Groundwater: Mining Pollution

Authored by: Jeff Skousen , George Vance

Managing Water Resources and Hydrological Systems

Print publication date:  July  2020
Online publication date:  July  2020

Print ISBN: 9781138342668
eBook ISBN: 9781003045045
Adobe ISBN:

10.1201/9781003045045-4

 

Abstract

Mining operations can influence groundwater quantity and quality by altering flow paths and underground water movement, and by introducing contaminants into the water. In the United States, the Safe Water Drinking Act was passed to protect groundwater quality by regulating the maximum contaminant loadings that can be discharged or released into groundwater resources. Substances that degrade groundwaters include nutrients, salts, heavy metals, trace elements, and organic chemicals, as well as contaminants such as radionuclides, carcinogens, pathogens, and petroleum wastes. If contaminants are detected, the extent and degree of the contamination must be determined and remediation strategies should be selected based on the type of contaminant, its location in the water system, and its potential hazards to human life and other organisms. Remediation strategies include (1) containment, (2) in situ treatment, and (3) pump-and-treat methods.

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Groundwater: Mining Pollution

Introduction

Mining activities can impact the quantity, quality, and usability of groundwater supplies. Underground mining for coal by longwall or room and pillar mining methods often interrupts and depletes groundwater, and can also alter its quality. Surface mining can enhance the introduction of surface water with dissolved solids into groundwater systems through fractures or other conduits. The type and nature of the mining activity, the disturbed geologic strata, and alteration of surface and subsurface materials will determine how groundwater supplies will be impacted. As waters contact and interact with disturbed geologic materials, constituents such as salts, metals, trace elements, and organic compounds become mobilized [1,2]. The dissolved substances can leach into deep aquifers and cause groundwater quality impacts [3]. In addition to concerns due to naturally occurring contaminants from disturbance activities, mining operations may also contribute to groundwater pollution from leaking underground storage tanks, improper disposal of lubricants and solvents, and contaminant spills. Blasting and hydraulic fracking activities can provide additional connection to surface water inputs, and underground injection of wastes can also occur during these operations [4].

In the United States, the Clean Water Act (CWA) and its subsequent amendments establish the authority for all water pollution control actions at the federal level [5] and regulate discharges into surface streams, wetlands, and oceans. Mining operations must acquire National Pollutant Discharge Elimination System (NPDES) permits for discharges to surface waters. Groundwater quality in the United States is regulated by the Safe Drinking Water Act (SDWA), which was originally enacted in 1974 and amended in 1996. The SDWA was passed to protect drinking water supplies by requiring discharges into groundwaters to meet the use standard or the ambient condition, whichever is of higher quality [6]. This is done by legislating maximum contaminant levels (MCLs) above which waters are considered unsafe for human consumption. The Office of Water within the Environmental Protection Agency provides guidance, specifies scientific methods and data collection requirements, and performs oversight for entities that supply drinking water including groundwater. Examples of some water contaminants with specified MCLs associated with mining activities are listed in Table 1 [7].

Table 1   Selected Contaminants in Drinking Waters That May Be Influenced by Mining Activities [7]

Contaminant

MCL (mg/L)

MCLG

Inorganics

Arsenic

0.010

0

Cadmium

0.005

0.005

Chromium

0.1

0.1

Copper

LV

1.3

Cyanide

0.2

0.2

Fluoride

4.0

4.0

Lead

LV

0

Mercury

0.002

0.002

Nitrate (NO3-N)

10

10

Selenium

0.05

0.05

Sulfate

500

500

Radionuclides

Radium

5 pCi/L

0

Uranium

30 ug/L

0

Organics

Benzene

0.005

0

Carbon tetrachloride

0.005

0

Pentachlorophenol

0.001

0

Toluene

1

1

Xylenes

10

10

Microbiological

Total coliforms

LV

0

Viruses

LV

0

MCL, Maximum contaminant levels permissible for a contaminant in water that is delivered to any user of a public water system; MCLG, Maximum contaminant level goals of a drinking water contaminant that is protective of adverse human health effects and which allows for an adequate margin of safety; LV, Lowest value that can be achieved using the best available technology.

