Navigating Phosphate

There is plenty of water in the universe without life, but nowhere is there life without water.

Harmful Algal Blooms (HABs) continuously threaten the Great Lakes. As the Great Lakes account for 20% of the freshwater on Earth it is imperative to understand the effects of nutrient loading. Phosphorus is a naturally occurring, limiting nutrient that is vital for plant growth. However, in large amounts, it is ecologically harmful. This study focuses on the Sheboygan River, located in Wisconsin, as it has a history of high phosphate concentrations.

As the Sheboygan River flows toward Lake Michigan, various non-point pollution sources cause the phosphate concentration to increase, leaving the lower 13 miles of the River continuously impaired.

The naturally occurring form of Phosphorus travels through different chemical shifts as it is highly reactive with water and oxygen. In the presence of these two other elements, it becomes phosphoric acid and will later go through various connections to become phosphate (Wagner, 2024). While these terms are used interchangeably, the composition difference between the two terms are important. Phosphorus is the element itself, while phosphate will have oxygen attached to it. Under the massive umbrella of phosphates, you can get various orthophosphates that will depend on the level of reactivity and the type of cation it is (Rafique, F., & Rehman, T., 2023). Like all squares are rectangles but not all rectangles are squares, all orthophosphates are phosphates but not all phosphates are orthophosphates.

Phosphorus (P) is a naturally occurring nutrient that plays a vital role in sustaining life on Earth. Not only is it an essential nutrient for all living organisms, but phosphorus is essential to atmospheric and aquatic changes of oxygen and carbon dioxide. The best way to understand the importance of phosphorus is by studying the phosphorus cycle. The phosphorus cycle highlights the movement of phosphorus in our environment, as well as organic and inorganic sources. P is crucial to the energy transfer between membranes and provides the backbone of DNA genes (paper). In the human body, P is responsible for ensuring the health of our bones and maintaining our blood pH levels (Harvard, 2023). However, much like in the environment, excess phosphorus in the body can lead to hyperphosphatemia. While an incredibly rare condition, it can cause soft tissues in the body and grow calcium deposits (Harvard, 2023).

The different types of phosphorus are easily divided into organic (Po) and inorganic (Pi). Po is primarily found in manure and plant residues as well as soils that can contain more than 50% of P (Feng, 2023). Another 15-74% is found in aquatic plants and algae and more than 50% can be found in particulate matter (Feng, 2023) Microbes in the soil will take Po and convert it to Pi that has a low solubility and will present a high soil pH. For surface water that contains both Po and Pi, the Pi is the main source of orthophosphate as it is released by those inorganic molecules (Feng, 2023).

Algae is a key indicator into the life of an aquatic ecosystem. It can provide an extensive habitat for fish, and shows the health of a river.

However, algae is vital in moderation. Too much can lead to harmful algal blooms (HABs). HABs are defined as an increased amount of algae that becomes environmentally dangerous for the environment and the people surrounding it. It is often caused by an increased amount of nutrients, most commonly being phosphorus and nitrogen. In respects to our Great Lakes, the people within the watershed use the water as their freshwater source (homes, businesses, agriculture, etc.). Afterwards, human wastewater is treated through a wastewater treatment plant and returned to the lake.

However, runoff from precipitation (often called stormwater in the industrial/manufacturing field), is not treated. Any rainwater that hits the surface will runoff into surrounding watersheds and ultimately make its way back into the lake.

The Sheboygan River is a staple to the affected communities. The river itself is 81 miles long, coasting through Fond du Lac, Sheboygan Falls, Kohler, Calumet, and Sheboygan until it reaches the final destination of Lake Michigan. The Basin encompasses the majority of Sheboygan County, northeast portions of Ozaukee and Fond du Lac counties, as well as a small part of southern Manitowoc County, totaling close to 615 square miles. However, the Sheboygan River has an extensive history and continues to have a series of water quality issues. These issues date back to 1987 - when the Lower Sheboygan River (estuary to the Sheboygan Falls Dam, the last 13 miles of the river) was listed as an Area of Concern (AOC) by the Environmental Protection Agency (EPA) after failing to meet the water quality criteria under the 1987 Great Lakes Water Quality agreement.

