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Dust Suppressant Test in the Hieroglyphic Mountains Northwest of Phoenix, Arizona (TPD0806011)


Two liquid dust palliatives were tested by driving vehicles over treated test plots while a trained observer assessed the amount of dust generated. Observations were made periodically over a six month period. Data analysis was conducted to determine product effectiveness as compared to an untreated surface.

Executive Summary:

In 2007, a new Arizona state law requiring dust management within Maricopa County caused the county to promulgate new dust rules. To assist the county in achieving air quality goals, BLM conducted a test of two manufactured liquid dust suppressants. The test was funded by the Arizona State Parks Off-Highway Vehicle Fund. Two liquid dust suppressants were sprayed on an existing dirt road at the Boulders Staging and Camping Area northwest of Phoenix, Arizona.  They were tested from December 2007 through May 2008.

The test was simple. BLM and contract staff drove a truck, ATV and motorcycle over the three 450ft test sections at predetermined speeds while a Maricopa County dust inspector judged the amount of dust created. A control, or untreated section, was used to evaluate the effectiveness of the Durasoil and Soiltac treatments as compared to bare soil.

The amount of dust generated, also measured as opacity, must be lower than a rating of twenty percent to pass county air quality standards. The test revealed that the suppressant named Durasoil worked very well, while the other, Soiltac did not. Durasoil works well due to its non-drying properties. This inert chemical looks and feels much like baby or mineral oil and can be sprayed like water onto dirt roads and trails.

Periodic tests were conducted over six months. The data was recorded and analyzed. Cost per vehicle pass was determined by extrapolating the expected vehicle counts over the application lifespan and dividing it into material cost. Durasoil was the lowest cost application. It calculates to $0.335 per vehicle trip mile for a truck. The cost goes down by half for an ATV ($0.168), and significantly down again for a motorcycle ($0.028). Soiltac cost is extremely high at $3.29 per vehicle trip mile for a truck, $0.66 for an ATV, and $0.22 for a motorcycle due to poor lifespan and high product cost.

Although these cost calculations are simplistic, the estimates offer a fiscal approach to an engineering solution which permits recreation, including OHV, in air quality sensitive areas. The statistical results from this study confirm that vehicle generated dust can be suppressed without daily watering.

The use of dust suppressants is a principal option in managing all uses within the dust boundary. This test was deemed successful and a follow up test is being scheduled to test other dust suppressants in an effort to find a lower cost solution comparable to Durasoil.

Special thanks are given to BLM’s partners in conducting this test. Maricopa County Environmental Quality provided two dust inspectors for the duration of the test at no cost to BLM. Arizona Off-Highway Vehicle Coalition provided a qualified ATV operator with an ATV for the duration of the test.

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Purpose and need:

An area on Bureau of Land Management (BLM) land popular for off-highway vehicle use is within the air quality boundary for serious non-attainment of particulate matter smaller than ten microns, also known as PM10. The intent of the PM10 boundary around the metro Phoenix, Arizona area is to improve air quality and overall citizen health. Furthermore, Maricopa County has promulgated rules 310 and 310.01 to manage blowing dust, also known as fugitive dust. To help Maricopa county meet air quality standards, BLM decided to conduct a test to determine if dust from passing vehicles can be reduced by spraying commercially available dust suppressants on dirt roads and trails.

Overall Goals:

  1. Find a solution for reducing airborne dust caused by vehicle passage and blowing wind
  2. Comply with Maricopa County’s 20% opacity rule for fugitive dust on access roads and parking lots by ensuring vehicles create dust opacity of 20% or less.
  3. Improve camping and riding experiences for recreationists
  4. Positively contribute to citizen health
  5. Determine baseline application lifespan and costs for workable solutions

Test Parameters and experimental procedure:

Known / Given:

  • A dusty road used as entry/exit route, to trails, at a popular off-highway vehicle staging area in a PM-10 serious non-attainment area.
  • Soil type = Ebon very gravelly loam, 1 to 8 percent slopes, 5-10% clay, 0-4% silt
  • Heavy vehicle use primarily on weekends, October 1- May 15
  • Maricopa County Department of Environmental Quality (DEQ) stabilization tests (Opacity observation test method)
  • Maricopa county dust inspectors available to conduct opacity method observations as described in Maricopa county rule 310.01, Appendix C, section 2 Test methods for stabilization.


