Guest Post by Lauren Magliozzi, 2025-2026 Sustainability Leadership Fellow and Postdoctoral Fellow in the Department of Atmospheric Science and Natural Resource Ecology Laboratory (NREL) at Colorado State University 

On a chilly October morning in Colorado’s Rocky Mountains, shallow pools of orange-tinted water slowly thawed as the sun rose over an abandoned mine in the Peru Creek watershed. What emerged from intensive hourly sampling across a complete daily cycle challenged conventional thinking about how wetlands process metal pollution.

(Top photo) The wetlands below the Pennsylvania Mine, which operated from 1879 to the early 1930s, are stained with iron-rich acid mine drainage. This toxic legacy impacts thousands of abandoned mines across the American West, and warmer summers combined with lower streamflows are increasing metal concentrations in Colorado’s mineralized watersheds.

Nature’s water treatment plants

Wetlands have long been recognized as natural “water purifiers”. Dense vegetation, organic-rich soils, and slow-moving water trap sediments and can transform pollutants. This natural cleaning power is so effective that engineers sometimes deliberately construct “

The key lies in wetlands’ layered structure: wetland sediments are host to distinct vertical zones with different chemical conditions. At the surface, oxygen-rich waters allow iron and other metals to precipitate out as solid minerals. Just below, in the saturated sediments, oxygen becomes scarce. Here, bacteria create conditions that can immobilize certain metals or convert sulfate back to sulfide minerals. Deeper still, in the groundwater, conditions may be completely oxygen-free, and host to anaerobic bacteria that facilitate other distinctive chemical reactions.

This vertical stratification means wetlands aren’t uniform systems. They’re more like multi-stage chemical reactors, with each layer contributing differently to metal removal. But there’s another dimension to wetland chemistry that’s received less attention: what happens at the sunlit surface throughout the day.

An hourly transformation

We sampled water every hour from sunrise to well after sunset, tracking parameters including temperature, light intensity, and chemical concentrations. The patterns that emerged were unexpected. 

As sunlight intensity increased through the morning, hydrogen peroxide, which is produced when UV light breaks down dissolved organic matter, began to accumulate in the wetland pools. This triggered what chemists call the photo-Fenton reaction, a cascade of chemical transformations involving iron.

The reaction works like this: during daylight hours, UV light reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), releasing metals that had been stuck to iron oxide surfaces. But simultaneously, hydrogen peroxide oxidizes that ferrous iron back to ferric iron. It’s a photochemical tug-of-war, and the outcome determines whether other metals stay dissolved or become trapped as solid particles.

The findings for other metals varied considerably. Lead concentrations more than doubled during peak sunlight, a 113% increase. Copper rose 30% and cadmium 9%. Meanwhile, zinc, manganese, and aluminum all decreased by 9-19%.

Rare earth element cycling

Perhaps most intriguingly, we found higher than expected concentrations of rare earth elements. These elements are needed for manufacturing tech such as smartphones, electric vehicles, and wind turbines. In this mountain wetland, cerium, neodymium, and yttrium frequently exceeded 100 micrograms per liter, which are levels rarely reported in natural waters.

Surprisingly, these rare earths also showed daily cycling patterns, though their responses varied. Some elements like cerium, neodymium, and lanthanum increased during peak daylight hours, while others like yttrium and dysprosium decreased slightly. Samarium showed the most dramatic shift, decreasing substantially as the day progressed. These different responses suggest the rare earths elements, despite their chemical similarities, interact differently with the photochemical processes transforming iron throughout the day. This finding had not been observed previously. 

Why this matters

These findings have important implications for water quality management in acid mine drainage-impacted systems. For example, traditional monitoring that collects single grab samples once a day could over- or under- estimate metal concentrations depending on time of day. Lead levels, for instance, could vary by more than a factor of two between morning and afternoon.

The photochemical cycling also affects metal bioavailability. Dissolved metals are generally more toxic to aquatic organisms than those bound to particles. The daily pulses of dissolved metals during peak sunlight could create windows of heightened ecological risk.

This is particularly concerning for rare earth elements. While historically considered relatively benign, recent research shows they can disrupt biological systems at concentrations in the tens to hundreds of micrograms per liter, exactly the range we measured. These elements interfere with calcium in cell membranes, causing oxidative stress in organisms from algae to fish.

As mining for rare earths expands to meet surging demand for new technologies, understanding how these elements cycle in natural systems becomes increasingly urgent.

Looking ahead

Climate trends suggest that acid mine drainage impacts will intensify. Warmer temperatures and lower stream flows lead to higher metal concentrations, while more intense UV exposure, particularly at high altitude, may amplify photochemical processes.

The good news is that understanding these photochemical controls opens new management possibilities. Wetlands aren’t passive filters but active processors, and working with their natural chemistry could enhance treatment efficiency in mining-impacted waters.

Our study focused on one wetland system in the Colorado mountains, but the underlying processes, e.g., photochemistry, dissolved organic matter, iron, and other metals, are features of many other iron-rich, acid mine drainage-impacted systems. The daily transformations we documented likely occur in contaminated wetlands worldwide.

As the sun sets over Peru Creek each autumn evening and the wetland pools freeze, metal concentrations shift back toward their nighttime states. Wetlands aren’t just nature’s filters, they are living chemical laboratories where every sunrise begins a new experiment.