Surface runoff from rainfall event is an important indicator of metal mobility in soil, which may enhance non-point source contamination of soil. This study is designed to assess the mobility of soil-bound lead through simulated rainfall runoff experiment and its spatial distribution within the vicinity of a berm at a major military shooting range. Contamination was more significant at the impact area of berm, indicating threefold increase in Pb (17,500 ± 3811 μg/g) within a space of ten years. However, the non-impact area (459 ± 147 μg/g) was less contaminated. Other metals (Cu, Cd, Cr, Ni and Zn) analyzed were about background levels except for Cu at impact area. The enrichment ratio of Pb in runoff sediments was mostly high for the 0.43 mm sediment fractions independent of rainfall condition. Principal component analysis (PCA) biplot showed strong correlation between spatial distributions of metals around the vicinity of the berm (farmlands behind the berm) with concentrations on the impact berm soil. Surface runoff simulated on impact area soil had high concentrations of Pb (40.4 - 65.6 μg/mL) which could further lead to enrichment of soil-Pb levels within the vicinity of the berm. Decontamination measure is therefore required to minimize extensive contamination of surrounding soils of the impact berm due to rainfall runoff events.
Soil contamination with toxic trace metals, is a major environmental problem largely attributed to industrial activities and improper waste disposals. However, activities at shooting ranges are gradually accounting for a significant source of this toxic trace metal contamination of soils [
One important range environment that may elicit research interest in this regard is the Ibadan military shooting range. The range has been active for well over 60 years, but over the last ten years there has been a gradual cessation of shooting activity, bullet fragments and spent short abound around the berm soils. The sizes and conditions of these fragmented pellets which has not been established will to an-extend determine its level of migration during rainfall surface run-offs. The range is sited within the tropical humid climate with an average rainfall intensity of 100 to 300 mm/hr. Depending on soil chemistry; these climatic conditions could enhance mobility of fragmented lead shot from impact berm to surrounding soils mainly used for agricultural purpose, since there are no well-defined drainages within the range. Previous assessment studies carried out in 2004 on the range showed lead levels as high as 5680 ± 2700 µg/g on the impact berm [
This study is therefore designed to assess the mobility of soil-bound lead through a simulated rainfall runoff experiment on contaminated soils from the berm. The berm was classified into impact (front slope) and non- impact (back slope) areas. Secondly, to assess spatial distribution of lead at varying distances away from impact and non-impact area of berm through the determination of Pb concentrations in soils as well as properties such as pH, particles size distribution and organic carbon. Finally, to assess the current Pb levels at the berm in respect to previous study. An experimental system was setup to simulate the runoff pollution process on contaminated soils from the berm. Runoff simulations were performed on different rainfall intensities. Enrichment ratio and time series data of lead levels in different graded particle sizes of discharged sediment at different intensities were obtained and analyzed.
The sampling area consists of a 100 m long, 18 m wide and 20 m high berm. In front of the berm is a platform for target placement along with the 100 to 500 m firing lines. Behind the berm are scattered plots of farm lands (
obtained about 2 km away from the range. Sampling was carried out on dry days with no significant rainfall events. Sampling was done using a stainless soil scoop and mixed and subsequently stored in labeled plastic bags and transported to the laboratory for further processing and analysis.
The soil samples were air dried in the laboratory and large pieces of debris were removed. Thereafter, core polluted soil samples were well aggregated and sieved through 4mm sieve to simulate actual field condition while other soil samples were sieved using 2 mm sieve. The electrometric method (Jenway 3510 pH meter) was used for soil pH determination. Mechanical properties and organic carbon were determined using the hydrometer method [
The setups compose mainly of a soil box and a rainfall simulator as described by Hignett [
A rain module, peristaltic pump and a flow meter was used in the construction of the rainfall simulator. When the simulator was in operation, the pump delivered water (pH 5.32) with closely similar characteristic to rain water from a tank to the rain module. The flow meter was used to monitor and control the delivery rate of water (i.e. rainfall intensity) through the needles on the rain module. The rain gauge was then placed in the soil box to record rainfall intensity and the output was manually used to adjust the delivery rate of the pump to the desired rainfall intensities. Once the steadiness and spatial uniformity of the rainfall intensity was achieved, the plastic hood was removed and the experiment started. The experiment was run for three different rainfall intensities to simulate conditions for light rainfall-80 mm/hr, moderate rainfall-150 mm/hr and heavy rainfall-200 mm/hr (obtainable in tropical climatic regions). For each of the rainfall intensity, five different runoff samples were intermittently collected at roughly equal time intervals.
