Chapter 1. Introduction

Background and Statement of Problem

Out of the 3.8 million miles of road in the continental US, it is estimated that over 47 percent are aggregate and earth surfaced roads (Colorado Transportation Information Center. #5, 1989). Depending on which part of the country one is located the percentage of the road networks that is unpaved could vary between 30 and 70 percent. For example, here in Colorado approximately 65 percent of public roads are unpaved. Most of the unpaved roads are classified as low-volume roads and are located in the rural and forest areas of the country. While unpaved roads carry a small portion of the country's traffic, they provide a vital first link in the county's economy.

One major and obvious problem associated with all unpaved roads is dust. To residents living along unpaved roads the airborne dust can penetrate their homes causing a nuisance and health problems such as hay fevers and allergies. Dust can also be a conveyor of other diseases (United Nations, 1979). The fine suspended dust particles contribute significantly to the particulate loading in the atmosphere making road dust one major source of air pollution. The dust cloud formed when vehicles use these roads can impair the visibility and cause a hazard to motorists. The fine abrasive particles can also greatly increase the wear and tear on the moving parts of vehicles. The dust can also pollute nearby surface waters and stunt crop growth by shading and clogging the pores of the plants (Colorado Transportation Information Center. #3, 1989).

Besides polluting the environment, the generation of dust means the loss of fines which act as road surface binders. This represents a significant material and economic loss. The severity of the dust problem is determined primarily by the speed of vehicles, the abrasive resistance of the road aggregates and the amount of fines in the initial aggregate mix (Colorado Transportation Information Center. #3, 1989). The volumes of traffic and the climatic condition of the region are also important parameters of the equation. Long dry spells of semiarid and arid regions can also aggravate the road dustiness.

The lost of fines from the road surface as dust led to the coarser aggregates becoming loose and therefore can be thrown or washed away from the road surface. The resulting road is one that is full of corrugations and potholes that require maintenance with a high cost. One source quantifies the dust problem well: "For every vehicle traveling one mile of unpaved roadway once a day, every day for a year, one ton of dust is deposited along a 1,000-foot corridor centered on the road" (USDA Forest Service, 1983). The high maintenance cost in terms of aggregate replacement, with increased public awareness of pollution problems and increased road user cost, has lead highway agencies to have a renewed interest in dust control measures. The frequently used dust control methods include reduction of vehicular speed, application of water and the use of dust suppressing chemicals. Although dust control has been in practice for decades now, quantitative studies on the effectiveness of the dust suppressing methods and their environmental impact have virtually been nonexistent. The purpose of this research project therefore is to determine the impact of dust suppressants on the environment.

Objectives

The main goal of dust control is to stabilize the road surface; reducing the rate of aggregate loss and the money spent annually in replacement. Methods for dust suppressing ranges from the use of dust suppressing chemicals, chiefly chloride compounds and resinous adhesives to utilization of geotextiles in the road surface during construction. There are a number of these dust suppressants on the market now, notable among them lignin derivatives - waste from paper production operations, calcium, magnesium, and sodium chlorides - deliquescent and hygroscopic by nature, Cohorex - a proprietary product, bitumens/tars and many others. So far, many good claims on the effectiveness of these chemicals are being made and any assessments of these claims have only been qualitative.

The objectives identified for this research project therefore are:

1. Development of a device that would provide a standard, quantitative and reproducible method of measuring dust from unpaved road surfaces.
2. Measure the relative effectiveness of the different dust suppressants in controlling dust clouding taking into consideration the prevailing climatic condition also all possible field conditions.
3. Assess the environmental impact in terms of possible water quality effects resulting from the use of the various dust suppressants.

Scope of Study

Control of dust on gravel roads is an important operational and maintenance need. To enable the said objectives to be met, an extensive study of the pertinent literature is carried out to review any previous work done in this area of dust control. Five test sections of 1.25 miles each are constructed, four are treated with four different suppressants and one is untreated which serves as the control. The four suppressants used in the research are calcium lignosulfate, calcium chloride, magnesium chloride and calcium chloride special. The Colorado State University Dustometer, a device attached behind the rear wheel of the research vehicle and samples dust continuously as it is generated is used to measure the dust amount from each of the test sections during the dust production season - late May to October. A monitoring program is setup to sample and analyze runoff from the surface of the test sections as well as runoff from the non-road surface during rainfall events.

Some process variables in this research include, how much dust suppressing chemical is used, how much dust is reduced, what percentage of the chemicals stays on the road, and what is the concentration in the runoff. What is the varying distribution of dust suppressant as a function of the distance from the road and time. How much suppressant remains associated with the dust particles and how much becomes dissolved and mobile and finally what are the concentrations in the non-road runoff. In addition, the seasonal aspects of timing of treatment are of interest, particularly as related to prevailing dry and wet seasons.