In late September, the corn and soybean fields of the lower Missouri River floodplain are a lovely dull brown, nearly ready for harvest. The row crops sprawl as far as the eye can see, their regimental march broken only by levees, gravel roads, the occasional band of cottonwoods, and the endless tracks of the Burlington Northern and Santa Fe. The scenery is pastoral and soothing. But this abundance, and the security it evokes, has a darker underside. The nation's breadbasket, it turns out, is poisoning the water. Farmin' is harmin' the water. Photo: EPA The Mississippi River basin, which includes …