If the pile sits directly on the ground instead of on a concrete pad that is built to control runoff, it can contaminate groundwater and surface waters, and is the main element that links agriculture and water pollution. Along with nutrients and bacteria found in manure, runoff from agricultural land can also collect nutrients from fertilizers, and toxic substances from pesticides and herbicides that have been applied to croplands without using soil and water conservation practices. Nutrients that enter nearby rivers and streams can cause algae overgrowth that robs the water of oxygen needed by other organisms. Bacteria and toxic substances can pollute wildlife habitat, and result in fish advisories, shellfish bed closures, and beach closures. Although water testing may show especially high levels of bacteria at a cattle crossing or near a manure pile, the pollution source is difficult to confirm when it is discovered downstream, which is why it is referred to as nonpoint source pollution.
Soil erosion is a related issue that affects water quality and a farm's productivity.
Animals grazing on stream banks or in fields eat and trample the vegetation that helps keep soil in place. Water that runs off of a farm can pick up loads of soil, depositing the nutrient-laden sediment into nearby waters, contributing yet more to excess algae growth, filling in areas in rivers and streams where fish rest and feed, and blocking light needed by aquatic plants. Schloss trains watershed stewardship groups to work with farmers to identify and correct water pollution problems. Piecing together funds Reducing water pollution and erosion on farms can be as expensive as building a manure containment and treatment system or as comparatively inexpensive as building a fence to keep cattle from wading into a stream. Federal assistance programs for such projects have been pared down in Canada and the US, and providers of technical assistance to farmers in the provinces and states work continually to cobble together financial support.