In 1987, an amendment to the Great Lakes Water Quality Agreement of 1978 between the United States and Canada was accepted (International Joint Commission United States and Canada, 1987), which caused the development of Remedial Action Plans to address poor water quality in the Great Lakes. Remedial Action Plans were created for 43 Areas of Concern within the Great Lakes Basin, including the Grand Calumet River Area of Concern, comprising the Grand Calumet River and Indiana Harbor Canal in northwest Indiana (Snyder, 2012). These Areas of Concern in the Great Lakes ecosystem were identified as having at least 1 impairment of 14 beneficial uses (hereafter referred to as “Beneficial Use Impairments” [BUIs]). According to the Agreement, a BUI is a chemical, physical, or biological change to water in the Great Lakes system that can cause degradation or negative effects on aquatic life, habitat, recreational enjoyment, water quality, or costs to industry. BUIs are applied as a status to an Area of Concern and may be removed if the condition improves. Before 2011, all 14 BUIs defined in the revised 1978 Agreement (International Joint Commission United States and Canada, 1987, p. 24) were present in the Grand Calumet River Area of Concern (fig. 1), including “eutrophication or undesirable algae” (U.S. Environmental Protection Agency, 2023c). The Remedial Action Plan developed for the Grand Calumet River Area of Concern includes management actions necessary for addressing the BUIs and ultimately delisting the Area of Concern (Snyder, 2012). The Stage 2.5 Remedial Action Plan states that dissolved oxygen, nutrient, and algae monitoring are needed to evaluate eutrophic conditions, which will allow the Indiana Department of Environmental Management to assess whether the removal of the eutrophication BUI is appropriate (Snyder, 2012).

In 2021, the Indiana water-quality standard stated that “for the maintenance of a well-balanced aquatic community...concentrations of dissolved oxygen shall (A) average at least five (5.0) milligrams per liter per calendar day; and (B) not be less than four (4.0) milligrams per liter at any time” (Indiana General Assembly, 2020, p. 77). The removal target for the eutrophication or undesirable algae BUI was revised in January 2024 and states the BUI can be considered for removal when the following conditions are met: (1) community outreach activities specifically designed to reduce nonpoint source and stormwater contributions of nutrients to Area of Concern waters have been conducted within the Area of Concern, (2) biological controls on nutrients or excess algal growth have been implemented, and (3) all approved contaminated sediment management projects have been completed or planned (Michael Spinar, Indiana Department of Environmental Management, written commun., 2024).

Eutrophication, or undesirable algae and aquatic plant growth caused by excess nutrients, occurs throughout the Grand Calumet River and Indiana Harbor Canal. Algal growths are prominent in shallow or low-flow locations near bridges, culverts, or other infrastructure designed to control water in the basin. These structures are common because of the high density of industrial development within the Area of Concern (fig. 2; Snyder, 2012). There are many sources of contamination in the Grand Calumet River watershed, including landfills, urban runoff, dumpsites, sewage treatment plants, combined sewer overflows (CSOs), and industrial effluent (Nevers and others, 2002).

A combined sewer system collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe (U.S. Environmental Protection Agency, 2023b). Under normal circumstances, the sewer can transport all wastewater to a treatment plant, but sometimes the amount of runoff exceeds the capacity of the system. When the capacity is exceeded, untreated stormwater and wastewater is released, flowing into nearby waterbodies through permitted outfalls (CSOs) that act as relief points during wet weather. These events, referred to as CSO events, occur within the Grand Calumet River watershed in the cities of Gary, East Chicago, and Hammond, Indiana, and discharge into the Grand Calumet River or Indiana Harbor Canal (fig. 1; U.S. Environmental Protection Agency, 2023b).

CSO events can negatively affect surface water quality and ecosystem health by causing beach closures, shellfish bed closures, algal growth, reduced oxygen levels in waterways, and aesthetic effects from floating debris or oil slicks (U.S. Environmental Protection Agency, 2023a). Nutrients in legacy streambed sediments and nutrients released during CSO events influence the growth of algae throughout the water column (Crawford and others, 1995; Nie and others, 2018). The life cycle of algae affects the concentrations of dissolved oxygen and may impair the ecosystem, especially when excess nutrients cause expansive growth of algal colonies (Klose and others, 2012). CSO releases can push large quantities of sewage, sludge, and runoff materials into the water, which can cause a spike in nutrient availability, especially ammonia (Atauzzaman and Ali, 2022). Increased ammonia availability can lead to increased nitrification rates, which can decrease dissolved oxygen concentrations when dissolved oxygen is used by microorganisms during the conversion of ammonia into nitrate (Sliekers and others, 2002).

