United States Office of Research and EPA/600/SR-99/007 Environmental Protection Development February 1999 Agency Washington, D.C. 20460 Project Summary Characterization of Mine Leachates and the Development of a Ground-Water Monitoring Strategy for Mine Sites

Russell H. Plumb Jr.

The total number of active and effective ground-water monitoring inactive sites in the United strategy: States has been estimated to be as high as 82,000. The 20,000 active mining 1. The composition and environ- sites currently process an estimated 1.5 mental behavior of mine waste billion tons of per year and the leachates is poorly understood. cumulative quantity of mine waste that has been produced has been estimated 2. The problem of how to effectively to be 50 billion tons. These wastes are sample the large areas covered by usually discarded in waste piles, tailings tailings ponds, that range in size basins, and depleted from several acres to several pads. Although the adverse thousand acres, has never been environmental impacts of mine waste addressed. leachates and acid mine drainage on surface waters have been widely 3. The parameters that should be documented, the impact of mine monitored in ground water leachates on ground water is poorly adjacent to mine waste disposal understood. One of the factors that sites to detect fugitive mine waste contributes to this situation is the fact leachate have not been selected that the United States does not have a and evaluated. national strategy for monitoring of mining sites. The objective of this research project was to develop a better understanding When the Resource, Conservation, of the composition of mine waste and Recovery Act (RCRA) program was leachates and to identify cost effective initially developed, seventeen industries ground-water monitoring parameters were classified as generators of non- that could be incorporated into a hazardous wastes and exempted from monitoring strategy to reliably detect the the ground-water monitoring migration of contaminants from hard requirements. One of these industries rock mining operations. was mining because the wastes were considered to have a low toxicity Approach despite their large volume. It was subsequently determined that mining Information used in this study was wastes may pose an unacceptable obtained from reports and data environmental risk if they are not submitted to state regulatory agencies properly managed. However, several as part of routine, ongoing monitoring issues limited the development of an programs at mining facilities in the southwest United States. This effort screening levels. Third, a small set of feasibility of fingerprinting mine focused on mines in that geochemical parameters are always leachates with geochemical utilize cyanide heap leaching tech- the most abundant ions in the parameters. The results demonstrated nology and copper mines in Arizona. leachates and usually represent 90 to that the leachates at each mine had a These operations were selected 95 percent of the total dissolved solids distinctive geochemical composition because (1) cyanide heap leaching is concentration. that displayed the following properties: the fastest growing sector in the mining industry and the high concentrations of Since the leachate constituents are 1. The shape of the fingerprint varied cyanide used in this process are a all naturally occurring substances that from mine to mine but the same potential environmental concern and are not uniquely characteristic of mine small set of geochemical (2) approximately 80 percent of the waste, the project focused on the use parameters defined a consistent mining activity in the country is of multiple ion chemical signatures to pattern in 26 case studies. associated with gold and copper uniquely characterize the mine mining. This approach to data leachates. The parameters chosen for 2. Monitoring results from multiple collection produced a larger project this purpose were the common geo- locations at the same mine database that was more representative chemical ions of calcium, magnesium, produced identical geochemical of the hard rock mining industry than a sodium, potassium, chloride, sulfate, fingerprint patterns. detailed investigation at a single site. and alkalinity. These ions were selected because (1) they were 3. The reproducibility of individual The data collection effort produced repeatedly present in mine leachates at geochemical fingerprint patterns monitoring records, ranging from 1 to high concentration, (2) they ranged from 70 percent to 99 15 years in length, for 30 heap represented more than 90 percent of percent with an average of 91 leaching facilities in Nevada and five the total dissolved solids concentration percent across all case studies. copper mines in Arizona. Monitoring of mine leachates, and (3) they had locations varied between sites but previously been used to successfully 4. The mine leachate fingerprint was usually included several locations in the characterize ground water and other consistently and distinctly different recirculating heap leaching systems waste leachates. An example of the from the geochemical fingerprint (barren ponds and pregnant ponds), graphical fingerprint pattern developed of ground water upgradient of tailings disposal ponds, and ground- for the tailings leachate at the Cortez each mining operation. water monitoring wells upgradient and Gold Mine is shown in Figure 1. downgradient of each mine. More than Despite the variable concentrations that This set of observations suggests 300 quarterly surveys of mine had been reported, the normalized that the set of geochemical parameters leachates and 500 quarterly surveys of results (actual concentration divided by can be used in a monitoring program to ground water in the vicinity of mining total dissolved solids concentration) uniquely characterize mine leachates at operations have been compiled and from the pregnant pond, barren pond, their source and differentiate them from evaluated in this study. Each of the tailings solution, and tailings reclaim regional ground water. surveys generally included complete water at this site produced a consistent geochemical cation analyses, complete sulfate-rich, alkalinity-poor fingerprint Attempts were also made to geochemical anion analyses, a trace for these leachates. The reproducibility fingerprint mine leachates with trace metal scan, and cyanide (heap of this fingerprint was estimated by metals because of concern regarding leaching sites only). regression analysis to be 98 percent. these contaminants. However, it was Another property of the mine leachate not possible to define a consistent trace Results fingerprint illustrated in Figure 1 is that metal fingerprint in the leachates. The it is distinctly different from the regional reproducibility of trace metals The compiled monitoring data illus- ground water fingerprint. Based on distributions was usually less than 30 trated several properties of mine these properties, it was postulated that percent which is considerably lower leachates. First, the individual the geochemical fingerprint would than the 90 percent repeatedly constituent concentrations are highly provide a mechanism to uniquely observed for geochemical parameters. variable and frequently display a non- characterize mine leachates at their These results suggest that trace metals normal distribution. Second, as many source and monitor their migration into would not be useful for uniquely as 31 mine leachate constituents (8 the environment. characterizing mine waste leachates. geochemical parameters, 19 trace metals, phosphate, pH, total dissolved The monitoring data from each The information compiled during the solids, and cyanide) may be present at selected mine site were used as a study also provided insight into the concentrations above environmental series of case studies to evaluate the behavior of mine leachate geochemical

