There is a social networking site dedicated to property contamination and environmental risk professionals where you may be able to get feedback from an environmental consultant in the Walnut Creek area for advice. commonground.edrnet.com. You can also buy an inexpensive environmental report at http://greenpropertyreport.com.
The sales prices have not been effected that greatly due to the contamination but I always advise all buyers to get full reports on the property before making any purchase. I suggest you put in an offer you feel comfortable with.
There is litigation among some of the homeowners agaists the company that contaminated the underground years ago. It has adversely affected home values but not as much as the market readjusting itself.
I would factor this in if you are interested in writing an offer on a property there, however, it would be at your own risk of how the law suit will settle out. I would also recommend you do a soils analysis if you are considering a purchase there.
Please contact me offline if you would like more information.