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21 <br />• These current effluent requirements are called the Best <br />Practicable Control Technology Currently Available (BPCTCA). <br />The methods which the EPA considers will best control the <br />harmfulness of effluents are designated as the water quality <br />criteria that will be effective after July 1, 1984. These <br />1984 standards are called the Best Available Technology <br />Economically Achievable (BATEA). The criteria established <br />by the EPA are the same for all copper, zinc, lead,. gold and <br />silver mines and they are listed in Table III. <br />This EPA study and designation of effluent limitations <br />for the metal mining industry can be considered to be suc- <br />cessful because both the current and future limitations are <br />based on sound chemical principles. Specifically, the tech- <br />nology is based on the fact that natural waters will tend to <br />• be between a pH of 6 to 9 and between those pH's most harmful <br />dissolved species that would come from a mining operation <br />will be precipitated from the water. This tendency of water <br />to remain between pH 6 and 9 is dictated by the buffering <br />effect that occurs when silicate and carbonate minerals are <br />weathered and dissolved. This buffering concept was explained <br />in the previous chapter. If sulfide minerals are weathered <br />and the pH of the water falls below 6, the changes in water <br />composition with pH are obvious as shown by the mine drainage <br />data in Table II and figure 1. Thus, the generation of harmful <br />mine effluents is caused by natural processes and, given <br />enough time, it can be controlled by natural processes. The <br />industrial control of mine effluents works on the same basis, <br />•