Mine Tailings Waste Leaching & Groundwater Contamination

Each year the mining industry generates tremendous quantities of solid wastes as a byproduct of the mining and concentration or preparation of valuable ores, minerals and coal. In addition, the quantities of these wastes generated on a yearly basis is increasing due to:

increased production resulting from rising demand,
the depletion of near surface high-grade ores which makes necessary the mining of lower grade ores at greater depths,
changing mining methods, and
the promulgation of strict emission standards which ultimately require more extensive cleaning of coal.

Inherent to the disposal of mining wastes on land is the potential for groundwater contamination due to the leaching of these wastes by rainwater.

It is evident that the crushing and grinding of mined material during extraction or preparation processes and subsequent disposal of process wastes in surface disposal areas creates conditions which are favorable to the rapid weathering of these wastes. These conditions are especially favorable to the accelerated oxidation of pyrite and subsequent production of acid. This, in turn, may accelerate the rate of leaching of trace metals contained in mining wastes. The rapid release of acid and metals from mining wastes by natural leaching processes, therefore, presents a great potential for the contamination of groundwater.

Groundwater is used extensively for public and rural drinking water supplies, agricultural water supplies and in some industries. Therefore, in recent years, an increasing number of reported incidences of groundwater contamination resulting from solid waste disposal has been cause for great concern. Five major pieces of federal environmental legislation, all enacted since 1972, contain sections pertinent to groundwater contamination. They are:

The 1972 Federal Water Pollution Control Act (amended by the Clean Water Act of 1977),
The Safe Drinking Water Act of 1974,
The Resource Conservation Recovery Act of 1976,
The Toxic Substances Control Act (1976), and
The Surface Mining Control and Reclamation Act of 1977.

However, in spite of this legislation the general concensus is that a comprehensive national program for the protection of groundwater is lacking. In recognition of this fact, the Environmental Protection Agency is presently considering four options for groundwater protection:

non-degradation or protecting all groundwaters at their present level of quality;
drinking water standards that assure all groundwaters are protected up to some specified level;
protection of groundwaters to the levels necessary for projected future uses; and
protection of only those groundwaters that are used as drinking water sources, relying on monitoring or treatment for use of unprotected groundwater sources.

A recent report to congress has concluded that although specific case histories of groundwater contamination related to the disposal of mine wastes do exist, adequate documentation of the problem is unavailable. In addition, although the propensity of pyrite containing mine wastes to generated acid drainages has been well documented, few previous studies have investigated the heavy metal content of leachates from these wastes.

Although all mining wastes have a potential to contaminate groundwater, the scope of the present study was limited to an evaluation of coal mining wastes. The primary objective of this study, therefore, was to determine which types of, and to what extent, coal wastes can contaminate groundwater through leaching of acid-forming or potentially toxic constituents. The waste types examined for this purpose included surface mining overburdens, underground mining waste rock, and coal preparation plant refuse. In addition, the potential for groundwater contamination by leachates from clean coal stockpiles was also examined.

The approach taken to fulfill the study objective included investigation or review of the following:

(a) The amounts and geographic locations of coal wastes in the United States.
(b) The chemical character of coal wastes, especially with, regard to potentially acid-forming or toxic-forming constituents.
(c) The extent to which acid and/or toxic components are leached out of coal wastes by rainwater.

The quantities and geographic locations of coal wastes in the United States were determined on the basis of data published annually by the Bureau of Mines for the years 1960-75 inclusively and by the Department of Energy for the years 1976-1977. The chemical characterization of coal wastes was based both on data published in the literature and on analyses of coal waste samples collected during the present study. Although the primary focus of this study is on coal mining wastes, waste quantity and chemical characterization data are also presented for the metal ore and non-metal mineral mining industries to reflect, on this basis, the relative potential of these industry segments for contamination of groundwater.

To evaluate the extent to which the various waste components are leached from coal wastes by rainwater, a laboratory waste leaching method was developed during this study to specifically simulate this process over a long-term. This method, a column test, incorporated simulation of actual field conditions including leaching by the infiltration mechanism, depth of the zone of leaching in a waste pile, regional precipitation chemistry and amount, and intermittant application of simulated rain to allow alternate flushing and drying of the waste sample.

