Why is storing hazardous wastes in barrels and burying them deep in the ground a bad idea?

This due to the risk of contaminating the water table, especially where the water table is high. The water table is usual defined as the highest zone / horizon of high saturation of water in the soil, underground. The water table feeds river through springs. If contaminated, the water in springs will also be contaminated and affect any living things that consumes the water.