The growth of algae, duckweed, and other aquatic plants is affected by nutrient availability and is also a factor for oxygen concentrations; algae and aquatic plant growth increases with elevated nutrient concentrations, and their metabolic processes then increase dissolved oxygen levels in water bodies during the day. The dissolved oxygen levels decrease at night when the photosynthetic rates decrease (Gasith and Resh, 1999). Algal overgrowth leads to the microbial decay of excess algae and plant matter, which produces large amounts of carbon dioxide and depletes dissolved oxygen, creating a hypoxic environment (National Oceanic and Atmospheric Administration, 2023). All these factors show how CSO events may ultimately lead to hypoxic conditions that can kill fish and other aquatic animal species and reduce the amount of riverine habitat. Reduction of nutrient sources through contaminated-sediment management projects (dredging or dredging and capping of legacy sediment) and the reduction of CSO events may improve conditions (Nevers and others, 2002).

Sediment management projects in the Grand Calumet River Area of Concern have been undertaken by Federal, State, and responsible parties as part of a regulatory enforcement action or as part of a nonregulatory action related to the Great Lakes Legacy Act of 2002 (Public Law 107–303, 116 Stat. 2355; Indiana Department of Environmental Management, 2023a). Remediation projects typically use dredging or dredging and capping as the primary methods of cleanup. During dredging, the contaminated sediment is removed with excavating equipment, and care is taken to reduce mixing of the sediment with the water column. Dredging methods include using a bucket to scoop sediment or a suction-controlled system to remove it. At some locations, water is diverted from the river and dry excavation methods are used. Removed sediment is disposed of as solid waste in a permitted landfill or as hazardous waste in a containment area. For projects where the removal of all contaminated sediment from the riverbed is not possible, a cap is applied after the top layer is dredged. The cap is designed to prevent the river from leaching contaminants or transporting contaminated sediments downstream while bacteria and other organisms in the polluted sediment bed naturally decompose pollutants (Hupfer and Hilt, 2008; U.S. Environmental Protection Agency, 2023c).

To address concerns of eutrophication and its effects on the Area of Concern, the U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (EPA) and the Indiana Department of Environmental Management, collected data in 2021 and 2022 to document the effects of remediation processes and CSO releases on nutrient and dissolved oxygen concentrations in relation to eutrophic conditions. Sampling for algal community structure analysis occurred concurrently with measurements of dissolved oxygen and clarity of the water to evaluate river conditions.

The goals of this study were as follows:

Data were collected from April through October 2021 and May through November 2022 and provide a snapshot of the eutrophic conditions during the peak photosynthetic period of the year. In 2021, an initial selection of 13 sites was made throughout the Area of Concern to assess whether the current environmental conditions of the watershed are consistent with the conditions needed for persistent eutrophication or hazardous algal blooms to occur. In 2022, a subset of 5 of the 13 sites were selected that would target locations affected by eutrophication during the 2021 monitoring period and allow for comparison of data from unremediated and remediated reaches.

The Grand Calumet River and Indiana Harbor Canal in Lake County in northwest Indiana consists of the east and west branches of the Grand Calumet River divided by (and including) the Indiana Harbor Canal and the surrounding shoreline of Lake Michigan (fig. 1). This area is coterminous with the Grand Calumet River Area of Concern (Crawford and Wangsness, 1987; Snyder 2012). The east branch Grand Calumet River starts in Marquette Park, flows west, and discharges to the Indiana Harbor Canal, which typically flows to Lake Michigan (Renn, 2000). Streamflow in the west branch Grand Calumet River divides at a topographic high about 1.5 miles west of the confluence with the Indiana Harbor Canal near Columbia Avenue in Hammond, Ind. East of the divide, the west branch flows toward the Indiana Harbor Canal; west of the divide, the west branch flows toward Illinois (fig. 1). The location of the divide in the west branch can vary on the basis of changing conditions in the area, including Lake Michigan water levels, wind direction, and discharges to the Grand Calumet River.

