Lupito is the first person to contaminate the water.
Early in the story, Lupito attacks and kills a policeman and is then chased down, confronted, and killed by a mob of men. Antonio watches the chase and the killing and realizes the Lupito is scared and out of his mind.
Lupito is shot by several men in the mob when he fires his gun into the air, apparently trying to entice the men to shoot him.
Antonio sees Lupito die on the bank of the river after crawling toward him and asking for a blessing. Lupito dies without a blessing and it is his blood that contaminates the river.
Later, Florence drowns in a different part of the river and his blood too becomes part of the spiritual contamination, but by that time several people have died in the story.