Growing food crops on sludge‐amended soils: Problems with the U.S. Environmental Protection Agency method of estimating toxic metal transfer

Abstract

Abstract-The use of sewage sludges as farm fertilizers, encouraged in recent years by changes in U.S. Environmental Protection Agency (U.S. EPA) policy, has raised concerns among some scientists regarding food safety and long-term soil productivity. The U.S. EPA risk assessment for entry into the human diet of three of the most toxic metals, cadmium (Cd), mercury (Hg), and lead (Pb), utilized uptake coefficients (UCs) to calculate the amount of each metal that could enter food crops from the soil. Each UC was calculated as the increment of metal concentration in the edible part of the crop per unit increase of metal loading to the soil. However, the final UC estimates employed in the risk assessment are biased toward low values by a number of factors. These include the use of geometric means to obtain single-point averages of UCs for each crop group evaluated, rather than using arithmetic means or probabilistic methods, a systematic analytical or contamination error apparent in the reported metal concentrations of the control crops, and the fact that most of the UC values were derived for soils with pH 6 or higher. For more than 50% of all the soil and cropping conditions represented in the risk assessment, the geometrically averaged Cd UC values used by the U.S. EPA underestimated the actual risk posed by uptake into crops. The UC values for Pb and Hg are uncertain because of analytical or contamination errors, and because of the few data available for a number of crops. These uncertainties and biases in the risk assessment would advise a more cautious approach to agricultural and home garden use of sewage sludge than is permitted by the U.S. EPA 503 rule.

Phytoavailability of cadmium to cherry-red radish in soils applied composted chicken or pig manure

“…In this experiment, the Cd loading rate by composted chicken and pig manure was in the range of 0.225 to 2.25 kg ha − 1 in soil, much less than the maximum permitted cumulative Cd loading rate of 39 kg ha − 1 established by USEPA-503 regulations for agricultural land application of sewage sludge (McBride, 1995(McBride, , 1998, but in the range of acceptable limits of 0.9 to 9 kg ha − 1 in European countries (Matthews, 1996). The UC values were in the range of 1.11 to 1.28 for radish leaves, 0.29 to 0.63 for radish roots in Calcaric Cambisols and 0.21 to 11.1 for leaves, 0.10 to 6.8 for roots in Ferralsols (Fig.…”

“…These UC values were markedly higher than other studies for vegetables amended with sewage sludge. The U.S. EPA calculated a mean UC of 0.182 for leafy vegetables with cumulative Cd loading rate up to 22 kg ha − 1 in soil ( McBride, 1998). Other studies showed that the UCs for leafy vegetables is often in the range of 0.5 to 1.0 even when the soil pH was above 6, Cd was below the U.S. EPA cumulative loading limit, and a long time has passed since the Cd was applied (Alloway et al, 1990;Jackson and Alloway, 1991;Chang et al, 1997).…”

“…Each UC was calculated as the increment of metal concentration in the edible part of the crop per unit increase of metal loading to the soil (McBride, 1998). Uptake coefficients (UCs) were then calculated as: UC = (C t − C 0 )/M, where UC is the uptake coefficient (μg g − 1 /kg applied Cd ha − 1 ); C t is the metal concentration of the edible part of the plant grown in metal treated soil (μg g − 1 dry weight basis); C 0 is the metal content of the edible part of the plant grown without metal applied (μg g − 1 dry weight basis); and M is the amount of metal applied (kg ha − 1 ).…”

Land application of sewage sludges: an appraisal of the US regulations

“…For some metals (including lead and mercury), measurements of uptake coefficients appear to be compromised by either analytical limitations or physical contamination of 'control' crops (McBride, 1998a). A number of field plot experiments used in the EPA risk assessment showed low or even negative uptake of these metals.…”

Investigation du lessivage des stocks de boues d’épuration de Nador : Étude sur terrain et apport de l’expérimentation

“…Ces boues renferment des matières organiques, des éléments azotés et phosphorés et des éléments qui peuvent être nocifs tels que les métaux lourds (Zn, Cu, Pb, etc.) (CHAUDRI et al, 2000;KHAN et SCULLION, 2000;McBRIDE, 1998). Ces composés sont sujets à un lessivage par les eaux de pluies et peuvent être transférés vers les milieux aquatiques adjacents et par là, la qualité des eaux souterraines et superficielles peut bien être affectée.…”