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Journal of Environmental Quality 31:62-72 (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

Radionuclides as Indicators of Sediment Transport in Agricultural Watersheds that Drain to Lake Erie

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
 
An issue in evaluating the success of agricultural management practices is the speed that eroded particles make their way through the downstream waters. In this study at Old Woman Creek (OWC) and Rock Creek (RC), two largely agricultural watersheds in Ohio, the flux of sediment and radionuclides (7Be, 210Pb, and 137Cs) in thunderstorm runoff was examined to better understand transport of eroded agricultural soils. The hydrograph in an agricultural area under no-till was similar in timing, but of lesser magnitude, than the hydrograph from a similar-sized area under conventional tillage. The activities of 210Pb and 7Be are linearly correlated and are higher in suspended sediments derived from no-till subbasins than those derived from conventionally tilled subbasins. A suspended sediment plume, identified by its unique radionuclide signature, was traced through 17 km of OWC stream channel in approximately 13.4 h (0.35 m/s). The downstream exponential decrease of 7Be activities in suspended sediments 3 to 12 h after passage of the sediment plume was used to estimate transport distances of suspended sediment from 2 to 17 km, respectively. Transport distances of suspended sediments were also calculated from wave kinematics and indicate that at OWC suspended sediment transport distances were longer in streams draining areas of no-till (19–26 km) than in the streams draining areas of conventional tillage (6–15 km). Suspended sediments travel 7 to 22 km at RC. The transport distances are long relative to the lengths of the stream channel and indicate that erosion control methods implemented in the watershed should be reflected quickly in downstream waters.

Abbreviations: NERR, National Estuarine Research Reserve • OWC, Old Woman Creek • RC, Rock Creek


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
TWO IMPORTANT COMPONENTS in the understanding of sediment fluxes in watersheds are the identification of sediment sources and the determination of sediment transport distances. The identification of sources allows recognition of regions or landuse practices that produce large amounts of sediment or sediment with associated constituents such as pollutants and nutrients. The determination of sediment transport distances allows estimation of which parts of a watershed are contributing sediment to receiving waters. If toxicity of contaminants is an issue, the degree to which there has been dilution or degradation of contaminants can also be estimated. Previous work has determined that nonpoint agricultural sources of sediment are a major contributor of nutrients and other pollutants to the Great Lakes (International Joint Commission, 1978, 1983; Logan, 1982, 1987; Yaksich et al., 1982, 1985; Wall et al., 1982; Baker, 1985). However, the delivery ratio, the fraction of sediment eroded from the landscape that is delivered to the lake, ranges from 30 to 90% for watersheds less than 1 km2 to 5 to 10% for watersheds of 1000 km2 or larger (Piest et al., 1975). Grass buffer strips and no-till preparation of fields are known to reduce rates of soil erosion (Myers et al., 2000), but their effects on particle transit distances are not well known. Additional work has shown that the majority of the nutrient and metal transport (at Old Woman Creek, OH, and presumably throughout the Great Lakes region as well as in other coastal areas) occurs during storms (Klarer, 1988). Determining the locations in the watershed where nonpoint-source agricultural sediment is derived in response to thunderstorms and defining particle transit distances of these sediments are therefore important in effective targeting of pollution control measures.

Radionuclides have been successfully used as tracers for particle transport. Beryllium-7 has been used to investigate the residence and settling times of particles and particle-reactive substances in lakes and coastal waters (Olsen et al., 1986; Schuler et al., 1991; Wieland et al., 1991). In our previous work in an alpine region in Idaho (USA) we were able to trace suspended sediment tagged with radionuclides released from snowmelt (Bonniwell et al., 1999). We found that the fine particles were transported about 60 km during the peak of the hydrograph and about 12 km during base flow. Thus, fluvial transport rates can be determined from tracers by monitoring the downstream progression of suspended sediments with unique radionuclide signatures. Several studies have used radioactive tracers such as 7Be, 137Cs, and 210Pb to identify and characterize (fingerprint) sediment source regions and land use (Walling and Woodward, 1992; Olley et al., 1993; Owens et al., 1996, 1999; Walling et al., 1999; Matisoff et al., 2002). Beryllium-7, cesium-137, and lead-210 are all deposited on the surface through wet and dry fallout. However, because of differences in half-lives, delivery rates, delivery histories, and land use, each radionuclide is distributed differently in the soil (Matisoff et al., 2002). Therefore, sediments derived from a soil will have a unique radionuclide signature corresponding to the land use and the depth of erosion (Whiting et al., 2001).

