Its caused mainly by erosion resulting from bare land, poor farming practices, and construction and development. Nutrients are substances which help plants and animals live and grow. Fertilizer and animal waste are the main sources of these substances. Toxic substances are chemicals which cause human and wildlife health problems. They include organic and inorganic chemicals and metals, pesticides, formaldehyde, household chemicals, gasoline, motor oil, battery acid, roadway salt and so on.
Pathogens are disease-causing microorganisms present in human and animal waste.
Most pathogens are bacteria. Algae blooms can occur when too much nitrogen and phosphorus enter a waterway. Algae block sunlight needed by underwater plants that provide habitat, and algae's decay robs the water of dissolved oxygen needed by aquatic life. You're probably an NPS polluter - Just about everyone who owns or uses land contributes to nonpoint source pollution. In Virginia, most efforts concentrate on NPS pollution leaving farmland, urban areas, construction sites and forest land.