Dust on Bonaire
The main goal of this research report is to shed light on the problems surrounding dust on Bonaire, and to give recommendations on how to deal with these problems. To reach this goal three types of research are carried out. Firstly a literature research, that intends to draw a clear picture about what factors are of influence when dealing with dust problems on Bonaire. Secondly a field study, where deposition and emission of dust is measured at different sites on the island. And finally a survey in order to find out how residents experience dust as a problem. By combining these results the authors give a clear picture of the situation on Bonaire with respect to dust problems. Below a summary is given of all the components of the study.
The type of dust examined in this report is the so-called mineral dust, or soil dust. This dust consists of the smallest particles that can be found within the soil. The soils on Bonaire are mostly a mixture of sand and silt, with a low fraction of clay. The diabase soils from the Washikemba formation contain large quantities of iron oxides, the limestone soils consist mainly of calcium carbonate (CaCO3). Typically about 5% of mineral dust consists of particles with a diameter smaller than 10μm. This particulate matter (or PM10) easily becomes airborne and is seen as air pollution; research has shown that it has an adverse effect on human health. As a rule of thumb; the smaller the particles, the more dangerous they are. When these particles enter the lungs they can cause all kinds of respiratory complaints, ranging from asthma up to lung cancer.
At the basis of dust problems on Bonaire are human activities. The problems are mainly caused by intensive traffic on dirt roads and by dust producing industries like the stone crushers. The largest dust production factor seems to be the dirt roads. A very large proportion of roads on Bonaire are unpaved, even within neighborhoods. For example in Antriol about half of the roads are unpaved. This, in combination with a rapidly growing number of vehicles (about 40% increase in period 2002-2007), contributes to large quantities of dust in the air. A more local source of dust is the stone crushing and asphalt industry. Currently there are three active stone crushers of whom two are situated right next to the residential areas Antriol and Amboïna. Because of the prevailing wind direction these residential areas are covered by large quantities of dust when the crushers are in production.
Another factor contributing to dust problems is the overpopulation of livestock on the island. The exceedance of the carrying capacity results in land degradation that in combination with high wind velocities leads to wind erosion. The natural environment on Bonaire has been under pressure since the 17th century when logging and the introduction of goats and sheep led to strong degradation of the landscape and a significant reduction of the biodiversity. The original vegetation has been largely replaced by small bushes and cactuses. Because of this reduction in vegetation, wind and water erosion started to play an increasingly large role on Bonaire. These processes have led to a serious denudation of the soil that in term has led to a poor water retention capacity. Because of this, precipitation either runs off straight into the ocean or it evaporates. As a consequence the soils on Bonaire tend to be very dry. This dryness of the soil is one of the prerequisites for wind erosion. Nowadays land degradation is mainly caused by (1) a goat population that exceeds the carrying capacity of the island at least by a factor 4 and (2) neglect of the agricultural land. The agricultural sector on Bonaire is becoming more and more marginalized because of the low returns and better job opportunities in other sectors (e.g. tourism).
Apart from the local dust Bonaire also experiences an influx of Sahara dust, especially during the hurricane season. This dust contains pollen, microbes, insects and chemicals from herbicides and insecticides that could potentially have an adverse effect on human health and ecosystems like coral reefs. Sahara dust that reaches the Caribbean generally has a high percentage of particulate matter; about 30-50% of the dust that reaches Miami and Barbados has a diameter smaller than 2,5μm. Measurements of fine dust concentrations on Trinidad show a significant increase in times of Sahara dust influx; 130-150 μg/m3 compared to 30-40 μg/m3 on average days. Assuming similar conditions for Bonaire this means that the proposed daily average maximum of PM10 of 50 μg/m3 (<35 days per year) can be exceeded by a factor 3 on days with Sahara dust. Taking into account the frequency of Sahara dust reaching the Caribbean, it is assumable that this maximum is exceeded up to 35 times a year or more.
Very little legislation and policy regarding emission of dust and especially particulate matter is in place. In the Dutch Antilles only Curacao has some legislation concerning air quality (immission norms); daily averages of total suspended particles (TSP) levels may not exceed 150 μg/m3. Concerning dust emission there is no legislation at all for the Dutch Antilles; this means that companies that generate dust (mineral or other) are not limited by any norms whatsoever. For emission of dust from heavy industry and power plants (mainly SO2 and NOx) legislation can be implemented that already exists in the Netherlands. This legislation puts a maximum on dust concentration per emitted gas volume within a certain timeframe. This however is not applicable for the stone crushing industry as the type of emission and type of dust are very different.
The field survey has two main goals; (1) to find out what the opinions of people concerning dust in their environment are, and (2) to identify differences in severity of problems in relation to place of residence. The results concerning the first goal are fairly clear cut. More than 70% of the respondents indicate that dust is a problem for them. Many of the complaints can be summarized as housekeeping problems such as constantly dirty floors, windows etc. But more disturbing is the large category (40%) of people who say they experience health problems due to dust in the air. Complaints in this category are, among others: respiratory problems like asthma, allergies and eye infections. Also a somewhat smaller group of people indicate to experience problems with electric appliances due to dust in the air.
