Concentrations of lead in soil

As indicated above, mean concentrations of lead in unpolluted soils relate closely to concentrations in the parent geological material. Consequently in non-mineral-ized areas (most areas, in practice), concentrations range from 2 to 200 mg kg S with most samples being in the range 5 to 25 mg kg [1]. Concentrations in excess of these are generally indicative of lead pollution or mineralization. In areas previously worked for mineral deposits, concentrations may reach 20 000 mg kg , but are more typically within the range of 1000 to 2000 mg kg [11]. 

Since soils are strong accumulators of lead, the analysis of lead in soil is an excellent indicator of accumulated deposition in the vicinity of a source of the metal. In one survey around a secondary smelter [12], concentrations of lead up to 21 000 mg kg (dry weight) were found in the upper 5 cm of soil adjacent to the smelter with the levels decreasing exponentially with distance from the source. Mean concentrations of lead in soil around the Silver Valley lead smelter [13] (air lead concentrations in Fig. 2.3) are shown in Fig. 4.2. Changes in soil lead between the 1974 and 1975 surveys presumably arise mainly from random errors introduced by minor spatial variability in the lead concentration. 

There have been numerous reports on the concentrations of lead in roadside soils. Typical distributions with respect to the road are shown in Fig. 4.3, which shows analysis of soil collected alongside a road of low traffic volume in New Zealand [14]. Two features are worthy of note. Firstly, deposited lead is normally restricted to the first few centimetres of an undisturbed soil, and secondly concentrations faU to almost background values within 100 m of the road. The diminution of lead concentration with distance from the road is approximately exponential, and is shown in Fig. 4.4 for roads of differing traffic volume. 

In comparing data on lead concentrations in soils, it is important to take account of the techniques of extraction and analysis used. Some techniques are designed to extract only available lead, i.e. that supposedly available for plant uptake, and even those designed to determine the total lead are of substantially differing efficiencies [16]. 

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In environmental health studies conducted near four NPL sites (plus a comparison area for each), ATSDR collected lead concentration data from both environmental media and human body fluids to estimate low-level exposure risk and to document the magnitude of human exposure to lead near those sites. Environmental samples collected at participants homes included drinking water, yard soil, house dust, and house paint body fluids collected from participants included venous blood and urine specimens. For the four sites, mean concentrations of lead in soil ranged from 317 to 529 mg/kg, and mean concentrations of lead in dust ranged from 206 to 469 mg/kg (ATSDR 1995). 

We need to make a decision related to the disposition of soil that has been excavated from the subsurface at a site with lead contamination history. Excavated soil suspected of containing lead has been stockpiled. We may use this soil as backfill (i.e. place it back into the ground), if the mean lead concentration in it is below the action level of 100 milligram per kilogram (mg/kg). To decide whether the soil is acceptable as backfill, we will sample the soil and analyze it for lead. The mean concentration of lead in soil will represent the statistical population parameter. 

Example 2.2 describes lead-contaminated soil with the baseline condition stating that the true mean concentration of lead in soil exceeds the action level. If a false rejection decision error has been made, contaminated soil with concentrations of lead exceeding the action level will be used as backfill, and will therefore continue to pose a risk to human health and the environment. On the opposite, as a consequence of a false acceptance decision error, soil with lead concentrations below the action level will not be used as backfill and will require unnecessary disposal at an additional cost. 

Figure 2.4 illustrates the changes in the decision performance goal diagram for the opposite baseline condition (the true mean concentration of lead in soil is below the action level or the null hypothesis H0 i< 100 mg/kg). 

Concentrations of lead in soil litter ranged from 3200.0 mg/kg in locations near a zinc smelter in Palmerton, Pennsylvania, to... 

Paint from old playground equipment can fall into the soil and contaminate it. Old farm equipment or vehicles that are left to deteriorate on land can leave high concentrations of lead in soil. Specific areas of soil can also become heavily contaminated from lead paint on rain gutters and the leads that drain the water from the roof away from the house. 

Lead in soils, dusts and paints represents a potentially highly important source of lead intake.This is especially so for young children who frequently lick and chew contaminated objects. The source of high concentrations of lead in soils and dusts has been discussed previously (Section 4.3). Paints also exhibit high lead concentrations, for instance primers may contain 30 000-600 000 mg kg [5]. Painted surfaces wliich show high concentrations of lead, in particular those that are flaking, are known to result in enhanced lead intake in certain children. A limit of 2500 mg kg is now imposed on lead in paint applied to children s toys, [4] whilst a limit of 5000 mg kg" is applied in the US on paint used for residential surfaces accessible to children [5]. 

Guidelines for soil (see Testing Soil ) suggest that dust above 500 pg/g should be abated. In the context of total exposure to lead fi om all sources, it is possible that a level below 500 pg/g is reasonable to reduce exposure. Based on the assumption that most dust comes fl-om soil and given that the natural concentration of lead in soil is 10-25 pg/g, it is unreasonable to expect household dust levels below this level. When there is no apparent source of dust, typical household dust concentrations are about 100 pg/g. 

