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Influence of Simulated Rainfall on the Retention of Sludge Heavy Metals by the Leaves of Forage Crops¹

ABSTRACT

The land disposal of municipal liquid digested sludge by surface application to a growing forage crop may cause foliage to become coated with sludge. The ingestion of foliage contaminated with sludge could be detrimental to the health of grazing animals, since municipal sludges often contain potentially toxic amounts of various heavy metals. The influence of simulated rainfall, leaf morphology, and sludge solids concentration on the retention of sludge heavy metals by the foliage of grasses was evaluated.

Liquid digested sludges, containing 2 and 4% solids, were applied to greenhouse flats of common bermudagrass (Cynodon dactylon L.) and Bell rhodesgrass (Chloris gayana Kunth.), equivalent to depths of 2.50 and 1.25 cm, respectively. Sludge-treated grasses were subjected to simulated rainfall equivalent to 2.5 cm/hour either immediately following sludge treatments or after 24 hours of drying. Additional flats receiving the sludge treatments were sampled at regular intervals over a 2-week period to determine the foliar retention of sludge in the absence of rainfall.

The foliage of both grasses was visibly contaminated with sludge following application. The foliar concentrations of Cd, Cu, Pb, and Zn were measured for each sludge treatment. There appeared to be no preferential retention of any particular heavy metal. Bell rhodesgrass retained more sludge than common bermudagrass because of its larger leaf blades and overlapping sheathes. The sludge containing 4% solids was retained by the foliage to a greater extent than sludge containing 2% solids.

Sludge that was not allowed to dry on the leaves was removed by 1.25 cm of simulated rainfall, regardless of the grass species or sludge solids concentration. Sludge that was allowed to dry on the leaves, however, was not removed by 2.50 cm of simualted rainfall.

Key Words: sewage sludge disposal • forage contamination • Bermuda grass • Rhodes grass • cadmium • copper • lead • zinc

¹ Research supported by the Texas Agric. Exp. Stn., College Station, TX 77843.

² Graduate Student, Associate Professor, Research Scientist, and Technician, respectively, Soil and Crop Sci. Dep., Texas A&M Univ., College Station.

Received for publication January 26, 1978.