Julia, Cottonwoods are certainly water lovers. Depending on the size of the tree it may not reach that far at this point but could in the future. Older septic systems tend to be more prone to clogging by tree roots. If this is a newer system the pipes may have been covered with a barrier to keep roots out. you would have to check with your installer. So its up to you as to how much you want to save or get rid of the tree.