This previous work indicates that radionuclides can be an effective tracer for identifying sediment source regions, erosion depths, and transport distances during snowmelt-driven hydrographs. However, the length of transport of storm-derived sediments has not been investigated. The purpose of this work was to use 7Be, 137Cs, and 210Pb to determine length of transport of storm-derived suspended sediments in two streams that are tributaries to Lake Erie. The study also aimed to identify regions in the watershed that yielded the greatest amount of sediment and to relate these regions to particular agricultural practices. The approach used in this study was to use radionuclide signatures to trace the sediment plume as it traversed the watershed. Similarly, the water and suspended sediment transport distances were traced using the wave kinematics method developed in Verhoff et al. (1979).


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Study Areas
The Old Woman Creek National Estuarine Research Reserve (OWC NERR) estuary lies at the mouth of a 69.5-km2 agricultural watershed that is a tributary to Lake Erie. The area is described in Matisoff et al. (2002). The study region experiences an average annual rainfall of 840 mm. Agricultural practices vary throughout the watershed; one part of the watershed is primarily in no-till while another part is conventionally tilled. In this study, no distinction is made between "no-till" and "conservation tillage" and the sites are all collectively referred to as "no-till". The Rock Creek (RC) watershed (Fig. 1) drains into the Sandusky River, which is a major tributary to Lake Erie, and for which nonpoint-source pollution has been identified as a major problem. Rock Creek is a 90.5-km2 watershed located in predominantly agricultural (81.5% agricultural and/or open urban areas) Seneca County, Ohio. A United States Geological Survey (USGS) gaging station (#04197170) is located on Rock Creek near its confluence with the Sandusky River in Tiffin, Ohio (Fig. 1). Adjacent to the USGS gaging station, the Heidelberg Water Quality Lab maintains automated water sampling equipment, which is used for continuous water quality monitoring. The uppermost site (A-3) drains 18.1% of the watershed (16.4 km2) and has a slope (stream gradient) of 0.00060 while the lowermost site (B-1) drains 69.0% of the watershed (62.4 km2) and has a slope of 0.0021 (Table 1). Average annual precipitation in Tiffin, Ohio is 900 mm.



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  		Fig. 1. Sample collection locations in the Rock Creek (RC) watershed. Storm runoff was sampled at 12 locations (Sites A-3 though A-11, TA-2, and B-1 through B-3) in the watershed. The A stations were located along the main drainage channel, and TA-2 was located on a tributary to the main channel that enters the creek just downstream from A-11. The B stations were located on the main channel downstream of tributary TA-2.

 

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  		Table 1. Basin characteristics and transport distances in the Old Woman Creek and Rock Creek watersheds in Ohio.

 
Beryllium-7 in Atmospheric Fallout
Precipitation was monitored at the OWC NERR weather station (Matisoff et al., 2002) (Fig. 1). Monitoring of 7Be fallout commenced in early November 1996 at the OWC NERR and six other locations in the OWC watershed (Matisoff et al., 2002), and continued until August 1999. Fallout of 7Be was not monitored in the Rock Creek watershed. Wet and dry fallout was collected over periods of 3 wk to 4 mo in 20-L polyethylene buckets (641-cm2 surface area) mounted 1 m above the ground surface. Approximately 500 mL of 10% HCl was placed in the bottom of each bucket to prevent the sorption of 7Be onto the container. Collected samples were filtered and spiked with 10 mL of 10% (w/v) FeCl3. Addition of NaOH to a pH of 8.3 precipitated the dissolved iron and its sorbed 7Be. The floc was allowed to settle and compact in a separatory funnel, extracted, and placed into a 120-mL polyethylene specimen cup, which was directly analyzed for 7Be activity by gamma spectroscopy. Samples were spiked with 100 µL of 1000 mg/L stable Be before precipitating the iron to determine the recovery efficiency of the method. Analysis indicates >95% recovery of beryllium.

