Effects of Precipitation on Metals Loading in Boulder Creek

Section 303(d) of the Clean Water Act requires each state to develop Total Maximum Daily Loads (TMDLs) for surface waters that do not meet and maintain applicable water quality standards. A TMDL establishes the amount of a given pollutant that the waterbody can withstand without creating an impairment of that surface water's designated use. The TMDL by definition (40 CFR Part 130) is the sum of all point and non-point sources with the inclusion of a margin of safety and natural background considerations.Boulder Creek, located near Bagdad, AZ, in west central Yavapai County, appears on the Arizona Department of Environmental Quality's1998 List of Water Quality Limited Waters for violations due to water quality violations for arsenic, beryllium, copper, lead, manganese, and zinc water quality standards. Specific water quality standards for these parameters are listed in Title 18, Chapter 11 of the Arizona Administrative Code. For this TMDL investigation, samples were collected at locations to provide definition of pollutant sources and the extent of impairment. The sources of pollutants are three tailings piles, the upper, middle, and lower tailings piles, and an adit discharge from the abandoned Hillside Mine.The critical condition of Boulder Creek occurs during low flow (0.75 cfs). At, or below, this flow, the amount of metals in the water column rises. There is also a significant increase in the number of exceedances of Surface Water Quality standards.Stream flow in Boulder Creek responds dramatically to seasonal conditions. Flow ranges from spatially interrupted, independent pools in the summer to raging floods in response to large winter storms. The winter storms have a larger areal extent, contribute higher overall flows to Boulder Creek, but bring rain mainly to the higher elevations of the headwaters. Metals flushed into the system during these types of precipitation events are diluted due to the high flows in the creek. The monsoon storm driven flows are highly variable, dependent upon location and storm intensity. Monsoon storms also have a dramatic effect on metals loading in Boulder Creek. By providing brief, but intense, precipitation, monsoon storms flush large amounts of metals into the creek without greatly increasing the overall flow.