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WATER QUALITY COMPONENT RESULTS |
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What story do the water quality results tell?Combined water quality scores reflect results from all three input indices. Low Non-point Index scores from land application of chemicals and impervious surfaces influence results in the Minnesota, Red River and portions of the Upper Mississippi basins. Urban point sources and mining activity lead to lower scores in the metro areas and the northeast. The Water Quality Assessment Index reflects low scores in the south and west from turbidity, and low scores in northern lakes due to mercury impairments from airbourne sources.
Why is the mean score important?Comparing the mean (average) score reveals statewide trends in water quality, although it may mask extreme values. The mean can also be used to compare similar watersheds, such as upstream or downstream within the same basin. Water Quality Results (Mean) Why is the minimum score important?The minimum is the lowest of the three water quality index scores calculated for each watershed. Like the lowest grade on a report card, it may indicate an area in need of focus and effort to improve overall watershed health. It may also help identify the most impacted or limiting aspect of the system. |
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| CREATING RESULTS | SCORES AND PATTERNS | THE HEALTH STORY | NEXT STEPS |
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What water quality information was
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What story do the water quality results tell across Minnesota?
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Non-Point Source Data Layers: Impervious Land Cover Satellite Data (U of MN, 2000) Chemical and Nutrient application rates (National Ag. Statistics Service, 2007) 200m Riparian Zone based on MDNR Streams and Lakes data |
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Point Source Data Layers: Potential Contaminant Sites (MPCA, MES July, 2008) Superfund Sites (MPCA, MES July 2008) MN County Feedlot Inventory (November, 2010) Mines of Minnesota (MDNR, Lands and Minerals, 2008) Water Discharge Permits (MPCA, January, 2009) |
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WQ Assessment Data Layers: Stream and Lake WQ Assessment Database (MPCA, July 2009)
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The three water quality index values were combined into one mean (average) water quality score for each watershed. This mean value masks some of the variation found in each individual index, but serves to illustrate an overall gradient in results by basin and region.
The Point Source Index illustrates the challenge of reporting a wide range of statewide values on a single 0-100 scale. The very high density of point sources (low score) in the metropolitan region results in comparatively low density of pollution point sources (high score) in outstate Minnesota. Reviewing the results for each of the five point source metrics (wastewater discharge permits, feedlots, potential contaminant sites, superfund sites and mine pits) is necessary to assess watershed scale concerns in out-state Minnesota.
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Of the three water quality index values, the index with the lowest value was identified and mapped across the state. The map on the left shows the lowest water quality index value in each watershed. The map on the right identifies which index scored the lowest in each watershed.
The mean water quality scores show a pattern of slightly higher results in the north, declining toward the south and along the western border. The lowest mean scores are in the Twin Cities watershed and the watersheds in the Cedar River Basin near the Iowa border. The Non-Point Source index follows a similar pattern as it reflects the increase in agricultural and developed land use from north to south.
The Water Quality Assessment Index has low scores in some northern Minnesota River watersheds, the Red River Basin and the Minnesota River Basin. Higher scores are found in the Upper Mississippi Basin and in the extreme northeast and southeast.
The Point Source Index is heavily weighted toward the metropolitan area where the highest number of point sources are found. Out-state Minnesota scores comparatively high, with slightly lower scores in the mining region of the northeast and the mixed developed-agricultural area in the southeast.
The point source index is heavily weighted toward the urbanizing areas painting an overly optimistic picture for out-state Minnesota. This index combines 5 different point sources scores, resulting in low scores for populated areas due to the density of many types of potential contaminant sites. For out-state watersheds with high (good) scores, it is very important to view each type of point source to determine if one or more may be of concern in a particular watershed.
Low scores for the Water Quality Assessment Index in northern watersheds is due to impairments for mercury found in many northern lakes. This mercury is carried into Minnesota on air currents, primarily from western coal burning facilities. Impairments in the south are primarily due to turbidity (sediment in water), nd nutrients.
It is interesting to note that the Minnesota River Basin has consistently low water quality scores, but the Cottonwood watershed scores are higher than the rest of that basin. Further investigation is needed to determine the reason for this result.
A Water Clarity Index may be added based on statewide satellite data of water clarity in Minnesota's lakes and streams. A consistent scale measuring deviations from expected levels of water clarity for different types of stream and lake settings would be needed to create statewide values.
The Point Source Index could weight different point sources by their relative impact, and trends in outstate Minnesota could be scored differently to show trends away from the metropolitan areas.
The Water Quality Assessments Index could be updated as more assessments are completed. This would lead to more comparable results due to a more consistent level of data collection effort across the state.
The Non-Point Index could be improved with better information on the location of chemical applications to agricultural lands.