Because mining activities can result in poor-quality groundwaters, enforcement of regulations is needed to minimize and/or eliminate potential problems. The Surface Mining Control and Reclamation Act (SMCRA) of 1977 identifies policies and practices for mining and reclamation to minimize water quality impacts [8]. SMCRA requires that specific actions be taken to protect the quantity and quality of both on- and off-site groundwaters. All mines are required to meet either state or federal groundwater guidelines, which are generally related to priority pollutant standards described in the CWA and SDWA.

Groundwater Resources

Groundwater resources are the world’s third largest source of water behind oceans (97%) and glaciers (2%), and represent 0.6% of the earth’s water content [9]. Approximately 53% of the US population uses groundwater as a drinking water source, but this percentage increases to almost 97% for rural households. In areas of low rainfall, weathering and translocation of dissolved constituents are relatively slow compared to high rainfall areas. For example, only 12% of precipitation will recharge underground water supplies in a dry coal mining area like Gillette, Wyoming, while almost 47% of precipitation was available for recharge in coal mining areas of Tennessee [10]. Transport of contaminants from surface and subsurface environments to groundwaters is generally accelerated as the amount of percolating water increases.

Infiltrating water moves through the vadose zone (unsaturated region) into groundwater zones (saturated region). The upper boundary of the groundwater system (e.g., water table) fluctuates depending on the amount of water received or removed from the groundwater zone. Groundwater movement is a function of hydraulic gradients and hydraulic conductivities, which represent the combined forces with which water moves as a function of gravitational, osmotic, and pressure forces and the permeability of geologic strata. Groundwater moves faster in coarse-textured materials and where hydraulic gradients are high. Aquifers are groundwater systems that have sufficient porosity and permeability to supply enough water for specific purposes. For an aquifer to be useful, it must be able to store, transmit, and yield sufficient amounts of good-quality water. Important hydrogeological characteristics of a site that determine groundwater quantity and quality are listed in Table 2.

Table 2   Important Hydrogeological Characteristics of a Site That Determine Groundwater Quantity and Quality

Geological

Type of water-bearing unit or aquifer (rock type, overburden).

Thickness and areal extent of water-bearing units and aquifers.

Type of porosity (primary, such as intergranular pore space, or secondary, such as bedrock discontinuities, e.g., fracture or solution cavities).

Presence or absence of impermeable units or confining layers.

Depths to water tables; thickness of vadose zone.

Permeability and connectivity to other voids or conduits.

Hydraulic

Hydraulic properties of water-bearing unit or aquifer (hydraulic conductivity, transmissivity, storability, permeability, dispersivity).

Pressure conditions (confined, unconfined, leaky confined).

Groundwater flow directions (hydraulic gradients, both horizontal and vertical), volumes (specific discharge), rate (average linear velocity).

Recharge and discharge areas.

Groundwater or surface water interactions; areas of groundwater discharge to surface water or vice versa.

Seasonal variations of groundwater conditions.

Groundwater Use

Existing or potential underground sources of drinking water.

Existing or near-site use of groundwater.

Groundwater Contaminants

Several types of substances affect groundwater quality [1,11]. Water contaminants include inorganic, organic, and biological materials. Some have a direct impact on water quality, while others indirectly cause physical, chemical, or biological changes that make the water unsuitable for its designated use. Substances that degrade groundwaters include nutrients, salts, heavy metals, trace elements, and organic chemicals, as well as contaminants such as radionuclides, carcinogens, pathogens, and petroleum wastes (Table 3, [12]). Several types of organic chemicals entering groundwaters are less dense than water and tend to move to and along the surface of the water table. Changes can also occur in groundwaters due to temperature fluctuations and odors. Some groundwaters near coal seams contain natural organic substances (such as dissolved methane gas) and synthetic organic chemicals. Methane gas can be extracted from coal beds where underground and surface mining operations are projected, and this extraction can alter methane gas concentrations in groundwaters [10]. Organic contamination may also result from leaking gas tanks, oil spills, or runoff from equipment-serving areas. In these cases, the source of the contamination must be identified and removed. Gasoline, diesel, or oil-soaked areas should be immediately excavated and disposed of by approved methods.