The runoff samples for each experiment were collected using a 5 L clear plastic container. The collection started immediately after the runoff was observed. The time for collecting one sample, ranged between 2 to 5 minutes depending on the rainfall intensity. The sediments from each runoff samples were partitioned into four particulate sizes by filtering through a 0.425, 0.250, 0.180 and 0.150 mm sieves. The filtrate was stored in clean plastic containers for subsequent determination of heavy metals while the sediments were air dry in the laboratory and stored for heavy metal analysis. A 100 ml portions of the filtrates was concentrated to 25 ml using 2 ml concentrated HNO3 while 1.0 g of each sized sediment samples was extracted using 50.0 ml 2 M HNO3 [
Strict quality control measures were followed in all the analysis. The reagents used were of analytical grade. To check the reproducibility of the experiment; 80 mm/hr and 150 mm/hr rainfall intensities was duplicated for one polluted soil sample. The repeated experiment generated similar results but only one was reported (
Descriptions/Rainfall condition (mm/hr) | 80 | 80 | 150 | 150 |
---|---|---|---|---|
Total number of runoff collected | 5 | 5 | 5 | 5 |
Total volume of runoff collected (L) | 9.7 | 9.5 | 12.3 | 12.6 |
Time intervals (min) | 2 - 2.7 | 2 - 2.2 | 3 - 3.6 | 3 - 3.4 |
Total sediment mass (g) | 32.6 | 31.4 | 51.3 | 51.1 |
Number of sediment of 0.43 mm size fraction separated | 5 | 5 | 5 | 5 |
Number of sediment of 0.25 mm size fraction separated | 5 | 5 | 5 | 5 |
Number of sediment of 0.18 mm size fraction separated | 4 | 4 | 5 | 5 |
Number of sediment of 0.15 mm size fraction separated | 5 | 5 | 5 | 5 |
Average Pb concentration (µg/g) in 0.43 mm fractions only | 10,203 ± 7189 | 10,378 ± 8572 | 16,461 ± 4476 | 16,273 ± 4830 |
The Paleontological Statistic software (PAST version 1.38) and Microsoft Excel (2007 version) were used for statistical evaluation of data. Principle component analysis (PCA) and analysis of variance were performed on the data set to assess metal variability, categorization, and the relationship between sample data set at varying locations. Furthermore, the enrichment ratio (ER) and accumulation factors were derived. Enrichment ratio is the ratio between the average concentration of the metal in the discharge sediment and its concentration in the original soil surface layer.
where Co is concentration of each metals in the original soil sample, CS(Ti) is the average concentration of metals in the sediment for the duration of the rainfall Ti.
The accumulation factor is defined as the ratio of the heavy metal concentration at different sample locations to the geometric mean of the control concentration of the corresponding metal.