Stream Discharge
Stage plates (staff gages) were installed to monitor water level and determine stream discharges at all stream sampling sites. Sites C, F, G, and J at Old Woman Creek and A-9 and B-1 at Rock Creek were equipped with Telog (Victor, NY) pressure transducers and dataloggers that recorded water depth to the nearest 0.2 cm every 120 s. In addition, Site K is located at the USGS gaging station on Old Woman Creek (#04199155) and there is a USGS gage near the mouth of Rock Creek (#04197170) (Fig. 1). Average flow velocity was quantified at a minimum of 20 points across the stream channel using a current meter. Measured velocities and depths were integrated across the channel to yield discharge. At all sites except those monitored by USGS gages, cross sectional measurements were made at each sampling location at two to five different stages (in addition to several dye measurements, where available [see below]) and the discharge versus stage data were fitted with a power function to establish discharge rating curves. Suspended sediment concentrations were combined with discharge rating curves to establish sediment rating curves.

An experimental dye tracer method was employed at Old Woman Creek as a second means of determining discharge. Bright Dyes Fluorescent FWT red dye (Rhodamine WT) (Kingscote Chemicals, Piqua, OH) was injected at a constant rate 100 m upstream of Site C. Stage levels were recorded when collecting water samples at Sites C, D, and G downstream of the injection site. The samples were then filtered through Gelman (Ann Arbor, MI) 0.2-µm filter paper and the dye concentration measured by a spectrofluorometer at an emission wavelength of 588 nm. Discharge was then determined from dilution of the tracer (Dingman, 1994). Results of the dye method are in good agreement with current meter measurements and were added to the stage discharge rating curves.

Suspended Sediment
Suspended sediment sampling commenced at the initiation of the thunderstorm and continued for approximately 48 h from 18–20 May 1997 until stream levels decreased to near baseflow. There were 11 sampling locations in the OWC basin (for a map see Matisoff et al., 2002). Eight of the sampling stations were along a 17.1-km reach of the main stem of Old Woman Creek (A–D, G, H, J, K). One of the stations was located below the confluence of the main stream and a western branch of Old Woman Creek at the site of the USGS gaging station #04199155 (K). An additional sampling station was located on the western branch of Old Woman Creek (I) immediately upstream of its confluence with the main stem. Seven of the eleven sites (A–G) were located in the upper 12% (8.62 km2) of the watershed (Table 1). Sites A through D and G drain predominantly tilled agricultural land, whereas Sites E and F are located on a tributary that drains agricultural land that is predominantly no-till. The remaining four sites (H–K) drain no-till and tilled lands as well as the urbanized area of the town of Berlin Heights, Ohio. Approximately 18 L of stream water was collected and the stream stage was recorded at each site nine times during storm runoff. Samples were collected manually from bridges and stored in 20-L polyethylene buckets. A 120-mL subsample, preserved with Kathon CG/ICP (Supelco, Bellefonte, PA), was collected for grain size determination. Subsamples were analyzed on a Spectrex (Redwood City, CA) 2000 laser particle analyzer to determine the median grain size (D50). The samples were not subject to sonication before grain size determination because the effects of sonication on the natural, in-stream grain size are unknown. A Sharples continuous flow centrifuge (Penwart Corp., Warminster, PA) was used to separate the suspended solids until the water was visually free of suspended material. Collected solids were placed in pre-tared 47-mm-diameter polystyrene petri dishes, dried at 60°C for 24 h, and weighed. The concentration of suspended solids was computed from the mass of solids divided by the volume of sampled streamflow.

Suspended sediment monitoring following a thunderstorm in the Rock Creek watershed took place over two days (12–13 June 1998) at six locations along a 16.1-km segment of the watershed (A-3, A-5, A-7, A-9, A-11, and B-1) and at one location (TA-2) on a major tributary of Rock Creek (Fig. 1). Sampling methodologies, hydrograph data logging, rating curve methodology, and laboratory analyses were identical to those in the Old Woman Creek watershed.

Gamma Spectroscopy
Suspended sediment, soil, and precipitation samples were analyzed by gamma spectoscopy for 7Be, 210Pb, 137Cs, and 40K using methods detailed in Bonniwell (2001) and summarized in Matisoff et al. (2002).

