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publications > paper > fertilizer-derived uranium and sulfur in rangeland soil and runoff: a case study in central Florida > results and discussion > chemical and isotopic composition of runoff

4. Results and Discussion

Abstract
Introduction
Site Desc. & Land Use
Methods
Results & Discussion
- Soil Composition
- Uranium in Soils
- Sulfur in Soils
> Runoff
- U, S, and P Mobility
Conclusions
Acknowledgments
References
Figures, Tables, & Equations

4.4. CHEMICAL AND ISOTOPIC COMPOSITION OF RUNOFF

Runoff samples from improved pasture S5 and unimproved pasture W4 were collected after 4 rainfall events at different times of the wet seasons of 2000 and 2001 (Table III). Chemical analyses of the waters show that S5 samples are consistently more concentrated in all measured constituents (Table III). Higher dissolved solids in S5 runoff may indicate (1) more soil/water interaction along the flow path, (2) fertilizer-enhanced decomposition of soil organic matter, (3) more physical disturbance of the site related to drainage improvements or land use, and/or (4) a greater source of additional solutes such as manure or fertilizer. Quartz sand is an unlikely source of dissolved solids, but other natural sources include rainfall, dryfall, and organic matter. The acidic pH of these runoff waters is the result of reaction with atmospheric and soil CO2, and with soil organic acids. Acidity persists because of the low neutralizing capacity of these quartz-rich, sandy soils. Slightly less-acidic runoff from S5 compared to W4 may result from the selective, though infrequent (5-10 year interval), application of lime to improved summer pastures (P. Bohlen, written communication, 2003).

The delta34S of sulfate in S5 runoff ranges from approximately 19 to 32parts per thousand symbol (average of 25parts per thousand symbol), which is similar to values of approximately 18 to 24parts per thousand symbol (average of 20parts per thousand symbol) in the W4 runoff. Major addition of isotopically-lighter (delta34S = 3.5parts per thousand symbol) ammonium sulfate to S5 should produce a lower delta34S in runoff from S5 when compared to W4, but this is clearly not observed. In view of this inconsistency, and considering the previously discussed inconsistency in S isotope data for the uppermost S5 soils, the S isotope data do not indicate significant amounts of ammonium sulfate-derived sulfate in S5 runoff or S5 soil. In contrast to U, the detection of a small component of fertilizer-derived sulfate in soil or runoff is made more difficult by the relatively large inventory of soluble sulfate of indeterminate origin in the local environment.

The upper reach of Fisheating Creek, located approximately 23 km west of the study site (site FC-3, Figure 6), receives contributions of runoff from the predominantly undeveloped lands and pasturelands that lie within its drainage basin. The upper reach was sampled 3 times during 1999-2003 and dissolved sulfate had an average isotopic composition and standard deviation of 25 ± 3.9parts per thousand symbol (B. Orem, U.S. Geological Survey, written communication, 2005). A value of 25parts per thousand symbol is identical to average values in the runoff samples from S5. Any addition of ammonium sulfate-derived sulfate (delta34S = 3.5parts per thousand symbol) to S5 runoff is apparently too small to produce a "mixed" isotopic composition that is low compared to values observed in Fisheating Creek.

Map showing locations of surface water samples collected north and west of Lake Okeechobee and described in Table 4
Figure 6. Map showing locations of surface water samples collected north and west of Lake Okeechobee and described in Table IV. Abbreviations: KR, Kissimmee River; TC, Taylor Creek; FC, Fisheating Creek; C59, Canal; HPC, Harney Pond Canal; IPC, Indian Prairie Canal; C41A, Canal; NS, Nubbin Slough; MAERC, MacArthur Agro-Ecology Research Center. [larger image]

Uranium concentration in the runoff samples is typically less than 0.1 ppb (Table III) which is low compared to a mean value of 0.21 ± 0.28 ppb for 40 surface water samples collected at varying times from canals and streams of the surrounding area (Table IV, Figure 6). Accumulation of U in the organic-bearing horizons of the S5 soil profile (Table I) indicates that active sorption of U under ambient conditions may limit the amount of dissolved U in local waters, and particularly surface runoff. Uranium derived from mineral weathering is limited because of the low U concentrations in quartz and its low solubility. Wind transport of U-rich phosphate rock particles from areas outside of the watershed where phosphate rock is exposed is not likely to contribute dissolved or extracted U, because phosphate rock is also highly insoluble. Despite its observed low leachability, U in S5 runoff is consistently higher in concentration than in W4 runoff, paralleling the behavior of other dissolved constituents (Table III), and of soluble reactive phosphate (SRP) in runoff from these pastures (Capece et al., 2006).

