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Journal of Environmental Quality 31:54-61 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

SPECIAL SUBMISSIONS
Findings from the USDA-sponsored Lake Erie Agricultural Systems for Environmental Quality Project

Soil Erosion and Sediment Sources in an Ohio Watershed using Beryllium-7, Cesium-137, and Lead-210

Gerald Matisoff*, Everett C. Bonniwell and Peter J. Whiting

Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106

* Corresponding author (gxm4{at}po.cwru.edu)

Received for publication August 12, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Soil cores and suspended sediments were collected within the Old Woman Creek, Ohio (OWC) watershed following a thunderstorm and analyzed for 7Be, 137Cs, and 210Pb activities to compare the effects of till vs. no-till management on soil erosion and sediment yield. The upper reaches of the watershed draining tilled agricultural fields were disproportionately responsible for the majority of the suspended sediment load compared with lower in the watershed (2.0–7.0 metric tons/km2 [Mg/km2] vs. 1.2–2.6 Mg/km2). About 6 to 10 times more sediment was derived from the subbasins that are predominantly tilled (6.8–12.4 Mg/km2) compared with the subbasins undergoing no-till practices (0.5–1.1 Mg/km2). In undisturbed soils the 210Pb activities decreased with movement toward the bottom of the cores to the constant supported 210Pb value at a depth of about 10 cm. There was a subsurface maximum in 137Cs activity within the top 10 cm. In contrast, the 210Pb and 137Cs distributions in soils that are currently or were previously tilled were nearly homogeneous with depth, reflecting continuing or previous mixing by plowing. The activities of 210Pb and 7Be were linearly correlated and were higher in suspended sediments derived from no-till subbasins than those derived from tilled subbasins, indicating that the soil surface is the source of suspended sediment. This study demonstrates that no-till farming results in decreases in soil erosion and decreases in suspended sediment discharges and that those eroded sediments have a radionuclide signature corresponding to the tillage practice and the depth of erosion.

Abbreviations: OWC, Old Woman Creek


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
EVALUATING THE NATURE and extent of erosion in a watershed is fundamental to developing management strategies for sediment and sediment-bound nutrients and contaminants. Various portions of watersheds contribute sediment at different rates, of different size and composition, and with different associated nutrients and contaminants. Differences in rates of contribution and sediment properties result from differences in processes contributing sediment (surface wash, rills, landslides) and differences in land usage (forested, prairie, agricultural, or urban). Sediment budgets (Dietrich and Dunne, 1978) provide an initial indication of sediment production, but a more detailed discrimination of sediment sources can be achieved from mass balancing soil tracers such as radionuclides (Wallbrink et al., 1998). Thus, characterizing soils and suspended sediments using radionuclide tracers permits identification of sediment source areas and land uses that are problematic, and allows for proper targeting of sediment management practices.

Lead-210 has been used extensively to determine sedimentation rates (Robbins, 1978) but it has received little attention in soil erosion studies. The radionuclide 210Pb is naturally produced as a decay product of 238U . Terrestrial 238U decays through a series of short-lived nuclides to 226Ra , which in turn decays to the gaseous radionuclide 222Rn . 222Rn escapes to the atmosphere where it then decays through a series of short-lived isotopes, including 214Bi , to 210Pb . Lead-210 sorbs strongly (Kd = 103–106; Wang and Cornett, 1993; Balistrieri and Murray, 1984) to aerosols and particulate matter that are deposited on the surface of the earth through both wet and dry fallout. This atmospherically derived 210Pb is referred to as "excess" 210Pb. A portion of 222Rn produced from the decay of terrestrial 238U does not escape to the atmosphere, which results in an in situ produced level of 210Pb, referred to as "supported" 210Pb. Supported 210Pb can be differentiated by quantifying its grandparent 214Bi and assuming secular equilibrium with the in situ 226Ra source.

Cesium-137 in North American soils is mostly the result of atmospheric nuclear bomb testing during the 1960s and 1970s. Deposition of wet and dry fallout occurred after sorption onto aerosols and particulate matter (Kd = approximately 105; Robbins et al., 1979). Fallout peaked circa 1963, and there has been negligible fallout of 137Cs since 1976 in North America. Fallout from the 1986 Chernobyl accident was restricted primarily to Europe.

