The seagulls eat the refuse then comtaminate the water supply with their feces.
Do you want seagulls to not eat, never eat? Do you want seagulls to be killed?
Is the water supply adjacent to the landfills? Will the seagulls not poop in the water supply if they didn't eat from landfills, or is pooping in the water supply not a problem as long as the seagulls haven't eaten from a landfill?