Water Resources Research Act Program

Details for Project ID 2009MS84B

Assessing the effectiveness of measures to reduce sediment loads in surface waters using 210Pb activity in lacustrine sediments

Institute: Mississippi
Year Established: 2009 Start Date: 2009-03-01 End Date: 2011-01-31
Total Federal Funds: $16,805 Total Non-Federal Funds: $33,933

Principal Investigators: Gregg Davidson

Project Summary: Efforts to improve surface water quality in recent decades have focused on reducing contaminants, excess nutrients, and suspended sediment. Because the sediment in waterways is a result of erosion of adjacent lands, successful reduction of sediment loading ultimately means successful erosion control. While considerable attention has been given to erosion reduction efforts, the effectiveness of such efforts is difficult to quantify. Erosion is often localized in small gullies or at meander bends making it difficult to normalize across an entire watershed. Assessing erosion by measuring sediment loads in streams is also complicated by large spatial and temporal variability and the inherent difficulties in measuring sediments transported in channels. One way of getting around these difficulties is to quantify the rate of sediment accumulation in the lakes or wetlands that ultimately receive sediment laden runoff water. Lakes and wetlands serve as natural sinks for eroded sediment, and preserve a record of both ancient and modern rates. If a lake has not been subject to scour from high flow events or severe storm activity, the rate at which sediment has been accumulating can be readily determined using radioisotopes deposited along with the sediment. For sediments deposited within the last century, 210Pb is ideally suited with a half life of 22 years. A series of 210Pb measurements taken at different depths will yield a log-activity verses depth relationship from which a rate of sediment accumulation can be calculated. If the rate has been constant over time, the data plots on a straight line. If erosion has been reduced in recent years, sediment accumulation in downstream sinks should also be reduced and will be evident as a change in slope in the 210Pb data. The date of such a change can be determined by multiplying the sedimentation rate by the depth of sediment to the point of interest, which allows assessment of whether the reduced sediment loading corresponds in time with a historical erosion control measure. Several watersheds have been identified in collaboration with the Yazoo Water Management District, the Mississippi Department of Environmental Quality, and the USDA Agricultural Research Service. In each watershed, erosion control measures were implemented at known dates in the past. It should now be possible to assess the effectiveness of these projects using 210Pb measurements of newly collected sediment cores from downstream lakes and wetlands. Lake-wetland sites identified within the watersheds of interest include Bee, Washington, Wolf, Moon, and Beasley. Multiple cores will be collected in each lake using a floating vibracoring rig. Based on studies of other lakes in the Delta region of Mississippi, it is anticipated that historical sedimentation rates will range from 0.2 to as high as 7 cm/yr. Sedimentation rates will be determined using 210Pb and 137Cs in two cores from each site, one at a higher resolution (15 samples) to constrain rates and the timing of any observed changes, and one at a lower resolution (10 samples) to evaluate reproducibility. The high resolution core will be sampled at a low frequency initially, and the results used to determine what portion of the core additional samples should be taken from. The final results from the first core will be used to target specific depths in the second core. Where it is suspected that recent sediment accumulation rates have dropped below 0.5 cm/yr, shallow cores will be collected and divided into increments finer than 1 cm for analysis.