Urban development has brought notable changes to the study area since 1900. Marshlands that formed because of the poor drainage to the Grand Calumet River were drained (Moore, 1959). Canals dug from Lake Michigan to the Grand Calumet River for shipping caused changes in the local direction of flow in the river. As a result of the change in flow direction, the former mouth of the Grand Calumet River at Lake Michigan was closed off by decreased streamflow and sand-dune migration. The course of the river was locally altered to accommodate harbors and highway construction. In 1907, the Indiana General Assembly passed legislation that allowed the building of artificial land in Lake Michigan by use of slag, a byproduct of the steel-making industry. Many industries are currently located on such land north of the study area. Additionally, slag has been used extensively as fill material in depressions and marshes throughout the study area.

Almost all the flow in the Grand Calumet River and Indiana Harbor Canal results from industrial and municipal discharges, and substantial flow variations have occurred in the east and west branches of the Grand Calumet River and in the Indiana Harbor Canal (Crawford and Wangsness, 1987). Changes in Lake Michigan water levels can temporarily reverse the direction of flow in the Indiana Harbor Canal and parts of the east and west branches of the Grand Calumet River (fig. 1). Parts of the Grand Calumet River and Indiana Harbor Canal have been dredged, channelized, and lined with metal sheet pile. Ultimately, the contribution to the Grand Calumet River and Indiana Harbor Canal from surface water runoff is small. The drainage area of the Grand Calumet River Basin is indeterminate (Stewart and others, 1999) but is estimated to be less than 50 square miles (Renn, 2000). Fenelon and Watson (1993) reported that model-estimated groundwater discharges to the Grand Calumet River and Indiana Harbor Canal were minimal (10 cubic feet per second).

Site selection, data collection (including continuous water-quality parameters, algal community analysis by metabarcoding, and discrete water-quality sampling), and the analytical methods used to interpret data were chosen to assess if CSOs and differing remediation conditions throughout the Area of Concern influence factors that contribute to eutrophic conditions on the Grand Calumet River and Indiana Harbor Canal. The algal community analysis for cyanobacteria and eukaryotic algae community data collected for this project are available through three USGS data releases (Aunins and others, 2023; Byappanahalli and others, 2023; Byappanahalli and others, 2025). All water-quality data (continuous and discrete) collected as part of this study are published in the National Water Information System (U.S. Geological Survey, 2023).

Results are presented from the wide variety of data collected in the Grand Calumet River and Indiana Harbor Canal for this study. Water chemistry and biological data were collected including continuous dissolved oxygen, water temperature, and fChl (reported as chlorophyll), environmental DNA algal community analysis, discrete water-quality samples of nutrients, and stable isotopic compositions of nitrogen in ammonia, nitrogen and oxygen isotopic composition in nitrate and hydrogen and oxygen isotopic composition in water. Additionally, CSO releases, stream discharge, and precipitation are presented in reference to the continuous and discrete water-quality data.

Continuous water-quality monitoring was used to assess conditions between remediated and unremediated sites. Data from upstream and downstream sites were compared where flow and sampling regimes permitted. A description of the flow dynamics is available in the “Study Area” section of this report, and flow dynamics are shown in figure 1. A description of the remediation statuses is available in the “Remediation Efforts” section of this report. The remediated condition of each of the 13 monitoring locations is available in table 1.

In both the east and west branches of the Grand Calumet River, there was not a strong correlation in how the remediation status affected dissolved oxygen concentrations. Regardless of remediation status, 7 of the 13 sites had more than 10 percent of monitored days when instantaneous minimum dissolved oxygen concentration decreased to less than the 4.0 mg/L threshold. Dissolved oxygen concentrations at the most upstream site in the east branch Grand Calumet River (EB1, the reference site) ranged from 0.0 to 21.3 mg/L during the 2021 and 2022 study periods. For comparison, in unaffected freshwater at 1 atmosphere of pressure, dissolved oxygen concentrations should range from 7.43 mg/L at 14 °C to 10.31 mg/L at 31 °C (U.S. Geological Survey, 2020).

