Radioactive Fallout Provides Estimator for Soil Erosion

Roger Vesseth

Chapter 1 — Erosion Impacts, No. 11, Winter 1990

Extensive atmospheric testing of nuclear weapons in the 1950’s and 60’s resulted in fallout of radioactive fission by-products around the world. One of these was CS-137, which has a half-life of 30 years. Research has shown that once Cs- 137 contacts the soil, it is strongly adsorbed to soil clay particles and not released for uptake by plants nor leached from the soil. Movement of CS-137 is believed to be only by physical movement of the soil to which it is adsorbed. Consequently, Cs- 137 content in the soil is now being used as a tracer of soil movement to document the extent and patterns of soil erosion.

The major period of global deposition of CS-137 fall out was between 1958 and 1964. Because deposition was concentrated in these years and little has occurred since, the amount of soil erosion and deposition which has occurred since about 1963 can be estimated by using CS-137 content in soil to quantify soil movement, This is accomplished by comparing the quantities of CS-137 in erosional and depositional areas with those of undisturbed areas which have sustained neither erosion nor deposition.

Northwest Research

Researchers with Washington State University at Pullman are currently using this Cs- 137 technique to document soil erosion and deposition in the Palouse region of eastern Washington and northern Idaho. W SU soil scientist Alan Busacca and graduate research assistant Carolyn Cook are conducting the research effort. Don McCool, USDA-ARS agricultural engineer at WSU, was involved in establishing the project. Dave Mulla, WSU soil scientist, has been assisting in sampling design and statistical analysis of data for spatial variability on the landscape.

Initial project funding was obtained from the Washington Centennial Clean Water Fund through the Washington Conservation Commission, The CS-137 project is also an important component of Busacca’s research program on documenting and predicting soil-landscape changes caused by cropland soil erosion — part of the STEEP (Solutions To Environmental and Economic Problems) research effort in the Northwest. Since 1975, the STEEP research program has annually involved more than 100 scientists from at least 14 different disciplines at the USDA Agricultural Research Service (ARS) research centers and universities in Idaho, Oregon and Washington, The goal of the STEEP research effort is to develop production technologies for profitable, efficient farming systems which protect soil and water resources.

Busacca and Cook point out that the technique of using Cs-137 toquantify soil movement incorporates soil loss by water and wind erosion, as well as tillage erosion, which is the downslope movement of soil during tillage operations. The Universal Soil Loss Equation (USLE), which is used by the USDA-SCS for estimating soil erosion, does not include contributions from tillage erosion. Tillage erosion is believed to be a significant factor in the declining soil depth on cropland ridgetops and other convex landforms. Because the Cs- 137 method includes estimations of both erosion and deposition within a watershed, another advantage is that it can also provide estimates of net soil loss from a watershed or other selected areas.

Field Site

The field research site is a 113-acre watershed which is part of the Missouri Flat Creek watershed in the Palouse River drainage. It is located just north of Moscow, ID, in a 22-inch annual precipitation zone. Average slope in the watershed is about 15 percent, with slope ranges from 4 to 19 percent, The predominant soil is Thatuna silt loam, a deep soil of loessial (wind deposited) origin. Ridgetops and knobs include Tilma and Naff soils, which have subsoils with higher clay contents at relatively shallow depths. This heavier subsoil material is often exposed by erosion on these landscape areas.

The watershed study area is a “closed” watershed today because all rapid drainage is blocked by Highway 95. Consequently, nearly all of the sediment which would have been lost from the watershed (net loss) into the river system has been trapped and can be quantified.

Data Collection

A detailed topographic map was developed for the watershed (Fig, 1) using an electronic distance meter to provide a precise survey of sample points in three-dimensional space. The watershed was sampled along a regularly spaced grid, The sampling grid also included clusters of more closely-spaced samples. This sampling arrangement combination allows for ”geostatistical” analysis of the data to more accurately determine spatial variability in soils over short and long distances for prediction of soil conditions between sampling points within the watershed. A total of 143 sites were sampled.

Selected soil samples in the watershed were analyzed to determine the depth of Cs- 137 incorporation by tillage and burial by sediment deposition. Based on these preliminary findings, soil samples were collected to a depth of 16 inches on erosional areas, such as ridgetops and convex slopes, and a depth of 24 inches on depositional areas.

Three non-cropland control sites within 10 miles of the watershed were sampled to measure a base level of Cs- 137 in the soil. None of these sites had been disturbed since the early 1950’s.

