The second ability of this card is a replacement effect, not a triggered ability.

External effects that generate more mana still generate their additional mana as directed without being affected by this card. Effects which generate more mana based on what type of mana the land produces will be affected in color but not in amount.

You choose whether to sacrifice a creature or not on resolution. If not, then you sacrifice Contamination. You can choose to not sacrifice even if you no longer control Contamination on resolution.