Instantaneous minimum dissolved oxygen concentrations recorded for sites in each remediation condition were grouped to help identify any differences in dissolved oxygen range for the different remediated conditions in 2021 while all sites were being monitored (table 1; fig. 19). Sites with a dredged remediated condition (sites EB4, EB3, and EB2) had the smallest range of dissolved oxygen concentrations (about 2–10 mg/L), whereas sites with an unremediated condition (sites WB3, LG1, HC1, HC2, EB6, and EB5) had the largest ranges (about 0–16 mg/L). Sites with a dredged and capped condition (sites WB1, WB2, and EB7) ranged from 0 to about 13 mg/L dissolved oxygen. Dissolved oxygen concentrations at the background, unremediated site (EB1) ranged from about 2 to 16 mg/L.

Daily dissolved oxygen flux was calculated for each monitoring location during the 2021 monitoring period by subtracting the daily minimum recorded dissolved oxygen concentration from the daily maximum concentration (fig. 20). Site WB3 (unremediated) had the largest daily dissolved oxygen flux at 21.61 mg/L. Site EB2 (remediated) had the lowest range in daily dissolved oxygen flux at 4.03 mg/L. Sites in the U.S. Steel remediation area with a dredged remediated condition (EB2, EB3, and EB4) and the next downstream site in the east branch Grand Calumet River (EB5) had lower daily dissolved oxygen flux values than the other monitored locations. Site EB7 (east branch Grand Calumet River; dredged and capped) had the largest daily dissolved oxygen fluxes. Large daily differences in dissolved oxygen may indicate increased periods of photosynthesis during the day and increased respiration at night (Wetzel, 2001).

In the U.S. Steel remediation area, downstream from the reference site, the river has been dredged, leading to greater stream depths, which may affect eutrophication processes and water quality. For example, as depth increases, light penetration decreases, especially if the water is not clear. This decreases photosynthetic activity, regardless of nutrient availability. There are three monitoring locations (EB2, EB3, and EB4) and two active CSOs (Chase and Polk) in this remediation area (fig. 3; table 2). Dissolved oxygen concentrations were generally higher and less varied at EB2 (0 days of dissolved oxygen daily mean values below 5.0 mg/L in 2021; table 5) than at the reference site EB1 (29 days of dissolved oxygen instantaneous minimum values below 4.0 mg/L in 2021; table 5), possibly due to better mixing of the water column or increased water column depth and lower water temperature at EB2. Downstream from the U.S. Steel remediation area at EB5, dissolved oxygen remained primarily above 4.0 mg/L, except for one occurrence on June 25, 2021. The monitoring site EB6 is just upstream from the east branch remediation area and receives water from an about 2 mile long unremediated reach with two active CSOs (Alder and Colfax; fig. 3; table 2). The instantaneous minimum dissolved oxygen concentrations remained below 4.0 mg/L from June 24 to August 22, 2021, and again from August 31 to September 24, 2021 (the daily mean was below 5.0 mg/L for 93 days; table 5). Similar results were seen at site EB7 within the east branch remediation area.

The highest dissolved oxygen concentrations within the east branch Grand Calumet River, including supersaturated conditions, were measured at site EB7 and could be caused by high rates of photosynthesis during periods of high algal productivity (U.S. Geological Survey, 2020). Supersaturated conditions occurred periodically at 7 of the 13 sites monitored in 2021 (EB1, EB4, EB7, LG1, WB1, WB2, and HC1) and at all 5 sites monitored in 2022 (EB1, EB7, WB1, WB2, and HC1). The supersaturated dissolved oxygen conditions indicate that eutrophic conditions were present in both the east and west branches of the Grand Calumet River during 2021 and 2022 and occurred regardless of remediation status (table 5; Wetzel, 2001). This result indicates that elevated levels of some contaminants remain, despite remediation efforts in both branches of the Grand Calumet River and the findings of Steevens and others (2020) and despite the hydraulic dredging that occurred from December 2002 through November 2003 in the U.S. Steel remediation area.

