Lakes, rivers, reservoirs, ponds, etc., are termed surface waters. They receive water directly from precipitation and surface run-off. These various bodies of water also receive a portion of their total amount from underwater springs connected with the groundwater supply. The previous diagram (Groundwater Water Zones and Belts) shows how the bed of streams extends below the groundwater level.
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As we have seen, surface waters are generally lower in mineral content. On the other hand, they possess far more contamination and are unsafe to use for human consumption unless properly treated.
Pollution of water comes from many sources. Municipalities and industries sometimes discharge waste materials into bodies of water that are used as public sources of supply. This is a most serious source of contamination. Surface run-off also brings mud, leaves, decayed vegetation together with human and animal wastes into streams and lakes. In turn, these organic wastes cause algae and bacteria to flourish.
There is a belief that rivers and streams purify themselves in the course of their flowing 20 miles. This action should not be taken for granted, however.
Organic pollution of water is reduced by nature in many ways:
- Bacteria and algae consume large quantities of organic waste. Larger microorganisms devour the bacteria and algae. In turn, the microorganisms provide food for fish and other higher forms of animal life.
- Unless the rate of flow is too fast, mud and suspended matter will naturally settle to the bottom and oxidation will render organic matter harmless. Rough bed streams, riffles and spill-ways speed this process.
- Due to its ultraviolet rays, sunlight also has some germicidal effect on the water. Sunlight is not constant due to cloudy weather and its unavailability at night.
Algae: any of a group of one-celled or many-celled microorganisms which are found in water and damp places. Algae contain chlorophyl and have no true root, stem, or leaf structure. Included among the algae are seaweed and pond scum.
Riffle: is a shoal, reef, or rocky obstruction in a stream of water. As the water flows over and around the obstruction, a stretch of shallow, rapid, or choppy water is produced depending on the nature of the obstruction.
Rivers and streams also show great variations in their dissolved mineral content. Tests taken over a period of a year at both the Rock River and the Arkansas River showed that both had the same average bicarbonate content of 207 ppm. In contrast, the Rock River had a total chloride and sulfate content of 30 ppm while the Arkansas River contained 613 ppm of these ions, mostly present in the form of hardness compounds.
In general, lakes and reservoirs (especially large ones) show fairly constant dissolved solids content. Because they are relatively more quiet than moving bodies of water, lakes and reservoirs are very efficient settling basins. The result is they possess less turbidity. Large bodies of water are frequently subject to seasonal changes that cause the water to become quite turbid for a period of time. Our definition of water at the beginning of this lesson states that it achieves its maximum density at 39.2'F. As it becomes chilled to this point in the fall or warmed to it in the spring, the denser water cannot stay at the top. As it sinks, it causes convection currents to be set up. Sometimes these become so strong that they lead to a complete overturning of the water and bring about the turbid condition. Heavy storms will also churn up a lake or reservoir and make it turbid.