Heavy metals due to poisonous, accumulating traits and long longevity in organism’s body are considerably important. Many countries were infected to water and soil pollution crisis to heavy metals . Human beings cause this contamination type through different ways. For example, applications of metal-contaminated fertilizers, animal manures, and sewage sludge can result in high concentrations of heavy metals in agricultural soils .
Plants are the main way of cadmium (Cd) and lead (Pb) transfer from contaminated soils to humans. In a Cd and Pb contaminated soil, plants can uptake more heavy metal and accumulate it in different organs especially edible parts . This phenomenon is especially important for high consumption crops like rice. Rice is the dominant staple food crop in developing countries (including Iran) so that 96% of the world’s rice is produced and consumed in developing countries , making up over 70% of the daily energy intake . The protein component in rice (7–9% by weight) is relatively low , but it forms a major source of protein (50%) in these countries . Also rice is the second high consumption food among Iranian people. It is the most common crop grown in agricultural lands in the north of Iran .
In total, averages normal value of Cd and Pb concentrations in rice grains are 60 and 440 μg/kg, respectively [8–11]. Al-Saleh and Shinwari (2001), noted that the average levels of Cd and Pb in rice grains were 20 and 135 μg/kg, respectively [12, 13]. Shimbo et al. (2007) also reported that the geometric mean contents of rice produced in Japan in 1998–2000 were 50 and 2 μg/kg based on fresh weight for Cd and Pb, respectively [1, 5, 6, 9, 10, 14–16]. According to Bennett et al. (2000), the median values in wild rice seed from northern Wisconsin, USA, were 16 and 250 μg/kg for Cd and Pb, respectively [17–19]. Jung (1995) also reported that Cd and Pb concentrations in rice grown in various countries were in the range of 10–50 and 1–500 μg/kg, respectively .
Estimated daily intake (EDI) as a common index for metal transfer from plant to humans were calculated and used for rice in some studies [3, 6, 8, 19, 21, 22].
Although several studies were conducted about Cd and Pb concentrations in rice grains, however to our knowledge, up to now few studies have investigated Cd and Pb concentrations in rice grain in calcareous soils where metals were supposed to be less available for plants than other soils. Previous researches showed that in calcareous soils, Cd and Pb solubility and also plant availability decreased due to metal-carbonate precipitation in higher pH, and calcium competition with metal cations for plant uptake [1, 3–11, 13–19, 21–32]. Another problem in previous studies is the scale of the studies. Most previous studies were conducted in pot or small scale areas like experimental plots or small regions. The results of pot and hydroponic experiments may not predict the uptake of heavy metal by crops in field conditions. In addition, small scale studies could not into account spatial variability of soil properties and metal concentrations in soil and plants, which is very important in real field condition. Therefore this study was conducted in large scales areas where the mentioned issues were considered.
Cadmium and Pb were chosen for risk assessment because of their high toxicities or comparatively high levels in all of the collected rice samples in previous studies. Cadmium is toxic to the kidney and has a long biological half-life in human. Lead has shown to be associated with damnification of central nervous system . It is therefore necessary to determine the dose level for human, which is considered to be taken daily over a lifetime without adverse effect.
The objectives of this large scale study were to 1) determine Cd and Pb concentrations of rice grain of paddy fields with calcareous soils; and 2) assess Cd and Pb intake from rice based on daily intake.