Atmospheric nitrogen deposition from regional and more distant emission sources have polluted high elevations of Rocky Mountain National Park for decades. Excess reactive nitrogen (hereafter called nitrogen) in the park and in ecosystems in general is a problem because it contributes to biodiversity loss and eutrophication. Based on USGS research in 2007, the State of Colorado, the Environmental Protection Agency, and the National Park Service developed the Nitrogen Deposition Reduction Plan (NDRP) to reduce emissions, initially based on voluntary emissions reductions from fossil fuel combustion and agriculture. As emissions from combustion sources have declined, emissions from agricultural sources including dairies, industrial feedlots, and irrigated croplands have become more prominent. Recently, these local stakeholders of Rocky Mountain National Park requested an analysis of more local sources of nitrogen to the system.

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To get at this question, USGS scientists estimated the contribution of nitrogen from a very local source, that of human urine, related to intensive visitor use in a popular destination of the park: Loch Vale watershed. Not only does human urine convey hormones, pharmaceuticals, and antibiotic resistant bacteria and antibiotic resistant genes to the environment, it supplies nitrogen that contributes to loss of biodiversity and eutrophication.

Using caffeine as a specific marker for human urine, scientists compared the calculated worst case potential input of urine with that from wet atmospheric nitrogen deposition. Nearly 30,000 and 45,000 people hiked to the lowest lake in Loch Vale watershed during June to September 2019 and 2020, respectively. Calculations suggest 2% of the nitrogen in the waters of Loch Vale was from local human urine, which is small but not insignificant compared with the measured atmospheric deposition values.

Importantly, however, the results document an ecological disturbance from an unprecedented level of human activity in a protected and designated wilderness area. Leave No Trace Principles, widely presented as guides for visitor behavior in wilderness, do not address urine as a waste product at all. Local solutions to this local problem could include greater outreach to visitors of public lands on the consequences of their activities and installation of latrines.