Beryllium-7 Partition Coefficient (Kd)
Storm runoff collected from the Rock Creek watershed (Site A-11) was analyzed to determine the partition coefficient of 7Be between the solid and dissolved phases (Kd). A total of 57.7 L of stream water was centrifuged within 36 h of collection to separate the majority of suspended material and yielded 13.92 g of solids, which corresponds to a suspended sediment concentration of 241 mg/L. The sample was placed into a 28.7-cm3 petri dish, dried, and analyzed by gamma spectroscopy. The supernatant liquid was filtered through a pre-tared 203-mm x 254-mm x 0.20-µm filter paper to separate remaining solids. The filter paper was placed into a 28.7-cm3 petri dish, dried, and analyzed by gamma spectroscopy. The filtered liquid was passed through two in-sequence ion exchange columns to remove 7Be from the dissolved phase. Beryllium-7 sorbed to the resin was eluted with approximately 4 L of 10% HCl, spiked with 15 mL of 10% (w/v) FeCl3, and allowed to equilibrate over several hours at pH = approximately 8.2. The resulting floc was separated and analyzed by gamma spectroscopy to quantify 7Be in the dissolved phase.

Wave Kinematics
To evaluate the transport distance of suspended sediment over the runoff hydrograph, the methodology of Verhoff et al. (1979) was used. Briefly, the method treats the hydrograph as a kinematic wave that moves downstream at a velocity greater than that of individual parcels of water. This results in an individual parcel of water changing its relative position in the hydrograph as it falls further and further behind the leading edge of the kinematic wave. The potential travel distance of sediment at a single site is calculated from the velocity of the kinematic wave and the flow velocity associated with each collected sample. The velocity of the kinematic wave is determined from the stream discharge, which is evaluated from the stream cross sectional area. The derivative of this function is then defined as the wave velocity (Verhoff et al., 1979). The incremental transport distance of each portion of the hydrograph is then evaluated as:

[1]
where ds/dto is the differential downstream movement relative to the beginning of the hydrograph; V is the velocity (m/s) of water at any given position in the hydrograph; and Vw is the wave velocity (m/s). Integration of this function over the hydrograph yields the cumulative distance traveled for each parcel of water in the hydrograph. Just as each parcel of water has a particular travel distance, so does the associated suspended sediment. The suspended sediment concentrations are plotted versus the cumulative travel distance, and the resulting plot can be separated into discrete distances that are weighted by their relative suspended sediment contribution to determine the average travel distance of sediment over the hydrograph. The complex hydrograph at OWC was separated and a single simple hydrograph associated with the initial rain and runoff analyzed. Suspended sediment concentrations for the extrapolated portion of the hydrograph were determined from sediment rating curves.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Event Description
The runoff monitored at Old Woman Creek began in association with a thunderstorm on 18 May 1997. A total of 30 mm of precipitation fell over two days during three to four major pulses. Twelve millimeters of rain fell in about 4 h followed by another 12 mm of rain during moderate showers and then an additional 2 mm a few hours later the following day. Finally, over a 1-h period later the second day another 4 mm of rain fell during moderate showers. This resulted in a complex hydrograph (Fig. 2) . Antecedent conditions were wet: a total of 72 mm of precipitation had fallen in the basin during the prior 14 d, and 16 mm during the previous 7 d. A total of 195 mm of precipitation fell during the month of May at OWC NERR (Station O1), which is 227% of the monthly average.



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  		Fig. 2. Runoff hydrographs at Old Woman Creek (OWC). The top figure (A) illustrates the difference between the hydrographs at Site C (tilled) and Site F (no-till), which are located in similar subbasin positions in the watershed. The lower figure (B) illustrates the progression in time of the hydrograph as the kinematic wave translates downstream 10 km from Site C to Site G.

 
The runoff event at Rock Creek was associated with a summer thunderstorm on 12 June 1998, which delivered 28 mm of precipitation in the town of Tiffin, OH (near the USGS gage). An additional 19 mm of precipitation fell on 13 June 1998. As observed at OWC, this resulted in a set of complex hydrographs (Fig. 3) . Further contributing to the complex hydrograph was the fact that the storm progressed from west to east through the watershed, but the watershed drains from east to west. Nine to thirteen samples were collected at each of seven sampling locations during the runoff event.



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  		Fig. 3. Sedigraphs and hydrographs for sites in the Rock Creek (RC) watershed. The scale for discharge at Station A-11 is three times larger than at the other stations.

