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.