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<br />An intermittent tributary of the Rfto Seco enters the river from the south at a point adJacent <br />to the former location of the Earth Saences heap leach pad and within the permit <br />boundary. The point at which the tributary enters the permit boundary is designated as <br />surface water station RSA (Fgure 2). The flow rate of this tributary at the permit <br />boundary was measured to reach a maximum in 1988 of 0.5 cfs (BMG, 1988). <br />The mill ftailings facility is situated in a drainage that contributes flow to Culebra Creek <br />approximately 6 miles downstream. Overland flow originating from the area of the mill <br />facility is diverted into the tailings pond. Overland flow upstream of the mill fadlity is <br />diverted into the pre-existing channel at a downgradient location, thus minimizing the <br />potential for entry of flood waters to the facility. The diversion ditches carrying upgradient <br />flow around the facility were designed to carry the peak discharge from the 100-year, 24 <br />hour preapitation event (BMG, 1989). Measurable flows in this drainage are infrequent <br />(BMG, 1994). <br />3.0 SOURCE CHARACTERISTICS <br />The known hazardous substances that are associated with past and cun'ent mining <br />activity at the site include metals and cyanide. Mercury in particular was reportedly used <br />at the site during initial gold extraction operations at the tum of the century. Mercury is <br />also a trace contaminant in the wastewater generated from current milling activities; a <br />sample collected from the onsfte leachate collection pond on September 13, 1991 <br />contained 1.3 ppb total mercury. Cyanide was used historically and in the present active <br />gold extraction operation at the site (URS, 1993; CDMG, 1994). Tailings~produced from <br />the gold extraction process consist of finely ground rock of which 80 percent is minus No. <br />,,,,,:eoq.oemunc - 4 - m.. a o~~ <br />