(case involves claim that county violated CWA in its application of anti-mosquito pesticides and dredging of mosquito ditches; county's maintenance activity of ditches did not violate CWA because CWA has permit exemption for maintenance of drainage ditches and ditches had as primary purpose the drainage of surface waters, and record supported conclusion that county's activities did not bring an area of navigable waters into a use to which it was not previously subject so CWAs recapture provision not triggered; trial court did not adequately explain its basis for finding that county fully complied with pesticide's label instructions and trial court's conclusion that pesticides were not discharged from point source was in error).