Research Results

The researchers found that convex ridgetops and midslope knobs sustained the greatest degree of erosion. Conversely, concave areas at low elevations accumulated the greatest amount of deposition. Midslope areas were a transition zone, experiencing both erosion and deposition with little netchange —what the researchers described as a “conveyor belt” in the soil movement process, Approximately 77 percent of the 113-acre watershed sustained soil erosion, 13 percent accumulated depositional sediments, and the remaining 10percent of the watershed was a transitional zone, sustaining both erosion and deposition. Three-dimensional diagrams show the location and relative amount of soil erosion (Fig. l)and deposition (Fig. 2) in relation to landscape positions in the watershed.

Areas with the greatest amount of soil loss, illustrated as numerous peaks in erosion on the diagram (Fig. l), correspond with convex knobs and steep, upper segments of slopes on the perimeter of the watershed. The large peak in soil deposition on the lower left corner (Fig. 2) is where the watershed drainage way is blocked by Highway 95. Much of this sediment would have been carried further down stream if drainage had not been blocked. Deposition was otherwise concentrated in the watershed drainage way and other concave areas.

It should also be pointed out that, although most of the eroded sediment was retained in the watershed, overall soil productivity in the watershed has probably declined, At least a portion of the most fertile, organic matter-rich topsoil has been eroded on about 77 percent of the watershed, often exposing less productive subsoils. In addition, accumulations of sediment on the midslope transition zones and on lower depositional areas in the remainder of the watershed would not increase productivity of those areas. In fact, clayey subsoil material which eroded from ridgetops and knobs, and redeposited over topsoil lower on the slopes typically lowers productivity and creates a less-desirable seedbed. Furthermore, burial of growing crops by sediment deposition would add to the production loss of the watershed each year.

Two sample transects across the lower portion of the watershed in two different directions help to further exemplify the inverse (opposite) relationship between elevation/landscape position and soil erosion or deposition (Figs. 3 and 4). Soil erosion was predominant on higher elevations, midslope convex knobs and the upper portions of concave slopes. Deposition was predominant at lower elevations and on concave slopes with decreasing steepness. Midslopes were areas of transitions between erosion and deposition.

Average annual soil erosion from the 77 percent of the watershed landscape which had sustained erosion from 1963 to 1988 was estimated at 7.8 tons/acre/year. Total erosion during this period was estimated at 907 tons/acre. This is equivalent to roughly 6 inches of soil depth on each of the 88 acres (77 percent) of the watershed which sustained erosion.

Fig. 1. Schematic diagram showing the location and estimated annual amount of soil erosion (top) from 1963 to 1988 relative to topography (bottom) of the sample watershed near Moscow, ID (Busacca and Cook, WSU). For a metric-to English unit reference of elevation and distance 10 m=33 ft.

Fig. 2. Schematic diagram showing the location and estimated annual amount of soil deposition (top) from 1963 to 1988 relative to topography (bottom) in the sample watershed (Busacca and Cook, WSU). For a metric-to English unit reference of elevation and distance 10 m=33 ft.

Fig. 3. Estimated annual amount of soil erosion or deposition from 1963 to 1988 along a north-south sampling transect across the sample watershed near Moscow, ID (Busacca and Cook, WSU). For a metric to-English unit reference of elevation and distance: 10 m = 33 ft.

Research Implications and Plans

Busacca and Cook point out that the CS-137 technique for determining soil movement is useful in evaluating the effects of management practices over a long time frame. It is not, however, a good indicator of changes in the short-term, such as 1 to 5 years. The considerable time and expense involved in sample collection and analysis are significant limitations, in addition to the large natural variability on the landscape.

Because of the relatively short life of CS-137 in the soil (30-year half-life), the researchers stress the importance of taking advantage of this unique soil tracer to document soil changes and management effects while its concentration in the landscape remains conveniently detectable.

One of the technique’s special values will be in estimating net soil loss from open watersheds (sediment delivery ratios to streams). The second phase of the CS-137 project will be to repeat the study on a nearby “open” watershed, where drainage and sediment transport has not been blocked. By comparative analysis of the open and closed watersheds, the researchers hope to establish an estimate of net soil loss to the Palouse River drainage system. This phase is currently underway.

Fig. 4. Estimated annual amount of soil erosion or deposition from 1963 to 198S along an east-west sampling transect across the sample watershed near Moscow, ID (Busacca and Cook, WSU). For a metric-to-English unit reference of elevation and distance 10 m = 33 ft.

Jim Montgomery, soils graduate research associate, will be focusing on this second phase in cooperation with Busacca and the other WSU and USDA-ARS scientists involved in the first phase of the project. In addition to documenting soil movement over the past 30 years, the project goal is to develop predictive equations for estimating future changes in Palouse soils and landscapes under different rates and types of erosional processes, Montgomery’s research will include quantifying the rates of soil movement by common tillage operations — providing the first in-depth documentation of tillage erosion in the region.