WATER QUALITY Assessments
Of the lakes and streams assessed for water quality impairments, what percentage was found to be impaired in each watershed?
Why is this important for water quality?
The federal Clean Water Act requires each state to adopt water quality standards to protect waters from pollution. These standards define how much of a pollutant can be in the water and still allow it to meet beneficial uses, such as for drinking water, fishing and swimming. A water body is designated “impaired” if it fails to meet one or more water quality standards.
Minnesota is assessing the lakes and streams in each of its 81 major watersheds on a 10-year cycle. About half of Minnesota's major watersheds have been assessed as of 2014.
CREATING THE INDEX
Multiple physical, chemical, and biological parameters are measured and monitored to assess surface water quality. The most common parameters causing impairments in Minnesota include turbidity, mercury, total phosphorus, PCBs and other exotic chemicals, fecal coliform, impaired biota, and low dissolved oxygen (DO).
Minnesota's 2014 Proposed Impaired Waters List has been submitted to the EPA for approval. Detail about the assessment and impairment listing process can be found in the MPCA Guidance Manual for Assessing the Quality of MN Surface Waters.
The five categories for waterbodies are:
- Category 1: all designated uses are meeting water quality standards
- Category 2: some uses are meeting water quality standards and there are insufficient data to assess other uses
- Category 3: there are insufficient data to assess any uses
- Category 4: at least one use is impaired, but a TMDL is not required
- 4A: Impaired, but a TMDL study has been approved by EPA
- 4B: Impaired, but a TMDL study is not required because water quality standards are expected to be met in the near future
- 4C: Impaired, but a TMDL study is not required because the impairment is not caused by a pollutant
- 4D: An assessment unit is impaired or threatened but doesn't require a TMDL because the impairment is because of natural conditions with only insignificant anthropogenic influences.
- Category 5: at least one use is impaired and a TMDL is required.
- 5A: Impaired by multiple pollutants and no TMDL study plans are approved by EPA
- 5B: Impaired by multiple pollutants and at least one TMDL study plan is approved by EPA
- 5C: Impaired by one pollutant and no TMDL study plan is approved by EPA
Lakes, reservoirs, and streams were used to develop this index. The data set for impaired lakes and streams is from the 2006 Final 303(d) List of Impaired Waters in Minnesota, approved by the USEPA on June 1, 2006.
- Lakes and reservoirs are grouped in the 303(d) list and will be referred to as “lakes”. Two designated uses, aquatic consumption and aquatic recreation, are used to assess lakes by the MN PCA and were assessed separately to reflect the type of impairment.
- Three designated uses are defined by the MN PCA for streams and rivers: aquatic consumption, aquatic recreation, and aquatic life. If a stream reach was assessed for more than one impairment each assessment was counted separately.
For this index only lakes and streams in categories 4 and 5 were considered impaired. The percentage of assessments that generated an impaired use was calculated relative to the total number of assessments completed in a watershed.
Assessments with Category 4 and 5 impairment / Total # of assessments in watershed
The percentage of waters in each watershed that were impaired ranged from 0 to 100. There are insufficient data in the scientific literature to numerically rank the percent impaired waterbodies and there are no known threshold values, thus the resulting values were ranked in equal intervals. The inverse of this percentage was used to generate scores from 0-100, thus a watershed with no impairments would receive a value of 100.
Watersheds with the fewest number of combined impaired lakes and streams are in the center of the state. Most of the rest of the watersheds have a high to moderate number of lakes and streams that are impaired.
Only 14 watersheds received a score between 60-100, thus 67 of the 81 watersheds have a relatively low to moderate score for water quality impairment, which are broadly distributed across the state. Section 303(d) of the Clean Water Act requires states to publish and update every two years a list of waters that are not meeting one or more water-quality standards. Minnesota's 2010 draft TMDL list contains 1,774 impairments on 388 rivers and 647 lakes. Minnesota’s 2008 list contained 1,475 impairments on 336 rivers and 510 lakes. Impairments in Minnesota are categorized as toxics (31%), eutrophication (24%), turbidity (18%), bacteria (11%), biology (10%), and other conventional (6%), by MPCA.
Altered hydrology may either increase or decrease the movement of pollutants into aquatic systems. Removal of vegetation in riparian or upland areas can increase runoff, which can transport pollutants to streams or lakes.
