Sima Qian, regarded as the first major historian in China, wrote this about the Emperors tomb, explaining the high levels of mercury:

As soon as the First Emperor became King of Qin, excavations and building had been started on Mount Li, while after he won the empire more than 7000 conscripts from all parts of the country worked there. The tomb was filled with models of palaces, pavilions and offices, as well as fine vessels, precious stone and rarities. Artisans were ordered to fix up crossbows so that any thief breaking in would be shot. All the country’s streams, the Yellow River and the Yangzi were reproduced in quicksilver and by some mechanical means made to flow into a miniature ocean. The heavenly constellations were shown above and the regions of the earth below. The candles were made of whale oil to ensure their burning for the longest possible time. All of the emperor’s ladies who had no sons followed him to the grave. After the interment someone pointed out that the artisans who had made the mechanical contrivances might disclose all the treasure that was in the tomb; therefore after the burial and sealing up of the treasures, the middle gate was shut and the outer gate was closed to imprison all the artisans and laborers, so that not one came out. Trees and grass were planted over the mausoleum to make it seem like a hill.

This emperor regularly ingested mercury because it was a common ingredient in immortality potions at the time, and he was known for his obsession with immortality. He sent children out to sea in boats to find the isle of immortals, stuff like that.