To be clear I wasn't trying to leap on, "haha Mexican immigrants can't speak English". I was pointing out proximity to a primarily Spanish speaking country is going to lead to a greater population whose native language is not English, and therefore less fluent English speakers.

I grew up in an area of the US with tons of immigrants, most of whom learned Spanish before English. Going the other way I learned Spanish after learning English, and as such I probably have a less than 6th grade reading level in Spanish because it's not the language I learned from birth, nor the one I speak at home.

I also specifically mentioned Mexican immigrants because the other country we border also has a primary language of English, which is why our northern border has better English literacy rates.

It's a pretty easy correlation to make, and doesn't require a whole study to identify the trend. Spanish is also the second most spoken language in the country so naturally areas with low English literacy rates are likely to have higher populations speaking the second most spoken language in the country. Hell, if you look at a map of latinos in the US it's almost identical to the above map.