Speak Out in Solidarity
Partnership for the Sustainable Development of Digby Neck and Islands Society
This article was presented at the BoFEP Workshop in 2004, and is published in: "Proceedings of the 6th Bay of Fundy Workshop, Cornwallis, N.S. Sept. 29-Oct. 2, 2004, Session 1 B"
Environmental Impacts of Blasting for Stone Quarries near Bay of Fundy
M. Ashraf Mahtab (1), Kemp L. Stanton (2), and Vitantonio Roma (3)
(1) Mining-Civil Engineer, Sandy Cove, Digby County, NS, Canada
(2) Fisherman, Whale Cove, Digby County, NS, Canada
(3) Geotechnical Engineer, Golder Associates S.r.l, Turin, Italy
ABSTRACT
In addition to deteriorating the quality of life of the local residents, there are some major environmental impacts that can be generated by the operation of a stone quarry in the vicinity of the Bay of Fundy. Detonation of explosives near water creates compression waves that produce a rapid rise in the peak pressure and its rapid decay to the ambient pressure. This can damage the swim bladder of fish and their eggs/larvae.
Large whales orient to objects by passively listening to underwater noise. Their exposure to the intense noise generated by blasting near the shoreline may result in damage to hearing and death by accidents.
Sedimentation resulting from blasting, grinding, and washing of the aggregate may cover spawning areas or reduce bottom dwelling life form that the fish use for food. The explosive residue may pollute the groundwater and may be toxic to aquatic life. Groundwater will be drawn down from upstream of the quarry as a result of excavation with the potential for lowering the water table and drying up the wells in the neighbourhood, as well as reducing the base flow in regional streams.
If the aggregate produced from the quarry is to be shipped across the Bay of Fundy, the new invasive plants brought in by ballast water may displace existing plant life (such as kelp beds and rock weed) and provide inhospitable environments for critical marine species.
1. INTRODUCTION
The proposed projects for industrial production and shipping of aggregate from stone quarries near the Bay of Fundy shore, especially in Southwest Nova Scotia, have emerged only recently. The proposals for quarrying basalt, a common rock found along the North Mountain abutting the Bay, have precipitated the concerns of the community regarding the adverse effects of a quarry on the environment and on the socio-economic well-being of the local inhabitants.
This article addresses the environmental impact of blasting in stone quarries near the Bay of Fundy. An introductory explanation of blasting scheme for quarries is provided in Sec. 2. The direct impacts of blasting on fish and fish eggs; the setback distances of a blast from marine life, regulated by the Federal Department of Fisheries and Oceans; and the impact of noise on marine mammals are discussed in Sec. 3. Additional sources of environmental impact of blasting for quarries near the ocean, which are identified and discussed in Sec. 4. include: blast residue, sedimentation, drawdown of water, and importation of ballast water
2. MAIN ASPECTS OF BLASTING IN QUARRIES
2.1 Bench Excavation
The drill and blast technique used in quarries or open pit mines involves a sequential excavation of benches (or steps) of the rock. The geometric features (shown in Fig. 1) include a grid of drill holes with spacing (S) along the free face (the wall of the bench), and spacing (B) across the wall; height (H) of the bench and corresponding length (L) of the drill hole; and diameter (D) of the drill hole. Each hole can be thought of as having to break its own individual area AR which equals BxS as outlined by the dashed lines in the plan view of the bench in Fig. 1. The blast design takes into account the type of rock, the ratio of B to D, the type of explosive, the delay interval between successive explosions in the same blast, and the explosive charge weight per delay (Hustrulid, 1999).
2.2 Ground Vibrations Generated by Blasting
Ground vibrations from blasting are generated by the resulting seismic waves. The primary or compression wave has the highest velocity and arrives first at a point or particle. The next to arrive at the point are the secondary or shear waves. The compression and shear waves are collectively called the body waves. The slowest and last to arrive is the Rayleigh wave, which constitutes the main component of the surface waves (Siskind, 2000, Roma, 2001).
The velocities of compression and shear waves, VC and VS respectively, are (Kramer, 1996):
(VC)**2 = 2G(1+v)/d(1-2v)
(1)
(VS)**2 =G/d
(2) where G = E/2(1+v), E = Young’s Modulus, v = Poisson’s Ratio, and d = density of the medium.
Measurement of ground vibration at a point (or particle) is generally made in terms of the peak particle velocity, PPV.
