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Rock Art Network Tom McClintock
Rock Art Network Tom McClintock
Rock Art Network Tom McClintock
Tom McClintock
Studying the Source of Dust Using a Simple and Effective Methodology:
13 May 2021

by Tom McClintock
Research Associate, Getty Conservation Institute

A rock art site to the southeast of Gunbalanya Road. Paintings are barely visible under layers of accumulated dust. If left untreated, this dust can bond to the rock surface forming a crust, exacerbate erosion, or negatively impact the paintings in other ways.
Figure 1
A rock art site to the southeast of Gunbalanya Road. Paintings are barely visible under layers of accumulated dust. If left untreated, this dust can bond to the rock surface forming a crust, exacerbate erosion, or negatively impact the paintings in other ways.
A pernicious impact to rock art sites, especially those that experience regular visitation or are in the vicinity of roads, is accumulation of dust. Beyond obscuring the art itself, deposited particulates can undergo a variety of diagenetic processes that lead to the formation of crusts. These layers of solidified sediment can be physically and chemically stable making their removal difficult. In the case of paintings, mobilized particulates can exacerbate existing condition issues like exfoliation or aeolian erosion. Especially with finer particulates which are hygroscopic, a layer of dust can readily attract moisture, negatively impacting paint layers below or inducing chemical alteration of an otherwise stable rock surface. This range of detriments demonstrates that, whenever possible, sources of dust should be identified and eliminated (Fig. 1).

Gunbalanya is the first community one encounters driving east from Kakadu, one of Australia’s most visited National Parks. Access by car is restricted to the dry season from May to October; the unpaved road is submerged in the wet season.
Figure 2
Gunbalanya is the first community one encounters driving east from Kakadu, one of Australia’s most visited National Parks. Access by car is restricted to the dry season from May to October; the unpaved road is submerged in the wet season.
In 2015 as part of my graduate studies in conservation and site management I was enlisted by Njanjma Rangers, an indigenous ranger group dedicated to natural and cultural resource management based in the community of Gunbalanya in Australia’s Northern Territory (Fig. 2). Their concern was that, during the dry season of May to October, traffic on the dirt road leading into the community from Kakadu National Park to the west was generating a significant amount of dust, impacting hundreds of known rock art panels within 250 meters to the road’s southeast (Fig. 3). Njanjma Rangers were interested in undertaking a study that would clearly demonstrate what everyone in the community suspected, that the road was the source of this dust, in order to lobby the Northern Territory government to seal the relevant section. The constraints were that we had almost no budget, the project area was remote, and we had no access to any specialized equipment. An additional consideration was that we wanted to make the methodology relatively adaptable and easy to replicate should other ranger groups be interested in conducting similar studies.

Vehicles traveling to and from Gunbalanya produce clouds of dust which impact rock art sites in the escarpment to the southeast, seen here at right.
Figure 3
Vehicles traveling to and from Gunbalanya produce clouds of dust which impact rock art sites in the escarpment to the southeast, seen here at right.
We had two principal research questions: 1) whether the road was the primary source of airborne dust, and 2) how far is the dust travelling, and thus how many sites are being impacted? To answer these, we devised a system of dust collection using supplies found at any standard hardware and office supply store. Using Mylar (semi-ridged sheets of clear plastic) and double-sided tape, we were able to manage sample collection in two different formats.

To measure distance dust could travel, Mylar swatches of 4cm2 with tape on one side were mounted to garden stakes stuck in the ground every 50 meters in a perpendicular line from the road (Fig. 4a & Fig. 4b). Similar swatches were placed at select sites with rock images. An additional four sampling stations were placed in a flood plain approximately 500 meters to the northwest of the road, away from off-road driving tracks, to measure ambient airborne dust conditions.

For directional sampling, 10cm diameter PVC pipe with threads on one end was cut into 6cm lengths (Fig. 5). A PVC end cap was screwed to the top of a garden stake so the pipe could be mounted atop. 12cm x 30cm sheets of Mylar were covered evenly on one side with double-sided tape, which were then wrapped around the PVC pipe. These Mylar sheaths, once mounted, were marked for magnetic north.

