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Metals are most commonly considered to be those elements in the middle of the periodic table that can exist in several charged states (protons vs. electrons). Surface water normally contains trace amounts of metals that are either dissolved in the water or associated with minute solid mineral material that is suspended. Metal concentrations in solution are expressed in micrograms per liter (ug/L) or parts per billion (ppb). These are very small concentrations. Metals are transported within river systems in different forms. Two specific forms, total (not filtered) and dissolved (filtered) metals are sampled by the Conservancy's monitoring program. Total metals include forms that are bound up and not available to aquatic life in a potentially harmful way. For example, in the case of copper sulfate, CuSO4, Cu2+, and SO42- are paired. When the two molecules break up, Cu2+ is floating free and would be considered dissolved. Dissolved metals in water are determined using a filtered sample. Only single metals (dissolved) pass through the 0.45 micron filter. It is this form of the metal that is the most harmful to aquatic life if present in excessive amounts. All life forms require trace amounts of metals to carry out normal cellular functions. Iron carries oxygen from air to the cells in our bodies. Copper performs the same function for invertebrates such as in shrimp and snails. Zinc is essential for cell differentiation and growth. However, excess amounts of some dissolved or suspended metals can impair the ability of aquatic life to live and reproduce by interfering with oxygen uptake. Other metals like lead and cadmium are toxic to aquatic organisms. As with alkalinity, hardness, and pH, exposure to minerals in soils and rocks increases the concentration of dissolved metals in water. High concentrations of dissolved metals are often found in low pH (<5) waters that drain certain types of rocks, mined areas, and urban areas. Rapid runoff raises the concentration of suspended mineral material in water. Metals will always be present in some natural background state. Concentrations of metals may increase due to human-caused land and water use practices, such as mining and dumping. Metals can affect aquatic life in several ways. A sudden, high concentration can cause death and is called the "acute" exposure limit. A lower exposure over a longer period of time can cause a wide variety of behavioral and physical impacts and is called the "chronic" exposure limit. Many metals have a maximum standard classified by the WQCC. Yet a metal concentration that is found to exceed a standard does not necessarily imply that organisms are experiencing harm, as it is also important to consider the duration and frequency of the exposure. If a metal concentration were to exceed a maximum state standard for all sampling events over a three-year period, it can be concluded with a greater degree of certainty that organisms are experiencing the effects of toxicity. If one or two data points in three years exceed a threshold, it is less likely that significant toxic impacts are felt by organisms, although such information would be extremely valuable in investigating and addressing possible sources of metals' contamination. The State has standards for the more common metals, based on their potential to impact human or aquatic life. |
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