Total Maximum Daily Loads (TMDLs) for Dissolved Oxygen and Iron in the Waters of Duck Creek in Mendenhall Valley, Alaska

摘要

Duck Creek is listed on the 1998 303(d) list of impaired waters in Alaska for metals (iron) and low dissolved oxygen. The primary source of iron in the creek is groundwater inflow. Much of the Mendenhall Valley is underlain by iron-rich glaciomarine deposits. As the watershed has become more developed, channel modifications and land disturbances near the creek, including the removal of the thick layer of peat that previously filtered out much of the iron, have become more common. The primary cause of low dissolved oxygen in Duck Creek is the increased influx of iron, which becomes oxidized and forms iron floc when the groundwater flows into the creek. Because the dissolved oxygen and iron impairments are related, reductions in the inflow of ironrich groundwater to the creek will result in attainment of the dissolved oxygen criteria. Therefore, both the iron TMDL and the DO TMDL are represented by the loading capacity established for iron. The water quality standard for dissolved oxygen and the oxygen demand exerted by dissolved iron set the loading capacity for iron at 0.27 tons/yr to protect designated uses, representing a 93 percent reduction in current loading. The Duck Creek Watershed Management Plan recommended a restoration approach which would include capping sources of iron with organic fill, planting riparian/aquatic plants capable of oxidizing iron, mechanically aerating the water at sources of dissolved iron, and increasing the volume of flow to dilute the dissolved iron.