 
Precipitation and Runoff
At Old Woman Creek, discrete pulses of rainfall resulted in a complicated hydrograph with two main peaks (Fig. 2). While the general shape of the hydrograph was consistent throughout the basin, the magnitude and timing of the two runoff peaks varied by position in the watershed and with land use. Figure 2A illustrates the difference in the runoff hydrographs between the subbasin draining to Site C and that of the equal size subbasin draining to sampling Site F located 2.5 km away. Subbasin C is mainly conventionally tilled, while Subbasin F is mainly no-till; otherwise these basins are similar. This difference in tillage practice produced a hydrograph in the no-till subbasin, which was similar in its timing, but less flashy and of a lesser magnitude in its initial response, than in the tilled basin. This dampening of the hydrologic response has been observed in other no-till basins (P. Richards, personal communication, 1998). No-till practices increase the presence of organic matter and create a more porous soil, which in turn increases the infiltration capacity of the soil, and decreases the amount of direct runoff (Ward and Elliot, 1995). Figure 2B illustrates the differences of the hydrographs between Sites C and G. Site G is about 5 km downstream of C and is similarly tilled. The hydrographs are similar in shape, but the 5 km separation resulted in an approximately 3-h delay in the peak of the hydrograph. At Rock Creek the multiple precipitation events and the west to east path of the storm through the east to west drainage system resulted in a complex timing of the hydrograph peaks (Fig. 3).

Beryllium-7 Fallout Monitoring
The maximum fallout of 7Be (7.99 ± 0.37 Bq/m2/d) occurred during the period mid-April to late May. The minimum fallout (2.33 ± 0.22 Bq/m2/d) occurred in January. The average daily flux of 7Be for the Old Woman Creek watershed was 4.11 Bq/m2/d. Robbins and Eadie (1991) reported 7Be fluxes to Lake Michigan that were similar, with a maximum flux of 6.66 Bq/m2/d in May to June and a minimum flux of 2.52 Bq/m2/d in December to January. A summer maximum and winter minimum is consistent with seasonal patterns in precipitation and stratospheric exchange.

Beryllium-7 Partition Coefficient (Kd)
Centrifuging removed 99.9% of the suspended solids, which accounted for 92.8% of the total 7Be activity in the collected water sample. Filtered solids accounted for only 0.1% of the solids, but they contributed 2.3% of the total 7Be activity, and the dissolved phase contained 4.9% of the total 7Be activity. This yields a partition coefficient (Kd) of 7.6 x 104 L/kg. This value is comparable with that obtained using the expression of Hawley et al. (1986) at a suspended sediment concentration of 241 mg/L:

[2]

Suspended Sediment
At Old Woman Creek, the suspended sediment concentrations ranged from 12 mg/L in no-till at Site F to 1909 mg/L in conventionally tilled Site C. The median grain size ranged from 2.36 to 4.80 µm with an average of 3.40 ± 0.46 µm. Maximum measured suspended sediment concentration occurred immediately prior to peak discharge associated with the first pulse of rain at Sites C and F (Fig. 4) . There is a secondary peak in concentration associated with the second peak of the hydrograph, which was produced by additional, but less intense precipitation. Reduction in the intensity of precipitation decreases the erosive potential of the runoff (Ward and Elliot, 1995).



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  		Fig. 4. Sedigraphs and hydrographs at Sites C and F in the Old Woman Creek (OWC) watershed.

 
At Rock Creek, suspended sediment concentrations of collected samples ranged from 69.4 to 4690 mg/L. The maximum suspended sediment concentration occurred early in the hydrograph at Site A-9 and subsequent samples were less than 3000 mg/L. Maximum measured suspended sediment concentrations at all sites occurred immediately prior to peak discharge associated with the first pulse of rain (Fig. 3). The sedigraph of the upper sites (A-3, A-5, and A-7) was significantly flashier and with greater maximum suspended sediment concentrations compared with those of the lower sites (A-11 and B-1). The three upper sites differed in the timing of the sediment response to the runoff because of the pattern of rainfall. The suspended sediment concentrations increased between the second and third sample (2.2–3.2 h) at Site A-3, between the first and second sample (0.58–2.4 h) at Site A-5, after the third sample (3.7 h) at Site A-7, and before the first sample (1.1 h) at Site A-9. Also associated with Site A-9 is the premature occurrence of the maximum suspended sediment concentration relative to other sites. The suspended sediment concentrations increased between the first and second samples (1.5–4.3 and 1.6–4.4 h) at the downstream Sites A-11 and B-1, respectively.