Beryllium-7 is a naturally occurring radionuclide, which is produced continuously in the upper atmosphere by cosmic ray spallation of nitrogen and oxygen. In a manner similar to 210Pb and 137Cs, it reaches the surface through mostly wet fallout, especially from thunderstorms that scub 7Be from the stratosphere, and is sorbed to particulates (Kd = approximately 104–106; Hawley et al., 1986).

Several studies have used radioactive tracers to identify and characterize (fingerprint) sediment source regions and land use. Walling and Woodward (1992) examined 7Be, 137Cs, and 210Pb in sediments from two small watersheds and found that material derived from the surface of undisturbed soils is characterized by 137Cs activities that are several times higher than those of cultivated soils because cultivated soils are plowed (mixed) to depths of about 20 to 25 cm. Olley et al. (1993) used 137Cs, 7Be, and the 226Ra to 232Th ratio to fingerprint sediment sources in a gullied catchment. They found low 137Cs and low 7Be values in gully sediment that were consistent with sediment derived from the gully wall and gully floor in support of the 226Ra and 232Th findings. Owens et al. (1996) observed that most of the 137Cs fallout is still contained in the surface layers (7 to 20 cm) in uncultivated soil, while the 137Cs is mixed throughout the plow layer (20 to 30 cm) in cultivated soils. Walling et al. (1999) recently used environmental radionuclides, mineral magnetism, geochemical composition, phosphorus content, and organic matter in a multivariate mixing model to fingerprint suspended sediment sources from uncultivated topsoil, cultivated topsoil, and channel bank sources. Similarly, Owens et al. (1999) had some success characterizing the relative proportions of topsoil and subsoil–channel bank sources in floodplain deposits over the last century using a multivariate fingerprinting technique. Cesium-137 has also been used extensively for measuring soil erosion rates, a topic reviewed by Ritchie and McHenry (1990).

This previous work demonstrates that radionuclide signatures of soils can be used to estimate soil erosion rates and sediment source areas. However, those studies were based on erosion and transport processes of time scales of a decade or longer. The purpose of this study was to compare the effects of till vs. no-till management on soil erosion and sediment yield on a shorter timescale (following a single thunderstorm) by using fallout radionuclides (7Be, 137Cs, and 210Pb) as indicators of soil erosion and sediment transport. The approach used was to measure the radionuclide distributions (7Be, 137Cs, and 210Pb) in both tilled and no-till soils, characterize the radionuclide fingerprint of surface soils subject to erosion, and use this information to estimate soil erosion and to identify sediment sources.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Soils and suspended sediment were collected within a few days following a thunderstorm on 18 May 1997 from the Old Woman Creek (OWC) watershed, a 69.5-km2 agricultural watershed in Ohio that is a tributary to Lake Erie (Fig. 1) . The upper, southern half of the watershed is on a till plain in Bennington (fine, illitic, mesic Aeric Epiaqualf)–Cardington (fine, illitic, mesic Aquic Hapludalf)–Centerburg (fine-loamy, mixed, active, mesic Aquic Hapludalf) soils (Richards et al., 2002), whereas the lower, northern half of the watershed is on glacial lacustrine sediments in Conotton (loamy-skeletal, mixed, active, mesic Typic Hapludalf)–Conneaut (fine-silty, mixed, active, nonacid, mesic Aeric Epiaquept)–Allis (fine, illitic, acid, mesic Typic Endoaquept) soils (Richards et al., 2002). A narrow northeast trending escarpment (slope = approximately 0.011; Woods, 1987) divides these two regions. A more extensive discussion of the regional geology and geomorphology can be found in Buchanan (1983) and Woods (1987). The watershed lies within two different counties and different soil conservation districts, which has resulted in a distinct line through the watershed (Fig. 1) separating predominantly no-till agricultural practices (Erie County, OH) from predominantly conventional tillage practices (Huron County, OH). The watershed is mostly planted in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Soils from the tilled and no-till agricultural fields were collected from the upper portions of the watershed, which is a till plain. Sites A through D and G drain predominantly conventionally tilled agricultural land (plowed), whereas Sites E and F are located on a tributary that drains agricultural land previously tilled but predominantly converted to no-till during the last decade. The remaining four sites (H through K) drain no-till and tilled lands as well as the urbanized town of Berlin Heights, Ohio. A subset of these sites (B through G and K) was selected for detailed study (Table 1). Some additional details such as slope and basin drainage area for these sites are given in Table 1.



