Site Evaluation Worksheet A

Why is the site evaluation important?

What is involved in completing this evaluation?

How do soils affect the potential for ground-water contamination?

How do subsurface and geologic materials affect the potential for ground-water contamination?

A word of caution

Why is the site evaluation important?

How homestead practices such as pesticide handling or wastewater disposal affect your ground water depends in part on the physical characteristics of the soil and geologic materials at your home site. These characteristics will control the physical and chemical response of contaminants that are introduced into the subsurface.

Although the focus of Home*A*Syst is on protection of the ground water and drinking water, preserving surface water quality is also important. Implementing the best management practices (BMPs) recommended in the fact sheets can also help protect surface water in two ways. First, implementing some BMPs may reduce runoff, which often carries significant amounts of contaminants to surface waterbodies. Second, because ground water is connected to surface water, contaminants that are transported to an aquifer may end up in downstream rivers, lakes, or wetlands. The dynamic interaction between surface and ground water is called hydraulic continuity. More information concerning hydraulic continuity can be found in the accompanying materials.

What is involved in completing this evaluation?

This evaluation has four parts:

Part 1 (PDF): Evaluating the geologic material at your homestead
Part 2 & 3: Evaluating the soil at your homestead & Combining the soil and                   geologic risk ranking
Part 4 (PDF): Diagraming your homestead (optional)

Obtaining the information to complete parts 1 and 2 may require assistance from outside sources, such as your county Soil Conservation District (SCD), Natural Resources Conservation Service (NRCS) or Cooperative Extension System (CES) office. How long this takes will vary depending on availability of information in your county. Once you have the information, though, it should take about an hour to complete the first three parts of Worksheet A. The homestead diagram will take additional time.

How do soils affect the potential for ground-water contamination?

Soil characteristics are very important in determining whether a contaminant breaks down to harmless compounds or leaches into ground water. Because most contaminant breakdown occurs in the soil, a greater potential for ground-water contamination exists in areas where contaminants are able to move quickly through the soil.

While held to soil particles, contaminants are broken down by bacteria and other soil organisms and by chemical reactions with minerals and natural chemicals in the soil. Most of this chemical and biological breakdown takes place in the surface layers, where the soil may be warm, moist, high in organic matter, and well aerated.

Sandy soils have large pore spaces between individual particles, and dissolved contaminants can move rapidly through the soil and into ground water. Also, sandy soils provide little surface area onto which contaminants can become attached or "adsorbed." On the other hand, clay soils are made of very small particles which slow the movement of water and dissolved contaminants through the soil. Some contaminants become strongly attached to clay particles. And finally, soils that are high in organic matter provide an excellent environment for chemical and
biological breakdown of contaminants.

How do subsurface and geologic materials affect the potential for ground-water contamination?

Materials that lie below the soil, or geologic materials, vary depending on where you are located geographically. In Idaho, this material varies from deposits of clay, silt, sands, gravels, and cobbles (called unconsolidated alluvium), basalt, mixed layers of volcanic deposits and
sediments, granite, or in a few areas, limestone.

The nature of the materials can affect the potential for ground-water contamination. Contaminants can move very quickly through deposits of sand, gravel, and cobble, fractured basalt or granite, or limestone with connected solution cavities. On the other hand, deposits of clay and silt will slow the movement of contaminants.

In general, it can be said that all the major ground-water systems in Idaho have either medium high to high potential for becoming contaminated. The presence or absence of clay above the water table seems to be the leading factor that reduces the potential for contamination. Another factor, the depth to the water table, has often been observed to be a much less significant factor.

A word of caution

As with the results of the previous assessment worksheets, use the rankings from this worksheet cautiously. Many factors affect whether or not a contaminant will reach the ground water. There is no guarantee that a “low risk” site will be uncontaminated, or that ground water will become contaminated at a “high risk” site. The type of contaminant involved, how you handle and store potential contaminants, and many other factors can affect the potential for ground-water contamination.