The source of water in most ponds is rainfall. Water evaporates from water bodies and other moist surfaces as water vapor, and in the atmosphere, water vapor condenses into droplets that form clouds. These droplets coalesce, and if they grow large enough, they fall as raindrops, snowflakes, or other forms of frozen precipitation. In other words, rain and other forms of precipitation are produced by the distillation of water by natural processes.
The atmosphere consists of gases, but it also contains suspended solid contaminants. Spray propelled into the air from the tops of breaking waves in the sea evaporates. The salts that were in the spray that evaporated are swept into the air. Mineral particles on soil surfaces are also swept up along with other dust by the wind. Contaminants from the combustion of fuels and wildfires, such as nitrous oxide and sulfur dioxide, are emitted into the air along with soot and other fine particles. Because solid particles suspended in the atmosphere provide surfaces upon which water vapor condenses, they are called the nuclei of condensation, and these particles are dissolved or suspended in rainwater.
As water drops fall through the atmosphere, they also become saturated with the atmosphere's primary gases: nitrogen, oxygen, argon, and carbon dioxide. Rainwater at 77°F (about 25°C) contains around 13.64 parts per milligram (ppm) of nitrogen, 8.24 ppm of oxygen, and 0.57 ppm of carbon dioxide.
Water vapor enters the atmosphere as pure water (H2O), but as it condenses into water droplets, grows by coalescence into clouds and raindrops, and falls through the atmosphere to the earth's surface, it acquires dissolved and suspended solids and dissolved gases. Pure water has a pH of 7.0, but carbon dioxide in rainwater depresses pH to around 5.6. In some areas, combustion of fuels releases considerable sulfur dioxide and nitrogen oxides, which oxidize to form sulfuric and nitric acids. Rainwater in such areas may have a pH of 2-4.
Acid Rain.
Rainwater contains small but measurable concentrations of inorganic ions. The average concentrations of major ions in rainfall across the conterminous United States, given in a US Geological publication, are presented (Table 1). Pond water typically is more concentrated in ions than rainwater, and especially in bicarbonate, calcium, and magnesium (Table 2). This occurs because ponds are usually filled by runoff from watersheds. As it flows downslope into ponds, runoff contacts mineral matter in soil and dissolves both major ions and trace elements. Once impounded, runoff remains in contact with the bottom soil and typically becomes more concentrated in dissolved ions.
The average retention time of water in ponds typically varies from one month to one year in areas with 30-60 inches of rainfall per year and even longer in more arid regions. Of course, the retention time of water also varies with evaporation rate and the amount of runoff received. Runoff per acre in a watershed varies with soil type (less for sandy soils and more for loamy and clayey soils). Vegetative cover influences the amount of runoff, with the most derived from short grass cover and the least from woodland. The slope of watersheds also affects the amount of runoff, with the most from steep slopes and the least from nearly level land. As a rule, between 10 to 70% of rainfall becomes runoff, and 20% to 40% is normal.
The area of the watershed with respect to the volume of a pond greatly influences the hydraulic retention time. Small ponds with large watersheds are quickly flushed with new runoff. This favors lower concentrations of dissolved solids. On the other hand, ponds with small watersheds will be flushed less, and in dry weather their volume may decline, concentrating dissolved solids.
I know of an incident in which a pond with a plastic liner was built in a very arid area. The water level was maintained by additions of water from the municipal water supply. After three years, the water had become so saline that the freshwater fish which had been stocked in the pond died. Evaporation allowed salt to build up.
Pond waters vary greatly in chemical composition as illustrated for ponds in different physiographical areas in Alabama and Mississippi (Table 2) all of which have a rather relatively similar annual rainfall and evaporation regimes. I will not bother you with descriptions of the physiographical areas, but in humid areas, the composition of the pond water relates primarily to the chemical properties of the different soils. In arid areas, soil properties also influence the chemical composition of pond water, but the effect of greater evaporation than precipitation in concentrating dissolved ions usually overshadows the influence of soil properties. Some ions may reach saturation concentrations in pond waters of arid regions resulting in precipitation of certain salts.
The food web in most sportfish ponds is based on phytoplankton productivity. The two major nutrients usually in short supply are nitrogen and phosphorus. Ponds on most watersheds will require nitrogen and phosphorus inputs in fertilizer to have optimum fish production. Phosphorus is especially critical, because rainfall contains little phosphorus, and it usually is at low concentration in unpolluted surface water. Rainfall is a significant source of ammonia and nitrate nitrogen (Table 1).
Ponds also need water with at least 20-30 ppm total alkalinity to respond properly to fertilizers and to maintain a suitable pH range. Ponds in humid areas where soils do not contain limestone frequently require liming with agricultural limestone. For example, note in Table 2 that ponds in the Coastal Plain region of Alabama and Mississippi and along the Alabama Piedmont Plateau and Appalachian Plateau, where limestone is not common in soils, typically require liming. In humid areas with calcareous soils, Yazoo Basin of Mississippi, Blackland Prairie in Mississippi and Alabama, and the Limestone Valleys and Uplands of Alabama (Table 2), ponds will seldom require liming. In arid regions, liming seldom is necessary.
Sweeney and Texas Hunter automatic fish feeders. Cargill Fish Food. Aeration Systems. Artificial Habitat. Bulk Lime/Gypsum, Floating Docks
Dr. Claude Boyd is a professor emeritus in the School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, Alabama 36849. His work with water quality is internationally renowned. His most recent book, Handbook for Aquaculture Water Quality, is a must for anyone interested in learning about water chemistry and how it relates to your pond. It's technical, thorough, but easy to read and understand. Buy it at www.pondboss.com in the online store.
Reprinted with permission from Pond Boss Magazine