  • Durasoil liquid dust suppressant and the application rate on a light use road
  • Soiltac liquid dust suppressant and the application rate on a light use road
  • Three motor vehicles – truck, all-terrain vehicle (ATV), off-highway motorcycle
  • Weather conditions in Phoenix, Arizona in fall, winter and spring
  • Traffic level and type of use between test periods
  • Operator skill to keep tires from spinning on uneven terrain
  • Consistency of vehicle test speed
  • Availability of test vehicles to administer tests


– By applying a spray on dust palliative, dust emissions can be reduced to meet Maricopa county Department of Environmental Quality standards of less than 20% opacity.

– The length of time the dust palliative remains effective will depend upon the number and type of vehicle passes and the precipitation received. Other factors such as surface preparation and presence of spin turn marks may lessen the application’s effective time.

– It is possible that the dust palliative’s effectiveness will have a major drop off point based on number of vehicle counts or time.

– Receiving precipitation will have a positive effect or negative effect on effectiveness (i.e. reactivation may occur, or dispersal may occur).

Experimental Goals:

  1. Determine the relationship between dust suppression and elapsed time – if effective time span is longer than test duration, extrapolate expected product lifespan
  2. Determine relationship between application effectiveness and number of vehicle passes. Consider the severity of accumulated passes if possible.
  3. Determine the opacity improvement of an application as compared to bare soil over time.
  4. Determine cost per mile by vehicle type for all applications.


350 gallon spray trailer (water buffalo) towed behind truck

PPE (Long pants/shirt, eye/foot protection, work gloves)

Wood stakes for road sections to be tested

Two active infrared vehicle counters, installed between control and test sections.

Dust measurement described in Maricopa County Fugitive Dust Test Methods manual (Rule 310)

1 Truck, 1 ATV, 1 dirt bike for testing


Stake three 150yard sections with a minimum of 100yards separating each test section. See Appendix H for a layout map.

Apply the following treatments to the test sections:

  1. Durasoil applied to unprepared soil surface (1gal Durasoil undiluted covers 30sqft, total 150gal)
  2. Soiltac applied to unprepared soil surface (1 gal Soiltac diluted at 7:1 covers 70sqft, total 450gal)
  3. No application – Control section

Apply the palliative on a Monday, Tuesday or Wednesday to allow it to set up as recommended (24hrs required).   See Appendix C for specifications and MSDS sheets.

Install two vehicle counters. Locate a counter between Durasoil and Soiltac sections and between Soiltac and Control test sections. Active infrared counters will be used. Accumulated vehicle counts to be recorded before each test.  See Appendix E for counter specifications.

Spray water on the test section approach, before the start of each evaluation, on the area between sections and test exit area to avoid fugitive dust from being carried into the test sections. Additionally, wait 10minutes for surface water to soak in such that mud is not tracked into test sections.

Drive (1) truck (20mph), (1) ATV (25mph), (1) dirt bike (30mph) over the test sections of road and measure airborne particulates for each individual vehicle. Repeat three times consecutively for each vehicle type, each test day. Allow the dust to settle between each of the consecutive trips.  Use a Garmin GPS as a speedometer on vehicles that do not have one.

Timing – Conduct tests during the high use period between October 1 and May 15.

Test vehicles –2005 Ford F-250 4×4 (truck), 2007 Polaris Sportsman 500 (ATV). 2001 Suzuki DRZ 250 (Motorcycle)

Conduct Opacity test evaluations for fugitive dust evaluation using a qualified dust inspector.

Record the data from Opacity test using the data collection form.

Record vehicle trip data from both vehicle counters.

Record the weather conditions – current and looking back to the previous evaluation.

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Table. Test schedule for Durasoil and Soiltac

Analysis of data:

Plot an X vs. Y graph of Opacity (percent) vs. test duration (days)

Plot an X vs. Y graph of Time (days) vs. Vehicle passes (counts)

Plot a bar graph of Opacity difference between Durasoil/Soiltac compared to control



Product effectiveness:

Having tested two products against an untreated control area, it has been determined that Durasoil is an effective application to reduce fugitive dust from vehicles. Both Soiltac and Durasoil initially reduced dust opacity, but only Durasoil performed well throughout the entire test. Durasoil was effective for the six month duration of the test and showed low opacity at the end of the test indicating it would continue to work for another year. Soiltac was not effective beyond two months, thus making it unsuitable for widespread vehicle fugitive dust suppression. Analysis shows that reapplication cost and frequency make it cost prohibitive. The Durasoil test section opacity was reduced by at least 10-15% in most tests.  It effectively cuts the dust generated in half. While there are many factors in dust generation, the data shows that the truck created the most dust on all test sections including the control, while the motorcycle created the least in all cases. Graphs in Appendix A show the results in detail.

