Is greywater safe for horse pasture?

[oasis]

When we purchased this house, it came with greywater pumped from the washer to the back yard (at the crest of the slope before it goes downhill). It is this area I'd like to convert to turn out/pasture for my miniature horses. There doesn't seem to be a finite 'exit' for the water (guess I need to dig a bit?) Will this be safe for my horses? the grass ? What books do you recommend I read to help finish this project? I also have a HE washer and use eco-friendly soap when doing the laundry- how important is this and do you have any recommendations ? thanks ever so much!!