Try lowering the  IDL irradiance cache.  I'd also add a bounce or two.

 

As an aside, are you using an environment sphere?  I ask becausae the intensity of the sun is very high; looking at the lit area, it looks like an atomic bomb has just gone off.  I usually use an infinite light with intensity of around 65% for outdoor sun when using BB's environment sphere with a sky texture applied. 

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