2 fingerprints. One particularly useful Nevada and 4 copper mines in Arizona mobile, characteristic fingerprint of study was a column study performed demonstrates that mine leachates have the leachate is not detected, there with Twin Butte tailings leachate. The a characteristic geochemical finger- is no need to monitor for the less data from this laboratory study print. Furthermore, the geochemical mobile and less abundant demonstrated that the geochemical fingerprint was shown to migrate faster constituents of the leachate. The fingerprints for mine leachates will than the trace metals present in mine site would remain in a low level retain their unique chemical identity as leachates and to maintain its identity as detection monitoring mode. they migrate through the subsurface. In the leachates migrates under both addition, the geochemical parameters controlled laboratory and field con- 3. In the third phase, when the more that define the fingerprint migrated ditions. These results suggest the mobile, more abundant, and faster than the trace metals present in possibility of using a multi-phased characteristic geochemical ions mine leachate. The case study field ground-water monitoring strategy for have been detected, there is a monitoring data provided additional mine sites: reason to believe that a leakage corroboration of this observation. The event or spill has occurred. At that tailings leachate fingerprint was 1. In the first phase, mine leachates time, the program would be identified downgradient of four copper can be characterized at their expanded to monitor for the less mines and the heap leaching process source with a short list of abundant and less mobile solution fingerprint was identified geochemical parameters (calcium, constituents of mine leachates downgradient of eight gold mining magnesium, sodium, potassium, (trace metals). facilities included in this study. These sulfate, chloride, alkalinity, and results suggest that the geochemical total dissolved solids). These data This phased approach to monitoring fingerprint acts as an internal tracer will be sufficient to define the is functionally similar to that developed and can be used to monitor the fingerprint of the leachate. for use at RCRA sites. Based on migration of mine leachates in the properties illustrated in the case environment. 2. In the second phase of the studies, it would provide strategy, it is suggested that the characterization of the mine leachates Proposed Mine Site Monitoring set of geochemical parameters at their source and early detection of Strategy used to define the source their migration into the environment. fingerprint can function as This approach should also provide The assessment of routine moni- effective detection monitoring more effective identification of leakage toring data from 26 gold mines in parameters. As long as the more events and lower monitoring costs.

Figure 1. Comparison of the geochemical fingerprint for mine leachates at the Cortez Gold Mine with the regional ground-water fingerprint.

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