When asked about causes of the dust problem, traffic and dirt roads are mentioned most. Almost 80% of the respondents note traffic as the number one dust producing factor. Other mentioned sources are: building sites (29%), natural wind erosion from Kunuku’s (29%) and the stone crushers (13%). It seems that not so many people see the crushers as an important contributor, but this is due to the fact that not all respondents live in close proximity of the stone crusher. Those that do all indicate the stone crusher as the main problem, and have very serious complaints about it. More than 70% of the respondents feel that measures should be taken in order to reduce the dust problem. Kinds of measures mentioned are; paving more roads with asphalt, stricter legislation (concerning stonecrushers, building and traffic), but also education and raising awareness among people.
During the field study the dust emission measurements were done with the so called Modified Wilson and Cook dust catchers (MWAC). The MWAC were placed at several sites: two dirt roads, the stone crusher at Antriol and (degraded) agricultural fields. As the MWAC at the agriculture sites did not yield any measurable quantities of dust (assumed to be because of weather conditions), they are not further discussed. The applied method of dust measurement does not give the total emission for a dust source, instead it measures dust transport through a certain surface within a short distance from the source. The measurements are given in the units of kg/2m2/day: this can be seen as the daily amount of dust that is emitted from a source up to a height of two meters (maximum height of a person). To make a comparison with norms about air quality possible, these results are converted into a concentration of dust per air volume (μg/m3), by adding another dimension to the measurement results (m2 m3). This can be done by taking wind speed into account.
The result is an impression of the average total suspended particles (TSP) near a source within the measurement period. When we compare our results with the maximum allowed average TSP on Curacao (150 μg/m3), our test sites along dirt roads sometimes exceed this level with a factor 5 and at the stone crusher site even with a factor 11. By calculating the average fractions of particulate matter in the TSP (by a grain size distribution analysis of the source material) it is also possible to estimate particulate matter concentrations PM10 and PM2.5 in the suspended material. Although those norms were not grossly exceeded, it indicates that there is clearly a problem that needs to be addressed.
Another part of the field study was to spatially give an indication of dust deposition in residential areas, and to correlate this with factors such as presence of dirt roads and stone crushers. For this we used 30 so called marble dust collectors (MDCO) that give dust deposition as a mass per surface per time unit, in this case g/m2/week. Measurements are taken weekly within a period of 8 weeks in the neighborhoods Antriol and Tera Cora. The results generally show a very high amount of dust deposition throughout both neighborhoods, but Antriol especially stands out. During the whole period Antriol yields on average 2.65 times more dust deposition than Tera Cora. This can be explained by the difference in dirt road density (48% in Antriol against 24% in Tera Cora) and the fact that a stone crusher is situated right next to Antriol. When comparing dust deposition in nature areas with residential areas, it becomes clear that dust deposition in residential areas is 15 times higher than in natural surroundings. This clearly indicates a large human influence.
As there is of yet no legislation concerning dust emission (or any pollutants for that matter), new legislation can be fitted into the already existing hinderverordening (hindrance ordinance). This legal framework is designed to counter negative externalities (such as dust emission) by holding a company or person responsible. As stated this legislation already exists, however so called uitvoeringsbesluiten (loose translation; Implementing decisions) need to be designed and approved by the local government before anything can be done. This means indicating certain activities as environmentally polluting. Legislation concerning environmentally polluting activities, on which Bonaire could base its own, is already in place in The Netherlands. The Dutch legislation states that if a company or person is conducting activities that are polluting the environment, steps can and must be taken in order to reduce this to an acceptable level. These steps are (1) adjusting the (production) process in such a way that pollution is reduced. If this is not enough (2) steps must be taken to prevent the pollution from reaching the surrounding area, for instance by protective barriers. Finally if this doesn’t work (3) companies can be shut down and/or relocated to a more suitable location.
When addressing problems of erosion from degraded agricultural land, steps must be taken to stop the process of degradation. Land degradation is mainly caused by uncontrolled grazing of a too large population of goats that Bonaire simply cannot sustain. Controlling the goat population can be done in a number of ways but considering the situation on Bonaire the authors see financial incentives as the most suitable. These financial incentives can be both positive or negative. For instance, owners of goats that are not grazing on the owners land can be fined. Or, goat owners who actually keep the goats on their own fields can be rewarded by subsidies for cattle fodder, fences etc. But the problems don’t stop with the goats. Agriculture (and maintenance of land) is simply not profitable anymore. New opportunities need to be created to get people back on the Kunuku’s or otherwise the landscape will keep degrading. Examples for instance are new irrigated agriculture, agro-tourism and environmental schemes where land owners are paid to take care of their land (aimed at biodiversity).
Dust emission as a result of traffic can be greatly reduced by simply laying out more asphalt in residential areas. Another measure could be implementing legislation concerning industrial traffic (trucks with building materials etc.). For instance special routes avoiding certain residential areas, better speed control, and covering up of dust sensitive materials like plaster sand and grinded diabase. To conclude this report, it can be said that problems with dust need to be taken very serious as is can be quite hazardous for human health in the quantities sometimes present in the air at Bonaire.