A DIRECT SOLID-PHASE ASSAY SPECIFIC FOR HEAVY-METAL TOXICITY .1. METHODOLOGY

We have developed a direct toxicity assay for soils, sediments and slu dges that is specific for heavy-metal toxicity. In the assay, a beta-g alactosidase-producing strain of Escherichia coli is mixed with the so lids sample together with a small volume (1.0 ml/0.5 to 1.0 g of solid s) of eluent. Extraction of metals from the solids sample is not requi red. Controls run with the assay eliminate interference due to indigen ous beta-galactosidase activity or interaction between the solid matri x and the chromaphore. Use of 0.1 M sodium nitrate as eluent was found to yield somewhat higher sensitivity to heavy metals in solid-phase s amples than MilliQ water. Application of the assay to a diverse array of soils, sludges, and sediments indicated that samples from industria l sites were generally more toxic than those from residential or comme rcial sites. Heavy-metal toxicity was correlated with the copper and z inc content of solids samples, but toxicity varied considerably at the lower range of metal contents. The proposed solid-phase assay should prove useful as a screening test for heavy-metal toxicity in soils, se diments, and sludges. It can also help distinguish between heavy metal s and organic chemicals as the cause of toxicity in solid-phase sample s.