The clue for the second blank is on the earth's surface, where the processing of chemicals is less costly." Underwater recovery, in contrast, would be more expensive. The second blank describes what's not true of the procedure, so use a word that means low-cost Cost-effective is a close match. Use the same due for the first blank. If the processing of ores from the surface is less costly, those ores must be plentiful. Abundant is a good match.