A sequential extraction procedure was developed to identity and quantify the structural components of aquifer sediment that bind chromium. On the average, 65 percent (225 mug/g) of chromium is extractable. Of this amount, 25 percent (57 mug/g) is exchangeable, 11 percent (25 mug/g) is bound to organic matter, and 30 percent (67 mug/g) is bound to iron and manganese oxide surfaces. The remaining 34 percent of chromium is tightly bound to the soil matrix. Significant spatial variability in the extractable chromium was observed. Higher accumulation of chromium was observed in the soil samples obtained closer to the source than in those away from the source. The significant amount of chromium tightly bound to the soil matrix suggests that a slow, kinetically controlled attenuation mechanism is predominant at this site. The strength of the extractants indicates that only a fraction of the total chromium accumulation is readily exchangeable. The presence of occluded chromium has significant implications to pump-and-treat remediation technology.
