DUST CONTROL

STANDARD OF REVIEW

According to the CBJ Mining Ordinance (49.65.135(a)), the CBJ shall require that:

The CBJ Land Use Code does not provide specific emission standards relating to dust, but focuses on air quality and mitigating adverse effects of dust on the public or adjacent property.

Dust is regulated as particulate matter within the regulatory framework described in the Air Quality report. In brief, air quality in Alaska is regulated by the Alaska Department of Environmental Conservation (ADEC) with oversight by the U.S. Environmental Protection Agency (EPA). Under that arrangement, ADEC sets and enforces air quality standards (and other types of pollutant limits) subject to the approval of the EPA.

This staff report analyzes the potential impacts due to dust from the Kensington Gold Project, proposed measures to control the amount of dust generated, proposed measures to mitigate dust impacts, and the extent to which air quality will be affected by dust. The full range of particulates (of which dust is one type) to be emitted during mine construction and operation is discussed under the Air Quality report. The mining ordinance, however, requires a separate finding with respect to dust. Consequently, this report examines issues associated specifically with dust.

BACKGROUND

As one form of particulates, dust emissions must comply with two types of state regulatory controls: Air quality standards, or limits on the total concentrations of pollutants in ambient air intended to protect public health and the environment; and emissions limits, or maximum emissions allowed from particular sources.

ADEC issued an Air Quality Control Permit to Operate on January 17, 1997 that authorizes air emissions (including dust) from mine construction and operation as currently proposed by Coeur.

ANALYSIS

Sources. Ten types of air pollutant emission sources are identified in the application for the January 1997 ADEC permit application prepared for Coeur Alaska Inc. by TRC Environmental Corporation (TRC, 1996). Within each of these general categories, more specific sources are identified. Potential sources of dust are highlighted in the following inventory of sources of all types of air pollutants:

(*These sources are greatly reduced by the Final SEIS preferred alternative to transport tailings via a slurry pipeline from the mill to the dry tailings facility.)

Pollutants. Dust is the common term for a type of particulates. For this analysis, dust is equated to all particulates that are not products of combustion. Total particulate concentrations are often referred to as "total suspended particulates," or TSP. The fraction of smaller particulates -- those less than 10 micons in size -- are considered inhalable and have particular health significance. This smaller particulate fraction is often designated PM10. Particulates can have both health as well as visual impacts.

Certain chemical constituents of inhalable particulates may cause toxic effects. Toxic constituents of interest are primarily metals, such as lead. Some metals are known carcinogens.

Control and Mitigation. The project applicant proposes three primary types of direct controls for dust: reducing the area of exposed erodible materials, water spray, and baghouses. In some cases, incidental control is also provided by operating conditions, such as wet conditions within the mine, and enclosure afforded underground operations.

Water spray, where an area is blanketed with a fine mist, will be used to control particulate emissions from coarse ore storage and transfer (conveyor, transfer to and from coarse ore stockpile, and apron feeder), and transfer to the SAG mill. A 90 percent reduction in emissions is projected (TRC, 1996).

Road surface water application will be used to control fugitive dust emissions from the haul road on dry, above-freezing days. Percent control efficiencies of 50 to 85 percent are expected (TRC, 1996). Limited vehicle speeds will also help to control dust.

Baghouse-type dust collectors will be used to control particulate emissions from the primary crusher (including associated ore transfer processes), laboratory crushers, and lime and cement loading silos. Baghouse collectors either suck or blow dust-laden air through bag-like filter membranes where the dust is removed. Percent reductions in particulate emissions are projected at 90 to 99 percent (TRC, 1996).

The applicant proposes to control wind erosion from the dry tailings facility (DTF) by limiting the size of active, exposed areas, and establishing or maintaining non-eroding cover over inactive areas in accordance with the DTF operating plans. Residual tailings moisture content will also help to reduce erosion. For modeling purposes, it was assumed that one-third of the total 115-acre facility (38 acres) is exposed to wind erosion -- while the other two thirds have not been constructed, or have been stabilized.

Predicted Emissions. Mathematical modeling was used to predict particulate emissions (with the proposed controls in place) associated with the applicant's proposal corresponding to Alternative B of the SEIS. Those figures were then adjusted to reflect the effects of eliminating sources associated with trucking tailings from the mill to the dry tailings facility as called for in the SEIS preferred alternative. Dust and total particulates emissions for the SEIS preferred alternative are predicted as follows.

Predicted Emissions in Tons per Year

  Construction Phase Operation Phase
Type TSP1 PM102 TSP1 PM102
Dust n/a n/a 51.6 23.6
Other Particulates n/a n/a 56.6 56.6
Total Particulates 9.033 n/a 108.2 80.2

1Total Suspended Particulates.

2Particulate matter less than 10 microns in size.

3From FSEIS page 4-6 (USDA, 1997). All other data derived from TRC, 1996 (Table 3.15).

The above table suggests that particulate emissions during the construction phase are predicted to be substantially less than during the production phase.

CBJ staff analysis included examining the relative contribution of predicted dust emissions from sources. Even with water application, road dust during production is expected to be the largest source accounting for 76 percent of the total dust emissions and 75 percent of the very fine dust (PM10). Windblown tails from the dry tailings facility will amount to 18 percent of the total dust and 19 percent of the PM10.