Mean and minimum daily dissolved oxygen concentrations were compared for two site pairs each consisting of one site receiving waters from a remediated reach and one site receiving waters from an unremediated reach. The east branch remediation area sites, EB6 and EB7, and the U.S. Steel remediation area sites, EB2 and EB5, showed few similarities in changing dissolved oxygen concentrations. Between EB6 and EB7, minimum dissolved oxygen was lower at the remediated site (EB7) than the unremediated site (EB6) on about 62 percent of days when minimum dissolved oxygen was calculated at both sites. Between EB2 and EB5, minimum dissolved oxygen was lower at the remediated site (EB2) than the unremediated site (EB5) on about 95 percent of days when minimum dissolved oxygen was available at both sites, and of the days when it was lower at EB5 it was below the Indiana Department of Environmental Management threshold of 5.0 mg/L on about 5 percent of days. This complicates the expectation that remediation should cause dissolved oxygen values to be higher on average.

The west branch Grand Calumet River experiences flow reversals and CSO releases, both of which affect dissolved oxygen levels at west branch sites (figs. 1 and 3). The waters received by WB1 and WB2 vary as a result of the flow reversals. During normal flow conditions water flows from just east of WB2 to WB1, but during flow reversals waters from Lake Michigan flow back up the Indiana Harbor Canal and push Grand Calumet River waters east from the confluences of the Indiana Harbor Canal and Grand Calumet River past all west branch sites (fig. 1). These flow reversals also change which CSOs affect sites in the west branch. During normal flow conditions the CSO Storage Basin flows reach WB1 and no CSOs affect WB2. During flow reversals the Magoun CSO affects WB1 and WB2, whereas the CSO Storage Basin continues to exclusively affect WB1. WB1 was below the Indiana Department of Environmental Management daily mean and instantaneous minimum thresholds for dissolved oxygen on a larger number of days than WB2 in 2021 (table 5). In addition, WB1 had more supersaturation events than WB2. These extremes are evidence of eutrophic conditions occurring at WB1, despite the water having flowed through a remediated reach. This could relate to the CSO events that affect waters at WB1. At both WB2 and WB1, dissolved oxygen concentrations decreased in June and July 2021. At WB1, during one CSO event dissolved oxygen reached about 4.0 mg/L in July 2021. Dissolved oxygen concentrations at WB2 fluctuated with instantaneous dissolved oxygen concentration transitioning quickly from nearly 0 mg/L to close to 5.0 mg/L three times in June and July 2021 (fig. 6). These transitions all occurred on days when the CSO Storage Basin was releasing, suggesting that CSOs can have a strong effect on dissolved oxygen concentrations that overrides the effect of temperature or remediation status. After July 2021, dissolved oxygen concentrations increased to above 5.0 mg/L and remained between 5.0 and 10 mg/L for the rest of the 2021 monitoring season (U.S. Geological Survey, 2023).

Eight CSOs were active within the monitored reaches of the Grand Calumet River and Indiana Harbor Canal during the monitoring periods within and outside of remediated areas (figs. 1 and 3; table 2). The remediated reach monitoring sites subject to potential CSO effects were WB2, EB4, EB3, and EB2 (table 2). The remaining sites, EB6, EB5, WB3 and HC1, were in unremediated reaches and received water from nearby CSOs. There was a CSO near EB7, a remediated site, but it was downstream from the monitoring site and therefore was not expected to affect water quality at the site (fig. 3). CSO releases can push large quantities of sewage, sludge, and runoff materials into the water, which can cause a spike in nutrient availability, especially ammonia, and can increase nitrification rates and decrease dissolved oxygen (Atauzzaman and Ali, 2022). Storms can also resuspend sediment from the bottom of rivers which could change nutrient concentrations regardless of CSO proximity or releases. Discrete sampling results reflect monthly sampling and sampling before, during, and after one storm event. The following descriptions are what was noticed for this specific sampling. Selected monitoring periods were graphically analyzed to determine the effect of storm events and CSOs on receiving waters (figs. 6 and 7).

