Hi
As has already been mentioned arseinc can be either due to a previous industrial use or as a result of the natural geology - certain areas of the country including parts of Northamptonshire and Lincolnshire have high natural levels.
The permitted level according to the CLEA guidelines for a residential use is 20mg/kg of soil. See the Environment Agency website for more info. That is what a site developer would normally be required to clean up a contaminated site to for housing.
Not so easy if you live in Northampton where the levels can often be 40-60mg/kg of soil sometimes more. It might be unreasonable to expect you to cleanup what nature has dealt you.
You will need to seek advice from the Contaminated Land officer at the District Council - they will most likely have some guidance they can give as this will be a common query in your area. Something like this
http://www.corby.gov.uk/docs/suppor...evelopers guide definitive version feb 06.doc
The fact that the arsenic is there is not necessarily a major problem it is relatively easy to deal with. What you most likely need to do would be to ensure that garden areas have a suitable depth of clean soil so that people, particularly children who might eat it, will not come in contact with the soil on a regular basis. It might be up to a metre of soil, depends on local requirements, some will allow 500 or 600mm.
Make sure you know the source of any soil bought onto you site and get it tested if in doubt - chances are it will also be local - so you might just be importing soil with the same level of arsenic - or worse if the supplier is a bit dodgy.
Poirot and Miss Marple have given arsenic a particularly bad name - I can assure you that there are a lot worse things to have contaminating your garden.