Environmental Compliance
In the United States, the Clean Water Act (CWA) is the Federal statute
that governs water quality. Under the CWA, each state is permitted to
set their own standards, provided that they are at least as stringent as
the standards set in the CWA (Federal Water Pollution Control Act,
1972). In Colorado, the state has adopted Regulation 31, which
establishes standards for classifying surface waters within the state of
Colorado using a range of parameters. For warm-water streams, such as
the South Platte River in the Denver metro area, the interim total
phosphorus maximum allowable concentration is 0.170 mg/L and the interim
total nitrogen maximum allowable concentration is 2.01 mg/L (Colorado
Departmet of Public Health and Environment, 2017).
Over most of the Denver metropolitan area, nitrogen and phosphorus
concentrations measured in this study were above the limits set by
Regulation 31. Nitrate-N concentrations in the South Platte ranged from
a low of 0 mg/L at the 0 km control site in August of 2016 to a high of
25.8 mg/L at the 14 km site in February of 2018. Ammonia-N
concentrations in the South Platte ranged from a low of 0 mg/L at nearly
every site at some point during the study to a high of 3.4 mg/L at the
33 km site in February of 2018. The nitrate-N concentrations were higher
than the ammonia-N concentrations in 99.9% of the samples. In many
cases, nitrate-N was an order of magnitude higher than ammonia-N. When
considered together (ammonia-N + nitrate-N), the concentration exceeded
the maximum allowable level of total nitrogen in more than 76% of the
samples.
Orthophosphate concentrations in the South Platte ranged from a low of 0
mg/L at the 0 km site on 14 different sampling dates, representing all
months of the year, to a high of 14.3 mg/L at 47 km site in October of
2017. Over 85% of the samples tested had concentrations of
orthophosphate that exceeded the maximum allowable total phosphate
concentration of 0.170 mg/L.
In addition to Regulation 31, the South Platte River and several of its
tributaries in the eastern plains of Colorado are managed according to
state Regulation 38 (Colorado Departmet of Public Health & Environment,
2012). According to Regulation 38, a Tier I Warm Stream, such as the
South Platte River, has a maximum daily temperature limit of 29.0oC for March – November and 14.5 oC
for December – February. All of the temperature readings collected were
below these limits; however, while the summer temperatures are within
the Regulation 38 limits, they are often too high to support healthy
populations of several species of desirable cold-water fish includingOncorhynchus mykiss (rainbow trout) and Salmo trutta(brown trout). The optimal summer temperature for growth and
reproduction of O mykiss is generally reported to be between 17
and 19 oC (Hokanson, Kleiner, & Thorslund, 1977).
Almost 29% of our samples from March – November exceeded 19oC. Trout Unlimited, an American non-profit has been
working with local municipalities to restore trout populations to the
South Platte in the Denver metro area. By building cold water refugia,
with deep pools and riparian vegetation for shade, the group has created
cooler conditions that support small pockets of trout (Trout Unlimited,
2020). However, it is unclear whether these isolated populations will be
self-sustaining in the long-term due to their limited population size
and low genetic diversity.
Regulation 38 also establishes a minimum DO of 5.0 mg/L May 1 – July 15
and 4.5 mg/L for the rest of the year. The reasoning is that higher DO
concentrations are necessary for spawning, which generally occurs May 1
– July 15. The South Platte River is generally in compliance with only
0.32% of our measurements below this threshold.