Back then, the Sheboygan River was not heavily monitored and had an extensive list of contaminants. These contaminants include, but are not limited to: polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), heavy metals, fecal coliform bacteria, and heavy amounts of phosphorus. These contaminants come from a wide variety of sources: unregulated discharges from manufacturing and industrial facilities, sewage treatment plants, spills and illegal dumping of household hazardous wastes, and rural non-point sources.

When the EPA registers a site as an AOC, they use Beneficial Use Impairments (BUIs) to measure the environmental damage as well as track the environmental restoration efforts. Initially, the Sheboygan River had caused damage to 9 or the 14 BUIs. These BUIs can be removed by the EPA when there is sufficient data to determine that the BUI is no longer impaired. Sheboygan River has had 4 removed, but still have 5 BUIs listed for the lower 13 miles of the river. 

The BUIs of the Sheboygan River not only are ecologically harmful, but propose great risk to humans through consumption. The WDNR has imposed lengthy consumption warnings covering the entire county, splitting it into sections based on location.

During the study, there were ecological indicators that show that Upstream of the Sheboygan River is in relatively good health, starting with the aquatic plants.

Bladderwort and Duckweed were the most prominent in this area based on visual observations. Both of these species are key to the ecosystem, providing food and habitat for insects and fish. Duckweed, in particular, prevents the growth of algae. It grows heavily on surface water, blocking out sunlight and keeping dissolved oxygen levels in the water on the high end.

To run a comparative estimate, two 1-mile long reaches of the Sheboygan River were selected and mapped. The first section, here and after named Downstream, is a one mile section in the center of the lower 13 mile AOC. The site was between Kiwanis Park and the famous 8th Street drawbridge. The other site, here and after named Upstream, was 30 miles west out of town and north of Elkhart Lake. The starting point was marked at the dock, close to the Sheboygan Marsh Water Tower, and stretched a mile west, ending at the start of Sheboygan Lake. The Upstream section was 47 miles from the headwaters, and 33 miles from the mouth at Lake Michigan. The Downstream section was a mile from the mouth. 

For optimal maneuverability, the surface water sampling was performed on an inflatable stand-up paddleboard (iSUP).

Prior to sampling, a straight-forward questionnaire was created on Survey123. The on the survey collected the following: 

• Date & Time
• Reach (Downstream or Upstream) 
• Coordinates 
• General Observations 
• Phosphate Concentrations (ppm)
• pH 
• Surface Water Temperature (℉) 

For each site, samples were collected every 0.2 km to equal 8 samples total on each site. The distance on the water was tracked using the Paddle Logger app on a cellphone.

To measure the phosphate concentrations, a monitor was purchased from Hanna Instruments It was a Low-Range Phosphate Monitor, as well as extra cuvettes and extra phosphate reagent packets. The monitor had to be zeroed out prior to every sample with a provided control cuvette. With each sample, the cuvette was dunked into the surface water until it was filled at the line. A packet of phosphate reagent was added to the cuvette and was shaken until it was all mixed in. After that, the sample cuvette had to sit for a couple of minutes prior to being placed in the phosphate monitor in order to get the most accurate reading. The regent is imperative to the sample as it will turn the sample water blue, and it is a slight difference, it is still visible to the naked eye.

The use of continuous variables calls for regression analysis to determine the statistical significance of the difference between phosphate concentrations. A regression test was used for every piece of phosphate concentration data, dividing the variables by location (upstream vs. downstream). In addition to the regression test, multiple t-tests were used to also determine statistical significance.

The results of the upstream reach ended with an average phosphate concentration of 0.72 ppm. , there are two individual days that the phosphate concentration exceeds the maximum allowable concentration by the Phosphorus Rule. However, the average over the three sample days is not exceeding that limit of 0.1 The pH shows an average of slightly acidic, but the variance is high, showing that pH values had an extensive range across the sample reach. Because of the high p-value, it fails to accept the null hypothesis, showing that there is no statistically significant difference between phosphate concentrations and pH levels.