2.3 Guidelines for Use of Explosives
In the guidelines for use of explosives (e.g., Hustrulid, 1999, and Wright and Hopky, 1998), the main variables of interest are the PPV; the shock pressure in water, PW; the setback distance, SD, from the blast to the position of interest; and the explosive charge weight, W, per delay. Equations have been developed for inter-relating the main variables. For instance, the following equation (from Wright and Hopky, 1998) relates PPV to SD and W, when body waves are considered.
PPV = 100(SD/W**0.5)**-1.6
(3)
where the units of the variables are as follows: PPV in cm/sec, SD in m, and W in kg. In order to account for soil characteristics (i.e., layering, mechanical properties, surface degradation), more refined models are required to select the relevant values of the constants used in equation (3).
3. DIRECT IMPACTS OF BLASTING
3.1 Effects on Fish
The compressional seismic waves from the detonation of the explosives produce a high peak pressure (Pmax) which rapidly decays to below the ambient hydrostatic pressure. This rapid pressure drop induces serious impacts on fish. As discussed by Wright (1982), the primary site of damage in finfish is the swimbladder, the gas-filled organ that permits most pelagic fish to maintain neutral buoyancy. The kidney, spleen, and sinus venous may also undergo rupture and haemorrhage. Smaller fish are more susceptible to damage than the larger fish.
The Canadian Guidelines (Wright and Hopky, 1998) require that “no explosive is to be detonated in or near fish habitat that produces, or is likely to produce, an instantaneous pressure change (i.e., overpressure) greater than 100kPa (14.5 psi) in the swimbladder of a fish. The simplified formula for calculating the minimum setback distance (SD) of the onshore (in rock) blast from the fish is
SD = 5.03
(W)**0.5
(4)
Therefore, a 100kg charge of explosives detonated in a stone quarry requires a setback of 50.3m from the fish in order to limit the Pmax to 100kPa.
3.2 Effects on Fish Eggs
The Canadian Guidelines (Wright and Hopky, 1998) state that “vibrations from the detonation of explosives may cause damage to incubating eggs” and that “no explosive is to be detonated that produces, or is likely to produce, a peak particle velocity greater than 13mm/sec in a spawning bed during the period of egg incubation”.
In reference to the vibrations resulting from the compressional waves, the Guidelines provide the following simplified equation for determining the setback distance, SD, from an explosion for a limiting value PPV of 1.3cm/sec.
SD = 15.09
(W)**0.5
(5)
Therefore, detonation of a 100kg charge of explosives requires a setback distance of 150.9m from the fish eggs in order to limit the PPV to 1.3cm/sec.
Figure 2. Description of body and surface Waves resulting from blasting near a shoreline (after Roma and Mahtab, 2004)
3.3 Significant Increase in Setback Distance Resulting form Rayleigh Waves
In subsections 3.1 and 3.2, the setback distances from the explosives were calculated in reference to Compressional (or body) waves as the source of vibrations. However, a blast in a quarry will also generate surface waves (or Rayleigh waves) as illustrated in Figure 2.
The amplitude of the vibrations from body waves is inversely proportional to the distance. On the other hand, the amplitude of the vibrations from Rayleigh waves is inversely proportional to the square root of the distance. Therefore, the Rayleigh waves attenuate more slowly than body waves. Figures 3 and 4 (after Roma and Mahtab, 2004) depict the significant increase in the setback distance with reference to the limits of Pmax and PPV, respectively.
Figure 3. Setback distances for body and Rayleigh waves using the limit Pmax=100kPa for fish habitat (after Roma and Mahtab, 2004)
Figure 4. Setback distances for body and Rayleigh waves using the limit PVV=1.3cm/sec for spawning habitat for fish habitat (after Roma and Mahtab, 2004)
For example, using a W of 100kg and a Pmax of 100 in Figure 3, the setback distances for body and Rayleigh waves, are 50m and 300m, respectively. For a PPV of 1.3cm/sec, and a W of 100kg, the setback distances associated with Rayleigh and body waves (and shown in Figure 4) are 150m and 1460m, respectively.
3.4 Impact of Noise on Marine Mammals
Decibel
As an introduction, it would be useful to define the term “decibel” as a common unit used to express noise, or loudness of sound. Decibel, or dB, is a measure of a single power source with respect to a reference source.
dB = 10[log(sound power/reference power) (6)
Some examples of the source, loudness, and qualitative nature of loudness are given in Table 1.
Table 1. Sound Loudness (after Lindeburg, 1982)
The level of noise from a given source is a non-linear function of the distance of the observation point from the source. A rule of thumb for noise propagation is to reduce the noise level by 6 dB for each doubling of the distance. For instance, if the level of noise at 25m from a bulldozer is 80 dB, the noise level at 50m will be 74 dB. To bring the noise level to a (moderate) 50 dB, the required distance would be 800m.
