Transparency effects?

Hi there,

Hopefully you can help me here. i am trying to do an effect on a drawing that I think really should be simple (and COREL easily capable), but I just can't quite get. It is doing my head in.
Im a bit of a Luddite, so simple explanations needed please!.

Ive used COREL for work for years, but only for relatively simple things. Essentially we digitise and add effects to field drawings/plans of sites or building elevations identified during our surveys.

At the moment I am working up a plan of a large cairn of stones. This cairn has numerous features within it that I have drawn on different layers, but on the whole comprises hundreds of stones, carefully recreated at 1:20 scale. As the dull drawings we do go, it looks pretty awesome.

What I am trying to do, is put a transparent effect across the whole cairn where I can show areas that have been disturbed in a deep transparent red (so the drawings/stones are visible beneath), but with this fading to clear where the cairn has been undisturbed. The pink areas of the gradient between the red and the clear will be where we think there has been disturbance, but cant say with certainty. Essentially, the more red an area, the more disturbed it is, the clearer or whiter, the less disturbed.

I have one closed outline around the entire drawing where I want this effect to take place. Obviously I can make it all transparent red to varying degrees. Fountain transparency seems to have vaguely the right idea, but I cant get the deeper, less transparent area in the centre of the cairn where I know it is disturbed, running to clear around the edges.

If you are still following me.... (well done, I am not sure Im following me)

What I want to know, is if I have one large closed object, is there a way to make it transparent, but fading from a centre point towards the edges? If so, can you add other areas to the same object where transparency is less?

Any help much appreciated.