How vulnerable is each watershed to water pollution from known point sources?

Why is this important for water quality?

Point Source Health Scores

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 The presence of point source contributions of contaminants are a threat to surface water and groundwater quality. A wide variety of chemical, nutrient and temperature impairments occur from point sources. Pollutants may move into streams as contaminanted runoff, infiltrate into groundwater, or discharge directly from wastewater or industrial processing.

• Wastewater treatment plants may serve populations that exceed their design capacity and become a major contributor of phosphorus to streams, which may cause excessive plant growth (Johnes et al. 1996).
• Open-pit mines that remove sulfur-bearing waste rock contribute acid mine drainage, which may result in the death of fish and insects in receiving streams. Nitrous oxides, sulfides, and mercury may be released during processing of ore. Mercury, cadmium, and nickel may occur in industrial wastewater and aluminum is associated with industrial discharges (Welch et al. 1998).
• Feedlots present a risk to water quality because of potential contamination from excess nutrients, microbial pathogens and pharmaceuticals present in the animal waste (Burkholder et al. 2007).










Input Data


Calculating the index


Ranking and scoring


Overview of results


Interpreting the results


Relationship to other health components

Scientific literature support


Confidence in index

Future enhancements



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Calculating the watershed scale index

This index quantifies the density of potential pollution point sources in the watershed. This index combines potential sources rather than reporting existing water quality impairments. In this way, the index reflects the potential for additional impacts to occur to the water quality in each watershed. Five point sources considered for this index are industrial and municipal wastewater discharge permits, feedlots, potential contaminant sites, superfund sites and mine pits. Known potential contaminant sites and identified superfund sites (current and remediated) are based on sites in the EPA toxic release inventory. All identified sites present some level of risk for releasing substances known to impair water quality. Sites that have been remediated are included, as they still present a low-level threat to water quality. Point source locations for each of the five types of input were summarized on a watershed basis. Site totals for each contaminant type within a watershed were divided by watershed land area to generate densities. These densities were then each scaled to a range of 0-100. The five values were then combined and averaged for a final point source score.


Watershed scale ranking and scoring

There is a significant scientific literature that supports a qualitative relationship between contaminant point sources and water quality within a watershed. However, no scientific literature defines a quantitative relationship or threshold between the density of point sources and impacts to watershed water quality. Thus, we scaled the density of each point source type from 0 (highest density or least desirable condition) to 100 (no point source present or most desirable condition) for each type of point source. We then combined the 5 point source rankings for each watershed.


Calculating the catchment scale index

Two of the five point source types have been calculated for the catchment scale.  These are: Potential Contaminants and Feedlot (Animal Units). 

Potential Contaminants were identified from a database of Program Interests managed by the Minnesota Pollution Control Agency.  Using data from October 2014, the sites that were included vary in the severity of the risk but were all deemed to be a threat to water quality. The site types included air pollution sources, hazardous waste producers and disposal sites, petroleum tanks, tank leak sites, solid waste dumps and landfills, contaminated sites that are under remediation, and storm water discharge sites. Site counts were summed by catchment and that total was normalized by the catchment land area. These values were scaled from 0 to 100 using a 0 threshold of 1.87 points/km2 (the 95th percentile of the dataset). This threshold prevented some of the very high outliers from skewing the scaled metric scores.

Feedlot (animal units) were identified using the Minnesota Pollution Control Agency database of animal feedlots. This database contains information on all feedlots that are capable of raising 50 animal units (AU) or more.   An AU normalizes the data across animal types and is equivalent to 1,000 pounds of animal, about the size of a mature dairy cow. Using data from September 2014, the total AU per catchment was calculated and then normalized by the land area within that catchment. The sites were scaled from 0 to 100 using a density of .75 Animal units per acre as a threshold to scale 0 values to. The ability to recycle manure on agricultural lands reduces the risk of contamination to surface waters. Research suggests that when cow manure is spread at rates exceeding .75 AU excess phosphorous is being applied and there is a heightened risk for contaminating surface waters (Saam, Mark Powell, Jackson-Smith, Bland, & Posner, 2005). This value is considered a conservative threshold as the density is scaled to all land area not only croplands.


Overview of results

Open pit and taconite mines are found in northeast Minnesota. Each pit is counted as a potential contributing point source.

Wastewater discharge sites include municipal and industrial discharge and are most concentrated near the metropolitan areas of the Twin Cities, Rochester and Duluth, as well as the mineral processing areas in the northern Iron Range.

Feedlots are heavily concentrated in the southern half of Minnesota, as well as in a corridor toward the northwest, particularly in the Sauk River watershed. A feedlot was counted as a single source regardless of the number of animals for the watershed scale results, but animal units/land area was applied for creating catchment level scores.

The highest density of potential contaminant sites and superfund sites are clustered around metropolitan areas and are primarily manufacturing and industrial facilities. The more dispersed contaminant sites found throughout Minnesota include un-permitted dumps, city landfills, industrial and manufacturing sites. 


Interpretation of results

The distribution and density of pollution point sources indicates where potential water quality impairments from nutrients, chemicals, temperature, and other by-products of human activities are most likely to occur. The highest density condition had 2106 point sources in a watershed, or 2.3 point sources per square mile, in the Twin Cities Metropolitan area. The lowest density had seven point sources in a watershed. Individual point source inputs also tell a story about the vulnerability of the northeast related to mineral extraction and processing; and the vulnerability in the southern two-thirds of the state because of a high density of animal feedlots. The southeast corner of the state faces additional risk due to the karst landscape, steep topography and valley streams, which increases the likelihood for both ground water contamination and overland runoff into surface waters. 


