Determining a Site-specific Objective and Tmdl for Copper in a Historical Mining District, Arizona

Pinto Creek is a predominately intermittent stream that drains approximately 183 mi2 in Gila, Pinal, and Maricopa counties in east central Arizona. The stream extends approximately 33 miles from its headwaters in the Pinal Mountains to Roosevelt Lake. Although much of the creek length is ephemeral or intermittent, it contains several perennial reaches, which are uncommon in this arid region. The watershed is located in the Globe-Miami mining district and has numerous historical mining-related disturbances include including open pits, tunnels, waste rock and tailings piles, leach dumps, and milling facilities. The Arizona DEQ has placed a 20-mile segment of Pinto Creek on its 2004 list of water quality limited waters due to exceedances of acute and chronic criteria for dissolved copper. A Phase I TMDL was completed in 2001, but was considered only a placeholder for several reasons. A new mining open-pit operation was proposed that would affect a perennial segment of Pinto Creek, causing the TMDL to be of heightened interest to environmental groups, mining interests, and local citizens. Second, it was recognized that the effects of natural mineralization were probably sufficient to cause exceedances of Arizona's copper criterion in Pinto Creek. During 2000-2005, ADEQ conducted a monitoring program to collect water quality, stream stage, and meteorological data in the Pinto Creek basin in support of a site-specific objective (SSO) and Phase II TMDL for copper. Special sampling efforts were made to determine the background copper concentrations associated with runoff from various lithologies, as well as to characterize loads from known mining-related sources. The Hydrologic Simulation Program-Fortran (HSPF) was chosen as the model framework for the Phase II TMDL. Water quality monitoring data were used to developed copper loading factors for background and discrete mining-related sources in the model. After hydrologic and water quality calibration, the Pinto Creek model was applied to predict changes in dissolved copper concentrations and loads for a variety of mine remediation and development scenarios. Remediation of several discrete mining-related sources was predicted to reduce copper loading to Pinto Creek by over 90 percent. Development of the proposed open pit mine, with associated hydrologic controls, was not predicted to increase copper concentrations in the stream. However, copper concentrations were predicted to remain above Arizona's water quality criterion under all remediation and development scenarios. The modeling framework was used to set the SSO as the ambient background copper concentration assuming remediation of major mining-related sources to about ten times the ambient background levels. This SSO proposal is currently being formulated for public review.