Overall, cyanobacteria and eukaryotic algae showed distinct spatial and temporal distributions at the sampled sites (figs. 11 and 12) that were similar to observations from other studies of riverine systems (Hamilton and others, 2011; Banerji and others, 2018; Manier and others, 2021; Sindt and Wolf, 2021). The spatial differences and temporal shifts in the algal communities were likely associated with abiotic factors such as nutrient inputs, CSO events, water temperature, sunlight, land use, and water flow (Gast and others, 1990; Hamilton and others, 2011; Zhao and others, 2017; Bao and others, 2022). However, biotic factors, which can be influenced by abiotic factors, may in turn influence algal community shifts (Wang and others, 2021). Nuisance algal blooms potentially influence other algal species: for instance, certain cyanobacteria can fix atmospheric nitrogen (Fay, 1992; Byappanahalli and others, 2019; Kim and others, 2020; Wang and others, 2021), which can promote growth of such species (Kim and others, 2020). For example, in a mesocosm study, Kim and others (2020) found that non-nitrogen fixing cyanobacteria were more stimulated by nitrogen treatments than phosphorus treatments. Furthermore, cyanobacteria have inherent traits that promote their dominance over eukaryotic algae (Huisman and others, 2018; Wang and others, 2021).

The sampling sites were chosen because they were geographically distinct with their own potential microclimate, nutrient inputs, juxtaposition to CSOs, remediation status, and flow regime. Therefore, it is reasonable to assume algal communities would also be different among sites and over time. Although there were several CSO events that occurred during this study, any direct relationship between these events and our observed patterns of algal communities in terms of both abundance and spatial distributions is tenuous because of the limited sampling regime. However, it should be noted that nutrients from CSO releases can lead to increased cyanobacterial and algal activity, including occurrences of algal blooms (Gast and others, 1990; Gleich, 2019). For example, Gast and others (1990) sampled algae at 63 locations that received different combinations of sewer system discharges and found that locations with a combined sewer system had an immediate decrease in algal biomass and species diversity due to flushing; however, this decrease was followed by an increase in algal biomass and sometimes algal blooms.

One of the overarching goals of this study was to assess whether potential bloom-forming taxa were found at these sites. Algae have the potential to become more abundant given the right conditions. Much research has demonstrated that algae populations increase, and community shifts occur in response to environmental changes, increased nutrient loads, increased water temperature, and altered flow regimes (Giblin and Gerrish, 2020; Song and others, 2020). Research has also shown that increased algal abundance and blooms have the potential to deplete oxygen and further alter algal communities (Sun and others, 2022; Li and others, 2018). Several of the taxa identified in this study have the potential to form blooms or reach nuisance levels. Microcystaceae (cyanobacteria), which are well known bloom-formers, were found in higher abundance at EB1 (reference site) but were found in low abundance at the other Grand Calumet River sites. Site LG1, which like EB1 provided key conditions for Microcystaceae blooms, had a lower abundance than EB1 during all sampling events. Among eukaryotic algae, Chrysophyceae, Cryptophyceae, and Bacillariophyceae taxa all have the potential to form blooms and some, or all, of these taxa were present during sampling at all sites. Viridiplantae was abundant at many sites, particularly at EB1 in July and September. Viridiplantae is a large clade containing more than 400,000 species, some of which are known to form blooms when nutrients are abundant.

Nitrogen and oxygen isotopic composition comparisons were used to determine if nitrate at WB2 and EB6 is from precipitation, synthetic fertilizer, reduced nitrogen fertilizer, soil organic nitrogen, or from CSO events (fig. 15B; Kreitler, 1979; Clark and Fritz, 1997; Zhang and others, 2018). Nutrient abundance and isotopic data were compared with continuous water quality to better understand the source of nutrients and how they relate to dissolved oxygen concentrations.

Continuous nitrate plus nitrite data were available from the streamgage at site WB2 during the 2022 study period (U.S. Geological Survey, 2023). Minimal or no consistent patterns occurred between continuous daily mean nitrate plus nitrite concentrations and daily mean dissolved oxygen concentrations; however, there is a statistically significant weak negative correlation between chlorophyll concentration and nitrate plus nitrite, which indicates periods of increased daily mean chlorophyll concentration generally correspond with decreases in nitrate plus nitrite at the site (figs. 22 and 23). A significant example of this occurred from June 11 to 18, 2022, when daily mean chlorophyll concentrations of less than 10 µg/L increased to more than 40 µg/L, indicating an increase of algal biomass. In that same time period, daily mean nitrate plus nitrite concentrations decreased from 9.5 mg/L to 4.0 mg/L. Additionally, daily mean dissolved oxygen concentrations (that were at a stable concentration of about 8 mg/L the week prior), increased to consistent values greater than 10 mg/L and as much as 13.7 mg/L from June 11 to June 18 (fig. 22). At site WB2, dissolved oxygen concentrations increased during periods of increased chlorophyll concentrations, and nitrate plus nitrite concentrations seemed to decrease in response to increased chlorophyll concentrations; however, further analysis and more data could help draw definite conclusions about this relationship (fig. 22).

