Yes, I have heard of it years ago. A lady who has land next to a motorway junction locally to me had some mares that kept throwing abnormally deformed and still born foals. She was at a loss as to what was causing it as all her stock was from good bloodlines, she had her ground tested, it was contanimated with heavy metals etc, it was the run off off the motorway causing it.
I think Al Fayed had the same problem with run off onto his land too but he successfully sued the highways dept responsible. Do you have any large or busy roads near you that could be the cause?
Or a drain coming onto your land anywhere?
How interesting. I didn't consider those aspects & I will check it out ASAP. For want of sounding naive, how would such contamination come off motorways & drains? Are things not regulated stringently enough to prevent such occurances?