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Pacific Southwest, Region 9

Serving: Arizona, California, Hawaii, Nevada, Pacific Islands, Tribal Nations

Test Methods for Unpaved Roads and Unpaved Parking Lots

The FIP rule requires owners/operators of unpaved roads and unpaved parking lots to comply with two standards -- a 20% opacity standard and a silt content standard. Silt content is not to exceed 6% for unpaved roads and 8% for unpaved parking lots or silt loading must be less than 0.33 oz/ft2. Described below is an opacity test method to be used to test compliance with the 20% opacity standard and a silt content test method to be used to test compliance with silt content/loading standards. If a source passes the opacity standard but fails the silt content/loading standard, or vice versa, it is not in compliance with the FIP rule.

For heavily trafficked sources, EPA recommends that the opacity test method be viewed as "Tier 1" and the silt content method as "Tier 2." If the source fails the opacity test method, it will not be necessary to conduct the silt content method at that time because the source is already deemed noncompliant. However, if not all of the conditions specified in the opacity test method can be properly met, or when a source passes the opacity test method, the silt content test method should be done to determine whether the source is in compliance with the appropriate silt content standard.

Opacity Test Method

The purpose of this test method is to estimate the percent opacity of fugitive dust plumes caused by vehicle movement on unpaved roads and unpaved parking lots.

Note: This method can only be conducted by an individual who has received certification as a qualified observer. Qualification and testing requirements can be found in [FR 63 41353, August 3, 1998 (Appendix A)].

Step 1:
Stand at least 16.5 feet from the fugitive dust source in order to provide a clear view of the emissions with the sun oriented in the 140-degree sector to the back. Following the above requirements, make opacity observations so that the line of vision is approximately perpendicular to the dust plume and wind direction. If multiple plumes are involved, do not include more than one plume in the line of sight at one time.

Step 2: Record the fugitive dust source location, source type, method of control used, if any, observer's name, certification data and affiliation, and a sketch of the observer's position relative to the fugitive dust source. Also, record the time, estimated distance to the fugitive dust source location, approximate wind direction, estimated wind speed, description of the sky condition (presence and color of clouds), observer's position to the fugitive dust source, and color of the plume and type of background on the visible emission observation form both when opacity readings are initiated and completed.

Step 3:
Make opacity observations, to the extent possible, using a contrasting background that is perpendicular to the line of vision. Make opacity observations approximately 1 meter above the surface from which the plume is generated. Note that the observation is to be made at only one visual point upon generation of a plume, as opposed to visually tracking the entire length of a dust plume as it is created along a surface. Make two observations per vehicle, beginning with the first reading at zero seconds and the second reading at five seconds. The zero-second observation should begin immediately after a plume has been created above the surface involved. Do not look continuously at the plume but, instead, observe the plume briefly at zero seconds and then again at five seconds.

Step 4:
Record the opacity observations to the nearest 5% on an observational record sheet. Each momentary observation recorded represents the average opacity of emissions for a 5-second period. While it is not required by the test method, EPA recommends that the observer estimate the size of vehicles which generate dust plumes for which readings are taken (e.g. mid-size passenger car or heavy-duty truck) and the approximate speeds the vehicles are traveling when readings are taken.

Step 5:
Repeat Step 3 and Step 4 until you have recorded a total of 12 consecutive opacity readings. This will occur once six vehicles have driven on the source in your line of observation for which you are able to take proper readings. There is no limit as to when the 12 consecutive readings must be taken. Observations immediately preceding and following interrupted observations can be considered consecutive.

Step 6:
Average the 12 opacity readings together. If the average opacity reading equals 20% or lower, the source is in compliance with the FIP rule's opacity standard.

Question and Answer - Opacity Test Method

If not all of the procedural conditions specified in Step 1 or Step 3 can be met, can the test method still be done?

The conditions specified in the test method are necessary for the test to be done properly. EPA recommends that, if the conditions can only be met at a certain time of day or a certain location, then the test method should be conducted at the appropriate time and/or location. If the test method cannot be done under the correct conditions due to logistics, EPA recommends that the test be done as consistently as possible with the specified conditions and that the source also be tested for silt content.

If the plumes of the first six vehicles for which opacity readings are taken do not seem to represent the majority of traffic that typically travels on the source, are the test method results valid?