Data Discussion:

This test occurred in the field where many variables were possible. Attempts were made to avoid unnatural variation by controlling several variables, namely the vehicle types and condition, speed, observers, and operators.  Some data points raised question of their validity.  The major data variations are discussed here.

One of the hypotheses was that there is a relationship between elapsed time and effectiveness. By taking data over time, the relationships could be determined by graphing. Opacity was expected to rise over time due to a variety of factors, yet it was never expected to decrease. An opacity decrease across all vehicle types was observed around 110days (Appendix A, Graph A1). There was no rain around this time, but there was a high wind advisory day which likely removed fine particles from all three test surfaces. Since this was a natural event, the data was retained. The opacity observation data could be deemed somewhat subjective since it is a visual observation except that there were two dust inspectors at most tests, and both inspectors noted similar opacity percentages on all days. Based on this, the observations were deemed reliable. Another notable observation is that the Durasoil section appeared to be increasing in surface compaction over the life of the test. It would appear that the non-drying properties allow for increased compaction and reduced dust generation.

Another hypothesis was that significant rainfall would either diminish or improve the performance of the dust suppressants. Only two days after the products were applied, almost two inches of steady rain fell. Rain continued to fall regularly throughout the first three months of the test, totaling 5.31 inches. Rain data is shown in Appendix F. When the ground dried out enough to make dust, tests were conducted on the scheduled test days. Since there was a control section, differences in opacity relative to soil moisture could be observed. Soil moisture content was not measured in this test. Product performance without rainfall may have produced different results. The test was originally scheduled for three months during the highest use season, but the data would have been more difficult to evaluate had the test been stopped at three months. Extending the test to six months allowed for more data points and the opportunity to see how Durasoil performed with more vehicle passes and extended dry weather.  There was no measurable rainfall for the last three months of the test.

Some equipment and procedure variations are worth noting.   During the test, the same equipment was used with relatively low wear showing on tires between tests. The vehicles were used occasionally between tests, so tire / knobby wear was minor, possibly insignificant.  Pictures and vehicle specifications are shown in Appendix E. On one occasion, the Polaris ATV was unavailable so a similar design Honda was used.  After reviewing the data from this day, the opacity observations were as comparable to previous tests, so the data was retained. On two occasions, only one dust inspector was available. The inspector making observations was an experienced person.  The data appeared to be in line with other days and was retained.

Only one data point was thrown out. During one of the passes in the Soiltac section, the truck was driven onto the untreated road shoulder. The opacity spiked to 45%. Since we were attempting to test the Soiltac performance and not driving skills, this data point was thrown out. There were two other truck Soiltac passes that were retained on this day.   Similarly, a Durasoil test using the truck showed high readings around 80 days. This data was retained because it fell under the 20% opacity limit and it was unclear as to the reason for the higher than usual reading. The observed opacity for the Durasoil section was the same as the control section. This was unexpected. One cause could be only one dust inspector was available and the opacity observations are made in 5% increments, making the difference between the data appear to be zero on paper. Rainfall had occurred only two weeks prior and number of vehicles passes only numbered about 4000 at the time.

Vehicle counter data was gathered in an attempt to determine how many passes the test sections were receiving in the highest use period of the year. The LP6 road is the main exit route to trails from the newly constructed Boulders Staging and Camping area northwest of Phoenix, eight miles west of Lake Pleasant on BLM land. It is suspected that sunlight and/or instability in the mounting caused one or both of the counters to have questionable reliability during the first two months of the test. The counters were remounted from trees onto pipes driven into the ground. Sensitivity was also adjusted to more accurately record multiple vehicles in a group. Staff observed the counts on a high use day to confirm the changes positively affected the counting. Data counts after two months were improved, yet memory limitations caused the counters to fill up at approximately 1,800 counts. In some instances, counters were checked and found to be maxed out. They were reset and the count recorded on the data sheets for the upcoming test. Closer observation of counters in the future and using the date stamp capability could improve the quality of data from the counters.

Opacity Data points for the graph datasets were input from the test data sheets. Opacity observations on all but two test days were conducted by two dust inspectors.  The method used to achieve a single opacity data point for graphing was to average the two observations per pass.  Diagram 1 below shows how the data was condensed down into a usable form for graphing. On the two tests where only one inspector made observations, there was no need to average the two readings. On these two occasions, Step 1 was simply bypassed and the three opacity observations were averaged as in step 2. Having a single inspector reduced the robustness of judging opacity.