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  		Fig. 1. Sample collection locations in the Old Woman Creek National Estuarine Research Reserve (OWC NERR) watershed.

 

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  		Table 1. Basin characteristics, sediment fluxes, and erosion depths in the Old Woman Creek watershed in Ohio (USA).

 
Soil cores were collected before planting in mid-May 1997 at three locations in the OWC watershed (Fig. 1) using a 3.8-cm2 AMS stainless steel soil recovery probe (Art's Manufacturing and Supply, American Falls, ID) fitted with plastic liners. One set of two cores was collected from Star Island, located in the middle of Old Woman Creek Estuary on the lake plain. The area has remained forested and undisturbed since the 1930s, which provides a good reference site for soil radionuclide inventories. The two cores, approximately 30 cm long, were collected within 5 m of one another, sectioned at 2-cm increments, placed in pre-tared 47-mm-diameter polystyrene petri dishes, dried, weighed, and analyzed by gamma spectroscopy. A second set of cores was collected from a conventional till agricultural field adjacent to Site C. Four 16-cm-long cores were collected in an area of approximately 5 m2. Two cores were collected in the furrows and two collected on the adjacent ridges of the row-cropped field. The cores were sectioned at 0.5-cm increments, and the corresponding sections of each core were combined to yield sufficient mass for analysis by gamma spectroscopy. The pooled sections of the cores were dried, weighed, and analyzed to a depth of 14.5 cm by gamma spectroscopy. Because we pooled multiple cores, soil characteristics were spatially averaged, producing a profile characteristic of the tilled areas as a whole. Furthermore, there was enough material to generate a sufficient number of decays in about 24 h of counting. The third set of soil cores was collected adjacent to Site E in an agricultural field where no-till is practiced. Three 16-cm-long cores were collected within 2 m of one another and sectioned in 0.5-cm increments to a depth of 14.5 cm. The corresponding sections were pooled and analyzed by gamma spectroscopy.

Methods for the collection of precipitation (fallout monitoring) and suspended sediments in the stream are described in detail in a companion paper (Matisoff et al., 2002).

Radionuclide analyses were conducted by gamma spectroscopy using methods detailed in Bonniwell (2001). Each sample was counted for a period of 23 to 48 h. Samples and standards used the same geometry to minimize counting errors, and the method of Cutshall et al. (1983) was used to correct the low energy 210Pb photon emission (46.52 keV) for sediment self-attenuation. The standard reference materials (SRM) used for quality assurance–quality control (QA–QC) consisted of the Amersham radionuclide solutions QCY44 and RBZ24 (AEA Technology QSA, Oxfordshire, UK) for standard construction, and the Amersham disk sealed source QCRB4136 for attenuation analysis. National Institute of Standards and Technology Columbia River Sediment (NIST 4350B) was also used to certify that the activities of 137Cs were within the 10% variance reported for the standard.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Soils
The distributions of 210Pb in the undisturbed, no-till, and tilled soils are presented in Fig. 2 . Both Star Island (undisturbed) soils exhibit a surface maximum of 210Pb activity (0.22 and 0.15 becquerels/gram [Bq/g]) decreasing to 0.05 and 0.06 Bq/g by 11 cm. Both cores exhibit a near constant activity of 0.051 ± 0.009 and 0.059 ± 0.010 Bq/g between a depth of 11 and 25 cm, which is the value of the supported 210Pb. Minor increases in 210Pb activity below 25 cm presumably reflect a slight downward migration by leaching from desorption (Kd = approximately 103–106; Wang and Cornett, 1993; Belistieri and Murray, 1984) or diffusion and translocation in response to increases in acidity and/or oxidizing–reducing conditions (Von Gunten and Moser, 1993). Except for subtle differences in the absolute activities, the two cores are in excellent agreement with one another as the cumulative downcore 210Pb inventories are within 2% of each other.



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  		Fig. 2. Downcore activities of 210Pb in soil cores collected from undisturbed (Star Island), no-till (Site E, Fig. 1), and conventionally tilled (Site C, Fig. 1) locations in the Old Woman Creek watershed.

