Still don't know why they can't have a roof garden.
The Kensington Roof gardens, for example, cover 6,000 m² (1.5 acres) on top of the former Derry and Toms building on Kensington High Street, London. Wikipedia notes that it includes:
a Spanish garden, in a Moorish style based upon the Alhambra in Spain, with fountains, vine-covered walkways and Chusan palms;
a Tudor style garden, characterised by its archways, secret corners and hanging wisteria. Roses, lilies and lavender contribute the rich summer scent to the garden;
an English woodland garden, with over 100 species of trees, a stream, and a garden pond that is the home to pintail ducks and four flamingos called Bill, Ben, Splosh and Pecks. There are over 30 different species of trees in the woodland garden, including trees from the original planting over sixty years ago, despite having only a metre of soil in which to grow.
All you need is supporting columns, and put an extra level above the existing car park - no need to take out a lot of the contaminated soil - make sure the sides are high enough for safety, there is a gentle ramp for wheelchair / child buggy users, and hey-presto - no car park problems - ok some places will be lost, but marginally less than losing the whole area. And the cost would surely be far less than taking out all that tarmac, bad soil etc.