Continuous water-quality monitoring was limited by site access constraints. To complete the field work in a timely, orderly, and repeatable fashion it was necessary for the sites to be accessible under all hydrologic conditions. Seasonal continuous water-quality monitoring only for 2 years limited the ability to assess long-terms trends within the study area and limited the range of conditions captured because yearly conditions can vary greatly.

The sampling frequency of the metagenomics component of the study (three events within one season) limited the identification of clear spatial differences and temporal shifts in communities. Bloom-forming taxa were abundant in some samples, which made it difficult to draw clear conclusions about the relation between algal communities and eutrophication. Continuous dissolved oxygen and chlorophyll concentrations during the week before and after algal community analysis showed that conditions before and after sampling events had about the same range as the full range through the period of record. Although the ranges were representative, the sampling events rarely occurred during periods of low dissolved oxygen. Future studies could benefit from including a lengthier, more robust sampling regime than the one in this study so that yearly comparisons of community analysis, water chemistry, CSO events, and other nutrient inputs can be made.

The lack of discrete nutrient concentration results during the period of community sampling limited the ability to perform algal community analysis for the identification of nitrifying bacteria such as Nitrosomonas, Nitrosospira, Nitrospira, and Nitrobacter. The identification of the presence and abundance of nitrifying bacteria communities, the identification of nitrate or ammonia and nitrite as the most dominant nitrogen compound species, and a comparison of the dissolved oxygen concentrations during the sampling events could identify the communities and associated abundance that control dissolved oxygen concentrations and nutrient concentrations in stream waters.

Future studies to assess the eutrophication impairment in the Grand Calumet River could be aided by more frequent and event-based metagenomics paired with water-quality sampling. Additionally, long-term dissolved oxygen studies and nutrient-data collection (continuous and discrete), could improve understanding of the complex dynamics that can lead to eutrophication.

The 1972 Great Lakes Water Quality Agreement between the United States and Canada identified the Grand Calumet River Area of Concern, which includes the Grand Calumet River and Indiana Harbor Canal, as having all 14 of the Agreement-defined Beneficial Use Impairments present, including Beneficial Use Impairment 8: eutrophication or undesirable algae. Remedial Action Plans were developed to address the Beneficial Use Impairments. At the onset of this study, the Area of Concern Beneficial Use Impairment removal matrix in the Stage 2.5 Remedial Action Plan indicated that dissolved oxygen, nutrient, and algae monitoring were needed to evaluate conditions to consider removal of the Beneficial Use Impairment. The removal target for Beneficial Use Impairment of eutrophication or undesirable algae was revised in January 2024 and states that it can be considered for removal when community outreach activities specifically designed to reduce nonpoint source and stormwater contributions of nutrients to Area of Concern waters have been conducted within the Area of Concern, biological controls on nutrients or excess algal growth have been implemented, and all approved contaminated sediment management projects have been completed or planned. Many sources of contamination that can contribute to excess nutrients within the system and cause eutrophic conditions are present in the Grand Calumet River watershed, including landfills, urban runoff, dumpsites, sewage treatment plants, combined sewer overflows, and industrial effluent.

To evaluate whether eutrophic conditions or undesirable algae were present, several sites within this area were monitored to determine the effect of combined sewer overflows on surface water quality and ecosystem health, the relationship between dissolved oxygen concentration and nutrient concentrations, how combined sewer overflow releases affect these relationships, and how different remediation processes (such as dredging, or dredging and capping) along the Grand Calumet River and Indiana Harbor Canal have affected water quality. One site (EB1) was selected as a reference site that had natural conditions unaffected by combined sewer overflows or remediation.