Water quality impairments may reflect the presence of structures in streams, rivers and lakes. Streams that are dammed or have culverts that were not appropriately designed create artificial impoundments that collect fine sediment, which can lead to high turbidity. Many impoundments are relatively shallow so pollutants, such as phosphorus, accumulate and are resuspended from the bottom into the water column by wind and waves, which may result in an impairment.
Lack of riparian connectivity limits the interaction between a water body and its floodplain. During high water events, the sediment, water, energy and contaminants will be directed downstream rather than being distributed and deposited into the floodplain.
There is an indirect relationship between landscape geomorphology and water quality impairments. For example, potential pollutants, such as sediment or phosphorus are more readily delivered to streams in areas with higher slopes or fine soils. The amount and timing of sediment input to a stream can alter the shape of a stream channel. For example, too much sediment (one of the primary causes of impairment) may cause a stream to aggrade (the stream becomes shallower and wider).
There is a direct relationship between altered stream geomorphology and water quality impairments. When a stream has been altered, its meander pattern no longer fits the slope and size of its valley. This mis-match leads to headcutting, streambank erosion, and general stream instablity that usually increases stream turbidity.
Many water quality impairments affect aquatic organisms. For example, many heavy metals can be lethal to fish, aquatic invertebrates, or mussels at very low concentrations. High sediment loads may be lethal to aquatic organisms, but more often sediment results in chronic effects, such as reduced growth rates in fish.
Section 303(d) of the Clean Water Act requires states to publish and update every two years a list of waters that are not meeting one or more water-quality standards. Minnesota's 2010 draft TMDL list contains 1,774 impairments on 388 rivers and 647 lakes. Minnesota’s 2008 list contained 1,475 impairments on 336 rivers and 510 lakes. Impairments in Minnesota are categorized as toxics (31%), eutrophication (24%), turbidity (18%), bacteria (11%), biology (10%), and other conventional (6%), by MPCA.
Most of the listings under toxics are fish consumption advisories related to mercury, Polychlorinated Biphenyls (PCBs), or Perfluorooctane Sulfonate (PFOS). All three of these toxic pollutants are primarily airborne and most often not directly related to anthropogenic changes on the landscape. These airborne pollutants usually do not result in death of aquatic organisms, but are taken up and concentrated in the tissues of fish, which are then eaten by humans.
Eutrophication, characterized by algal blooms, is related to nutrient pollution (usually nitrogen or phosphorus), which are applied as fertilizer to agricultural fields or lawns. Nutrients typically do not cause direct mortality of aquatic organisms, but often cause excessive plant growth. Actively growing plants may remove dissolved oxygen (DO) from the water. Decaying plant material may also remove DO from the water. There are 100 listings on the 2010 303(d) list for low DO. In either case the reduction of DO can cause mortality in aquatic organisms when the concentration falls below 5 mg/L (Davis 1975). Low DO may not cause immediate mortality but can reduce metabolic function of fish (Jobling 1994: 233-235).
Turbidity is usually related to fine sediment that enters waterbodies from agricultural practices, road or home construction, timber harvesting, or other anthropogenic activities that alter the landscape by removing vegetation and exposing soil to erosion. Fine sediment can reduce a number of physiological functions of aquatic organisms, such as growth rate, or may lead to death at very high concentrations (Newcombe and Jensen 1996).
Bacteria enter streams from a number of sources, but most often from cattle grazing near a waterbody, manure lagoons, or leaky septic systems (Baxter-Potter and Gilliland 1988, Krysel et al. 2003, Sherer et al 1988, Tiedemann et al. 1987, 1988). Bacteria usually do not result in mortality of aquatic organisms, but can affect human health.
The MPCA is the state agency charged with the collection of water quality information in Minnesota. The information is collected following standard published procedures by trained biologists and environmental chemists. Information on impaired waters is complied into the section 303(d) list, which is publically available.
Carlson’s Trophic State Index (TSI) is used as the basis for estimating the trophic status of Minnesota lakes. Trophic status ranges from oligotrophic to hypereutrophic (and is viewed as a continuum). Carlson’s TSI is based on the interrelationships of Total Phosphorus (TP), chlorophyll-a, and Secchi transparency. The individual TSIs are very useful for understanding the relationship of TP, chlorophyll, and transparency for a given lake and provide the best information on the trophic condition of the lake. However, data for TP, chlorophyll-a, and Secchi transparency are not always available for each lake, thus effort could be focused on collection of the variables for all lakes that are visited by MN PCA or other agencies.