up the hill from the little river (that floods alarmingly often)
What are they made of? If there all synthetic, you may be able to put them in the washing machine on the gentle cycle and then let them dry in the sun. Sunlight is great for killing nasties.
If they do have leather, you can still probably hose them off (they're winter boots, so they've got to be somewhat OK with getting wet/muddy) and then let them dry in the sun. That's what I did with my Ariat winter paddocks last year (need to do it again soon too, actually).