We planted six 6' emerald green arborvitae a few months ago. They took great at first, but in the past month or so, they're starting to struggle. I'm noticing 3 main things:

1) toward the bottom of the trees, some of the needles are turning a dark, tar black. This seems to be limited to the bottom of the trees.
2) some of te internal branches are turning a little brown.
3) There are some spider webs that have recently formed in a few trees.

Regarding the black and brown, there are two things that I think might be causing this. I've noticed my dog peeing on the trees lately and our neighbors have an irrigation systems that they seem to put on every single night (the trees) are just inside the property line) and the ground around the trees is sopping wet and swampy. Does anyone know what the black needles are from and do either of these 2 suspected causes make sense?

Also, what should I treat the trees with the get rid of the spiders?

I apologize for the basic questions, but I am just learning about gardening and tree care! Thanks in advance for your help!