Remediation status was not shown to affect dissolved oxygen concentrations, because many of the sites, regardless of remediation status, had days where daily mean dissolved oxygen concentrations fell below the daily mean threshold of 5.0 milligrams per liter (mg/L) or had days when instantaneous dissolved oxygen concentrations fell below the 4.0 mg/L minimum threshold. However, monitoring locations in unremediated reaches had the largest daily difference in dissolved oxygen concentrations, indicating that more algal growth and decomposition could be taking place in these locations than at remediated reaches in the study area. In 2021, 11 of the 13 monitored locations had at least 1 occurrence of daily mean dissolved oxygen falling below 5.0 mg/L, and 12 of the 13 monitored locations had at least 1 occurrence of instantaneous dissolved oxygen values falling below the minimum standard of 4.0 mg/L. In 2022, analysis at four of the five sites showed daily mean dissolved oxygen values below 5.0 mg/L and instantaneous readings below 4.0 mg/L. Low dissolved oxygen concentrations commonly followed combined sewer overflow events.

Samples were collected for algal community analysis in 2021 at five geographically distinct sites within the study area during three sampling events. Algal communities in these samples were determined by deoxyribonucleic acid (DNA) sequencing that targeted 16S ribosomal ribonucleic acid (RNA) for cyanobacteria and 18S ribosomal RNA for eukaryotic algae to characterize the spatiotemporal differences in these algal communities and whether communities capable of becoming nuisance species or those associated with eutrophication were commonly found during this period. Sampling identified 42 main cyanobacteria and 20 main eukaryotic algae taxa. Several of the taxa capable of becoming nuisance species were identified, including the cyanobacteria Microcystaceae (predominantly found at the reference site EB1) and the eukaryotic algae Chrysophyceae, Cryptophyceae, and Bacillariophyceae.

After completion of the 2021 monitoring, a subset of 5 of the 13 original sites were selected for continued monitoring in 2022. Discrete samples were taken at three sites for nutrients; nitrogen, hydrogen and oxygen isotopic concentrations; and biological oxygen demand and chemical oxygen demand to identify the source of nutrients that contribute to the eutrophic conditions observed in 2021. Site WB2 (in the middle of the west branch remediation area) had the highest concentrations of chemical oxygen demand, biological oxygen demand, nitrate, phosphorus, and orthophosphate for the sampling period in 2022. Site EB6 (just east of the east branch remediation area) had the highest concentrations of ammonia and nitrite, and reference site EB1 (the farthest east site) had the lowest concentrations, including nondetections, for all nutrients.

All isotopic sample results from sites WB2 and EB6 in 2022 fell within the isotopic range of nitrate from soil organic nitrogen or from combined sewer overflows for values of stable isotopic composition of nitrogen and oxygen in nitrate. Most stable isotopes of hydrogen and oxygen in water samples fell along the local meteoric water line except for the samples from EB1 that had heavier isotopic compositions of hydrogen and oxygen in water than the WB2 and EB6 samples, indicating evaporation of water at EB1.

Combined sewer overflow releases were found to affect both continuously monitored and discretely sampled parameters. After combined sewer overflow events, dissolved oxygen typically decreased and chlorophyll concentration typically increased, possibly because of nutrients being flushed into streams and increased growth and respiration by plants and algae. On the basis of the nitrogen isotopic analysis, combined sewer overflow releases were associated with higher nitrate concentrations in the Grand Calumet River. Additionally, biological oxygen demand was affected by combined sewer overflow events and could affect how combined sewer overflow events impacted water quality. Results from storm sampling at WB2 showed that ammonia concentrations first increase and then decrease during combined sewer overflow events.

Discrete sampling gave insight into diminished dissolved oxygen levels in the different reaches of the Area of Concern based on remediation condition. Nitrification may have been more efficient at sites where nutrient loading from combined sewer overflow releases initiated conversions from ammonia and nitrite to nitrate. Greater nitrate concentrations where biological oxygen demand and chemical oxygen demand are also high suggest that nitrification is occurring and that dissolved oxygen levels may diminish as nitrification continues.

Overall, the study showed that eutrophic conditions were present in the study area regardless of remediation status. Combined sewer overflow events have a substantial influence on water quality parameters measured during the study and appear to contribute to periods of eutrophication. Discrete water-quality samples collected in 2022 included parameters that identified nutrients from both sewage and soil. The information from this study can assist in identifying management actions that can help address the eutrophication Beneficial Use Impairment in the Grand Calumet River Area of Concern.