ral studies cited in the record showing that trivalent chromium can oxidize to hexavalent chromium, though the levels of conversion appear to be low.  A survey of these studies led one scientist to conclude that a "[trivalent chromium]-containing sludge could release low levels of [hexavalent chromium] over a long-period of time."30  Because it is not now possible to estimate the extent of hexavalent chromium formation, that scientist recommended regulating all chromium--hexavalent and trivalent--alike.31  As to the second complication, the Mortvedt & Giordano study on which the EPA relied to determine the chromium phytotoxicity threshold found that trivalent chromium is less toxic to corn, but nevertheless causes some phytotoxicity.32  There is, then, enough genuine scientific debate regarding trivalent chromium's potential harm and its potential conversion to the hexavalent state to warrant significant discretion on the part of the agency in its choice to use data on the harm posed by hexavalent chromium to regulate trivalent chromium.


56
More important, however, from our reading of the underlying reports, it appears that to the extent that trivalent chromium is less toxic to plants than hexavalent chromium, it is not because plants can absorb trivalent chromium with no harm, but because trivalent chromium is less available for plant uptake.  Thus, the hexavalent/trivalent distinction appears to be critical to Step 2, discussed below, but not to Step 1.  The Mortvedt & Giordano study, for instance, explains that the lower toxicity of trivalent chromium "suggests that [trivalent chromium] may have been fixed by the soil in forms which were less available than [hexavalent chromium] to plants."33  The Peer Review Committee likewise explains that "[h]exavalent chromium is more soluble and more bioavailable for plant uptake than the trivalent chromium usually found in sludges and field soils."34


57
The relevant distinction between hexavalent and trivalent chromium is that trivalent chromium is less available for