Baseline, Pre-Mine and Mine Operation Aquatic Monitoring; How Long-Term Data Track Response of Aquatic Systems for an Open Pit Copper Mine in Central Arizona

The Carlota Copper Mine is a new open pit mine in central Arizona, which opened in 2009 in the Pinto Creek watershed among several existing mining operations.  The mine is in an arid mountainous region at approximately 1000 m elevation within the Tonto National Forest.  Semi-annual sampling in 1993, 1994 and 1996 provides a baseline data set for comparison to semi-annual pre-mine sampling in 2007 and 2008 and semi-annual sampling since mine operation for 2009 – present.  Mitigation and monitoring for the mine was included in the 1995 EIS.  A standardized monitoring plan developed in 1993 is used for each sampling event.  The standardized sampling metrics provide a common data set for comparison of sampling periods.  Since standardized methods are used for the physical and biological components, analysis of either physical or biological components is possible over a multi-year time period.  Data collection includes habitat characteristics of mesohabitat, fish abundance, and benthic macroinvertebrate community from six permanent sampling locations upstream, within and downstream of the mine.  The watershed experiences wide inter- and intra-annual variations in hydrologic conditions from extreme flood events (greater than 200 m³/s) to zero flow conditions.  The multi-year baseline data set is used as the benchmark to compare each new sampling event.  Key characteristics are mesohabitat area, fish community metrics, and benthic macroinvertebrate community metrics. Mesohabitat components are known to change rapidly due to the large flood events, however, the multi-year sampling is used to track long term trends.  The biological components also change as a result of environmental conditions both natural and man made.  The fish community consists of three species, two native and one non-native species.  Non-native green sunfish is both a competitor and predator on the native longfin dace and desert sucker.  Native species are of particular interest due to the limited range where they occur compared to historical distributions.  The biological metrics can have a wide range within a single year as a result of natural conditions.  The values for the biological metrics from baseline, pre-mine and during mine operation are similar. The long-term data set has provided insight into the natural dynamics of biological communities in a rapidly changing environment. Further, it provides a basis to determine anthropogenic caused changes.  The mitigation measures implemented for the mine have protected the stream environment based on the monitoring results.