(plaintiffs brought citizen suit action under CWA claiming that defendant’s (U.S. Bureau of Reclamation) Grasslands Bypass Project in San Joaquin Valley of California illegally discharges polluted water (water containing naturally-occurring selenium from soil) into navigable waters of the United States via subsurface tile system under farmland without an NPDES permit; plaintiff directly challenges exemption of tile drainage systems from CWA regulation via “return flows from irrigated water” on basis that groundwater discharged from drainage tile systems is separate from any irrigation occurring on farms and is, therefore, not exempt; court initially refused to grant defendant’s motion to dismiss; in instant case, court dismissed case and specifically noted that parties agreed that only reason project exists is to enable growing of crops which require irrigation and drainage of contaminated water only occurs due to irrigated agriculture; court noted that “return flows” narrows type of water permissibly discharged from irrigated agriculture and covers discharges from irrigated agriculture that don’t contain additional discharges unrelated to crop production; plaintiffs failed to plead sufficient facts to support claim the some discharges unrelated to crop production).