LOW DISSOLVED OXYGEN TMDLs: CRITICAL THRESHOLDS ASSOCIATED WITH LAND-USE AND LANDSCAPE

Minnesota's abundant water resources are valuable for the economic well being of the State. They provide great natural beauty and supply water necessary for recreation, industry, agriculture and aquatic life. Minnesota submitted its first Section 303(d) list to US Environmental Protection Agency (USEPA) under the Clean Water Act in 1998 by utilizing, where available, ten years of monitoring data for various pollutants. The list showed that many of Minnesota's water resources failed to meet their designated uses given exceedances of individual numeric criteria. However, a comprehensive understanding of watershed systems and their associated ecological integrity were not directly incorporated into the listing process. The list was updated in 2002 using a similar listing methodology. As a result, several questions have been raised about the validity of some identified low dissolved oxygen impairments.This paper highlights the importance of utilizing a comprehensive understanding of watershed systems and their associated ecological integrity in the identification of various water quality impairments and the preparation of the Section 303(d) list. The identification of various impairments solely based on concentrations of dissolved oxygen may inadequately represent stream condition. The need for a systems approach for identification of dissolved oxygen impairments in the 303(d) listing process is demonstrated using the results from three case studies in Minnesota.Geochemical, isotopic and fluvial techniques were used to assess three TMDL watersheds. The results point to the importance of understanding the natural processes driven by climate, hydrology and geology. Land-use must then be superimposed over the natural phenomenon to characterize anthropogenic influences within a given watershed. There are water quality impairment thresholds between natural phenomenon and anthropogenic influences. The TMDL process must define water quality impairment thresholds to effectively implement water quality restoration and/or protection.