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Cooperative Extension Service |
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Agricultural
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Arbor Day
Dale Bumpers College
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Site EvaluationWhy is the site evaluation important? • What's involved in completing this evaluation? • How do soils affect the potential for groundwater contamination? • How do subsurface and geologic materials affect the potential for groundwater* contamination? • Part 1: Evaluating the Soil on Your Farmstead • Part 2: Evaluating Subsurface and Geologic Materials on Your Farmstead • Part 3: Combining Your Farmstead's Why is the site evaluation important? How farmstead practices affect groundwater depends in part on the physical characteristics of your farmstead site: soil type, bedrock characteristics, and depth to groundwater. That's why evaluating the soils and geologic characteristics of your farmstead is such an important step in protecting the groundwater you drink. What's involved in completing this evaluation? This evaluation has four parts:
Getting the information to complete parts 1 and 2 will require assistance from outside sources, such as your county Natural Resources Conservation Service (NRCS) or Extension office. How long this takes will vary depending on the availability of the information in your county. Once you have the information, though, it should take about an hour to complete the first three parts of Work Sheet #9. The farmstead diagram will take additional time. If some of the information you need isn't readily available, the work sheet contains instructions on how to proceed. The more information you can get, the better; but some information is better than no information. The definitions on the last page may help you better understand some of the words that are used. How do soils affect the potential for groundwater contamination? Soil characteristics are very important in determining how a contaminant breaks down into harmless compounds or moves through soil and into the groundwater. Because most breakdown occurs in the soil, there is a greater potential for groundwater contamination in areas where contaminants are able to move quickly through the soil.
While held securely to soil particles, contaminants are broken down by bacteria, other soil organisms, and by reactions with minerals and natural chemicals in the soil. Most chemical and biological breakdown takes place in the loose, cultivated surface layers, where the soil tends to be warm, moist, well-aerated, and high in organic matter. Soil organic matter is also important in holding contaminants. Soils high in organic matter provide an excellent environment for chemical and biological breakdown of these contaminants. The soil's natural purification capability is limited. Certain conditions, such as heavy rainfall and chemical spills, may exceed the soil's purification capacity, allowing the chemical to move through the soil relatively quickly. In such cases, the subsurface geologic material and the distance a contaminant must travel to groundwater are important factors in determining whether a contaminant actually reaches the groundwater. How do subsurface and geologic materials affect the potential for groundwater* contamination? Depth to groundwater is important because it determines two important factors: (1) the thickness of material through which contaminants must travel before reaching the local aquifer and (2) the length of time contaminants are in contact with the soil. Other factors being equal, the greater the thickness of material above the groundwater, the less chance of contamination from the ground surface. Bedrock geology influences groundwater pollution when the local aquifer is below the bedrock surface. Movement of contaminants in fractured rock is difficult to predict, and pollutants can travel as far as one mile from their source. Where bedrock material contains significant fractures that are in contact with the local aquifer, the thickness and characteristics of the soil above the fractures largely determine the potential for groundwater contamination. *Groundwater = local aquifer = the most shallow water-bearing strata that is used as a water source for human and/or animal consumption. A word of caution As with the results of the previous 8 work sheets, use the rankings from this work sheet cautiously. Many factors affect whether or not a contaminant will get into the local aquifer. There is no guarantee that a "low-risk" site will be uncontaminated - or that groundwater will become contaminated at a "high-risk" site. The type of contaminant involved, how you handle and store potential contaminants, the location and maintenance of your well, and many other factors can affect the potential for groundwater contamination. Part 1: Evaluating the Soil on Your Farmstead To complete your soil evaluation, you will need a copy of your county's soil survey report. This report is available at most county offices of NRCS or Extension Service. Step 1: Start by locating your farmstead on the aerial photos in the soil survey, note the soil mapping unit indicated on the photo, and look up information related to that soil in the written sections of the soil survey report. If you have more than one soil mapping unit on your farmstead, rank each soil individually (up to 3) using this work sheet. These rankings describe soil in native, undisturbed conditions. If your farmstead soil has been altered by human activities, such as clearing, tilling, or ditching, contact your county Extension agent or your NRCS office for assistance. Don't skip any parts of the work sheet. If you are not familiar with using soil surveys, you may need help completing Part 1. Ask your county Extension agent or your NRCS specialist to help you find the following information:
Step 2: Determing Soil Characteristics: With the information in hand from Step 1, you are now ready to rank your soil(s) according to seven characteristics. For each of the seven characteristics highlighted in the left column, find information about your soil in the soil survey. Then, match your soil description to the description in the middle column to determine your score in the right column. For example: The soil survey says that the texture of one of your soils is a clay loam. Your score for that characteristic would be 8. Enter your scores for up to 3 soils in the spaces indicated.
