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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
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 34S of sulfate in S5 runoff ranges from approximately 19 to 32 (average of 25), which is similar to values of approximately 18 to 24 (average of 20) in the W4 runoff. Major addition of isotopically-lighter (34S = 3.5) ammonium sulfate to S5 should produce a lower 34S 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 ± 3.9 (B. Orem, U.S. Geological Survey, written communication, 2005). A value of 25 is identical to average values in the runoff samples from S5. Any addition of ammonium sulfate-derived sulfate (34S = 3.5) to S5 runoff is apparently too small to produce a "mixed" isotopic composition that is low compared to values observed in Fisheating Creek.
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).
U.S. Department of the Interior, U.S. Geological Survey
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