Wednesday, November 15, 2006 - 11:05 AM

The Answer is in the Lakes: A New Application of Pb-210 for Fingerprinting Riverine Suspended Sediments.

Shawn P. Schottler and Daniel Engstrom. St. Croix Watershed Research Station, 16910 152nd St. N., Marine, MN 55047

Atmospherically deposited radioisotopes have been used to discriminate between upland field erosion versus streambank erosion as contributors to riverine suspended sediment.   These traditional fingerprinting methods are limited by the spatial variability and correction factors necessary to accurately characterize the upland (or agricultural field) source fingerprint.  To get around these obstacles, this study utilized excess 210Pb in the sediment of seepage lakes, with no channelized inputs, as reference systems to ascertain a temporally and spatially integrated upland field fingerprint, encompassing sheet, rill and gully erosion.  The relationship between 210Pb flux and modern sedimentation rate was plotted for 15 such reference lakes in the Minnesota River watershed.  The slope of this relationship, 4.8 pCi/g, (p<0.001, r2=0.58) predicts the average activity of 210Pb on sediments eroded from upland fields. The relationship of 210Pb flux to sedimentation rate was also applied to 20 cores collected from Lake Pepin (Mississippi River, MN and WI).  Lake Pepin receives greater than 85% of its sediment burden from the Minnesota River and acts as a basin wide integrator of bank and upland erosion sources. A plot of 210Pb flux to sedimentation rate at two core intervals, pre 1965 and 1965 to 1996, yielded slopes of 3.16 and 1.87 pCi/g respectively (r2 > 0.94, p<0.001).  The ratio of these slopes to the slope determined in the reference lakes predicts the proportion of sediment derived from upland sources.  This comparison estimates that the relative contribution from upland sources to Lake Pepin has decreased from approximately 65% to 38% in the past 30 years.  During this period, the overall sediment accumulation rate remained 8 to 10 times greater than historic rates, thus highlighting the significant contribution (>60%) from non-upland sources to the current sediment loading.