The results of the downstream reached ended with an average phosphate concentration of 0.44 ppm. Downstream also showed a lower variance, sitting at just 0.31. The average pH for the downstream surface water showed slightly basic at an average of 8.48, and a low variance of 0.85. Because of the high p-value, the study fails to reject the null hypothesis, stating that there is no statistically significant difference between the phosphate concentrations and pH levels.

The overall results displayed the differences between the reaches. Upstream displays a low average phosphate concentration, but a much larger variance. Downstream shows a much higher phosphate concentration, but remains similar between the different days. This ultimately brings the variance for this reach down.

The red line on the graph represents the Wisconsin DNR limitations for phosphate for rivers. The limit is set at 100 ug/L, which converts to 0.1 ppm.

Multiple regression tests were performed to determine any statistical significance between phosphate concentrations and pH, phosphate and temperature, and finally, phosphate by location. All of these regressions tests calculated a large p-value ranging from 0.4-0.9.

The results of the study fail to reject the null hypothesis at the alpha, showing that there is no statistical significance between these two reaches of the Sheboygan River. The Lower Thirteen miles of the Sheboygan River has been a focal point for freshwater fishing and access to freshwater, however, the Lower Thirteen has been a designated Area of Concern (AOC) within the Environmental Protection Agency. High levels of phosphorus continue to threaten the health of the river, leaving ecosystems impaired and imposed fish consumption restrictions.

The results of the data showed that even though the Total Phosphorus BUI has been removed from the AOC, high levels of phosphate still persist in the water. This can be a result of residential phosphate use, that ultimately comes from runoff, or industrial sources. Further East of the mouth, but still within the AOC, there is a huge industry and manufacturing presence. These companies should have the necessary stormwater permits to limit their outputs into the River. However, there are still non-point pollution sources that the Upstream reach of the Sheboygan River does not experience.

Over the course of this study, there were multiple adjustments that were made. The ExTech probe uncalibrated in the middle of the river, resulting in missing temperature and pH data points in the Upstream portion. To combat this, HTH pH strips were purchased and utilized for the remainder of the study. Temperature readings in the Downstream reach were supplemented from a surface water thermometer on the boat, but the Upstream portion was too shallow for the vessel, resulting in missing data on July 27th and August 3rd.

The Low Range Phosphate Checker is also a relatively inexpensive option. To improve the accuracy of the study, water samples could be collected and stored for lab analysis. Samples for this study were collected and analyzed on the water, immediately after sampling. Since I was handling the cuvettes, finger prints could have smudged the glass, resulting in inaccurate readings.

These two reaches of the Sheboygan River were chosen for accessibility. The Upstream reach is within the state-protected marsh, while Downstream is channelized through the city of Sheboygan. The Upstream section may be too ecologically different from Downstream to draw a true comparison.

The purpose of this study is to serve as a baseline and reference point for future research. Other methods can be used to increase the accuracy of the study, such as adding more sample points through each reach of the river. The more data that is available, the accuracy of statistical analysis increases. It would also be beneficial to collect data over time in order to show the difference in phosphate concentrations better. By sampling in different seasons, the data would support the increased use of phosphate fertilizers and the true effects on plants and wildlife in these areas. 

With the increase in sample sizes, different measurable variables can be added to further show the risks of high phosphate concentrations. Accurate measures of turbidity levels would be incredibly useful for this study. Total phosphorus and phosphate molecules are known to attach to solids. A rough estimate can be used from turbidity measurements, but further chemical testing would have to be done to determine the concentration of phosphates. With that, soil samples from visible runoff points could also be used to determine the phosphates entering the Sheboygan River.

I am most grateful for my professors: Dr. Zorn, Dr. Gartner, Dr. Sun, Dr. Mast, and Dr. Rubinfeld for their endless support throughout my college career. From class and meetings to coffee and drones, I appreciate you three for sharing your knowledge, wisdom, and humor over the last 4 years.

I would also like to thank my father, Rodney, my sister, Katrina Gilbank '13, and my dear friend and colleague, Alyssa Davis, for their unwavering support and encouragement, both on and off the water. Their belief in me has been a constant source of strength and inspiration throughout this journey!

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