House dust. Houses are enclosed spaces and tend to accumulate dust from the outside. There are also internal sources of house dust. The concentration ratio [MJhouse dust/[M]soil has a mean of 0.33 (standard deviation = 0.09) for the ten elements Mn, Fe, La, Sm, Hf, Th, V, Al, Sc and Ce suggesting that around 33% of house dust is soil (93). The concentration ratio for the two surface dusts, [M]house dust/[M]street dust is >1 for the elements Cu, Co, As, Sb, Zn, Cd, Au, Cl and C suggesting these elements also have an internal component. All of these elements, as well as Pb and Br, are enriched in house dust relative to their concentrations in soil. Lead and bromine originate mainly from outside the house, and probably from street dust and motor vehicle emissions and, in the case of lead, from paint. When the concentrations of lead in house dust are very high this generally signifies an internal source of lead paint, especially in older houses. 

Figure 4 shows an isomap of the lead pollution in the first smelter area of the Dallas Lead Study. The round symbol in the center represents the smelter. The lines are isopleths of lead in soil in pg/g. Note the cluster of closed isopleths encircling the smelter. The large number of concentric isopleths encircling the smelter shows a steep gradient or rapid change in a short distance between a low (200 ig/g) outside and a high (3,000 pg/g) inside. 

Numerous observations of non-linear relationships between PbB concentration and lead intake in humans provide further support for the existence of a saturable absorption mechanism or some other capacity limited process in the distribution of lead in humans (Pocock et al. 1983 Sherlock et al. 1984, 1986). However, in immature swine that received oral doses of lead in soil, lead dose-blood lead relationships were non-linear whereas, dose-tissue lead relationships for bone, kidney and liver were linear. The same pattern (nonlinearity for PbB and linearity for tissues) was observed in swine administered lead acetate intravenously (Casteel et al. 1997). These results suggest that the non-linearity in the lead dose-PbB relationship may derive from an effect of lead dose on some aspect of the biokinetics of lead other than absorption. Evidence from mechanistic studies for capacity-limited processes at the level of the intestinal epithelium is compelling, which would suggest that the intake-uptake relationship for lead is likely to be non-linear these studies are discussed in greater detail in Section 2.4.1. 

In soils, lead concentrates in organic-rich surface horizons (NRCC 1973). In one instance, only 17 mg of soluble Pb/kg was found in soils 3 days after the addition of 2784 mg of lead (as lead nitrate)/kg (NRCC 1973). The estimated residence time of lead in soils is about 20 years complete turnover in topsoil is expected every few decades (Nriagu 1978a). In forest litter, however, the mean residence time of lead is lengthy estimates range from 220 years (Turner et al. 1985) to more than 500 years (Friedland and Johnson 1985). 

The low mobility of lead is reflected in the vertical concentrations of lead in the soil and the distribution of lead within the plant system. Lead seems to preferentially concentrate in roots rather than aerial parts of the sampled species. This is agreement with the frequent behaviour of most species in relation to this heavy metal. 

These long-term small catchment study results suggest that stream water concentrations are very low and not a water quality concern. In addition, a study of lead in soil solution and stream water following a commercial whole-tree harvest at Hubbard Brook showed that Pb was not released to drainage waters from clearcutting activities (Fuller et al., 1988). 

To use what is termed simple kriging, only the assumption that the random function is intrinsic needs to be made. The problem with this assumption is that the expected value of the phenomena of interest is rarely a constant. For example, the expected concentration of lead in the soil around a smelter would decrease as the distance from the smelter increased. If this decrease (or trend) is gradual enough, it is often assumed that within a limited neighborhood the random function has a "local stationarity" and then simple kriging is used, since generally only the observations within the limited neighborhood are used in the estimation process. 

At a battery recychng facihty in Iowa, an uncontrolled dump site was treated using both in situ and ex sitn applications of MAECTITE technology. Concentrations of lead in the untreated soil were as high as 80 mg/liter. According to the vendor, approximately 52,000 yd of soil were treated at an average cost of 14.75 per ton. Lead concentrations in treated soil were below 5.0 mg/liter (D15712K, pp. 8-11). 

De Muynck, David, Christophe Cloquet, Elisabeth Smits, Frederik A. de Wolff, Ghylaine Quitte, Luc Moens, and Frank Vanhaecke. Lead Isotopic Analysis of Infant Bone Tissue Dating from the Roman Era Via Multicollector ICP-Mass Spectrometry. Analytical and Bioana-lytical Chemistry 390 (2008) 477-486. The researchers used isotope analysis to show that high concentrations of lead in the bones of Roman infants probably did not come from the soil or other objects in the graves. 

Results For the St. Louis data, the target transformation analysis results for the fine fraction without July Uth and 5th are given in table 6. The presence of a motor vehicle source, a sulfur source, a soil or flyash source, a titanium source, and a zinc source are indicated. The sulfur, titanium and zinc factors were determined from the simple initial test vectors for those elements. The concentration of sulfur was not related to any other elements and represents a secondary sulfate aerosol resulting from the conversion of primary sulfur oxide emissions. Titanium was found to be associated with sulfur, calcium, iron, and barium. Rheingrover ( jt) identified the source of titanium as a paint-pigment factory located to the south of station 112. The zinc factor, associated with the elements chlorine, potassium, iron and lead, is attributed to refuse incinerator emissions. This factor could also represent particles from zinc and/or lead smelters, though a high chlorine concentration is usually associated with particles from refuse incinerators ( ). The sulfur concentration in the refined sulfate factor is consistent with that of ammonium sulfate. The calculated lead concentration in the motor vehicle factor of ten percent and a lead to bromine ratio of about 0.28 are typical of values reported in the literature (25). The concentration of lead in... 

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