Delaware, 1975. ( 2 ) Southern California Coastal Water Research Project, Annual Report, 1500 East Imperial Highway, El Segundo, Calif. 90246, 1977. ( 3 ) Evans, H., Milton, C., Caho, E., Adler, I., Mead, Co., Ingram, B., Berner, R., U.S.G.S. Professional Paper 475-D, 1964, pp D64D69. (4) Berner, R., J G e o l , 72,293 (1964). ( 5 ) Goldbaber, M. B., Kaplan, I. R., in “The Sea”, Vol. 5, Goldberg, E., Ed., Wiley-Interscience, New York, 1974, p p 569-655. (6) Roberts, W., Walker, A,, Buchanan, A., Miner. Deposita, 4, 18 (1969). (7) Berner, R., Am. J . S e i , 268, 1 (1970). (8) Sweeney, R., Kaplan, I. R., Econ Ceol., 68,618 (1973). (9) Stumm, W., Morgan, J. J., “Aquatic Chemistry”, U’iley-Interscience, New York, 1970, p 102.
(10) Pankow, J.F., Ph.D. Thesis, California Institute of Technology, Pasadena, Calif., 1978. (11) Smith, R., Martell, A,, “Critical Stability Constants”, Vol. 4, Plenum Press, New York, 1976. (12) “The Handbook of Chemistry and Physics”, 51st ed., The Chemical Rubber Co., Cleveland, Ohio, 1970, p D122. (13) Berner, Am. J . Sci., 265,773 (1967). (14) Berner, R., Science, 137,669 (1962). (15) Parsons, R., “Handbook of Electrochemical Data”, Butterworths, London, 1959, p 79. (16) Haussuhl, S., Muller, W., Krist. Tech., 7, 533 (1972). (17) Barton, A., Wilde, N., Trans. Faraday Sot., 67,3590 (1971). (18) Locker, L. D., deBruyn, P. L., J . Electrochem. Soc., 116,1659 (1969). Receiued for reuieu: March 21, 1979. Accepted J u n e 22, 1979.
Migration of Applied Lead in a Field Soil Frank J. Stevenson” and Louis F. Welch Department of Agronomy, University of Illinois, Urbana, 111. 61801
Field plots amended with variable amounts of P b in the spring of 1969 were resampled after 6.5 years to ascertain the extent to which the applied P b had moved from the application zone. The soil, Drummer silty clay loam, was shown in laboratory studies to effectively bind P b in nonexchangeable forms over the range a t which Pb had been applied (none t o 3200 kg/ha). Downward movement of P b occurred (to 90 cm) and was attributed to leaching as soluble chelate complexes with organic matter, transfer of soil by earthworms and other faunal organisms, translocation in plant roots, or a combination of these. Significant horizontal movement also occurred, due primarily to physical transfer of soil through tillage operations. Evidence was obtained for migration of trace amounts of P b as wind-blown soil or plant particles. H
Considerable attention has been given to the distribution of P b in soil along busy highways and in the vicinity of leadproducing industries. In general, P b levels decrease with distance from the highway and changes in traffic volume (1-5). Soils near I’b smelters are particularly high in Pb, often exceeding 1000 pg/g in the top layer (6, 7). Lead has been added to soil as the insecticide lead arsenate and as an impurity in fertilizers (8). Because P b is tightly bound by most soils, and since most investigations have shown that P b is concentrated in the top few centimeters, the assumption has been made that P b is immobile and that little if any movement will occur beyond the deposition zone. This aspect of the environmental impact of soil contamination with P b is the subject of the present communication. Some experimental P b plots located a t the University of Illinois offer a unique opportunity to study the migration of P b in contaminated soil. These plots were established on a typical prairie grassland soil in the spring of 1969, the primary objective being to determine the effect of the applied P b on emergence, growth, and P b content of corn, Zea mays L. ( 9 ) . Six and one-half years later, in the fall of 1975, the plots were resampled to determine the extent of P b movement from the application zone. Experimental
Field Plots. The experimental design has been given by Baumhardt and Welch (9). Lead acetate was applied in the spring of 1969 to a Drummer silty clay loam soil, a Typic Ha0013-936X/79/0913-1255$01 .OO/O @ 1979 American Chemical Society
plaquoll, a t rates of 0, 50, 100, 200, 400, 800, 1600, and 3200 kg/ha. Incorporation was accomplished by disking. Each treatment was replicated three times. Prior to the P b additions, the area was fertilized with N, P, and K a t rates of 340, 70, and 130 kg/ha, respectively. The plots, which measured 6.1 m square, were used in 1969 and 1970 to determine P b uptake by corn, after which they were abandoned and the area used for other experimental purposes. From 1971 on the area was planted to soybeans, Glycine m a x L. The original plots were arranged in two parallel strips running in a north-south direction, each strip being 6.1 m wide. In 1973 and 1974, ground corn cobs a t the rate of 22.4 metric tons/ha were applied to the western strip and fertilizer N at rates up to 560 kg/ha were applied across all plots. During these years, the