if everything is lowering IQs at all time, wouldn't it just be reflected in the mean then? This would imply that IQ tests would have to get easier over time to keep the average at 100.
Beyond that, given rural areas have been subject to over a century of severe brain-drain and intelligence is fairly heritable, I'd expect that to overwhelm any benefits of country living, if you tried to measure the effect for a general rural population. You'd need a twin study or something like that to sort it all out, i.e. a bunch of twins where one grew up in the country, one in the city or 'burbs or whatever you're comparing it to.