 
The soil distributions of 210Pb in tilled and no-tilled soils differ markedly from those of the undisturbed soils. Both the tilled and no-tilled soils exhibit relatively homogeneous profiles, but statistically significant (p < 0.01 and p < 0.05), although subtle, linear decreases in 210Pb activity with depth (tilled soils: slope = 3.4 x 10-4 [Bq/g]/[g/cm2 cumulative dry mass]; no-tilled soils: slope = 2.8 x 10-4 [Bq/g]/[g/cm2 cumulative dry mass]). This modest linear decrease in activity with depth over the entire no-till core is less significant (p < 0.01) than the greater decrease in activity in the upper 6.25 cm (slope = 1.8 x 10-3 [Bq/g]/[g/cm2 cumulative dry mass]) of the same core. The greater decrease in activity with depth is comparable with the rapidly decreasing activity near the surface of the undisturbed soils (Star 1: 1.6 x 10-2 [Bq/g]/[g/cm2 cumulative dry mass]; Star 2: 4.8 x 10-3 [Bq/g]/[g/cm2 cumulative dry mass]).

The 137Cs activities vary by land use (Fig. 3) . In Star Island profiles, subsurface maxima in 137Cs activities at 3 and 7 cm depth decrease to negligible activities by 17 and 21 cm, respectively. Like the 210Pb profiles, both cores exhibit increasing activities below 25 cm, which presumably are caused by leaching from desorption (Kd = approximately 102–104; Wauters et al., 1996) or diffusion and translocation in response to soil acids or humics (Owens et al., 1996) from higher in the profile and redeposition lower in the profile. Deeper sampling would be required to determine the extent of this migration. Although the 137Cs activity profiles in the two cores appear to be in fairly good agreement, the cumulative downcore 137Cs inventories differ by about 50% between the two cores. This is caused by downward migration of 137Cs and a slight downward shift in the Star 2 profile relative to Star 1. This is apparent from the difference in cumulative dry mass down to a depth of 19 cm (Star 1: 29.95 g/cm2; Star 2: 24.7 g/cm2). The 137Cs distributions in the no-till and tilled soils are markedly different from those of the undisturbed soils of Star Island, as well as from one another. Linear trends in no-till and tilled soils are not statistically significant (p > 0.1 and p > 0.2), indicating that the 137Cs distributions are nearly homogeneous in both soils. The average 137Cs activities in the no-till and tilled soils are 0.010 ± 0.002 and 0.0064 ± 0.0016 Bq/g, respectively. In other words, the average 137Cs activity in the no-till soil is 1.6 times greater than that of the tilled soil.



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  		Fig. 3. Downcore activities of 137Cs in soil cores collected from undisturbed (Star Island), no-till (Site E, Fig. 1), and conventionally tilled (Site C, Fig. 1) locations in the Old Woman Creek watershed.

 
Because of its short half-life and atmospheric source, 7Be activities are highest at the soil surface and decrease rapidly with movement toward the bottom of the core (Walling and Woodward, 1992; Bonniwell et al., 1999). Unfortunately, the vertical scale of sampling of the cores was too coarse to determine 7Be activity profiles. However, it is possible to estimate the inventory or standing crop (total activity per unit area) of 7Be in a vertical column of soil by calculating the equilibrium achieved between the influx and decay of 7Be. The average daily flux of 7Be from the atmosphere for the Old Woman Creek watershed is approximately 4.11 Bq/m2/d (Matisoff et al., 2002). This depositional flux would support a soil inventory of 0.032 Bq/cm2, assuming all fallout 7Be is retained in the soil and none is lost by runoff or erosion.

Suspended Sediments
The runoff began in association with a thunderstorm on 18 May 1997. Thirty millimeters of rain fell during several moderate showers over a period of 2 d. Sixteen millimeters of precipitation had fallen in the basin during the previous 7 d and a total of 72 mm during the prior 14 d. A total of 195 mm of precipitation fell during the month of May, which is 227% of the monthly average (86 mm). Suspended sediment concentrations and their radionuclide activities were measured in runoff leaving the field following the thunderstorm.

