Lake Pepin sediment over time. The results they found were groundbreaking. Of the total sediment entering Lake Pepin, 90% (+/- 1.4) of it is contributed by the Minnesota River (Kelly 2000). More importantly, this proportion has risen since the early 1800’s and the onset of European settlement, indicating anthropogenic causes to the increased sediment load. Because the Minnesota River is the primary source for sediment entering the lake, recent efforts have been focused on the river and its tributaries. According to the National Center for Earth Surface Dynamics at the University of Minnesota, 33% of sediment in the Minnesota River comes from the Le Sueur River even though it is only 7% of the total watershed (fig. 1). With such a large input for its size, there is high probability that human activity is cause to the increased sediment load. Located in southern Minnesota, the Le Sueur River drains parts of four counties before flowing into the Blue Earth River near Mankato, Minnesota in Blue Earth County. Draining largely agricultural land, these rivers contribute to the greater Minnesota River watershed that empties much of southern and southwestern Minnesota (Lorenz D., 1990). Within the river, bluffs, ravines, and topsoil have been cited as the three possible sources of sediment contributing to the river turbidity. Defined as a steep riverbank or cliff, bluffs are steep slopes where active erosion or steep slopes undermine vegetative growth. Bluffs along the Le Sueur river are composed primarily of till deposited by the glaciers during the last glaciation. Ravines are erosional features that feed into the river system. The fluvial deposits within ravines are in contrast to the sides of ravines which presumably have similar composition to the unchanged till in bluffs. Originally, the land 2 was covered uniformly with till, but the processes of erosion and creation of ravines may accumulate specific parts of the soil, giving ravines their own distinct chemical signature. Similar to the river valley, ravines are cut by running water but are narrower with steeper slopes. In the Le Sueur River watershed, these features contain water seasonally, but still could contribute to the large influx of sediment into the river. Both bluffs and ravines are often a river’s response to a lowering of base level, the lowest point to which a river can flow. Because rivers have an ideal profile of efficiency, a drop in base level causes the river to erode away sediment until it reaches achieves a more efficient slope. Bluffs and ravines can indicate that a river is actively eroding sediment to reach equilibrium. Finally, topsoil is simply the sediment horizon closest to the land surface. Farms in southern Minnesota thrive off of the region’s thick, nutrient rich top soil. The disturbance of the land by farming could mobilize this resource and increase sediment load into the Le Sueur River. However, other initial studies indicate that suspended sediment in the river is not contributed by top soils (Schottler, personal communication, 2008) making this sediment source a non-factor in this study. With down cutting and erosion in the river valley, it is possible that while the overlying till was once uniform in composition, weathering processes due to ravine formation and topsoil development could have affected it differently as sediment makes its way to rivers and eventually downstream. These processes could create measurable differences in the geo-chemical signatures of ravines and bluffs. Using a similar method to Kelly et al (2000), we attempted to geochemically distinguish bluff and ravines and compare them to suspended sediment in the river. 3 Successfully differentiating the sediment sources can help in deciding what actions need to be taken in cleaning up the watershed to protect Lake Pepin. Bluffs, for example would take different stabilization techniques than ravines would. To best study these sources, we tested two different components of each sediment sample, the “whole” sediment, and the “heavy mineral” sediment. Testing the heavy mineral fraction of the sediment is helpful in that it eliminates some of the “background noise” potentially added to the sediment by other components. It is possible that organics, carbonates, iron oxides, and light minerals can enter the various sediment sources via random processes such as organismal activity or groundwater percolation. These processes could mask the true signature of the sediment sources we are trying to study because they are not necessarily the actual “fingerprint” of the source. Testing heavy minerals for their chemical content could serve as a control to each sediment source. On the other hand, rivers are not selective with what they carry, so it is of equal importance that each source is tested for chemical composition as a whole component as well. The section of the Le Sueur River watershed that contributes most of the sediment to the river is below the town of St. Clair, MN to just upstream of Blue Earth county road 90 (Minnesota Pollution Control Agency, personal communication, 2008). St. Clair is approximately the location of a knick point in the Le Sueur River (Patrick Belmont, NCED, personal communication, 2008), which is defined as an abrupt change in slope. Knick points are a river’s in response to a lowering of base level or other hydrological changes. By actively down cutting and eroding along the knick point, a river is attempting to reach equilibrium with the new conditions. 4 Because the amounts of sediment in the Le Sueur and Minnesota Rivers are unnaturally high, anthropogenic activity is undoubtedly a large contributor regardless of the sediment source.
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