The 210Pb activities in suspended sediments at Rock Creek ranged from 0.078 to 0.701 Bq/g with an average of 0.203 Bq/g, and exhibited an inverse relationship with suspended sediment concentration (Fig. 5A) . This inverse relationship was probably a consequence of a finite quantity of rain-derived 210Pb (excess 210Pb) delivered to the watershed regardless of the amount of dilution by suspended sediment deficient in excess 210Pb. The 7Be activities ranged from 0.11 to 2.42 Bq/g with an average of 0.37 Bq/g, and also exhibited an inverse relationship with suspended sediment concentration (Fig. 5B). The 137Cs activities ranged from 0.006 to 0.063 Bq/g with an average of 0.016 Bq/g. The 40K activities (an indicator of the clay content) ranged from 0.80 to 1.47 Bq/g with an average of 1.08 Bq/g.



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  		Fig. 5. The 210Pb (A) and 7Be (B) activities in the suspended sediment as a function of suspended sediment concentrations.

 
The activities of 210Pb and 7Be in the suspended sediments from Rock Creek and OWC were linearly correlated (Fig. 6) . At OWC radionuclide activities were higher in suspended sediments derived from no-till subbasins than those derived from conventionally tilled subbasins. Rock Creek sediments displayed the same trend as OWC sediments, with the majority of samples exhibiting activities lower than most OWC sediments. If the demarcation between till and no-till was the same in Rock Creek as in OWC, then the majority of the suspended solids at Rock Creek were derived from tilled fields even in a watershed that is >75% no-till. At OWC tilled parts of the watershed produced about five times the amount of sediment as no-till areas (Fig. 4).



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  		Fig. 6. The 210Pb and 7Be activities in suspended sediments from Rock Creek (RC) and from conventionally tilled and no-till subbasins at Old Woman Creek (OWC).

 
Sediment Transport—Radionuclides
Plots of suspended sediment concentration along a longitudinal transect at different times (Fig. 7) suggest that the suspended sediment transport can be viewed as a slug or plug flow that became attenuated and diffused as it moved downstream through the OWC stream system. This sediment slug was identified as a suspended sediment concentration maximum, which progressed through 17 km of stream channel in approximately 13.4 h (0.35 m/s). While the definition of the slug was subtle downstream, changes in the velocity of the slug coincided with changes in the stream gradient. Furthermore, slug velocities (approximately 0.28–0.40 m/s) were also similar to stream velocities in the watershed (approximately 0.1–1.0 m/s). The radionuclide activities per gram of the suspended sediments in the slug remained relatively constant in the downstream direction (Fig. 8) , despite decreases in sediment concentration. This supports the interpretation that these were samples of the same slug as it progressed downstream. This relatively constant radionuclide composition within the slug was different from the radionuclide activities at other suspended solids concentrations (Fig. 5) and indicates that some sediment within the slug was transported through the entire length of the channel. An exception to the consistent composition of the sediment slug was at Site C (the dashed line assumes that the signature of the sediment slug at Site C is anomalous). This sample had the greatest suspended sediment concentration of all samples analyzed (1910 mg/L), which suggests significant erosion upstream of the site. The radionuclide signature of this sample, relative to other samples, also indicated that deeper erosion was taking place near Site C than near other sites. The 7Be and 210Pb activities decreased downstream, while the 137Cs activity increased. Because (i) 7Be was concentrated at the soil surface, (ii) the 210Pb activity increased toward the soil surface (Matisoff et al., 2002), and (iii) the 137Cs activity was homogeneous in the soil, the only means by which to create these relative changes was to erode deeper into the soil profile near Site C.



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  		Fig. 7. Suspended sediment concentrations along the length of Old Woman Creek (OWC) during and after the rainfall event.

 


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  		Fig. 8. The radionuclide signature of the sediment slug (identified in Fig. 7) as it moved downstream.

 
The downstream exponential decrease in 7Be activity of suspended sediments after passage of the sediment slug was evaluated following Cushing et al. (1993):

[3]
where F(x) is the 7Be activity, Fo is the 7Be activity at the most upstream site, and 1/k1 is the characteristic transport distance. The exponential decrease is from settling out of particles, not radioactive decay (Mundschenk, 1996). In a manner analogous to radioactive decay, a "half-distance" (x1/2) may be calculated as x1/2 = 0.693/k1. The characteristic transport distance was used here. The analysis was limited to sampling locations A, B, C, D, and G in order to eliminate the influence of tributaries from areas of no-till, since no-till produces a different radionuclide signature (Fig. 6).