The general physicochemical properties of soil (
Parameter | Impact area | Non impact area | Impact area 10 to 75 m | Non impact area 10 to 75 m |
---|---|---|---|---|
pH | 6.25 ± 0.25 | 6.43 ± 0.21 | 6.03 ± 0.16 | 5.96 ± 0.29 |
% Organic carbon | 2.94 ± 0.70 | 3.42 ± 0.41 | 3.23 ± 0.51 | 3.02 ± 0.30 |
% Organic matter | 5.08 ± 0.23 | 5.91 ± 0.03 | 5.58 ± 0.36 | 5.23 ± 0.42 |
% Sand | 66.8 ± 4.10 | 74.7 ± 0.91 | 71.8 ± 6.84 | 76.3 ± 6.00 |
% Clay | 14.1 ± 0.77 | 10.9 ± 1.41 | 13.0 ± 3.46 | 11.2 ± 4.03 |
% Silt | 19.0 ± 0.30 | 14.4 ± 0.51 | 15.2 ± 3.38 | 12.5 ± 3.25 |
Distances (m) | Direction | Pb | Cd | Cr | Cu | Ni | Zn |
---|---|---|---|---|---|---|---|
0 | *Front | 17,500 ± 3811 | 2.93 ± 0.69 | 30.8 ± 1.04 | 816 ± 293 | 5.50 ± 0.87 | 72.4 ± 20.2 |
10 | 370 ± 347 | 3.16 ± 1.38 | 7.83 ± 6.66 | 152 ± 164 | 5.50 ± 2.35 | 37.3 ± 20.6 | |
20 | 220 ± 191 | 2.01 ± 1.35 | 2.00 ± 3.08 | 416 ± 149 | 5.75 ± 4.13 | 104 ± 83.9 | |
30 | 63.8 ± 46.4 | 1.51 ± 0.63 | 18.9 ± 9.28 | 32.0 ± 8.94 | 6.88 ± 2.66 | 54.6 ± 15.6 | |
50 | 86.3 ± 40.4 | 1.10 ± 0.29 | 9.25 ± 8.55 | ND | 5.88 ± 1.89 | 60.6 ± 48.7 | |
75 | 405 ± 329 | 3.36 ± 1.67 | 23.8 ± 1.19 | 15.3 ± 15.9 | 7.13 ± 1.70 | 56.4 ± 30.6 | |
0 | **Back | 459 ± 147 | 2.38 ± 0.74 | 25.8 ± 5.3 | 31.8 ± 6.8 | 4.00 ± 0.71 | 31.2 ± 36.6 |
10 | 263 ± 125 | 1.93 ± 1.74 | 5.63 ± 3.84 | 7.50 ± 9.90 | 6.63 ± 3.71 | 47.8 ± 25.7 | |
20 | 249 ± 45 | 3.21 ± 2.05 | 7.25 ± 6.65 | ND | 8.75 ± 6.51 | 31.4 ± 19.1 | |
30 | 85.3 ± 33.1 | 3.26 ± 2.00 | 18 ± 11 | ND | 3.13 ± 0.75 | 23.2 ± 19.9 | |
50 | 103 ± 65 | 3.61 ± 1.61 | 11.5 ± 3.85 | ND | 8.00 ± 3.89 | 22.4 ± 9.57 | |
75 | 250 ± 22 | 2.41 ± 0.25 | 26.1 ± 5.7 | 165 ± 14 | 4.75 ± 2.22 | 22.3 ± 8.84 | |
CTR | 30 ± 8 | 7.85 ± 0.42 | 15.9 ± 4.91 | 8.25 ± 2.47 | 3.75 ± 0.35 | 21.7 ± 20.2 |
CTR-Control; *Front-Impact area of berm; **Back-Non-impact area of berm.