plots were used for an N2 fixation study by soybeans. The plots were sampled in the fall of 1975. The Drummer soil has a cation-exchange capacity of 30.3 mequiv/100 g and a p H of 5.9 & 0.1. Soil composition (A horizon) was 9.5% sand, 60.2% silt, and 30.3% clay. Sorption capacity for P b (0-15 cm layer) exceeded 20000 pg/g (10). Carbon contents of the soil a t the time of sampling ranged from 3.64 to 3.89% for the plots amended with corn cobs to from 2.94 to 3.27% for those receiving no cobs. Soil Sampling. A total of 120 surface samples (0-15 cm) were taken from within the experimental site, five from each plot. The plots were quartered and samples were taken from the midpoints of both the main plot and the four subsections. Additional samples were taken from soil surrounding the experimental area. Each sample represented a composite of six separate cores. Depth samples were taken from nine locations within the experimental area, namely, from the midpoints of the 0,800, and 1600 kg/ha plots of reps. 1and 3 and from all three 3200 kg/ha plots. A pit 45 cm square was prepared with a stainless steel spade to a depth of 45 cm, from which soil was taken from the exposed surface a t intervals of 0-15,15-30, and 30-45 cm. Samples between 45 and 90 cm, also taken a t 15-cm intervals, were secured with a stainless steel hand auger, special precautions being taken to prevent cross-contamination of soil a t succeeding depths. Depth samples from the grass border area (0-15,5-10, and 10-15 cm intervals) were taken with a stainless steel hand auger. The surface “dust” was also sampled by gently removing the top 1-2 mm of dry soil from between the sward with a small hand brush. VOlUme 13, Number 10, October 1979 1255
Table I. Mean Pb Contents of Depth Samples from Experimental Plotsa applied Pb, kg/ha
depth interval,
600
0
cm
rep. 1
rep. 3
rep. 1
0-15 15-30 30-45 45-60 60-75 75-90
40.0 17.3 18.0 18.3 18.3 16.7
34.3 16.3 16.7 18.7 16.7 18.0
250' 36.0'. 22.0 19.7 19.3 18.0
1600 rep. 3
300"" 28.0. * 23.7" 22.7' 20.3' 18.0
3200
rep. 1
rep. 3
rep. 1
370' 51.0" 20.7 19.3 19.0 18.0
280" 41.0" 19.7 19.0 18.0 17.0
660*' 430" * 30.7' 22.0' 21.0' 21.3'
rep. 3
640" 210"' 25.0' 23.7' 22.3' 19.7"
Each item represents an average of three determinations. Values indicated by an asterisk (*) are significantly higher than for control soil at the same depth ( p = 0.05). A double asterisk (") indicates significance at p = 0.01. *Results for the 3200 kg of Pb/ha plot of rep. 2 were 610", 57.5"* , 23.3', 21.7', 20.3', and 20.0', respectively.
Laboratory Methods. All soils were thoroughly dried a t 60 "C and ground to pass a 100-mesh screen, using an agate mortar. Lead determinations were carried out in triplicate by the analytical laboratory of the Institute for Environmental Studies a t the University of Illinois, as follows: a 2-g sample of soil was dry-ashed a t 490 "C to remove organic matter, following which the residue was digested with 10 mL of 6 N HCl a t 85 "C to bring the P b into solution. This extraction procedure was found to give 96 to 97% recovery of the total P b in the Drummer soil, as determined by analysis of select samples after digestion of the dry-ashed material with HFHC1 to destroy silicate minerals. Analysis of P b was by atomic absorption spectroscopy a t a wavelength of 283.3 nm. Precision and accuracy were typically 1-3%, but improvements were obtained by taking multiple readings on samples and standards. The standard error of the mean, s?, for P b determinations in the 17-32 pg/g range (triplicate analyses) was found to be 0.34. Specific adsorption, defined as adsorption of a cation in the presence of an excess of a second exchangeable cation ( 1 1 ) ,was determined by shaking a 5-g sample of soil for 24 h a t 23 "C with 50 mL of a 1m KCl solution containing a known amount of P b (0-80 pg/mL). Precipitation of P b was extremely unlikely a t the pH of the Drummer soil (pH 5.9). The soil was separated from the solution with a high-speed centrifuge, following which the supernatant was analyzed for P b by atomic absorption. The difference between the initial and equilibrium concentration of P b was used to compute the amount of P b adsorbed by the soil, from which specific adsorption was calculated from the Langmuir adsorption equation:
c _ - 1 + -c --
x/m a b a where C is the equilibrium concentration in the solution (pg/mL), x/m is the amount adsorbed per unit weight of soil (pg/g), a is the adsorption maximum (pg/g), and b is the Langmuir "bonding term". The objective of the study was to determine whether specific adsorption was the dominant process responsible for retention of the applied Pb, and no attempt was made to maintain pH a t a constant value or to correct for small amounts of nonspecific adsorption a t the higher P b levels. Equilibrium concentrations over the range of P b applied were low (
