Contaminated substances found in the investigation of 800,000m3 of soil excavated at the former industrial site include oil, petrol, tar, arsenic and lead.
Experts have overseen the soil investigation and washing to separate the soil from contaminants ready for reuse on site. Four soil washing machines at two large remediation plants are continuing to wash and shake out contamination.
Meanwhile, 60 geotechnical engineers and technicians are testing treated earth at a soil analysis laboratory.
Up to 80 samples a day are being tested to ensure it is ready for reuse.
The ODA is also monitoring ground and surface water and air across the site, and an offsite laboratory has been set up to monitor water and air samples. To date, more than 1000 surface and ground water samples, and 5400 atmospheric samples have been taken.
ODA director of infrastructure and utilities Simon Wright said the decontamination project was one of the "most challenging ever undertaken in the UK".
"With this laboratory on site we are able to use cutting edge technology to quickly tell when soil needs to be cleaned, and when it's clean enough for on-site reuse," he said. "This rapid process is helping us prepare former industrial land for the construction of world class venues and a brand new urban park."
Morgan Est has lowered a tunnel boring machine (TBM) into a shaft at Ross-on-Wye to allow tunnelling to start as part of a flood alleviation scheme in the Herefordshire town. The 50t, Lovat 101 TBM, nick-named "Victoria", will now begin boring a 385m-long, 2.1m-diameter tunnel 15m below ground. The scheme, due to be completed this summer, is worth £2.1M to Morgan Est.