After passage of the sediment slug (Fig. 7), 7Be activities in the suspended sediment decreased exponentially downstream (Fig. 9) . From Eq. [3], estimates of transport distances for the four time periods (3.3, 5.3, 8.1, and 11.6 h) were 2.2, 5.2, 15.0, and 17.4 km, respectively. These distances are as long or longer than the distance between stations over which the transport was calculated and indicate that sediment eroded from the landscape is likely to be delivered to the estuary at OWC NERR.



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  		Fig. 9. Exponential decrease of 7Be in suspended sediments at various times after passage of the sediment slug and the transport distances calculated from Eq. [3].

 
Radionuclides could not be used to calculate transport distances at Rock Creek because the storm traveled upgradient from west to east as it passed through the westerly draining watershed, making it impossible to calculate longitudinal transport.

Sediment Transport—Wave Kinematics
Verhoff et al. (1979) proposed a kinematic model to evaluate the ability of a stream to transport suspended sediment based upon the difference between the velocity at which the hydrograph (kinematic wave) progresses downstream and the velocity of the streamwater. Mean transport distances calculated using the method of Verhoff et al. (1979) for the storm event at OWC are presented in Table 1. Because these transport distances were calculated from the separated hydrograph, it is reasonable to expect that use of the larger, complex hydrograph would result in longer calculated sediment transport distances. Cumulative probability distribution curves of transport distances for Sites B, C, D, and G indicate an increasing sediment transport distance in the downstream direction (Fig. 10) . Along the main tributary, mean transport distances increased downstream from 6 km at Site B to 20 km at Site G. Median transport distances also increased downstream, but were 1.5 to 2.5 times lower (3 to 13 km) because of the skewness of the cumulative probability curves. Sites E and F in the no-till subbasins had mean transport distances 19 and 26 km, and median transport distances of 14 and 19 km, respectively.



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  		Fig. 10. Probability distributions of suspended sediment transport distance at Sites B, C, D, and G.

 
Suspended sediment was transported farther in the no-till subbasins. The percentage of suspended sediment that would travel to the estuary is 3, 11, 27, and 51% for Sites B, C, D, and G, respectively. Probability curves for Sites E and F (not plotted in Fig. 10) indicate that 48 and 68%, respectively, of the suspended sediment would be transported to the estuary. The probability distributions also illustrate that some proportion of the sediment at each site will travel short distances. For instance, approximately 39% of the sediment passing Site B will travel a distance less than the 2.0 km to Site C. However, because transport distances are increasing in the downstream direction, shorter transport distances are not necessarily indicative of deposition.

While transport distances based upon the Cushing et al. (1993) or Mundschenk (1996) approach (Eq. [3]) could not be calculated at Rock Creek given the upstream track of the storm, a kinematic estimate was possible because it relies on more local cross-sectional and hydrographic information. At Rock Creek, kinematic wave-derived suspended sediment transport distances were calculated at sites for which discharge and sediment rating curves have been developed (A-3, A-5, A-7, A-11, and TA-2). The median and mean transport distances are reported as distance upstream of the Sandusky River confluence (Table 1). Sites A-3 and A-5 exhibit similar mean (13 and 13 km) and median (9 and 7 km) transport distances. Further downstream, Site A-7 exhibits the greatest mean transport distance (22 km) and median transport distance (10 km) for the Rock Creek watershed. Site TA-2 exhibited mean and median transport distances of 18 and 12 km, respectively.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results lead to several observations. First, no-till subbasins generate less sediment per unit area (Matisoff et al., 2002) than tilled basins. This has been reported for other basins. For example, Myers et al. (2000) demonstrate that a decrease in suspended-sediment discharge from streams in the Maumee Basin was related to an increase in conservation tillage in the basin. Second, suspended material is transported about three times farther in the no-till subbasins as compared with conventionally tilled basins. Increased porosities and organic content, resulting from no-till management techniques, increase the infiltration capacity and inhibit runoff and erosion (Fetter, 1994; Ward and Elliot, 1995). Thus, the no-till sites produce less flashy hydrographs (Fig. 2). However, the water moving in the stream in a no-till basin travels at a relatively high sustained velocity for a greater period of time. Therefore, a controlling factor in transport distance is the shape of the hydrograph, which is significantly influenced by the agricultural management practice as it alters the infiltration capacity of the soil.