The 7Be activities in the suspended sediments at OWC ranged from 0.20 to 1.97 Bq/g. Highest activities of 7Be were found in the no-till subbasin (E and F), where activities were almost uniformly >1.0 Bq/g. In the rest of the watershed activities were <1.0 Bq/g. Total 210Pb activities ranged from 0.19 to 0.69 Bq/g, and again the no-till subbasin exhibited overall higher activities (> approximately 0.4 Bq/g) than the remainder of the watershed (< approximately 0.4 Bq/g). The supported 210Pb ranged from 0.054 to 0.209 Bq/g with an average of 0.087 ± 0.036 Bq/g, which accounts for 29% of the total 210Pb activity. The 137Cs activities ranged from 0.013 to 0.108 Bq/g with higher activities at Sites E and F. Plotting the 210Pb activity versus the 7Be activity for all samples analyzed (Fig. 4A) reveals that 210Pb and 7Be are linearly correlated . Further inspection reveals there are distinct differences in the radionuclide signature of sediments derived from no-till subbasins (E and F) compared with those from tilled subbasins (B and C). Fig. 4B subdivides the data by conventional tillage (Sites B and C) and predominantly no-till (Sites E and F). This plot illustrates that these two basins of the same size but with differing agricultural practices produce suspended sediments with distinctly different radionuclide signatures.



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  		Fig. 4. (A) The 210Pb vs. 7Be activities in suspended sediment for all sites. (B) Subsets of the data from no-till and conventionally tilled subbasins.

 
Soil Erosion
A minimum estimate of soil erosion rates can be made by quantifying the suspended sediment flux over the storm hydrograph (Table 1). Sediment fluxes were calculated as the product of suspended sediment concentration and discharge (Matisoff et al., 2002) and are available for Sites B, C, D, E, F, G, and K. The flux is a minimum estimate of soil erosion because some sediment redeposits along the transport path (i.e., downstream delivery is typically less than 100% of upstream erosion rates). This is especially true at the upstream sites, where sediment can be seen to accumulate in the channel, although some bank erosion occurs at the most downstream sites. There is no evidence that resuspension of sediment from the bottom is significant. The cumulative flux of suspended sediment over the 48 h of the hydrograph generally increased downstream along the main branch from Site B (4.9 Mg) to Site K (80.9 Mg). However, a decrease in sediment flux was observed between Sites D and G (33.2 and 22.0 Mg, respectively). This apparent loss of sediment coincides with an apparent loss in the flux of water. This may indicate that the stream exceeded its banks at some point in the hydrograph, that the pond stored water, or that the refinement of one or more flow-rating curves is necessary.

Soil erosion from runoff averaged 1.2 Mg/km2 over the basin as calculated at the most distal monitoring site, K. The greatest erosion (7.0 Mg/km2) occurred in the Site D subbasin in an area of conventional tillage, while the lowest erosion (0.5 Mg/km2) occurred in the Site E subbasin in an area of no-till agriculture. The area draining to Site D accounts for 41% of the sediment flux through Site K, despite draining only 11.3% of the watershed above K.

The influence of agricultural practice on erosion can be isolated by comparing yields in subbasins of similar size, position in the watershed, gradient, and general morphology, but different management practices (Table 1). Sites B and E as well as C and F are similar, except that Subbasins B and C are tilled while Subbasins E and F are no-till. Tilled basins (Sites B and C) had higher sediment yields (2.0 and 6.8 Mg/km2, respectively) than their no-till counterparts, E and F (0.5 and 1.1 Mg/km2, respectively). This difference in erosional loss cannot be attributed to differences in basin or subbasin drainage area or in slope (stream gradient), as these paired subbasins (B vs. E and C vs. F) are very similar (Table 1). These calculations show that the upper reaches of the watershed draining tilled agricultural fields are, disproportionate to their area, responsible for the suspended sediment delivered to the estuary.