Source Summary

  TSP2 PM103
  tpy1 (% of total) tpy (% of total)
Road Dust 39.2 (76%) 17.6 (75%)
Windblown Tails 9.4 (18%) 4.5 (19%)
Other Sources 3.0 (6%) 1.5 (6%)
Total Dust 51.6 23.6

1tons per year

2total suspended particulates

3inhalable particulates

Source: Data extracted by CBJ staff from TRC, 1996 (table 3.15)

Ambient Air Quality. Modeling was used to predict impacts of total PM10 (dust plus fine particulate products of combustion) emissions on ambient air quality at the periphery of the facility where air quality standards and increments apply, as well as within the facility to determine worker exposures. All modeling was conducted using the applicant's original dry tailings alternative (Final SEIS Alternative B). No modeling was conducted for the Final SEIS preferred alternative (Alternative D), but because dust emissions are less under the preferred alternative, air quality will be better than that predicted for alternative B. The following table compares the predicted concentrations of total PM10 for SEIS alternative B with ambient standards.

Predicted Pollutant Concentrations

Pollutant Averaging

Period

 Background

Concentration

(µg/m3)

 Max Periphery

Concentration

(µg/m3)

 Max Camp

Concentration

(µg/m3)

 Air Quality

Standard

(µg/m3)
Total PM10 24-hour 401 65.8 88.2 150
  annual 221 25.4 36.7 50

1Assumed by TRC based on ADEC recommendations

The modeling results presented in the above table suggest that total PM10 concentrations will be within State standards during operation of the mine. Since dust makes up only a portion (a little less than half) of the total PM10, dust levels will also be within state standards. Particulate concentrations during construction will be substantially less than during operation.

Modeling was also conducted to determine potential exposure to metals in dust. The calculations were based on Final SEIS Alternative B and the conservative assumption that the metal mass fraction of all dust was that of the highly mineralized ore. In fact, most dust, of course, will be derived from other sources that do not have the high metals content of the ore. Since Alaska has not adopted standards for metals, TRC compared calculated concentrations with the most restrictive of standards adopted by other states.

Predicted Metals Concentrations (all units are µg/m3)

  24-hour Annual
  Predicted Standard Predicted Standard
Zinc 0.0037 12.0 0.00024 6.55
Nickel 0.00053 0.002 0.00003 0.002
Arsenic 0.00053 0.39 0.00003 0.0002
Antimony 0.00106 8.0 0.00007 1.19
Chromium 0.00739 0.068 0.00048 0.07
Cadmium 0.00106 0.0056 0.00007 0.000435
Selenium 0.00021 0.27 0.000014 0.26
Mercury 0.00002 0.08 0.0000014 0.01
Barium 0.00053 8.0 0.00003 11.9
Manganese 0.0844 17.0 0.00542 0.24

The above table suggests that levels of metals contained in dust under Final SEIS Alternative B would meet the most restrictive of the standards adopted by other states. Dust metal levels will be less under the Final SEIS preferred alternative.

With prolonged exposure, four of the metals, Nickel, Cadmium, Chromium and Arsenic, are believed to cause cancer. Using standard risk factors and some very conservative assumptions, TRC calculated a cancer risk of six deaths per one million people for a 70-year exposure to maximum predicted particulate metal concentrations. The TRC analysis should be viewed as overly conservative for several reasons. For example, the metal mass fraction of all particulates was assumed to be that of the highly mineralized ore. In addition, a person would have to be physically located outdoors at the place of maximum metals concentrations for a period of 70 years ­ well in excess of the expected mine life. In fact, the actual cancer risk would be far less.

Visual Impacts. The Final SEIS discusses visual impacts associated with predicted emissions. A screening model (VISCREEN) was used to predict visibility impacts due to particulates and nitrogen oxides. Modeling from the standpoint of an observer aboard a cruise ship in the middle of Lynn Canal, and using worst-case meteorological conditions, showed "slight impact from some plume visibility." The Final SEIS concludes that impacts would be consistent with U.S. Forest Service visual quality objectives (VQOs).

Permit Conditions. Stipulations of the January 1997 Air Quality Control Permit to Operate issued by ADEC that apply to dust include:

Discussion of SEIS Alternatives. Projected dust emissions from the applicant's proposed operation (Final SEIS Alternative B) are higher than those predicted for the wet tailings project configuration (Alternative A). While staff are unable to account for all of the difference, the increase appears to be due in large part to the dry tailings facility and associated hauling activity. The U.S. Forest Service's preferred alternative (Alternative D) would pipe tailings to the dry tailings facility, eliminating a substantial source of dust and and resulting in lower total particulate and total PM10 emissions.

Comparison of Projected Emissions (in tons per year)

Pollutant Projected Emissions

Alternative A

 Projected Emissions

Alternative B

 Projected Emissions

Alternative D
TSP1 33.3 159.0 108.2
PM102 23.6 103.1 80.2

1Total suspended particulates

2Particulate matter less than 10 microns in size

While the above table suggests that the preferred alterative in the DSEIS would reduce total particulate emissions (a fraction of which would be dust) by a little more than 30 percent, and total PM10 emissions by a little more than 20 percent, particulate levels would meet air quality standards under either the applicant's alternative or the U.S. Forest Service's preferred alternative.

STAFF FINDINGS

STAFF RECOMMENDATION

Staff recommends approval of this aspect of the project.

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Last revised on 06/28/99 - bgb