Application to cropland is rapidly becoming the major method of sewage sludge disposal in the United States. This is especially true in Ohio. Sewage sludges are good sources of nitrogen, phosphorus and, to a lesser extent, potassium and trace nutrients, and can be significant sources of nutrients to farms in the vicinity of the sewage treatment plants producing the sludge. Land application costs are generally borne by the producer, and sludge is usually provided free of charge to the farmer. In most cases, the sewage treatment authority (e.g., the municipality) also has the responsibility for spreading the sludge.
In addition to valuable nutrients and organic matter, sludges contain variable quantities of trace elements, including the so-called "heavy" metals. Most elements can be found in sludge, but the ones which have received the most attention are: cadmium (Cd), copper (Cu), nickel (Ni), zinc (Zn), and lead (Pb). Of these, copper, nickel, and zinc arc phytotoxic, at high concentrations, i.e., they reduce the growth of plants, while cadmium and lead are food-chain contaminants. Copper is also toxic. to certain livestock. Of the food-chain contaminants cadmium and lead, only cadmium can readily enter the food-chain via plant uptake. It is more plant-available in soils than most metals and uptake is not restricted by plant phytotoxicity. Lead, on the other hand, is very insoluble in soil and is not readily taken up by plants. The major pathway by which lead enters the human food-chain is by direct ingestion of lead-contaminated soil or lead-containing materials. Lead poisonings of children in urban areas from ingestion of contaminated soil or lead-based paints have been reported. Soil contamination has been attributed to accumulation from paint debris in urban renewal areas and atmospheric deposition of lead from automobile emissions in high intensity transportation areas.
As metal additions to soils increase with planned or unknown applications of wastes containing metals, the testing of soils for metal concentrations will increase. Analyses may include total metals or some measure of plant-available metals. In order to interpret the results of soil tests in terms of previous metal contamination, the natural metal levels in the soil (background concentrations) must be known. While published data for other regions may be helpful in establishing background metal levels, it is important to have information that is specific for Ohio.
Background (uncontaminated) levels of heavy metals in soils are low (in the µg/g range or less) and are related to the geochemistry of the parent materials. Worldwide values have been published, and arc given later in this paper for comparison with Ohio data. Major sources of localized heavy metal contamination of soils include:
As part of a 5-year project on land application of sewage sludges sponsored by the Ohio Farm Bureau Federation and the U. S. Environmental Protection Agency, a number of farms in several regions of the state were studied. These farms were selected only on the basis of the participants' willingness to cooperate in the study and only farms that had never previously received sludge were included. A total of 239 fields were sampled in seven counties (Figure 1 ), with the largest number (96) in Pickaway County. These counties arc located in most of the major parent material regions of the state: Defiance in the Lake Plain; Clark, Madison, and Pickaway in the High Lime Glacial Till; Medina in the Low Lime Glacial Till; Muskingum in the Sandstone and Shale Unglaciated Region; and Franklin in both the High and Low Glacial Till Regions. Since these sites were selected at random with respect to those factors which might affect heavy metal levels (prior use of phosphate fertilizers, proximity to transportation corridors or industrial atmospheric sources, etc.), heavy metal levels for these 239 soil samples should provide a reasonable estimate of background metal levels for Ohio soils.
|Figure 1. Number of farms sampled by county.|