Zinc and Cadmium in Soils and Plants Near Electrical Transmission (Hydro) Towers Roger Jones" and Magdalena S. E. Burgess Department of Biology, Trent University, Peterborough, Ontario, Canada KQJ 7B8
Concentrations of Zn and Cd were determined in planta and soils around and beneath corroding galvanized electrical transmission (hydro) towers located in different habitats near Peterborough, Ontario. High concentrations of Zn occurred in a well-drained, uncultivated drumlin soil around and beneath a tower. The pattern of contamination indicated spread of Zn by runoff and by wind-driven spray and water droplets from the tower. Plants growing close to this tower accumulated Zn but apparently were not adversely affected, probably because of low Zn availability in the soil. In a cultivated field, the distribution of Zn around the base of a tower seemed to be affected by soil cultivation and by crop removal. Concentrations of Cd were not elevated in plants or soils beneath or near towers in this study.
Introduction Approximately 50% of the Zn produced in the world is used as a corrosion-resisant coating on other metals, particularly on iron and steel (1). Metals are protected because the Zn coating is preferentially corroded (sacrificial protection), but once this coating is breached, then the metal beneath begins to corrode, rusting in the case of galvanized iron or steel. In many countries, the towers (hydro towers or pylons) supporting high-voltage electrical transmission lines are constructed with galvanized steel, which in Ontario has to have a Zn coating at least 0.076 mm in thickness (2)to provide protection against rusting. A lattice tower supporting a 230-kV line may be covered with 23-133 kg of Zn, depending upon the structural configuration of the tower (2). The rate of corrosion of Zn from galvanized steel depends on factors such as temperature, acidity of precipitation, and duration of wetness (3). In an highly industrialized area such as Hamilton, Ontario, atmospheric pollution causes approximately two-thirds of the protective Zn coating to be lost from hydro towers in 5-10 years (2). Once the Zn coating has been reduced to 0.025-mm thickness, then a tower takes on a brownish appearance because of rusting. Zinc lost by sacrificial protection enters the habitat in which the tower is located so that galvanized steel hydro towers present innumerable point sources of Zn contamination in a wide variety of environments and habitats. This study determined concentrations of Zn and Cd in planta and soils around and under 30 m tall,galvanized steel, lattice hydro towers in different habitats near Peterborough, Ontario, since excessive concentrations of Zn in soils can be toxic to plants (4-6). The towers were constructed in 1950 to support a 230-kV line and are beginning to rust. Analyses 0013-936X/84/0918-0731$01.50/0
for Cd were included because it may be present in concentrations up to 0.2% in the Zn used in hot dip galvanized (7) and because it is particularly toxic to organisms (8).
Materials and Methods The distribution of Zn and Cd in plants and soils was studied most extensively around a hydro tower situated on a slope of 2-5% on a well-drained drumlin. The soil at this site is an Otonabee loam (an Orthic Melanic Brunisol) derived from calcareous, moderately stony loam till (9). Soil was collected from the 0-5-cm interval along transects A, B, C, and D shown in Figure 1to determine if elevated concentration of Cd and Zn occurred in the surface soil around the tower. Once this was established for Zn, then contiguous, 5-cm-thick segments of soil were collected to a depth of 35 cm from pits along transects E and F (Figure 1)to determine the distribution of Zn with depth in the soil profile. Samples were also collected from the 0-5-cm depth interval 1 and 2 m from the four sides of the concrete block supporting the southeast leg of the tower (Figure 1). Vegetative and flowing shoots of yarrow (Achillea millefolium), St.