The 234U/238U activity ratio in 4 samples of runoff from S5 (1.025 to 1.051; Table III) is low compared to the runoff from W4 (1.066, 1.109; Table III) and to the mean value of 1.064 ± 0.065 for 37 surface water samples from the area (Table IV). The average value for S5 runoff (1.037) is closer to the value of 1.020 for phosphate fertilizer. A contribution of fertilizer-derived U in runoff is likely and not surprising, considering the U isotopic evidence for fertilizer-derived U in shallow layers of S5 soil (see above).

TABLE III
Composition of runoff water from fertilized (S5) and unfertilized (W4) pastures
Sample pH
(lab.)
Specific Cond.
(µS/cm)
PO43-
(ppm)
NH4+
(ppm)
NO3-
(ppm)
Cl-
(ppm)
F-
(ppm)
Br-
(ppm)
HCO3-
(ppm)
SO42-
(ppm)
34S of Sulfate
(per mil)
Uranium
(ppb)
234U/238U
Activity Ratio
Date: 9/18/00
S5 5.20 225 n.d. n.d. 0.47 n.d. 0.70 0.57 10 40.7 32.17 0.065 1.025
W4 4.50 150 n.d. n.d. 2.08 11.6 0.54 0.17 0 32.4 24.13 0.022 n.d.
Date: 7/16/01
S5 5.70 310 n.d. n.d. n.d. n.d. n.d. n.d. 40 77.8 19.22 0.066 1.036
W4 4.40 140 n.d. n.d. n.d. n.d. n.d. n.d. 0 32.2 18.82 0.030 1.066
Date: 9/18/01
S5 5.60 370 0.78 0.16 0.16 51.2 0.48 0.01 35 55.4 26.56 0.091 1.036
W4 4.40 50 0.12 0.02 0.12 4.65 0.17 < 0.01 0 3.62 17.86 0.021 1.109
Date: 11/13/01
S5 6.20 620 0.38 0.03 0.47 71.4 0.25 0.30 68 81.3 22.43 0.087 1.051
W4 4.30 180 0.16 0.01 0.08 22.8 0.08 0.14 0 24.4 18.84 < 0.001 n.d.
n.d.: not determined


TABLE IV
Dissolved U concentration and U isotope composition of surface waters collected north and west of Lake Okeechobee
Sample location Sample site (Figure 6) Date sampled U (ppb) 234U/238U activity ratio (AR)
Upper Taylor Ck. at Hwy. 68 TC-3 5/00 0.03 1.092
8/00 0.03 1.070
Middle Taylor Ck. at Hwy. 441 TC-2 5/00 0.20 1.109
8/00 0.13 1.128
Taylor Ck. above fertilizer plant TC-4 8/00 0.72 0.918
Lower Taylor Ck. at Hwy. 70 TC-1 5/00 1.54 0.951
8/00 0.80 0.952
Taylor Ck. at Lake Okeechobee TC-5 7/98 0.28 n.d.
5/99 0.43 0.965
Upper C59 Canal at Hwy. 441 C59-3 5/00 0.09 1.151
8/01 0.34 0.968
8/00 0.29 0.957
C-59 Canal at Hwy. 710 C59-2 8/00 0.12 1.100
C-59 Canal at Lake Okeechobee C59-1 8/00 0.09 1.117
Nubbin Slough at Hwy. 710 NS-1 5/00 0.39 1.074
8/00 0.23 1.121
Kissimmee R. at Ft. Basinger, FL KR-3 5/00 0.22 1.039
8/00 0.28 1.146
Kissimmee R. at Hwy. 70 KR-1 8/00 0.21 1.135
8/01 0.11 1.086
Kissimmee R. at Lake Okeechobee KR-2 7/97 0.10 1.005
8/97 0.03 1.026
7/98 0.15 n.d.
5/99 0.05 1.018
5/00 0.37 0.984
8/00 0.22 1.078
Upper Fisheating Ck. at Hwy. 70 FC-3 5/00 0.07 1.086
8/00 0.10 1.074
Upper Fisheating Ck. at Venus, FL FC-4 5/00 0.12 1.130
8/00 0.13 1.153
Middle Fisheating Ck. at Palmdale, FL FC-2 5/00 0.02 1.112
8/00 0.05 1.147
Fisheating Ck. at Lake Okeechobee FC-1 8/97 0.03 1.096
7/98 0.06 n.d.
5/99 0.07 1.056
5/00 0.10 1.063
8/00 0.06 1.099
Harney Pond Canal HPC 8/01 0.10 1.064
Indian Prairie Canal IPC 8/01 0.11 1.051
C41A Canal C41A 8/01 0.07 1.062
n.d.: not determined.

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