39    A SIMPLE MODEL FOR CADMIUM UPTAKE FOR CROPS
Scott M. Brown, Environmental Assessor
Safety and Environmental Management, 4S
General Mills, Inc.
Minneapolis, Minnesota 55440
Thomas J. Overcamp, Professor
Department of Environmental Systems Engineering
Clemson University
Clemson, South Carolina 29634-0919
INTRODUCTION
Safe and economical disposal of sewage sludge has become a major concern for municipal wastewater treatment plants. With landfill space for sewage sludges becoming increasingly scarce and
expensive, many municipalities are taking a new look at land treatment of sludges. The nitrogen and
phosphorous content of sludge as well as its ability to improve soil structure, texture, and buffering
intensity make sludges an attractive crop supplement.
On the other hand, sludges from wastewater treatment plants with industrial inputs may not be safe
for land application due to contamination by refractory organics and heavy metals. If these contaminants are taken up by food crops, transport through the human food chain may pose a threat to
health.
Cadmium emerged as a heavy metal of concern for sewage sludge disposal when it was discovered
that cadmium can be taken up by crops at concentrations that are zootoxic before becoming phyto-
toxic.1"4 The human health effects of cadmium include renal proteinuria due to kidney damage and
osteoporosis due to calcium displacement by cadmium.
The most common modeling technique used to predict crop cadmium uptake is linear regression.5,6
Although this method may successfully correlate data from a single study, it may be incorrect for a
site where important factors such as soil pH and cadmium distribution coefficient differ from those
for that study.
This paper presents a simple cadmium uptake model. Predictions using this model are compared to
published field data.
MODEL
A rigorous model for cadmium uptake should consider such fundamental processes as the transport
and chemistry of cadmium in the soil, water uptake and evapotranspiration, and physiological mechanisms within the plant. Such a model is beyond the current state of the art.
Because data on the annual cadmium uptake versus the applied cadmium often appear to fit a
rectangular hyperbola, this equation was chosen to model the annual cadmium uptake of a crop. The
annual uptake is assumed to be
Uptake =      Un~C'      + Ubg, (I)
in which Ca is the average concentration of cadmium in the sludge, Um is the maximum uptake
concentration, Sm is the maximum soil-sorbed cadmium, and Ubg is the background cadmium for
crops grown on soils with no sludge addition. These variables are expressed in parts per million on a
dry basis (ppm-d.b.). A background value was included since most studies have shown cadmium
uptake at zero sludge-borne cadmium application rate. The U.S. Food and Drug Administration
background cadmium levels in various foods were adopted in this study.2 This simple model is first
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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