about that of the control. Analysis of variance (p = 0.05) showed a significant difference in Pb, Cu, Zn, Cr, Ni and Cd levels distribution within the 0 to 75 m sampling points for both the impact and non impact areas of the berm. This indicates the degree of variability of soil metal levels within the locations. From the principal component biplot (
Soil Pb levels of impact (17,500 ± 3811 µg/g) and non-impact (459 ± 147 µg/g)areas reflected in corresponding high levels of Pb in runoff sediment with respect to other metals (
Rainfall intensity (mm/hr) | Particle Sizes (mm) | Pb | Cd | Cr | Cu | Ni | Zn |
---|---|---|---|---|---|---|---|
F-80 | 0.43 | 10203 ± 7189 | 6.53 ± 2.60 | 68.4 ± 30.4 | 225 ± 189 | 12.3 ± 4.21 | 62.7 ± 25.1 |
0.25 | 6520 ± 2552 | 6.04 ± 2.83 | 23.3 ± 9.11 | 211 ± 127 | 5.01 ± 2.67 | 40.9 ± 26.6 | |
0.18 | 6119 ± 5969 | 12.0 ± 4.96 | 44.5 ± 36.9 | 308 ± 157 | 9.07 ± 3.13 | 66.8 ± 30.9 | |
0.15 | 4031 ± 2951 | 6.28 ± 2.42 | 23.1 ± 6.01 | 231 ± 82 | 7.75 ± 5.81 | 63.4 ± 31.7 | |
F-150 | 0.43 | 16461 ± 4476 | 7.99 ± 1.39 | 38.5 ± 27.9 | 279 ± 211 | 14.5 ± 7.66 | 63.4 ± 47.7 |
0.25 | 7504 ± 7504 | 9.43 ± 3.29 | 28.5 ± 13.0 | 287 ± 187 | 5.10 ± 2.07 | 42.6 ± 22.8 | |
0.18 | 3973 ± 2641 | 8.62 ± 3.15 | 21.1 ± 16.1 | 159 ± 122 | 7.64 ± 5.50 | 47.5 ± 27.70 | |
0.15 | 2658 ± 760 | 6.68 ± 2.21 | 13.9 ± 1.02 | 157 ± 83.9 | 3.78 ± 0.69 | 35.59 ± 2.62 | |
F-200 | 0.43 | 10039 ± 6840 | 5.35 ± 2.94 | 39.5 ± 34.2 | 216 ± 114 | 11.0 ± 7.59 | 28.7 ± 32.8 |
0.25 | 9444 ± 6236 | 5.78 ± 2.17 | 26.7 ± 5.01 | 251 ± 207 | 5.02 ± 3.86 | 29.7 ± 24.6 | |
0.18 | 6894 ± 6373 | 5.42 ± 1.84 | 43.2 ± 20.15 | 253 ± 167 | 11.4 ± 6.64 | 32.4 ± 20.5 | |
0.15 | 4664 ± 4584 | 8.46 ± 7.52 | 50.7 ± 31.1 | 440 ± 347 | 5.77 ± 1.80 | 119 ± 113 | |
B-80 | 0.43 | 152 ± 58.2 | 7.36 ± 4.70 | 46.3 ± 30.7 | 24.6 ± 20.5 | 6.51 ± .99 | 58.0 ± 48.5 |
0.25 | 127 ± 39.3 | 13.0 ± 5.71 | 42.0 ± 16.2 | 45.4 ± 48.2 | 9.96 ± 8.34 | 38.3 ± 29.9 | |
0.18 | 90.3 ± 43.8 | 9.20 ± 3.64 | 36.7 ± 17.2 | 12.5 ± 22.7 | ND | 80.9 ± 18.4 | |
0.15 | 69.7 ± 30.3 | 26.0 ± 27.4 | 54. ± 37.18 | ND | ND | 41.3 ± 28.4 | |
B-150 | 0.43 | 207 ± 78.4 | 7.61 ± 1.40 | 2.90 ± 1.08 | 20.3 ± 12.9 | 7.40 ± 2.99 | 17.9 ± 13.3 |
0.25 | 145 ± 34.3 | 9.42 ± 2.96 | 11.9 ± 7.33 | 19.2 ± 8.73 | 8.96 ± 6.11 | 28.7 ± 24.3 | |
0.18 | 136 ± 40.0 | 6.35 ± 2.29 | 2.77 ± 1.42 | 43.4 ± 27.8 | 17.8 ± 22.8 | 47.3 ± 34.3 | |
0.15 | 111 ± 39.8 | 18.2 ± 56 | 9.27 ± 5.07 | ND | 9.55 ± 9.09 | 84.8 ± 28.2 | |
B-200 | 0.43 | 269 ± 82.4 | 5.57 ± 1.79 | 64.7 ± 16.6 | 125 ± 90 | 7.0 ± 3.7 | 48.3 ± 23.4 |
0.25 | 189 ± 48.9 | 4.52 ± 1.90 | 16.2 ± 8.74 | 287 ± 257 | 13.0 ± 18.1 | 87.1 ± 51.7 | |
0.18 | 182 ± 61.8 | 7.39 ± 4.46 | 35.7 ± 15.5 | 58.9 ± 53.5 | 9.17 ± 6.74 | 76.3 ± 69.9 | |
0.15 | 153 ± 66.4 | 11.8 ± 1.33 | 68.0 ± 72.2 | 103 ± 73 | 19.1 ± 14.6 | I23 ± 179 |
F-Impact area of berm, B-Non impact area of berm.