Table 3   Different Classes of Groundwater Pollutants and Their Causes [12]

Water Pollutant Class

Contributions

Inorganic chemicals

Toxic metals and acidic substances from mining operations and various industrial wastes

Organic chemicals

Petroleum products, pesticides, and materials from organic wastes industrial operations

Infectious agents

Bacteria and viruses

Radioactive substances

Waste materials from mining and processing of radioactive substances or from improper disposal of radioactive isotopes

The chemistry of groundwaters and potential levels of naturally occurring contaminants are related to (1) groundwater hydrologic conditions, (2) mineralogy of the mined and locally impacted geological materials, (3) mining operations (e.g., extent of disturbed materials and its exposure to atmospheric conditions), and (4) time. Movement of metal contaminants in groundwaters varies depending on the chemical of concern. Solubility considerations include metals such as cobalt, copper, nickel, and zinc being more mobile than silver and lead, and gold and tin being even less mobile [1]. As conditions such as pH, redox, and ionic strength change over time, dissolved constituents in groundwaters may decrease due to adsorption, precipitation, and chemical speciation reactions and transformations.

Acid mine drainage (AMD) is the most prevalent groundwater quality concern at inactive and abandoned surface and underground mine sites. If geologic strata containing reduced S minerals (e.g., pyrite (FeS2)) are exposed to weathering conditions, such as when pyritic overburden materials are brought to the surface during mining activities and then reburied, high concentrations of sulfuric acid (H2SO4) can develop and form acid waters with pH levels below 2 [2]. Neutralization of some or all of the acidity produced during the oxidation of reduced S compounds can occur when carbonate minerals in proximity to the acid-producing materials dissolve [3]. Neutralization can also occur when silicate minerals dissolve, but sometimes high levels of potentially toxic metals such as Al, Cu, Cd, Fe, Mn, Ni, Pb, and Zn may be released. For example, mining of coal in the Toms Run area of northwestern Pennsylvania resulted in groundwater contamination by AMD containing high concentrations of Fe and sulfate (SO4) that leached into the underlying aquifer through joints, fractures, and abandoned oil and gas wells.

The Gwennap Mining District in the United Kingdom contained numerous mines that operated over several centuries to extract various mineral resources. One of these mines, the Wheal Jane metal mine in Cornwall, extracted ores that included cassiterite (Sn-containing mineral), chalcopyrite (Cu), pyrite (Fe), wolframite (tungsten, W), arsenopyrite (arsenic, As), in addition to smaller deposits of Ag, galena (Pb), and other minerals. After closure in the early 1990s, extensive voids remaining in the Wheal Jane mine that contained oxidized and weathered minerals were flooded. Initial groundwater quality was poor with a pH of 2.9 and a total metal concentration of 5000 mg/L, which contained elevated levels of Fe, Zn, Cu, and Cd. Water quality worsened with depth, and at 180 m, the groundwater had a pH of 2.5 and a metal concentration of 7000 mg/L. Treatment of discharge waters originating from the mine involves an expensive process that will continue long term to preserve environmental quality in surface and groundwaters in the region. A similar situation occurred when a Zn mine in southwestern France was closed. In this case after flooding, discharge mine waters had a solution pH near neutral, but the water still contained high concentrations of Zn, Cd, Mn, Fe, and SO4.

Within the Coeur D’ Alene District of Idaho at the Bunker Hill Superfund site, groundwater samples were found to contain high concentrations of Zn, Pb, and Cd [13]. The contamination originated from the leaching of old mine tailings deposited on a sand and gravel aquifer. When settling ponds were constructed to catch the runoff from the tailings, water from the ponds infiltrated into the aquifer and caused an increase in metal concentration in the local groundwater system [14].

Gold mining operations have used cyanide as a leaching agent to solubilize Au from ores, which often contain arsenopyrite (As, Fe, and S) and pyrite [1]. Unfortunately, cyanide, in addition to being toxic on its own, is a powerful nonselective solvent that solubilizes numerous substances that can be environmental contaminants. These ore waste materials are often stored in tailing ponds and, depending on the local geology and climate, the cyanide present in the tailings can exist as free cyanide (CN, HCN); inorganic compounds containing cyanide (NaCN, HgCN2); metal-cyanide complexes with Cu, Fe, Ni, and Zn; and/or the compound CNS. Because cyanide species are mobile and persistent under certain conditions, a large potential exists for trace element and cyanide migration into groundwaters. For example, a tailings dam failure resulted in cyanide contamination of groundwater at a gold mining operation in British Columbia, Canada [1].