Double-sided tape mounted on Mylar was an effect means of capturing dust for measurement. Swatches were placed in a transect perpendicular to the road every 50 meters to measure how far dust was traveling.
Figure 4a
Double-sided tape mounted on Mylar was an effect means of capturing dust for measurement. Swatches were placed in a transect perpendicular to the road every 50 meters to measure how far dust was traveling.
 
Double-sided tape mounted on Mylar was an effect means of capturing dust for measurement. Swatches were placed in a transect perpendicular to the road every 50 meters to measure how far dust was traveling.
Figure 4b
Double-sided tape mounted on Mylar was an effect means of capturing dust for measurement. Swatches were placed in a transect perpendicular to the road every 50 meters to measure how far dust was traveling.
 
Sheets of Mylar with double-sided tape were wrapped around PVC pipe and mounted on garden stakes. These samples identified the direction from which dust was travelling.
Figure 5
Sheets of Mylar with double-sided tape were wrapped around PVC pipe and mounted on garden stakes. These samples identified the direction from which dust was travelling.
 
A USB microscope was a cheap and effective way to photograph the samples.
Figure 6
A USB microscope was a cheap and effective way to photograph the samples.
 
Microphotographs of samples were cropped and converted to greyscale then binary images (A), which allowed their analysis in the software ImageJ (B). The software identifies each separate particle of dust (in black) and calculates its size (in pixels), giving a percentage of the total area covered by dust (%AC).
Figure 7
Microphotographs of samples were cropped and converted to greyscale then binary images (A), which allowed their analysis in the software ImageJ (B). The software identifies each separate particle of dust (in black) and calculates its size (in pixels), giving a percentage of the total area covered by dust (%AC).
 
Because each microphotograph captures a tiny fraction of the overall sample, 50 images were taken of each sample to stabilized the %AC value.
Figure 8
Because each microphotograph captures a tiny fraction of the overall sample, 50 images were taken of each sample to stabilized the %AC value.

Both sampling methods were deployed for 1-week collection periods. When ready for collection, the exposed double-sided tape of each sample was covered with clean Mylar and labeled with permanent marker.

The results from directional samples were plotted as rosette diagrams, giving a clear indication that the Gunbalanya road was the primary source of dust when factoring prevailing wind direction.
Figure 9
The results from directional samples were plotted as rosette diagrams, giving a clear indication that the Gunbalanya road was the primary source of dust when factoring prevailing wind direction.
To measure dust accumulation, a USB microscope (widely available for ~$30 USD) was used to photograph the samples (Fig. 6). These photographs were then converted into black and white, and processed in ImageJ, a free and open-source software widely used for visual analysis (Fig. 7). By calculating the percentage of area covered (%AC) in black of the sample image, it was possible to develop a closed dataset that would yield internally quantifiable results. As can be seen in the plotted line, it was determined that %AC results for each sample stabilized after 50 images per sample (Fig. 8).

Directional sample sheets were delineated into sixteen columns corresponding to 22.5° each. %AC was calculated for each column and plotted on a rosette diagram (Fig. 9). %AC was calculated for accumulation samples and compared to the ambient dust collection samples from the flood plain.

The results demonstrate two clear conclusions. First, the directional samples consistently showed concentrations of dust in the direction of the road accounting for prevailing wind direction (measured by a directional sample placed in the floodplain), pointing to the road as the principal source of local mobilized dust. Second, when compared to the ambient collection samples placed in the floodplain, the accumulation samples all had measurably elevated amounts of dust, indicating that the dust mobilized by traffic on the road was traveling at least 250 meters (the greatest distance of a sample from the road), and rising 20 meters high (the elevation of a sample at one of the rock art sites).

Anecdotally, all these conclusions had been clearly articulated by Njanjma Rangers and other community members of Gunbalanya for years. With this simple study however, they had an evidence-based report that could be presented to officials and decision makers. By 2018, after consultations with the Northern Territory government and the Central Land Council, a decision was adopted to seal a 3km stretch of the road corresponding to the study area.

All photographs and diagrams © Tom McClintock

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