It is possible that the plumes observed from six vehicles may not represent typical vehicle movement on a source. For example, some drivers may slow down to observe an individual standing at the edge of a road or parking lot, thus creating a smaller, less dense plume than faster-moving vehicles. Or, the six vehicles observed may be passenger cars as opposed to heavier vehicles, such as sport utilities or trucks, which may also travel on the road. EPA suggests that the opacity test method be repeated as many times as necessary to gain a representative sample of vehicle movement on the surface. It is important for owners/operators to know whether the source will be deemed in compliance if it is tested for enforcement purposes under different vehicle use conditions.

Silt Content Test Method


  • A set of sieves with the following openings: 4 millimeters (mm), 2mm, 1 mm, 0.5 mm and 0.25 mm and a lid and collector pan
  • A small whisk broom or paintbrush with stiff bristles and dustpan 1 ft. in width. (The broom/brush should preferably have one, thin row of bristles no longer than 1.5 inches in length.)
  • A spatula without holes A small scale with half ounce increments (e.g. postal/package scale)
  • A shallow, lightweight container (e.g. plastic storage container)
  • A sturdy cardboard box or other rigid object with a level surface
  • Basic calculator
  • Cloth gloves (optional for handling metal sieves on hot, sunny days)
  • Sealable plastic bags (if sending samples to a laboratory)
  • Pencil/pen and paper

Obtaining Equipment for Silt Content Test Method

The purpose of this test method is to estimate the silt content of the trafficked parts of unpaved roads and unpaved parking lots. The higher the silt content, the more fine dust particles that are released when cars and trucks drive on unpaved roads and unpaved parking lots.

Step 1:
Look for a routinely-traveled surface, as evidenced by tire tracks. [Only collect samples from surfaces that are not wet or damp due to precipitation, dew or watering.] Use caution when taking samples to ensure personal safety with respect to passing vehicles. Gently press the edge of a dustpan (1 foot in width) into the surface four times to mark an area that is 1 square foot. Collect a sample of loose surface material using a whisk broom or brush and slowly sweep the material into the dustpan, minimizing escape of dust particles. Use a spatula to lift heavier elements such as gravel. Only collect dirt/gravel to an approximate depth of 3/8 inch or 1 cm in the 1 square foot area. If you reach a hard, underlying subsurface that is < 3/8 inch in depth, do not continue collecting the sample by digging into the hard surface. In other words, you are only collecting a surface sample of loose material down to 1 cm. In order to confirm that samples are collected to 1 cm. in depth, a wooden dowel or other similar narrow object at least one foot in length can be laid horizontally across the survey area while a metric ruler is held perpendicular to the dowel.

At this point, you can choose to place the sample collected into a plastic bag or container and take it to an independent laboratory for silt content analysis. A reference to the procedure the laboratory is required to follow is at the end of this section.

Step 2:
Place a scale on a level surface. Place a lightweight container on the scale. Zero the scale with the weight of the empty container on it. Transfer the entire sample collected in the dustpan to the container, minimizing escape of dust particles. Weigh the sample and record its weight.

Step 3:
Stack a set of sieves in order according to the size openings specified above, beginning with the largest size opening (4 mm) at the top. Place a collector pan underneath the bottom (0.25 mm) sieve.

Step 4:
Carefully pour the sample into the sieve stack, minimizing escape of dust particles by slowly brushing material into the stack with a whisk broom or brush. (On windy days, use the trunk or door of a car as a wind barricade.) Cover the stack with a lid. Lift up the sieve stack and shake it vigorously up, down and sideways for at least 1 minute.

Step 5:
Remove the lid from the stack and disassemble each sieve separately, beginning with the top sieve. As you remove each sieve, examine it to make sure that all of the material has been sifted to the finest sieve through which it can pass; e.g. material in each sieve (besides the top sieve that captures a range of larger elements) should look the same size. If this is not the case, re-stack the sieves and collector pan, cover the stack with the lid, and shake it again for at least 1 minute. (You only need to reassemble the sieve(s) that contain material which requires further sifting.)

Step 6:
After disassembling the sieves and collector pan, slowly sweep the material from the collector pan into the empty container originally used to collect and weigh the entire sample. Take care to minimize escape of dust particles. You do not need to do anything with material captured in the sieves -- only the collector pan. Weigh the container with the material from the collector pan and record its weight.