The column leach method developed for this study is unique in that it allows for the oxidation of pyrite in much the same manner that this process occurs in nature. The significance of this design feature is related to the profound impact that pyrite oxidation has on leachate quality. In this regard, it has previously been reported that a short-term elution test, in which the waste sample was mixed with the leaching solution by stirring or shaking under saturated conditions, failed to accurately predict leachate quality from a coal pile. This failure was attributed to the inability of the elution test to simulate the pyrite oxidation process. For this reason, a secondary objective of the present study was evaluation of the short-term elution leach test which has been developed by the environmental Protection Agency ( EPA) under the Resource Recovery and Conservation Act of 1976 (RECRA) to identify hazardous wastes on the basis of waste leachability. This method has been published in the Federal Register and is to be used for purposes of regulation of the disposal of hazardous wastes. Whether mining wastes will be regulated under RECRA is the subject of an on-going study and is uncertain at the present time.

As a result of their objection to the EPA extraction procedure developed for regulation of hazardous wastes, the American Society for Testing and Materials (ASTM) Subcommittee D 19 has developed a short-term shake test which they have proposed as an alternative to the EPA method. Therefore, the ASTM method was also evaluated during this study. For purposes of this evaluation, each coal waste sample collected during this study was split into three parts for leaching by the three leach tests (that is, the column, EPA, and ASTM methods. Results of the EPA and ASTM methods were compared with the results of the column method to determine the extent to which the former methods accurately predict the leachability of coal mining wastes.

The primary types of solid wastes generated by mining activities include overburden and waste rock from surface mining, waste rock from underground mining, refuse from coal cleaning processes, and bulk tailings from the milling of metal ores and non-metal minerals.

During the period 1960-1977, 53,566 million cubic yards of overburden were generated by surface mining for coal, and 75 percent of this in just nine states-Kentucky, Pennsylvania, Illinois, Indiana, Alabama, West Virginia, Virginia, Tennessee, and Missouri. Likewise, during the period 1966-1976, 1215 million short tons of preparation-plant refuse were produced, and 97 percent of this in just eight states-West Virginia, Pennsylvania, Illinois, Kentucky, Alabama, Virginia, Ohio, and Indiana.

Waste Tailings Characterisation & Disposal Practices

The quantity of coal mining overburden has been rapidly increasing during recent years due to increasing production of coal and to the trend for increasing production by surface mining methods. The quantity of preparation-plant refuse produced annually is also increasing due both to an increasing rate of coal production and to the trend for an increasing percentage of refuse production per unit of raw coal cleaned.

Reported production of anthracite is entirely from northeastern Pennsylvania. Interestingly, the accumulated volume of anthracite preparation-plant refuse has actually been decreasing during recent years due to rapidly decreasing production of anthracite by strip mining and underground methods and an increased percentage of the production resulting from the recovery of coal from existing refuse piles (that is, bank recovery).

During the period 1960-1977, metal-ore mining was conducted in 23 states-10 east of the Mississippi River, 17 west of the Mississippi River, and Alaska. Non-metal mineral mining is ubiquitous, occurring in all 50 states. More than 62 different mineral commodities have been mined in the United States, including 21 metal ores and 41 non-metal minerals.

Approximately 95 percent of all metal ore and non-metal mineral production is by surface mining methods. As a result, 67 percent of all the metal-ore and non-metal mineral mining wastes generated during the period 1960-1977 was overburden and waste rock from surface mining, while only 0.8 percent was waste rock from underground mining. The remaining 52.2 percent was tailings from ore processing operations.

A total of 51,191 million tons of overburden, waste rock, and tailings was generated by the metal-ore and non-metal mineral mining industries during the period 1960-1977. Of this total, approximately 40 percent was generated by the copper mining industry alone. Approximately 81 percent was generated by the copper, iron, uranium, and phosphate mining industries combined including 84 percent of the overburden and waste rock from surface mining, 42 percent of the waste rock from underground mining, and 76 percent of the tailings. The copper and uranium, iron and phosphate industries are also highly regionalized occurring predominately in the Southwest, Minnesota, and Florida, respectively. As a result, approximately 75 percent of all mining wastes generated in the metal-ore and non-metal mineral mining industries is generated in these same locations.

Few previous studies have been conducted to characterize mining wastes. Analysis of coal mining wastes collected during this study reflected that the trace-element compositions of refuse and overburdens are very similar. In comparison to average elemental abundance of some sedimentary rocks, some coal mining wastes were enriched by a factor of 10 or more in the following elements: arsenic, cadmium, cobalt, mercury, selenium, and thallium. In most instances, however, the coal waste compositions were very similar to the average crustal elemental abundance in sedimentary rocks. A notable geographic difference is the generally lower levels of trace elements in Western region coal mining wastes than in wastes from the Appalachian and Eastern Interior regions.

