Disappearance and crop uptake of PCBs from sludge‐amended farmland

Abstract

ABSTRACT: The Madison Metropolitan Sewerage District (MMSD) has conducted a five‐year (1985‐1990) field experiment to obtain data that provide a scientific basis for regulating land disposal of sludge contaminated with polychlorinated biphenyls (PCBs). Over 1 400 samples of soil and crop tissue were analyzed. This paper presents all the field data and interpretation. The experimental design included eight different sludge treatments and controls, making it possible to study the effects of sludge concentration, sludge loading rate, and sludge application pattern on the fate of PCBs in the soil system. Each treatment was replicated three times, giving thirty test plots in all (24 sludge applications and 6 control plots). Seventy‐nine PCB congeners were detected during the study and the concentration of each PCB congener was measured in the surface soil, corn stover, and corn grain. Surface runoff runoff studies were conducted in 1988 and 1990 and congener‐specific PCB analyses were made on these as well. A first‐order disappearance model, C = C0 exp(− Kt), was appropriate to evaluate the disappearance of PCBs from the surface soil layer. AT is the rate constant for PCB disappearance. Results indicate that the rate constant for disappearance of each individual PCB congener in the soil was independent of the initial PCB sludge concentration, the sludge loading rate, and sludge application pattern. Most of the 2‐, 3‐, and 4‐chlorinated PCB congeners showed significant decreases in their soil concentration with half‐lives in the range of 4 to 58 months. The more highly chlorinated PCBs were more persistent in the sludge amended farmland, but many of them did disappear. The concentration of PCBs in the runoff samples was correlated with surface soil concentration. The PCBs were associated with the runoff sediments and there was no measurable PCB in the liquid portion of the runoff. It was concluded that there is no PCB translocation into either corn grain or corn stover samples. The bioconcentration factor for PCBs in the crop tissues is zero.