The average depth of erosion was estimated from the mass balance of stream fluxes for four parameters (210Pb, 137Cs, 7Be, and sediment mass). Stream fluxes of 210Pb and 137Cs for Sites B, C, and D were mass-balanced with soil profiles from tilled fields, while stream fluxes of 210Pb and 137Cs for Sites E and F were mass-balanced with radionuclide profiles from no-till fields. Subbasins G and K are composed of mixed tillage practices, so 210Pb and 137Cs erosion depths were not calculated. Beryllium-7 provided a third estimate of the erosion depth. The 7Be distribution in the soils was assumed to decrease exponentially (Bonniwell et al., 1999; Walling and Woodward, 1992) from the surface downward with a total depth of penetration of 1 cm, as observed in soils in Idaho (Bonnewell et al., 1999). The inventory supportable (0.032 Bq/cm2) by the average measured 7Be fallout in the precipitation samples (4.11 Bq/m2/d) was used to calculate the preexponential factor. The erosion depth was calculated from the constructed 7Be profile in a manner similar to the 210Pb and 137Cs estimates. The fourth estimate of erosion depth resulted from a mass balance on stream sediment fluxes, and an average soil bulk density of 1.2 g/cm3, by dividing the sediment flux by the basin area and the bulk density. Tilled basins (Sites B and C) had greater erosion depths than their no-till counterparts, E and F (till vs. no-till ; B vs. E ; C vs. F ). Subbasins B (5.2 ± 3.4 µm) and C (11.4 ± 5.1 µm) exhibited average erosion depths approximately 2.5 times greater than their no-till counterparts, Subbasins E (2.1 ± 2.1 µm) and F (4.0 ± 3.4 µm). The estimates may not be technically meaningful for 7Be since it was not measured in profile at the sites, but instead it was calculated from the total soil inventory. In addition, average erosion depths are not truly independent measures, as the sediment flux is used in the calculation of all four estimates. Among sites for which all four estimates were possible, Subbasin D exhibited the greatest average depth of erosion, and Subbasin E exhibited the lowest. The depth of erosion calculated from the bulk density of the soil gave uniformly the lowest values.

The depth of erosion during a single storm event would produce annual erosion rates of 6 to 580 µm/yr assuming that about 15 runoff events of similar magnitude or larger occur per year. Given a bulk density of 1.2 g/cm3, this corresponds to 0.1 to 9.29 Mg/ha/yr, with the average value equal to 0.50 Mg/ha/yr. Evans and Seamon (1997) estimated 7.26 Mg/ha/yr of sediment detachment from the Revised Soil Loss Equation (RUSLE; Soil and Water Conservation Society, 1993) and a GIS database. To compare their number to our results, a delivery ratio must be applied to their gross erosion rate. In the nearby larger Sandusky and Maumee basins the delivery ratio is about 10 to 20% (Baker, 1985). Applying this delivery ratio to the Evans and Seamon (1997) gross erosion rate yields a net erosion of 0.73 to 1.45 Mg/ha/yr, which is similar to that derived from the single storm.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The radionuclide profiles in the no-till and tilled soils exhibit considerable differences from the profiles in the undisturbed soils of Star Island. The soils on Star Island show either a surface peak (210Pb) or a near surface peak (137Cs) followed at depth by a decrease to a constant activity (210Pb) or to near zero (137Cs) (Fig. 2 and 3). The distribution of radionuclides in the tilled soils is largely homogeneous (Fig. 2 and 3) because plowing to a depth of 20 to 30 cm mixes surface soils, which are enriched in radionuclides, with deeper soils, which are depleted in both 210Pb and 137Cs. The no-till agricultural soils also appear to have been significantly mixed by previous plowing, as inferred from the largely homogenized radionuclide distributions. The trend in 210Pb in the top 6.25 cm of the soil, however, indicates that the 210Pb distribution is being rebuilt since the cessation of conventional tillage (Fig. 2). The adoption of no-till agricultural methods in Erie County did not take place until the mid to late 1980s as a result of the 1985 Food Security Act (16 U.S.C. 3801-3862) (Erie County Soil Conservation District, personal communication, 1998). It is likely that the no-till agricultural region was tilled until the mid-1980s. The fallout of anthropogenic 137Cs had largely ceased by the mid-1970s; consequently, a soil that was tilled until 1985 and then no-tilled thereafter would probably exhibit a 137Cs distribution similar to a continuously tilled soil. However, the continued fallout of 210Pb would result in its buildup at the surface in the absence of plowing.

Despite similar trends in the 137Cs distribution between tilled and no-till soils, the absolute activities are greater in no-till soils. The tilled soils could have been plowed to a greater depth and the 137Cs activities diluted by mixing with a greater amount of depleted soil. An alternative explanation is that the soil could have undergone more erosion during the continued years of tillage that removed more 137Cs from the profile. The difference between these possibilities would be resolved by comparing the total 137Cs inventories in deeper cores.