Data available from the literature reflect that metal-ore tailings are highly enriched in their metal content. Due to crushing and grinding of ore for milling purposes, the tailings also consist of small sized particles. On this basis, the character of tailings favors the rapid leaching of metals under certain conditions.

Methods Commonly Employed for Disposal of Wastes

The acid-forming materials present in many mine wastes are the iron sulfides, the most common of which is pyrite (FeS2). When exposed to the atmosphere under humid conditions, such as is generally found in mine dumps, coal refuse piles, spoil banks, and abandoned tailing disposal areas pyrite is oxidized by a complex series of chemical reactions. As this process proceeds, the oxidation products (that is, acid and hydrous iron complexes) accumulate in the waste. Subsequently, when rainfall or snow melt occurs, these products are flushed out of the waste by percolating water resulting in production of a contaminated leachate or drainage.

Other metal sulfides are subject to the same oxidative processes as are the iron sulfides. A notable difference, however, is that acid is not produced by oxidation of the other metal sulfides. In addition, in pyritic mining wastes, the generation of acid that accompanies pyrite oxidation greatly increases the rate of dissolution of metals present.

A column leaching method was developed during this study to evaluate the leachability of coal wastes. This method was specifically designed to simulate real-time leaching by artificial rainwater applied to the wastes in a manner to allow infiltration much the same as it occurs under actual field conditions. The coal wastes collected during this study were also leached by the Environmental Protection Agency (EPA) extraction procedure developed by that agency to identify hazardous wastes and by an American Society for Testing and Materials (ASTM) procedure developed by a panel of scientists and industry representatives as a proposed alternative tn the EPA extraction procedure. The results of this study demonstrated that the column method was successful in simulating the conditions necessary for the oxidation of pyrite. In addition, the column method also was determined to be capable of simulating conditions necessary for the dissolution of salts when pyrite oxidation was not a major factor.

Paired t-tests were used to compare the results of the column leach method to the EPA and ASTM methods for the purpose of evaluating the extent to which these latter two methods accurately predict the leachability of coal mining wastes. Results of the comparison of these methods indicate that the inability of the EPA and ASTM methods to accurately predict the leachability of pyritic mining wastes is due to the failure of these methods to replicate the pyrite oxidation process. When compared on the basis of non-acid producing wastes, however, the three leach test methods produced similar results.

The results of the laboratory leaching experiments (that is, the column leach method) conducted during this study indicate that all coal mining wastes have the potential to cause significant degradation of groundwater quality. The threat presented by overburden spoils results primarily from the leaching of soluble salts from these spoils, resulting in high concentrations of sodium, magnesium, calcium, and sulfate ions. Preparation plant refuse and clean coal stockpiles represent the greatest potential sources of acid leachates containing high concentrations of heavy metals. However, Individual overburden strata may also have a net acid producing potential.

A number of previous studies have provided evidence that a cone of weathering exist in the top 6-15 inches of coal refuse piles or spoil banks and that below this cone little or no oxidation of pyrite or leaching of metals occurs. The rates at which waste components were leached from coal wastes during laboratory simulation of this process suggest that over a period of 5 to 10 years the reactive pyrite and leachable metal present in this surface cone would be totally removed by leaching. Subsequently, coal refuse piles or spoil banks would cease to have an adverse impact on groundwater quality after this period of time. However, if significant erosion of the surface of the refuse pile or spoil bank occurred, this situation could change dramatically as fresh material would be exposed to oxidation and leaching. Therefore, the long-term potential of an inactive refuse pile or spoil bank to contaminate groundwater by leaching of acid and metals will he a function of the rate of leaching (that is, “depletion”) relative to the rate of erosion of the surface of the pile (that is, “renewal” of unleached waste). In the case of an active pile, the continuous disposal of fresh material on the pile would provide the “renewal” process and sustain the waste potential for leaching of acid and metal components.

The impact of rainfall volume on a refuse pile or spoil bank was also evaluated during this study. It was determined that although the infiltration of rainfall is required for transportation of contaminants from these coal wastes, the volume of rainfall which infiltrates the waste is not necessarily related to the quantity of acid, metals, or salts leached from the waste (except in arid regions where rainfall may be a limiting factor). This is true since the quantity of contaminants leached is a function of the rate processes of chemical oxidation and diffusion controlled dissolution of salts. These rate processes are not directly a function of the volume of water percolating through the waste. As expected, therefore, no correlation was found between the quantity of acid or metals leached and regional annual precipitation rate.