Also you say the chickens have a large run on a separate property. Has this property previously been used in such a way in the past that could have deposited something deadly in the soil?
I see it looks wooded, some things are toxic to chickens, namely oak leaves and acorns. There are more plants. Just search tixic plants to chickens and see the big list.
Im not sure what csv is but i think untill you know for sure what you are dealing with fresh water would be best.
I am sorry for the trouble you are having!
You obvisously love your chickens and are trying to do your best for them.