Sample collection and analysis
In May 2016, we identified 14 sampling sites along the South Platte
River, spanning the length of the Denver metropolitan area (Figure 1).
The first point was located at the mouth of Waterton Canyon, where the
South Platte River emerges from the foothills. From preliminary work, we
knew that water quality at this location was similar to points higher in
the watershed and for this reason, we used it as our upstream control
point (Heilman & Schliemann, 2015). From Waterton Canyon, water flows
into Chatfield Reservoir. We placed our second sampling point
immediately downstream of the reservoir, 14 km from the first point. The
remaining points were spaced approximately 4.8 km (3 mi) apart with
negligible variation due to sampling accessibility. Additional sampling
sites were included above and below two of the main tributaries to the
South Platte River: Cherry Creek and Clear Creek (Figure 1).
From May 2016 – May 2019, every two weeks (March – November) and every
four weeks (December – February), we assessed a range of water quality
parameters at every site. We measured water temperature, pH, dissolved
oxygen (DO) in situ; and collected grab samples to measure the
concentrations of nitrate-N (NO3-N), ammonia-N
(NH3-N), and orthophosphate
(PO43-) in the lab within 3 hours of
collection. In addition, we performed a BOD5 test using
a DO probe and standard methods (Eaton, Clesceri, Rice, Greenberg, &
Franson, 2005). Temperature, pH, DO, BOD, and the concentration of
NO3-N were measured using Hach probes and a Hach HQ40d
meter (Hach CO, Longmont, Colorado, USA). The concentrations of
NH3-N and orthophosphate were measured colorimetrically
using a Hach DR900 colorimeter (Hach CO, 2020).
From November 2017 – February 2018, we systematically inventoried all
visible storm drains, effluent points, and natural tributaries that join
the South Platte River from the 14 km site to the 63 km site. We did not
sample between the 0 km and 14 km sites due to access limitations.
Furthermore, this area is a state park, with a little urban development.
Daily, we walked segments of the river, marking each point using a
handheld Garmin etrex 20x GPS unit (Garmin Ltd., Olathe, Kansas, USA) .
We also recorded whether the outfall point had flowing water, the type
of outfall (storm drain, effluent point, or natural), the outfall
structure (i.e., pipe or culvert), and the relative size of the outfall.
After collecting 18 months of data, we noticed that two of the areas we
were sampling (28 km – 37 km and 47 – 48 km) seemed to have
consistently high concentrations of nutrients. Consequently, in November
2017, we initiated an intensive study of these two “hotspot” areas.
Over six months (November 2017 – April 2018), we sampled each flowing
outfall from the beginning of the hotspot (i.e. 28 km and 47 km,
respectively) to 5 km upstream (i.e. 23 km and 42 km, respectively). In
addition, we sampled the river in this same section intensively, taking
samples every 100 m. At each sampling point, we measured the parameters
discussed above.