Masking
Noise generated by blasting of rock and associated activities, such as grinding and shipping of the aggregate, can affect the marine mammals in various ways. Noise can mask communication signals that play a role in social cohesion, group activities, mating, warning, or individual identification. Noise can further interfere with environmental sounds that animals might listen to. Noise also affects the direction of sounds of predators and prey (Erbe and Farmer, 2000).
Behavioural Disturbance
Noise has the potential of disrupting normal animal behaviour. Reported animal reactions include a cessation of feeding, resting, socializing, and an onset of alertness or avoidance (Richardson et al., 1995). For many marine mammals, disturbance is known to have occurred at continuous noise levels of about 120 dB. In our opinion, the normal blast per delay in a rock quarry will generate a noise that exceeds 120 dB. If noise scares the animals away from their habitat for an extended period, the effect will have a biological significance (on foraging, mating, or nursing).
Hearing Impairment
Prolonged exposure to continuous noise, such as from shipping and grinding, can also bring about hearing loss. Audiologists call this impairment “threshold shift”. On exposure to damaging noise, one’s acoustic threshold rises by a few decibels. For a marine mammal, each additional dB can mean a loss of vital information: the call of a calf, a predator, or a prospective mate (NRDC, 1999).
Cumulative Effect
Repeated exposures to relatively low levels of noise may have a cumulative effect in inducing permanent hearing loss in mammals (as has been confirmed in humans and other species). Perhaps the most serious impact of noise is the debasement and depletion of habitat, as evidenced by the driving of gray whales, and possibly humpback whales, from traditional waters (NRDC, 1999).
4. ADDITIONAL IMPACTS OF QUARRYING STONE NEAR BAY OF FUNDY
4.1 Water Pollution from Explosive Residue
As indicated in Sec. 2.1 and Fig. 1, a pattern of drill holes is used to load and detonate explosives for breaking rock in a quarry. A fraction of the explosive may be left as “explosive residue” in the form of unexploded material after completion of the explosion.
As discussed by Kelleher (2002), there is evidence to support the suggestion that explosive residue is derived from a thin outer layer of the charge. The outer layer in a charged drill hole is the cylindrical surface. (The magnitude of the cylindrical surface for a given bench height is a direct function of the diameter of the drill hole.) As a general rule, the explosive residue will decrease as both the charge size and the velocity of detonation increase. However, in the case of a quarry near the ocean, the charge size per delay will need to be constrained to meet the DFO Guidelines for Peak Particle Velocity. In addition, the practical choice of the explosive for a quarry may not be associated with a high velocity of detonation. Therefore, the small charge (i.e., diameter of the drill hole) and/or the low velocity of detonation will tend to increase the percentage of the explosive residue.
The explosive residue will enter the surface and ground water through gravity flow and washing of the aggregate. The pollution potential of the explosive residue will depend on the chemical constituents of the explosive, such as nitrate and fuel oil. The potential hazard will be the contamination of the groundwater, its eventual flow into the Bay of Fundy, and the harmful impact on the marine life.
4.2 Sedimentation
Construction activities for a quarry near the Bay of Fundy shore may require clear cutting of the trees from the site, removing of top soil, and altering the water courses. All of these aspects will accelerate erosion, mainly by exposing large areas of soil or hills to faster flow of water during rainstorms. The rock formation along the shore is generally dipping toward the Bay. The silt-laden runoff from the site will end up in the Bay. Figure 5 shows an example of the silt being washed down the stripped hills from a proposed quarry site near the Bay of Fundy. Sedimentation or siltation from a rock quarry will also be generated by blasting, grinding, and transporting of the rock and aggregate. As indicated in Appendix A of NS DEL (1988), one of the most serious environmental effects of siltation is the destruction of fish and fish habitat. High turbidity may induce physiological stress which makes fish susceptible to infection by disease-causing micro-organisms. High turbidity levels reduce light penetration and inhibit photosynthesis, thereby affecting the food chain and dissolved oxygen content. Sedimentation may cover spawning areas or reduce bottom dwelling life form that the fish use for food.
Figure 5. Sedimentation runoff from stripped hills on a proposed quarry site near the Bay of Fundy.