Arsenic and uranium (U) contamination has resulted from extensive mining and smelting of ores containing various metals (Ag, Au, Co, Ni, Pb, and Zn) and/or nonmetals (As, P, and U). Arsenic-contaminated groundwaters have been a source of surface recharge and drinking water supplies. At one site, a nearby river had As levels 7 and 13 times greater than the recommended national and local drinking water standards, respectively [1]. Arsenic is known as a carcinogen and has been the contributing cause of death to humans in several parts of the world that rely on As-contaminated drinking water [11]. Waters from dewatering a U mine in New Mexico had elevated levels of U and radium (Ra) activities as well as high concentrations of dissolved Mo and Se, which were detected in stream water 140 km downstream from the mine.

Groundwater Analysis

Both the remediation and prevention of groundwater contamination by nutrients, salts, heavy metals, trace elements, organic chemicals (natural and synthetic), pathogens, and other contaminants require the evaluation of the composition and concentration of these constituents either in situ or in groundwater samples [2,10]. Monitoring may require the analysis of physical properties, inorganic and organic chemical compositions, and/or microorganisms according to well-established protocols for sampling, storage, and analysis [15]. For example, if groundwater will be used for human or animal consumption, the most appropriate tests would be nitrate-nitrogen (NO3-N), trace metals, pathogens, and organic chemicals. Several common constituents measured in groundwaters are listed in Table 4. However, other tests can be conducted on waters including tests for hardness, electrical conductivity (EC), chlorine, radioactivity, water toxicity, and odors [16].

Table 4   Groundwater Quality Parameters and Constituents Measured in Some Testing Programs [16]

Physical

Metals and Trace Elements

Nonmetallic Constituents

Organic Chemicals

Microbiological Parameters

Conductivity

Salinity

Sodicity

Dissolved solids

Temperature

Odors

Al, Ag, As, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, Se, Sr, Zn

pH, acidity, alkalinity, dissolved oxygen, carbon dioxide, bicarbonate, B, Cl, CN, F, I, ammonium, nitrite, nitrate, P, Si, sulfate

Methane

Oil and grease

Organic acids

Volatile acids

Organic C

Pesticides

Phenols

Surfactants

Fecal coliforms

Bacteria

Viruses

Recommendations based on interpretation of the groundwater test results should be related to the ultimate use of the water [2]. The interpretation and recommendation processes may be as simple as determining that a drinking water well exceeds the established MCLs for NO3-N and recommending the well should not be used as a drinking water source or that a purification system be installed. However, interpretations of most groundwater analyses can be quite complicated and require additional information for proper interpretation. If a contaminant exceeds an acceptable concentration, all potential sources contributing to the pollution and pathways by which the contaminant moves must be identified. In many cases, multiple groundwater contaminants are present at different concentrations. Because the interpretation of water analyses is a complex process, recommendations should be based on a complete evaluation of the water’s physical, chemical, and biological properties. Integrating water analyses into predictive models that can assess the effects of mining activities on water quality is needed in the long term to determine the most effective means to preserve and restore water quality.

Strategies for Remediating Contaminated Groundwaters

Mine sites that have been contaminated generally contain mixtures of inorganic and/or organic constituents, so it is important to understand these multi-component systems in order to develop remediation strategies. Therefore, a proper remediation program must consider identification, assessment, and correction of the problem [17,18]. Identification of a potential problem site requires that the past history of the area and activities that took place are known, or when a water analysis indicates a site has been contaminated. Assessment addresses questions such as (1) what is the problem, (2) where and to what extent is the problem, and (3) who and what is affected by the problem. Afterward, a remediation action plan must be developed that will address the specific problems identified. A remediation action program may require that substrata materials (e.g., backfill) and groundwater be treated.

If remedial action is considered necessary, then three general options are available: (1) containment, (2) in situ treatment, or (3) pump-and-treat method (Figure 1). The method(s) used for the containment of contaminants are beneficial for restricting contaminant transport and dispersal. Of the remediation techniques, in situ treatment measures are the most appealing because they generally do less surface damage, require a minimal amount of facilities, reduce the potential for human exposure to contaminants, and when effective, reduce or remove the contaminant so that the groundwater can be utilized again [18]. In situ remediation can be achieved by physical, chemical, and/or biological techniques. Biological in situ techniques used for groundwater bioremediation can either rely on the indigenous (native) microorganisms to degrade organic contaminants or on amending the groundwater environment with specialized microorganisms (bioaugmentation). The pump-and-treat method, however, is one of the more commonly used processes for remediating contaminated groundwaters [17]. With the pump-and-treat methods, the contaminated waters are pumped to the surface where one of the many treatment processes can be utilized. A major consideration in the pump-and-treat technology is the placement of wells, which is dependent on the contaminant and site characteristics (see Table 2). Extraction wells are used to pump the contaminated water to the surface where it can be treated and re-injected or discharged. Injection wells can be used to re-inject the treated water, water containing nutrients and other substances that increase the chances for chemical alteration or microbial degradation of the contaminants, or materials for enhanced oil recovery.