The transport distances calculated at Rock Creek are comparable with those at OWC. Kinematic wave analysis indicates that transport distances increase downstream from about 13 to 22 km. The transport distances at A-3 (13 km) and A-5 (13 km) are similar. This may be a consequence of the fact that this portion of the watershed has an engineered agricultural channel resulting in a uniform, nearly complete transport of sediment through the channel and yielding nearly identical estimates of transport distance. Transport distances calculated from wave kinematics and radionuclide signatures, as well as the rapid progression of a slug of sediment through the watershed, indicate that suspended sediments travel long distances (2–17 km at OWC [radionuclide signatures]; 6–26 km at OWC and 12–22 km at Rock Creek [wave kinematics]) relative to the 20-km length of the stream channel.

The distances determined in this study are intermediate to the 0.58 to 200 km reported for transport during a single event (Verhoff et al., 1979; Bonniwell et al., 1999; Cushing et al., 1993). Verhoff et al. (1979) determined transport distances in the Sandusky River Basin, Ohio, using the methods described in this study and found median transport distances on the order of 30 to 200 km for discharges ranging from 50 to 200 m3/s. Bonniwell et al. (1999) used 7Be, 210Pb, and 137Cs activities as tracers of suspended material in the Gold Fork River, Idaho. Beryllium-7 was delivered by snowmelt as a headwater source. An exponential downstream decrease in 7Be activity, corrected for changes in source material, indicated transport distances of 12 to 55 km over the 70-d snowmelt hydrograph. Cushing et al. (1993) found an exponential downstream decrease in concentration of radiogenically tagged (14C) organic material in an Idaho stream (gradient = 0.0075–0.0130), and from these data they determined transport distances of 0.58 to 0.80 km over 24 h at baseflow conditions (discharge = 0.25–0.75 m3/s). Meyers (1995) reports the downstream concentrations of lead and copper derived from a mine tailings dam failure. An exponential fit to his data using the technique of Cushing et al. (1993) yields transport distances of 19 and 15 km, respectively. Transport distances over a longer time period have also been determined using radionuclide activities in suspended and bottom sediments, and the reported transport distances exhibit a wide variation. Mundschenk (1996) calculated a mean range of the particles of 260 km for 3 to 6 yr of transport downsteam from a nuclear power plant in the Moselle River. Graf (1996) determined the downstream distribution of plutonium in Los Alamos Canyon, New Mexico (average gradient = 0.020). Applying the same technique of Cushing et al. (1993) to the data set of Graf (1996) yields a mean transport distance of 6 km. Sayles et al. (1997) reported 137Cs and 239,240Pu contaminated sediments of the Ob River (Siberia) have been transported approximately 1700 km over many years. Although these other studies encompass a wide variety of hydrologic conditions and transit times, the results indicate that fine particles, once they enter the stream system, tend to travel long distances (many kilometers).

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 from the no-till and tilled basins reveals that sediments derived from no-till basins are distinctly higher in both 7Be and 210Pb (Fig. 6). However, the majority of samples analyzed exhibit relatively lower 210Pb and 7Be activities, which correspond with sediments derived from conventionally tilled basins. Furthermore, at Rock Creek, the majority of the watershed is no-till, which implies that the radionuclide signature is dominated by sediments derived from the small fraction (<25%) of the watershed that is conventionally tilled. This supports the results from the sediment flux measurements that identify the conventionally tilled subbasins as the greatest contributors of sediment (Matisoff et al., 2002).

Transport distances calculated from all three techniques (sediment plume, longitudinal 7Be activities, kinematic wave analysis) are long relative to the lengths of the stream channels and indicate that the majority of fine sediment eroded from these watersheds will be transported the entire length of their channels and delivered into their receiving waters (OWC NERR at Old Woman Creek and the Sandusky River at Rock Creek). Since conventionally tilled basins had higher sediment yields than their no-till counterparts (Matisoff et al., 2002), these findings indicate that erosion control methods implemented in the portion of the watershed where erosion is greatest should be reflected quickly in the receiving waters.


   ACKNOWLEDGMENTS
 
David Klarer, Research Coordinator at Old Woman Creek NERR, provided assistance and 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 by grants from the Sanctuaries and Reserves Division (NOAA) Award #NA67OR0238 and from the USDA Lake Erie Agricultural Systems for Environmental Quality (ASEQ) Award #RF715539.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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