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Water Quality and Use

  MG Manual Reference
Ch. 16, pp. 2 - 4

Lawn and garden chemicals, such as fertilizers and pesticides enter groundwater in two ways. First, they can be carried by rainwater into a stream as runoff. The contaminants can eventually reach groundwater supplies if the stream loses water to the ground. The most common method of groundwater contamination is leaching. Leaching is the downward movement of a dissolved substance through the soil. Residual fertilizers in the soil that plants cannot use, leach through the soil along with rain or irrigation water. Pesticides too, can leach into groundwater, or they may vaporize into the atmosphere, or be broken down by microbial and chemical means into harmless substances.
The quantity of an applied chemical that reaches groundwater depends on two factors, persistence and solubility.
Persistence is the length of time a chemical remains active before it is broken down; it is measured in "half-life" units. Each half-life unit measures the time it takes for one-half of the original amount of a pesticide to break down. Chemicals with a long half-life remain active in the soil for a long time.
Solubility refers to how easily chemicals dissolve and mix with soil water. Chemicals entering soil will adhere to soil particles or dissolve into the soil water. Highly-soluble chemicals have greater potential to move into groundwater supplies than low-soluble ones. The combination of persistence and solubility measures a chemical's potential to contaminate groundwater. A very soluble chemical with a long half-life would have the greatest potential for contamination. On the other hand, a chemical with a short half-life and low solubility would not remain active long, and would be less likely to leach into groundwater. Slow release fertilizers remain in the soil longer for plant use than very soluble fertilizers.
As discussed before, soil texture, organic matter content, and pH all affect the movement of chemicals through soil. Also, the availability of water, either by rainfall or irrigation, directly effects solubility. Understanding these factors is important for developing sound management practices in lawn and garden care.

Water enters soil naturally through precipitation. Part of this water evaporates into the atmosphere; plants take up another part; the rest gradually percolates downward to become groundwater. This percolating water, called recharge, passes downward through the root zone until it reaches the water table. Immediately below the water table lies the saturated zone. In this zone, all pore spaces are filled with water.
Underground layers of sand, gravel, or permeable rock that contain enough water to serve as a water source are called aquifers. Aquifers vary greatly in size. Smaller ones may supply only a few wells. Some aquifers are large enough to supply water to thousands of households and business.
Groundwater can become contaminated from the chemical fertilizers and pesticides which we use on our lawns and gardens. As recharge water percolates through the soil, it can carry these chemicals downward to the water table. Once in the saturated zone, these pollutants move with the groundwater, forming a region of contaminated water called a plume. Groundwater moves slowly, usually only a few feet per month. Therefore, it may take years before a plume appears in a water supply some distance away from the source of contamination.
Soil can filter out some contaminants. The following characteristics of soil affect this filtration process and the movement of chemicals:
Soil Texture refers to the relative amount of sand, silt, and clay particles a soil contains. Water and chemicals move quickly through coarse, sandy soils because the pore spaces between the particles are large. Clay particles are fine in texture. Water and chemicals move slowly through clay soils because the pore spaces are very small.
Soil Organic Matter contains decayed plant and animal materials. Soils high in organic matter can hold more water than soils low in organic matter.
Soil pH refers to the relative acidity/alkalinity of a soil. The pH level can affect the movement of chemicals, especially fertilizers, through a soil. In acidic soils (low pH), nutrients such as nitrogen, phosphorous, and potassium can become unavailable to plants. This may result in nutrients being transported down through the soil into the water table; a process called leaching.
Large enough amounts of fertilizers, pesticides, and other chemicals in an aquifer can make the water unfit to drink. Cleaning up a water supply, whether by human activity, or by natural filtration through soil, can take years. Preventing groundwater contamination is the best approach to managing our water supplies.

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