Step 7:
If the source is an unpaved road, multiply the resulting weight by 0.38. If the source is an unpaved parking lot, multiply the resulting weight by 0.55. The resulting number is the estimated silt loading. Then, divide by the total weight of the sample you recorded earlier in Step 2 and multiply by 100 to estimate the percent silt content.

Step 8:
Select another two routinely-traveled portions of the unpaved road or unpaved parking lot and repeat this test method. Once you have calculated the silt loading and percent silt content of the 3 samples collected, average your results together.

Step 9:
Examine Results. If the average silt loading is less than 0.33 oz/ft&sup2, the surface is STABLE. If the average silt loading is greater than or equal to 0.33 oz/ft&sup2, then proceed to examine the average percent silt content. If the source is an unpaved road and the average percent silt content is 6% or less, the surface is STABLE. If the source is an unpaved parking lot and the average percent silt content is 8% or less, the surface is STABLE. If your field test results are within 2% of the standard (for example, 4%-8% silt content on an unpaved road), it is recommended that you collect 3 additional samples from the source according to Step 1 and take them to an independent laboratory for silt content analysis.

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Independent Laboratory Analysis

You may choose to collect 3 samples from the source, according to Step 1, and send them to an independent laboratory for silt content analysis rather than conduct the sieve field procedure. If so, the test method the laboratory is required to use comes from the from the following text:
Procedures For Laboratory Analysis Of Surface/Bulk Dust Loading Samples, (Fifth Edition, Volume I, Appendix C.2.3 "Silt Analysis", 1995), AP-42, Office of air Quality Planning & Standards, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina.
This method is available at the Technology Transfer Web site

Question and Answer - Silt Content Test Method

If I see dust escaping when I collect a sample and transfer it to the sieves, should I start over?

Not necessarily. A small amount of dust can escape without influencing the silt content results. In fact, it is very difficult to avoid having some dust escape. However, if you rush when collecting and/or transferring a sample to the sieves, you may cause too much dust to escape thus potentially causing an error in your results. Or, on a relatively windy day you may lose too much dust unless you set up a wind barricade. Avoid doing this test on very windy days.

Once I calculate the percent silt content for 3 samples collected on one segment of an unpaved road, can I assume the same result for the whole length of the road?

You may extrapolate results only to the extent that the rest of the unpaved road has the same average daily trips as the segment you tested and the surface condition on other segments of the road is the same.

If water is being used as a control measure on the source and this causes the surface to be damp, should I do the silt content test method on a damp surface?

Do the silt content test method when the surface is dry in between waterings. The condition of the surface immediately following watering is different than after the water has evaporated. Since sources are required to be in compliance with the rule at all times, test the surface when it is dry.

If speed limit signs have been posted along an unpaved road as a control measure, do I need to test the surface for silt content?

Answer: Yes. If speed limit signs have effectively lowered vehicle speeds on the road, the percent silt content may decrease. If signs have been ineffective in controlling speeds and no other controls are being applied, the source may be out of compliance. Either way, you should test to see whether the source meets the appropriate silt content standard.

Obtaining Equipment for Proposed Silt Content Test Method

A set of sieves with the following openings: 4 millimeters (mm), 2mm, 1 mm, 0.5 mm and 0.25 mm, a lid and collector pan can be purchased at a laboratory supply store. Such stores may be listed in telephone directories under "Laboratory Equipment & Supplies" or you may conduct an internet search using the key words "test sieve". When ordering sieves, specify the following:

  • Tyler sieve numbers 5, 10, 18, 35 and 60, respectively. (If you already have access to a set of sieves, but the Tyler sieve numbers are slightly different than those specified here, you may still be able to conduct the test with sufficient accuracy. Contact EPA with any questions of this nature.)
  • 8 inch diameter sieves as opposed to 12 inch diameter. (The 8 inch diameter sieves are easier to handle due to less weight and bulk.)
  • Full height (as opposed to half height) individual sieves.
  • While optional, special certification of the mesh sizes on each sieve is not necessary.

If you do not wish to purchase sieves, you may collect three samples according to Step 1 of the Silt Content Test Method and deliver them to an independent laboratory for silt content analysis. Testing facilities may be listed under "Laboratories - Testing" in telephone directories.

The remaining equipment needed for the Silt Content Test Method is commonly available at hardware stores, grocery stores, or office supply stores.

For more information:

Please contact Colleen McKaughan, Associate Director, air Division, U.S. EPA Region 9 at (520) 498-0118. Send questions and comments to r9.phoenixdust@epa.gov.

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