4.3 Drawdown of Groundwater
The quarrying operation will progressively remove one or more benches of rock, most likely advancing from close to the Bay and proceeding away from the Bay. Depending on the cumulative height of the benches, the pit will act as a dug well which will draw down the water from the hills (or land) behind the pit toward the Bay. The extent of drawdown will depend on the rate of advance of the quarry face, level of the water table in reference to elevation of the bottom of the pit, and the conductivity of the rock. The rock near the shore is well fractured and has high conductivity, in both horizontal and vertical directions. This conductivity would be enhanced by the effect of blasting.
The drawdown of water will adversely affect the level of water table and the use of aquifers by the neighbours. The wells in the vicinity of the quarry may run dry and the base flow in the regional streams may be reduced. The dust from blasting and grinding as well as the siltation carried by the drainage through the blasted rock will affect the quality of the groundwater.
4.4 Impact of Ballast Water
For practical reasons, a large-size stone quarry may need to ship the stone or aggregate using a marine terminal located near the quarry site and on the Bay of Fundy shore. A major concern regarding shipping the product across the Bay would be the new invasive plants brought by the cargo ships in the ballast water. As stated in the guidelines of Transport Canada (2001), ballast water has been associated with the unintentional introduction of a number of organisms in Canadian waters and several have been extremely harmful to both the ecosystem and the economic well being of the nation. When a new organism is introduced to an ecosystem, negative and irreversible changes may result, including a change in biodiversity. The imported plants may displace the existing plant life, such as kelp beds and rockweed, and provide inhospitable environments for critical marine species.
5. CONCLUSIONS
The environmental impacts of blasting for a stone quarry near the Bay of Fundy include: loss of marine and terrestrial habitat, impairment of water and marine habitat due to siltation from the site, and lowering of groundwater level.
An economically feasible quarry would require shipping of the product (most likely, the aggregate) across the Bay. The traffic of bulk carriers will disrupt the movement pattern of the whales. Creation of a marine terminal will jeopardize the safety of small craft that follow the shoreline. The importation of invasive species in ballast water may have a potentially lethal impact over a wide area.
Regardless of the size of a proposed quarry near the Bay, a detailed environmental assessment report needs to be furnished by the proponent of the quarry project. The report must be examined by the Federal and Provincial governments and the concerned community before the proposed project is approved.
6. REFERENCES
Erbe, C. and Farmer, D. M. 2000. A software model to estimate zones of impact on marine mammals around anthropogenic noise. J. Accoust. Soc. Am., Vol. 108, No. 3, pt. 1, pp.1327-31.
Hustrulid, W. 1999. Blasting Principles for Open Pit Mining. Vol. 1 – General Design Concepts. A. A. Balkema, Rotterdam, Netherlands. Kellehr, J. D. 2002. Explosive Residue: Origin and Distribution. Forensic Science Communications, Victoria Forensic Science Center, Melbourne, Australia, Vol. 4, No. 2.
Kramer, S. L. 1996. Geotechnical Earthquake Engineering. Prentice Hall, NJ, USA. Lindeburg, M. 1982. Engineer in Training Review Manual. Professional Publications Inc., Belmont CA, p.15-15.
NRDC. 1999. Sounding the Depths: Supertankers, Sonar, and the Rise of Undersea Noise. Natural Resources Council, New York, NY. (available on www.nrdc.org) Richardson, W. J., Green, C. R., Maime, C. I., and Thomson, D. H. 1995. Marine Mammals and Noise. Academic Press, San Diego, CA.
NS DEL. 1988. Erosion and Sedimentation Control Handbook for Construction Sites. Nova Scotia Department of Environment and Labour, Environment Assessment Division. Roma, V. 2001. Soil properties and site characterization by means of Rayleigh Waves, Ph.D. Dissertation, Structural and Geotechnical Engineering Department, Technical University of Turin, Italy.
Roma, V. and Mahtab, A. 2004. Use of Rayleigh Waves as Reference for Determining Setback Distances for Explosions near Shorelines, 6th Bay of Fundy Workshop, September 29th-October 2nd, Cornwallis Park, Nova Scotia
Siskind, D. E. 2000. Vibrations from Blasting. International Society of Explosives Engineers, Cleveland, OH, USA.
Transport Canada. 2001. Guidelines for Control of Ballast Water Discharge from Ships in Waters under Canadian Jurisdiction. Transport Canada Marine Safety, TP 13617E.
Wright, D. G. 1982. A discussion paper on the effects of explosives on fish and marine mammals in waters of the Northwest Territories. Can. Tech. Rep. Fish. & Aquat. Sci. 1052:V+16p.
Wright, D. G. and Hopky, G. E. 1998. Guidelines for the use of explosives in or near Canadian Fisheries waters. Can. Tech. Rep. Fish. & Aquat. Sci. 2107:IV+34p.
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