Complex sentence: The holding pond was not lined, in violation of standards in the Uranium Mill Tailings Radiation Control Act of 1978 that required that holding ponds be impermeable, which allowed tailings solution to seep into the ground, weakening the foundation of the dam and contaminating the groundwater.
Split into simpler sentences: The holding pond was not lined, in violation of standards in the Uranium Mill Tailings Radiation Control Act of 1978 that required that holding ponds be impermeable. This lack of lining allowed tailings solution to seep into the ground, weakening the foundation of the dam and contaminating the groundwater.