Page 212 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 201

Table 6.6 Typical background and unsafe soil levels of heavy metals
Typical background levels Unsafe for leafy Unsafe for gardens
for non-contaminated soil or root vegetables and children
−1
−1
−1
Heavy metal (mg kg ) (mg kg ) contact (mg kg )
Arsenic 3–12 >50 >200
Cadmium 0.1–1.0 >10 >50
Copper 1–50 >200 >500
Lead 10–70 >500 >1,000
Nickel 0.5–50 >00 >500
Selenium 0.1–3.9 >50 >200
Zinc 9–125 >200 >500
Source : http://www.aleastern.com/forms/Heavy%20Metal%20Interpretation.pdf

Table 6.7 Threshold levels of heavy metals in soil

Threshold levels in soils Threshold levels in soils
of concern for human of concern for human
−1
−1
Heavy metals health (mg kg ) Heavy metals health (mg kg )
Arsenic 0.07 Molybdenum 380
Cadmium 1.70 Nickel 1,600
Chromium 1,00,000 Selenium 380
Cobalt 660 Silver 380
Copper 3,000 Vanadium 530
Lead 80 Zinc 23,000
Mercury 18
Source : California Office of Human Health Assessment, http//:oehha.ca.gov/risk/chhsltable.html

6.2.12.2 Safe Limits of Heavy Metals in Soil
The term “safe level” needs to be explained. A level safe for growing vegetables
may not be safe for children’s contact. A level safe for human may be toxic for
crops. Moreover, getting a universally accepted safe limit of heavy metals in soil is
diffi cult. Table 6.6 gives some typical background levels in non-contaminated soils
of some common heavy metals and their unsafe levels for vegetable growing and
gardening.
Zhou et al. ( 2008 ) proposed that “maximum allowable concentrations” of Cd,
− 1
Cu, and Hg in Chinese agricultural soils are 0.30, 50.00, and 0.30 mg kg , respec-
tively. According to Coskun et al. ( 2006 ), “world limits” of Cd, Hg, Pb, and As in
− 1
soil are 0.35, 0.30, 35.00, and 10.00 mg kg , respectively. The toxic limits in soils
of As, Cd, Cr, Cu, Pb, Hg, Mo, Ni, Se, and Zn are 1–4, 5–20, 5–30, 20–100, 5–30,
− 1
1–3, 10–100, 30–300, 5–10, and 100–400 mg kg , respectively (Alloway 1995 ).
Threshold levels of heavy metals in soils of concern for human health are given in
Table 6.7 .