Both human activities and natural activities can change the physical, chemical, and biological characteristics of water, and will have specific ramifications for human and ecosystem health. Water quality is affected by changes in nutrients, sedimentation, temperature, pH, heavy metals, non-metallic toxins, persistent organics and pesticides, and biological factors, among many other factors (Carr and Neary 2008). Following are brief discussions of these major contaminants. Many contaminants combine synergistically to cause worse, or different, impacts than the cumulative effects of a single pollutant. Continued inputs of contaminants will ultimately exceed an ecosystem’s resilience, leading to dramatic, non-linear changes that may be impossible.