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Diagram 1 – Methodology to Condense Data for Graphing

The number of data points could have been increased, thus adding accuracy to the test. Due to rain and moist soil conditions, three of the ten test days were cancelled. Rain initially was a factor in running the tests, yet when the tests were extended from three months to six months, the true nature of the materials could be observed.  Furthermore, the rainfall received in the beginning turned out to be an excellent test of solubility, presenting possibly the worst case scenario. In the end, both solubility and performance were observed and evaluated.

Opacity vs. Elapsed Time graphs discussion and results: (Graphs A1, A2, A3)

These graphs show the three test vehicles and their associated opacity observations as compared to the test duration measured in days. While there was considerable rain during the test period, the control section was evaluated at the same time the Soiltac and Durasoil sections were evaluated. This gives a baseline for whether or not remaining soil moisture was the main mechanism for dust suppression. Relationships between opacity and duration were sought using the standard graphing software in Microsoft Excel. The data points shown on the Durasoil graph A1 could point to an ever decreasing opacity as time increases, yet that seems unlikely. More data points over time would answer the question of what the slope of the curve should be. A least squares fit line relationship appeared most suitable and is shown in graph A1. Conversely, an exponential relationship was easily shown with the Soiltac test data. Soiltac loses effectiveness quickly at first, and then gradually continues to lose effectiveness as time increases as shown in graph A2. Graph A3 is simply the daily averaged data points from the control section. Since there was significant rainfall during the test, the control section opacity data did not have a trend. While no trend line was assigned, the data serves as a comparison for use against the other test sections as shown on graphs A6-A11.

Lifespan graphs discussion and results: Graphs A4, A5

The extrapolated lifespan graphs A4 and A5 show a projection of a curve or best fit line of the observed opacity percentage data to a point where all three of the vehicle types pass the 20 percent opacity threshold. The county dust standard is 20% opacity or less, so this was used as the extrapolation limit. The Soiltac graph shows short lifespan for Truck (one month) and ATV (2.5months), but a much longer duration for motorcycle (1.5years). Durasoil, however, shows a very long lifespan for the initial application of 1.5years for both trucks and ATVs and 3.5years for motorcycles.

The Soiltac motorcycle extrapolation might make it look like it could be a good application for motorcycle trails. Considering that the difference in opacity between the control and Soiltac sections for the motorcycle is minimal, the cost is not justified. A single pass by a motorcycle will meet or exceed 20% opacity even in the driest conditions according the observations made in this test. The following conditions were not tested, but likely have a negative effect on application lifespan: multiple vehicles in a group, driving with wheels spinning (or intermittent traction), higher speeds. Further testing could identify product limitations or the need for behavioral changes to meet dust standards. It should be recognized that solving the air quality issue will take equal parts of engineering, education and enforcement.

Opacity Differences graphs discussion and results: Graphs A6-A11

The bar graphs showing a particular vehicle types opacity data for Durasoil or Soiltac vs. the control section offers a visual explanation of amount of dust reduction observed. Since opacity as a percentage is difficult to describe, the bar graphs offer a means of showing visually the true difference in the amount of dust generated during the respective test passes. Each graph shows only one vehicle type and compares Durasoil or Soiltac to the untreated control section.

The Soiltac graphs A9-A11 show the dust suppressing ability of this product as used in this application.  For this test’s sprayed on method of application, the amount of dust reduction is not very high at only 2-7% opacity. On an instance shown on graphs A9 and A10, the Soiltac plot had a higher opacity than the control. This is an immediate failure since the purpose of applying the product is to reduce dust. Furthermore on Graphs A9-A10, the control section had a passing mark of 15% opacity, while Durasoil failed at 20-30%.  Perhaps if Soiltac was mixed in to a recently graded road and compacted, its performance would be higher. This could be tested at other sites in the future.

The Durasoil graphs A6-A8 shows the dust suppressing ability of this product as used in this application. Its non drying properties work very well when sprayed onto the unprepared road surface. Average opacity reductions were 6-15%.  The opacity observations were commonly half of the untreated control plot. Furthermore, the application maintained this level of performance for the six month duration of the test.

