Minnesota Impaired Waters

Lake4What does “impaired waters” mean? People are concerned, as they should be, about what this means for their lake. When you understand the details about why each lake is on the Impaired Waters List, it is much easier to digest.

The Clean Water Act requires states to publish a list every two years (even years) of the lakes not meeting certain water quality standards. This list is called the Impaired Waters List, and currently the 2008 version is in draft form, and will get submitted to the Environmental Protection Agency (EPA) for approval in April.

The words “impaired” and “polluted” can sound scary and are very general, so it is extremely important to understand the different types of impairment and the criteria used to determine these types. Remember, the term “water quality” is subjective and depends on how you intend to use the water body. A lake that is good for duck hunting is not necessarily good for water skiing. In turn, a lake with natural sandy beaches that is great for swimming may not be great for bass fishing.

Therefore, data for each lake is assessed and tied to a designated use. For lakes, the most common types of impairment are mercury in fish tissue for the use of aquatic consumption (fish) and excess phosphorus for the use of aquatic recreation. These are explained in more detail below.

In all, there are 23 types of impairments and 3 different designated uses. The other types of impairments mainly apply to rivers and streams and are not covered in this description.

The standard for excess phosphorus impairment in Ottertail County and the western half of Becker County (North Central Hardwood Forest Ecoregion) is 45 ppb total phosphorus. To be considered impaired for phosphorus, a lake must have at least 10 data points of each phosphorus, chlorophyll-a and secchi disk in the past 10 years over these standards. Each lake is carefully considered by the MPCA and the data quality is checked.

The reason that this amount of phosphorus is considered impaired, is because phosphorus is food for plants and algae so the more phosphorus there is, the more algae there is that makes the lake undesirable for recreation. In fact, the criteria for phosphorus were in part determined through lake-user surveys, which summarized user perception of water quality. MPCA researchers found that when there was a certain amount of algae in the water, people didn’t want to swim in the lake anymore, and this corresponded to a certain level of phosphorus.

For excess mercury in fish tissue, the MPCA says “A water body will be considered impaired when the recommended fish consumption frequency is less than one meal per week for any member of the population.” Mercury is a different type of pollutant in that most of it comes from outside Minnesota and is deposited here from the air. Approximately 30% of the mercury deposited by air in Minnesota originates from natural sources, such as volcanoes; 60% of mercury comes from human activities outside the state such as coal-fired power plants and mining. The remaining 10% originates in the state.

There are lakes that were on the 2006 Impaired List for mercury, but they are now part of the state-wide mercury TMDL study.To see a list of these lakes, visit: http://www.pca.state.mn.us/water/tmdl/tmdl-mercuryplan.html.

Just because a lake is not named on list that it is impaired for mercury, does not mean that it is safe to eat unlimited amounts of fish. It may just mean that it hasn’t been tested yet. For fish consumption advice, please visit: http://www.health.state.mn.us/divs/eh/fish/index.html.

So once a lake is determined to be impaired, what comes next? The next step is a TMDL study. TMDL stands for Total Maximum Daily Load. A TMDL defines how much of a pollutant can be in the water and still allow the lake to meet designated uses such as fish consumption and aquatic recreation.

For more information about this topic and to see the full Impaired Waters Lists, visit: http://www.pca.state.mn.us/water/tmdl/tmdl-303dlist.html.