Preliminary research shows traces of a rocket fuel ingredient in milk across California, prompting a debate about safety standards for the pervasive pollutant perchlorate. In a report to be released today, the Environmental Working Group said its samples of supermarket milk generally confirm unpublished tests done by the California Department of Food and Agriculture, which in April sampled milk at a few dozen processors statewide. Both the state and the Environmental Working Group said adults and children should keep drinking milk - which offers calcium and important vitamins - because little is known about risks of perchlorate in milk.