Relationship to other health components


Withdrawal and/or discharging water into streams and other surface water can change flow patterns and impact the timing and duration of high and low flows.


Water quality degradation due to chemical or pharmaceutical contaminants can result in populations of aquatic organisms that are unable to reproduce. These contaminants effectively disconnect aquatic systems by creating chemical “barriers” or locations within which some organisms cannot survive and disconnects the remaining populations.


Point source pollutants interact in different ways based on the pH, temperature, sediment load, available sunlight and other characteristics. The soil type and slope of the landscape have a large influence on the sediment load in streams and the way in which contaminated sediment moves and deposits within surface waters. Thus, the geomorphic setting influences the impact of different pollutants on the health of the system.


 Pollution can impact biological communities by directly affecting the health of plants and animals. Chemicals and hormone disruptors can reduce fertility and feminize male fish. Toxins in the air, water and soil that bioaccumulate through the food chain have broad unforeseen consequences. Heavy metals, e.g., cadmium or nickel, released in industrial or mining discharges can result in mortality of aquatic organisms at very low concentrations.


Scientific literature support

 The influence of point source pollutants on water quality were well known when the Clean Water Act was authorized (Karr 1981). Although the number of point sources has been reduced, a number of contaminats are still dicharged and pose a threat to surface water and groundwater quality. Aquatic life (class 2) standards for water quality are often more stringent than drinking water standards (class 1) for many pollutants, so class 2 standards help protect drinking water as well. A wide variety of chemical, nutrient and temperature impairments occur from point sources including wastewater treatment plants, feedlots, landfills, mines. Moreover, many older sewage treatment plants may now serve much larger populations than their design capacity with consequent reduction in treatment efficiency, which are a major contributor of phosphorus to streams (Johnes et al. 1996).

Pollutants, sources and impacts:
Excess phosphorus increases algal blooms and productivity (Moore 2007), which ultimately leads to reduced dissolved oxygen concentrations when plants die and decompose. Low dissolved oxygen can affect fish growth and give competitive advantage to tolerant species (Annear et al. 2004). The threshold for impairment in Minnesota for warmwater streams (Class 2Bd, B, C, D) is 5 mg/L (Moore 2007). The effluent limit where point sources discharge directly into the water is 1 mg/L.

Elevated levels of ammonia, such as from wastewater can deplete the dissolved oxygen in the water and cause fish kills. Un-ionized ammonia (NH3) is toxic to aquatic biota at elevated levels where sensitive species and early life stages of fish are affected first (Moore 2007). The chronic standard for coldwater streams in Minnesota (Class 2 A) is 0.016 mg/L unionized ammonia and 0.04 mg/L for warmwater streams (Class 2Bd, B, C, D). In rural streams, ammonia nitrogen and nitrate nitrogen comes from leaky septic tanks and effluent from wastewater treatment plants (Gary et al. 1983).

Chloride enters waters from industrial and wastewater treatment plant effluents (Moore 2007). Chloride may interfere with the osmoregulatory capacity of organisms and is considered a pollutant. The Class 2 chronic standard for chloride is 230 mg/L.

Heavy metals, such as mercury, aluminum, copper, zinc, lead, nickel, selenium, chromium or cadmium, may be found in waterways and at elevated concentrations they can bioaccumulate in fish. Copper, lead and zinc are the most common metals found in the water and mortality of aquatic organisms may occur at very low concentrations. Sources for these metals include industrial wastewater, discharge from old landfills, or mining discharges, while aluminum is associated with industrial discharges (Welch et al. 1998). Metal concentrations increase in storm water flow in urbanized areas from corrosion of car parts and pipes.

Feedlots present a risk to water quality because of potential contamination from excess nutrients (primarily nitrogen and phosphorus), microbial pathogens, endocrine disruptors, and pharmaceuticals present in the animal waste (Burkholder et al. 2007).  A threshold of .75 animal unit per acre of land was used as the "0" threshold value for scoring this metric at the catchment scale.  This threshold is a conservative interpretation of a study estimating manure recycling ability of Wisconsin dairy farms. (Saam et al. 2005)

Thermal pollution from heated water discharge without regard to other pollutants can reduce growth in trout and other coldwater species. Fleming and Quilty (2007) found an increase from 15.2 to 15.9°C in daily mean stream temperature doubled the risk to aquatic biota and this risk grew exponentially with increasing stream temperatures.


Confidence in index

There is a well documented relationship between point source pollution and impacts to water quality from Minnesota and multiple locations around the world. The contaminants in effluent from point sources can be measured directly in receiving waters and many studies have quantified these inputs.

The ranking process is a straightforward count of potential contaminant sites to create a density per watershed land area. There is no attempt to weigh the relative risk associated with different types of pollutants or different size sites. The number of each type of point source was ranked from 0 to 100 prior to combining into an overall score. This approach allowed the different input types to have equal weight in the final score despite greatly different range in site numbers. Feedlot counts per watershed ranged from 0 to 1,702; whereas mine pits ranged from 0 to 210. In reality, the risk from each point source varies greatly. Much more information about specific site characteristics, as well as location relative to surface and ground water, would be needed to quantify the variability of risk.


Future enhancements

This index contains a very wide variety of potential contaminant sites and weights them equally. Points could be ranked based on site type, location, remediation actions, proximity to water, volume of discharge, and type of contaminant. Additional contaminant sources that are currently not scored, such as incinerators that distribute contaminants into waterways through the air, could be included.

This index could be improved by quantifying the difference in impact from sites distributed over a large watershed to clustered sites covering a small portion of the watershed.