Cost Analysis:

Cost per mile for applying Durasoil is just over $10,000 per mile at the tested application rate.  By the end of the test, over 10,000 vehicle counts were recorded. The extrapolated length of service that might be expected from Durasoil is 1.5 years to 3.5 years, depending upon the type of use. (I.e. mostly truck, ATV or motorcycle). Assuming 20,000 vehicle passes per year based on the observed 10,000+passes over the six month test period and the shorter lifespan of 1.5years from graph A5 for truck traffic, treatment cost per vehicle would be $0.335 per vehicle mile traveled. The lowest cost vehicle to manage dust for is the motorcycle at a per vehicle mile traveled cost of $0.028.

Soiltac initial application cost per mile is lower than Durasoil due to a 7:1 dilution with water, yet the reapplication frequency would need to be much higher, thus raising its per trip cost tenfold. Soiltac cost per vehicle mile is extremely high at $3.29 per vehicle trip mile for a truck, $0.66 for ATV and $0.22 for motorcycle due to poor lifespan and high product cost. This makes it an unattractive solution for dust suppression. Soiltac is best used as a “dust cap” for open areas that need to have the surface stabilized or crusted over for dust compliance.

The bigger question of whether or not controlling dust from vehicles was even possible without daily watering has been answered positively. Dust from vehicles can be managed. Further study is warranted to achieve lower costs.  Durasoil should be considered the standard at which other products are tested against.

Cost calculations can be found in Appendix B.

Vehicle Counts Results: Graph A12

Two digital, active infrared vehicle counters were installed.  The accumulated counts totaled 11,169 counts over the six month period. Due to counter reliability concerns, a round number of 10,000 counts was used for calculations. The counts were questionable early in the test. Insufficient sensitivity, low mounting rigidity and sensor overloading by afternoon sun blinding were problems believed to have been solved by mounting the counters on steel pipes sticking out of the ground 24inches and re-aiming in a northwesterly direction, away from direct sunlight.   A round number of 10,000 counts was used in cost calculations.

Wear Factors and other data:

Durasoil, a soil wetting agent, exhibited excellent durability against water dilution and churning by knobby tires. There was no noticeable loss of effectiveness after receiving over five inches of rain. Rain and knobby tires do appear to be main factors in the early failure of Soiltac. Testing on the Soiltac section was discontinued after only eight weeks.  The Soiltac test section actually produced more dust than the control section before being discontinued.   Since Durasoil does not evaporate or wash away, its mechanism for dispersal is most likely dilution into the surrounding soil as tires grind it into the soil. The Durasoil remains in the soil where additional applications should have an additive effect.

Physical breakdown of the chemicals due to ultraviolet light is not known. This could be an unknown wear factor that needs consideration, especially for summertime use in Arizona. This warrants mentioning since water and knobby tires are suspected to be the mechanisms that caused Soiltac to fail, yet the failure mechanism is not completely understood.  This was not the focus of the study.

Appendix A: Test data analysis

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Graph A1 Durasoil Opacity (%) vs. Time (Days)

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Graph A2 Soiltac Opacity (%) vs. Time (Days)

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Graph A3 Control (Untreated) Opacity (%) vs. Time (Days)

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Graph A4 Soiltac Lifespan Projection – Opacity (%) vs. Time (Days)

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Graph A5 Durasoil Lifespan Projection – Opacity (%) vs. Time (Days)

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Graph A6 Comparative Opacity % (Durasoil compared to Control (Truck))

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Graph A7 Dust Opacity % Reductions (Durasoil compared to Control (ATV))

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Graph A8 Dust Opacity % Reductions (Durasoil compared to Control (Motorcycle))

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Graph A9 Opacity % Reductions (Soiltac compared to Control (Truck))

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Graph A10 Opacity % Reductions (Soiltac compared to Control (ATV))

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Graph A11 Opacity % Reductions (Soiltac compared to Control (Motorcycle))

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Graph A12 Vehicle Counts vs. Time (Days)

Appendix B

Cost Calculations

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Various Tables. Trip cost calculations

Appendix C

Dust Suppressant Chemicals

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Table. Durasoil Product Information and MSDS

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Table. Soiltac Product Information and MSDS.

Appendix D

Test Vehicles

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Various Photos. Truck, ATV, Motorcycle.

Appendix E

Traffic Counters – 2 used


Image of Trailmaster TM1050 Infrared Trail Monitor

Appendix F

Rainfall Data


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Table. Rainfall Data.

Appendix G


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Photo. Watering between test sections. 

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Photo. Durasoil test section with truck.

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Photo. Rider on motorcycle on Durasoil section.

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Photo. Rider on ATV on Control section.

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Photo. Dust inspector observing for opacity.

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Photo. Dust inspector observing for opacity.

Appendix H

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Test Location Map (Not to Scale)

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