-Johnswort (Hypericum perforatum), and white sweet clover (Melilotus alba) were collected along transects E and F (Figure 1). A second tower was in a poorly drained, interdrumlin swale of slope 0-5% where the soil is a Foxboro silt (an Orthic Humic Gleysol) derived from calcareous fine sandy and silty deltaic deposits (9). Samples were collected from the 0-10-cm depth interval in the organic soil along a transect (Table I) set 90° to the direction that water flowed through the swale during spring snow melt. Rhizomes of cattail (Typha Eatifolia) were dug up along the same transect. A third tower, also on Otonabee loam, was in a cultivated field (slope 2-5%) in which alfalfa (Medicago sativa) and red clover (Trifolium pratense) were growing. Soil from the 0-10-cm depth interval, so as to be from the plough layer, was collected along two transects set at 90° to one another and originating at the same concrete supporting block. Red clover plants were collected along these transects. At another site, located 120 km east of Peterborough on the Canadian Shield, shoots of the grass Deschampsia flexuosa were collected. Isolated turfs of this grass were the only plants growing in a channel -1 m wide and -4 m long draining from one corner of the tower. For comparative purposes, shoots of D. flexuosa were collected 13 m upslope from this tower. Soil samples were air-dried and sieved through a 5-mm
0 1984 Amerlcan Chemical Soclety
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Table I. Distribution of Total Zn (jtglg of Oven-Dried Soil) with Distance (m) from Hydro Towers Situated on a Drumlin, in an Interdrumlin Swale, and in a Cultivated Field near Peterborough, Ontario
drumlin"
distance, m, from hydro tower 5 10
soil touching concrete base
1
2
11 480 f 2966
10431 f 7511
869 f 339
1
362 f 240 distance, m 4
2
160 f 38
25
50
70 f 4
54 f 16
16
8
32
interdrumlin swaleb 2907 f 400 2482 f 76 1272 f 46 358 f 28 146 f 8 109 f 8 cultivated fieldc east transect 256 170 107 86 88 south transect 258 222 101 73 "Soil collected from 0-5-cm depth interval (mean and SD;n = 4). bSoil collected from 0-10-cm depth interval (mean and S D n = 3). cSoil collected from 0-10-cm depth interval (n = 1). Table 11. Concentrations of Zn and Cd ( p g / g ) in Plants Collected near Hydro Towers Zn, bg/g, at these distances from hydro tower, m
1 Melilotus alba, shoots 158 Achillea millefolium, shoots Hypericum perforatum, shoots Typha latifoliab rhizomes (n = 10) roots (n = 8) Trifolium pratenseb shoots (n = 6) roots ( n = 5) "n = 3. bData combined as mean and SD.
J
2 261 226
\
Figure 1. Positions around the drumlin hydro tower of soil sampling sites (A)along transects A, B, C, and D and soil pits (0)along transects E (north transect) and F (south transect).
polyethylene screen prior to grinding with an agate mortar and pestle and oven-drying a t 110 "C for 48 h. Total Zn and Cd were determined by atomic absorption spectrophotometry (AAS) after acid digestion (IO). Analysis of five replicates of a reference sediment sample (SRM 1645, obtained from the National Bureau of Standards, Washington, DC) gave concentrations that were in the certified ranges of 1720 f 169 pg/g for Zn and 10.2 f 1.5 pg/g for Cd. The pH of fresh soil was determined by the water saturation method ( I 1) using a glass-calomel combination electrode. Ground, oven-dried soil was muffled at 450 OC for 1 h to determine loss on ignition. Plants were washed with tap water and deionized water, then oven-dried at 110 O C for 24 h, ground in a Wiley mill, and digested in an acid mixture as described by Jones (10). Cadmium and Zn in the digest were determined by AAS. Results and Discussion Corrosion of galvanized hydro towers leads to considerable increases in the concentrations of Zn in soils beneath 732 Envlron. Sci. Technol., Vol. 18, No. 10, 1984
5 160 f 100" 176 131 f 87O 72 f 18 297 f 157 65 f 41 35 f 17
12 17 51 55
25 10 35 f 1" 50 f 34"
Cd, r g l g 1.5 f 0.6b (n = 7) 2.1 f 0.4b ( n = 6 ) 2.4 & 0.4* (n = 5) 1.6 f 1.3 0.24 f 0.17