Step 4: From the box below, find your score in the listed ranges in the left column. Then identify your soil's "potential to protect groundwater" and find the rank number assigned to your score.
Step 6: Understand your soil(s) rankings. A soil with more than 50 points (ranking #4) probably is a deep, medium- or fine-textured, well-drained soil which contains 4 to 10 percent organic matter. Potential contaminants move slowly through the soil, allowing them to become attached to soil particles. Sunlight, air, and microorganisms then have time to break down the contaminant into harmless compounds. The groundwater contamination risk level is low. A soil with a score of 30 or less (ranking #1) is probably a coarse, sandy, extremely well-drained soil with less than 1 percent organic matter. Such a soil would allow most contaminants to move rapidly down toward the water table.
Part 2: Evaluating Subsurface and Geologic Materials on Your Farmstead This part looks at the subsurface and geologic materials beneath your farmstead's soils. Completing the work sheet will give you a much clearer picture of your site's potential for keeping pollutants out of groundwater. For example, the soil evaluation might have indicated a moderate potential for protecting groundwater. However, if the soils are fairly thin and lie over fractured bedrock, the potential for groundwater contamination at the site is probably higher than indicated by the soil evaluation alone. This part requires two items of information: (1) your site's subsurface geologic material and (2) depth to the local aquifer. Unfortunately, information on subsurface geologic material, as well as depth to water, is often difficult to obtain.
Try not to skip any steps in this part. Ask your county Extension agent or NRCS personnel to help you gather the information and provide assistance in completing Part 2. Step 1: Find the information you need - from the soil survey, well construction reports, Geological Commission reports, or Extension and NRCS personnel to identify (1) the geologic materials beneath your farmstead and (2) depth to local aquifer. Step 2: Match the information on your site's geology to one of the descriptions in the table that follows. You will be choosing only one description from the entire table. If your well construction report describes more than two types of geologic material below 5 feet, ask for help in filling out this section from your county Extension or NRCS office. Step 3: When you have chosen the description that best matches your site's geology, read across to the right until you get to the appropriate "depth to local aquifer" for your site and circle that score for your farmstead. For example, you may determine from your well construction report that geologic material beneath your farmstead consists of 30 feet of coarse-textured, unconsolidated material over fractured bedrock, and that depth to local aquifer is 15 feet. Looking down the left column to find your category, and then going across to the right, you see that your rank is "1."
The table below shows what your subsurface geology
ranking means.
A ranking of "4" shows that the subsurface material has the best potential to protect groundwater. This material has small pore spaces, groundwater is at least 10 feet from the soil surface, and the risk of groundwater contamination is low. A ranking of "1" indicates a material with poor potential to protect groundwater. Its large pore spaces allow contaminants to move downward easily, increasing the risk of groundwater contamination. In highly fractured rock or in very coarse-textured, unconsolidated materials, the depth to groundwater doesn't seem to matter, because some contaminants will flow through the pore spaces with very little slowdown.
Part 3: Combining Your Farmstead's Soil and Subsurface/Geologic Rankings Combining the rankings from parts 1 and 2 will provide you with a good overall ranking of your farmstead site's potential to keep pollutants from moving down to groundwater. Step 1: Transfer your boxed rankings from the soil evaluation (Part 1, step 5,) and the subsurface/geologic evaluation (Part 2, step 4) to the boxes below:
Step 2: The table below shows the overall level of
groundwater contamination risk associated with your farmstead site conditions.
Find your two numbers from step 1 on page 11, written in the correct
sequence of "soils rank-subsurface rank," and circle the sequence in
the following table.
Step 3: Look above the sequence you circled to find your risk level and your ranking. (For example, if your numbers are 3-2 "soils rank-subsurface rank," your site is in the low-moderate risk column and your ranking is 3.) Step 4: Enter your combined ranking here. (If you calculated more than one soils ranking, calculate a combined ranking for each soils ranking.)
Step 5: Understand your combined ranking. In general, a site with a combined ranking of 4 (low groundwater pollution risk) will have a soil with a good capacity to hold and break down contaminants. Its subsurface conditions will also keep contaminants from reaching the water table. Under certain conditions, however, such as spills, poor management, and heavy rainfall, contaminants may reach groundwater. On the other hand, if you carefully manage a site with a combined ranking of 1 (high groundwater pollution risk), you may not affect your drinking water. Both site characteristics and your management practices are of equal importance. Your three site ranking numbers (soils ranking, subsurface ranking, and
combined ranking) will be used again in Work Sheet #10. If you have more than
one soil on your farmstead, you will need to transfer individual soil rankings
and combined rankings to Work Sheet #14. It will be especially important for you
to complete Part 4 of this work sheet if you have more than one soil on your
farmstead, so that you can match particular site concerns with each farmstead
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University of Arkansas • Division of Agriculture |
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