Hotspots
Although there was a general trend of decreasing water quality through the study area, it is evident that certain locations had a disproportionate influence on the overall water quality. In this study, we identified three hotspots (Figure 11). Nitrate-N reached its highest concentrations at the 14 km site (max 25.8 mg/L) and remained high through the 19 km site (max 20 mg/L) (Figure 7). These elevated nitrate-N values are surprising given that most of the area immediately upstream of the 14 km site is contained within Chatfield State Park (Figure 12). This park has little development and most of the area is natural, consisting of the Chatfield Reservoir, shortgrass prairie ecosystems, and wetland ecosystems. Nitrogen is generally removed from the fluvial system within lakes and reservoirs through denitrification, sediment burial, and uptake by vegetation (Harrison et al., 2009). Given this, we would expect to see nitrogen levels decrease downstream of the reservoir, but in this study, we found the opposite. While it is not possible to conclusively identify the source of this nitrate-N pollution, in urban areas, wastewater treatment facilities often contribute to nutrient pollution (Bernhardt et al., 2008; Brown et al., 2009). Approximately 2.3 km upstream of the 14 km site is the discharge point for Marcy Gulch, which drains the Marcy Gulch Wastewater Treatment Plant (Figure 12).
There was a second, large hotspot located between 28 km and 37 km (Figure 13). This area had high BOD (max 10.5 mg/L) (Figure 4) as well as elevated concentrations of ammonia-N (max 3.4 mg/L) (Figure 6), nitrate-N (max 11.9 mg/L) (Figure 7), and orthophosphate (max 9.9 mg/L) (Figure 8). Additional testing in 2017-2018 upstream of this area revealed several potential point sources of this pollution (Figure 13). Two small tributaries: Little Dry Creek (24.3 km) and West Harvard Gulch (25.2 km) had elevated concentrations of nitrate-N (mean 2.9 mg/L and 8.1 mg/L, respectively). In addition, the effluent from the South Platte Water Renewal Partners Facility, a wastewater treatment plant (25.5 km), had elevated concentrations of nitrate-N (mean 6.2 mg/L), ammonia-N (mean 3.1 mg/L), and orthophosphate (mean 10.5 mg/L).
This hotspot is also located in the most urbanized part of the city, with considerable areas of impervious surface. Over 90 storm drains flow into the river over this stretch, which amounts to 11.11 storm drains/km and accounts for more than 35% of all drains identified. It is likely that discharges from the storm drains contribute to the high levels of pollutants detected. All measured parameters improved at the next site (38 km). Cherry Creek, one of the river’s main tributaries, joins the South Platte just upstream of this point, at 37.2 km. It is likely that added flow from the confluence at Cherry Creek improved water quality by dilution.
There was a third hotspot between 47 km to 48 km (Figure 14). This area had elevated BOD (max 15.0 mg/L) (Figure 4), elevated ammonia-N (max 1.8 mg/L) (Figure 6), elevated orthophosphate (max 14.3 mg/L) (Figure 8), and reduced DO (min 3.11 mg/L) (Figure 5). This area has more pervious land cover and fewer outfalls (0.43 outfalls/km) than the other hotspots. In addition, two large tributaries join the South Platte River in this area: Sand Creek at 45.5 km and Clear Creek at 47.2 km. However, the city around this hotspot is highly industrialized, which could be contributing to the diminished water quality. Two potential sources of pollution in this area are the Robert W. Hite Treatment Facility, the largest wastewater treatment facility in the metro area, and the Suncor Commerce City oil refinery.
The Robert W. Hite Treatment Facility discharges approximately 130 MGD of treated wastewater into the river at 45 km. Through much of the year, this effluent makes up a significant portion of the downstream flow in the South Platte River (Metro Wastewater Reclamation District, 2020). However, additional testing in 2017-2018, revealed that the wastewater treatment effluent had relatively low concentrations of nitrate-N (mean 3.1 mg/L), ammonia-N (mean 0.16 mg/L) and orthophosphate (0.3 mg/L). From this data, we can presume that while the wastewater treatment facility is contributing to the diminished water quality, it is not the main source of the pollution we see in this area.
The Suncor Commerce City oil refinery is located on Sand Creek, approximately 1 km east of the South Platte River. Through its normal operations, Suncor discharges wastewater effluent into Sand Creek. Oil refinery discharge can contain a range of pollutants including hydrocarbons, sulphides, ammonia, suspended solids, nitrogen compounds, and heavy metals (Diya’uddeen, Daud, & Aziz, 2011; Wake, 2005). While we did not directly sample the Suncor effluent, we did sample Sand Creek, about 0.75 km downstream from Suncor. This area had elevated concentrations of nitrate-N (mean 4.9 mg/L), ammonia-N (mean 0.17 mg/L), and orthophosphate (mean 3.8 mg/L). Without more intensive sampling, it is not possible to conclude that Suncor is the source of elevated nutrient concentrations. Given the other industries in the area, there could be other sources of pollution. During our intensive sampling, we found elevated nitrate-N levels at three additional points along the South Platte River in this area: two outfalls at 43.4 km and 43.6 km (mean 8 mg/L and 11.1 mg/L, respectively) and one industrial effluent point at 44.2 km (mean 13.1 mg/L).