Remedial options to consider if cleanup of contaminated
                              groundwater is deemed necessary.

Figure 1   Remedial options to consider if cleanup of contaminated groundwater is deemed necessary.

Treatment techniques can be grouped into three categories, namely, physical, chemical, and biological methods [2,18].

Physical methods include several techniques. Adsorption methods physically adsorb or trap contaminants on various types of resins. Separation treatments include physically separating contaminants by forcing water through semipermeable membranes (e.g., reverse osmosis). Flotation, or density separation, is commonly used to separate low-density organic chemicals from groundwaters. Air and steam stripping can remove volatile organic chemicals. Isolation utilizes barriers placed above, below, or around sites to restrict movement of the contaminant. Containment systems should have a permeability of 10- 7 cm/s or less.

Chemical methods are also numerous. Chemical treatment involves addition of chemical agent(s) in an injection system to neutralize, immobilize, and/or chemically modify contaminants. Extraction (leaching) of contaminants uses one of the several different aqueous extracting agents such as an acid, base, detergent, or organic solvent miscible in water. Oxidation and reduction of groundwater contaminants are commonly done using air, oxygen, ozone, chlorine, hypochlorite, and hydrogen peroxide. Ionic and nonionic exchange resins can adsorb contaminants, thus reducing their leaching potential.

Biological methods for contaminant remediation are less extensive than physical and chemical techniques. Land treatment is an effective method for treating groundwaters by applying the contaminated waters to lands using surface, overland flow, or subsurface irrigation. Activated sludge and aerated surface impoundments are used to precipitate or degrade contaminants present in water and include both aerobic and anaerobic processes. Biodegradation is one of the several biological-mediated processes that transform contaminants, and it utilizes vegetation and microorganisms.

References

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2
Pierzynski, G.M. , J.T. Sims , and G.F. Vance . 2005. Soils and Environmental Quality. 3rd Edition. CRC Press, Inc., Boca Raton, FL. 584 pp.
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U.S. Congress.1977. Surface Mining Control and Reclamation Act. Public Law 95-87. Available at https://www.osmre.gov/lrg/docs/SMCRA.pdf.
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10
National Academy of Sciences.1990. Surface Coal Mining Effects on Ground Water Recharge. Committee on Ground Water Recharge in Surface-Mined Areas, Water Science and Technology Board, National Research Council. 170 pp. Available at http://www.nap.edu/catalog.php?record_id=1527.
11
Manahan, S.E. 2017. Environmental Chemistry. 10th Edition. Lewis Publishers, Chelsea, MI.
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Nielsen, D.M. 2005. Practical Handbook of Groundwater Monitoring. 2nd Edition. Lewis Publishers, Boca Raton, FL.
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Todd, D.K. , D.E.O. McNulty . 1975. Polluted Groundwater. Water Information Center, Inc., Huntington, NY.
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National Research Council.2005. Superfund and Mining Megasites: Lessons learned from the Coeur D’ Alene River Basin. The National Academies Press, Washington, DC. https://doi.org/10.17226/11359.
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Rice, E.W. , R.B. Baird , and A.D. Eaton (Eds). 2017. Standard Methods for the Examination of Water and Wastewater. 23rd Edition. American Public Health Association, Washington DC.
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U.S. Environmental Protection Agency. 2009. Drinking Water Contaminants. Available at https://www.epa.gov/sdwa/drinking-water-regulations-and-contaminants
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Interstate Technology & Regulatory Council.2005. Overview of Groundwater Remediation Technologies for MTBE and TBA. Available at https://www.itrcweb.org/GuidanceDocuments/MTBE-1.pdf.
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Hyman, M.H. 1999. Groundwater and Soil Remediation, pp. 684–712. In: Meyers, R.A. , (ed.), Encyclopedia of Environmental Pollution and Cleanup. Volume 1. John Wiley & Sons, Inc., New York, NY.
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