Radionuclide signatures of the suspended sediments also provide an indication of the source of the particles. Comparison of 7Be and 210Pb activities in suspended sediment (Fig. 4B) between the no-till and tilled basins reveals that sediments from the no-till basin are distinctly higher in both 7Be and 210Pb. The no-till sediment also generally contained higher activities of 137Cs. The higher radionuclide activities in sediments derived from the no-till may be due to recent radionuclide fallout. It is also possible that the radionuclides are associated with the fine particles and there is greater winnowing of fines in the no-till soils. In this case, if the no-till soils are winnowed to produce a finer-grained sediment richer in radionuclides, eventually the soil surface would become covered by a coarser-grained lag that produces little sediment. Finally, it is also possible that the higher organic content in the surface of no-till soils sorbs more radionuclides than soils with lesser organic contents.

The radionuclide activities in the soil profiles illustrate that radionuclide signatures of the suspended sediments may be viewed as a two-component mixing model in which the two tillage practices serve as end members. Refining the description of these two components would make it possible to characterize the relative contributions of sediment from various regions according to their radionuclide signature. The majority of samples analyzed exhibit relatively low 210Pb and 7Be activities that correspond with tilled sediments. This supports the results from the sediment flux measurements that identify the tilled subbasins as the greatest contributor of sediment (Table 1).

Figure 5 illustrates a generalized model of how 7Be, 137Cs, and 210Pb can be used to examine soil erosion and sediment movement in a watershed. All three isotopes are deposited on the surface through wet and dry fallout. Each radionuclide is distributed differently in the soil because of differences in half-lives, delivery rates, delivery histories, and land use. An undisturbed soil will exhibit higher radionuclide activities near the soil surface, which reflects their surficial input and slow downward transport. The shorter half-life of 7Be compared with 210Pb results in less downward migration of 7Be. Hence, 7Be is found only at the soil surface but some 210Pb will migrate down the core. In addition, some 210Pb is produced in the soil by in situ decay of 222Rn resulting in some 210Pb activity at all depths in the soil. Because 137Cs had its peak delivery in the early 1960s and almost no delivery before 1955 or since 1975, its activities have a distinct peak at approximately 10 cm depth in the soil. Plowing homogenizes 210Pb and 137Cs within the plowed layer, but because of its short half-life and constant input, 7Be activities are highest at the surface and are homogeneous only immediately after plowing. Because there has been no 137Cs fallout since the 1970s, its distribution will remain homogeneous within the soil profile, even after the cessation of plowing. On the other hand, 210Pb, like 7Be, is continuously deposited on the land surface. Its distribution will remain homogeneous if the soil is plowed annually, but it will accumulate at the surface and slowly rebuild a profile with decreasing activity with depth if no-till practices are implemented.



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  		Fig. 5. Generalized sketch illustrating the distributions of 7Be, 210Pb, and 137Cs in soils under different tillage practices. In the radionuclide soil profiles, the shading and the sketches indicate radionuclide activity with depth in the 10- to 30-cm-deep soil profiles.

 
Sediment eroded from a soil will have a radionuclide signature corresponding to the tillage practice and the depth of erosion. Thus, radionuclide signatures in suspended sediments can provide a means of tracing particles eroded from the landscape and can identify soil sources and be used to quantify the erosion.

In conclusion, tilled basins had higher sediment yields and average erosion depths than their no-till counterparts in response to runoff from a thunderstorm. The upper reaches of the watershed draining tilled agricultural fields are disproportionately the major source of suspended sediment delivered to the estuary. These findings indicate that erosion control methods may be most beneficial at upland locations near the head of the drainage in the watershed where erosion is the greatest, and that the effects of improved land management practices in these areas should be reflected quickly in the receiving waters.


   ACKNOWLEDGMENTS
 
David Klarer, Research Coordinator at Old Woman Creek National Estuarine Research Reserve, provided assistance at facilities at the estuary. Chris Wilson assisted in the field sampling and in processing samples in the laboratory. A graduate research fellowship to Chris Bonniwell was provided by the National Estuarine Research Reserve System (NOAA) #NA77OR0217. This research was supported in part by grants from the Sanctuaries and Reserves Division (NOAA) Award #NA67OR0238, NOAA Cooperative Institute for Estuarine Environmental Technology (CICEET) Award #99-297, and from the USDA Lake Erie Agricultural Systems for Environmental Quality (ASEQ) Award #RF715539.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
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