vicinity of the berm. These particulates could be carried long distances from source by the heavy rainfall runoff, which is evident in the high Pb concentration in the 0.43mm fractions, thereby affecting uncontaminated soil ecosystem. These could further explain the high levels of Pb recorded in surrounding farmland soils (10 - 75 m) behind the impact berm (
Analysis of variance (p = 0.05) showed no significant differences in metal distribution across the sediment fractions of the three rainfall intensities at the impact area. However, for the non-impact area, there was a significant difference in the distribution of Pb, Cu and Cr for the various sediment fractions. This further confirms that Pb Cu and Cr are anthropogenically derived through spent shot deposition in the soils.
Surface rainfall runoffs obtained from the experiment similarly showed higher levels of Pb compared to other metals (
Berm | Rainfall intensity (mm/hr) | Pb | Cd | Cr | Cu | Ni | Zn |
---|---|---|---|---|---|---|---|
Front | F-80 | 40.4 ± 20.0 | 0.01 ± 0.01 | ND | 2.41 ± 1.31 | 0.02 ± 0.02 | 0.38 ± 0.12 |
F-150 | 46.9 ± 24.3 | 0.02 ± 0.01 | ND | 1.68 ± 0.48 | 0.04 ± 0.01 | 0.62 ± 0.38 | |
F-200 | 65.6 ± 32.1 | 0.02 ± 0.02 | ND | 2.25 ± 1.75 | 0.04 ± 0.01 | 0.76 ± 0.37 | |
Back | B-80 | 2.24 ± 0.61 | 0.04 ± 0.02 | 0.01 ± 0.01 | 0.02 ± 0.01 | 0.03 ± 0.01 | 0.30 ± 0.11 |
B-150 | 2.69 ± 1.51 | 0.13 ± 0.14 | ND | 0.26 ± 0.12 | 0.03 ± 0.01 | 0.51 ± 0.37 | |
B-200 | 3.28 ± 2.05 | 0.04 ± 0.02 | 0.07 ± 0.04 | 0.14 ± 0.14 | 0.02 ± 0.01 | 0.32 ± 0.33 |
Elevated concentrations of Pb were found in soils at the impact area of the berm. This translated to higher enrichment ratio of Pb in sediment, particularly the 0.43 mm fractions obtained from the rainfall runoff experiment. The presence of metallic bullet fragment significantly explains the variations in enrichment ratio for the different fractions. The enrichment ratios of Pb in sediment increased with higher rainfall intensity but showed no appreciable increase for prolong rainfall duration. Although reducing pH condition, organic matter and silt content of the soils may limit migration of Pb, increasing rainfall intensity characterized by the tropical humid climate of the study site will invariably lead to migration of corroded metallic Pb particles. This clearly accounts for the high Pb levels recorded in surrounding farmlands few meters away from the non-impact area of the berm. Surface runoff from the impact area was obviously contaminated with levels of Pb far exceeding TCLP limits of 5 mg/L which possess even greater risk of contaminating surrounding farmlands. It can be concluded, that Pb levels within the cultivated farmlands behind the berm and the general surroundings areas were strongly related to mobility of fine metallic Pb particulates from the impact berm soil as a result of heavy and continuous rainfall runoffs event. These could have extensive implications in terms of availability and possible bioaccumulation of Pb in agricultural production obtained within the area. It is, therefore, imperative for closer monitoring and possible remediation to void further and prolong contamination of arable farmlands around the range.
Effiong Ukorebi Etim, (2016) Distribution of Soil-Bound Lead Arising from Rainfall-Runoff Events at Impact Berm of a Military Shooting Range. Journal of Environmental Protection,07,623-634. doi: 10.4236/jep.2016.75056