Hydrological, Geochemical and Microbiological Controls on Iron Mineralisation in an Intermittent Stream
Iron is one of the most abundant elements on Earth, widely present in the Earth’s crust, water bodies, and living organisms. The redox reactions of iron in natural cycles, especially in water bodies, play a crucial role in nutrient cycling and pollutant degradation in ecosystems. However, the mechanisms of iron cycling and its interaction with microbial activities in intermittent streams within agricultural watersheds have not been fully studied. In particular, the occurrence of iron mineralization (such as iron flocs, iron films, etc.) in stagnant pools of streams may be closely related to groundwater inputs, microbial activities, and hydrological conditions. Understanding these processes not only helps reveal the ecological role of iron in agricultural streams but also provides scientific insights for water pollution control and nutrient cycling.
Therefore, researchers led by Zackry Stevenson conducted a comprehensive study combining hydrology, geochemistry, and microbiology in Clear Creek, a stream located on the campus of Iowa State University, aiming to uncover the controlling mechanisms of iron mineralization in intermittent streams.
Source of the Paper
This paper was co-authored by Zackry Stevenson, Mia Riddley, Tamara McConnell, and Elizabeth D. Swanner, all from the Department of Earth, Atmosphere, and Climate at Iowa State University, USA. The paper was published in 2025 in the journal Geo-Bio Interfaces, titled Hydrological, geochemical and microbiological controls on iron mineralisation in an intermittent stream.
Research Process and Results
1. Study Site and Observations
The study site was Clear Creek, located on the campus of Iowa State University in an agricultural watershed. The researchers selected four areas with intermittent iron mineralization along the stream, located upstream (Bridge Top, BT), midstream (Log Jam, LJ and Beaver Dam, BD), and downstream (Dead End, DE). Through a year of observations, the researchers recorded iron mineralization phenomena in the stream, including iron flocs, iron films (Schwimmeisen), and solid mineral precipitates.
2. Hydrological and Geochemical Measurements
The researchers used a YSI ProDSS multiparameter probe to measure physicochemical parameters of the stream water, including dissolved oxygen (DO), temperature, conductivity, pH, and oxidation-reduction potential (ORP). Additionally, piezometers were installed to monitor the hydrological characteristics and iron concentrations of groundwater. The results showed that the conductivity in the midstream areas was significantly higher than upstream, indicating additional water inputs, likely from groundwater. Groundwater had higher iron concentrations and low oxygen conditions, providing suitable conditions for iron redox cycling.
3. Quantification of Iron and Phosphorus
The researchers quantified total iron concentrations in the water using two methods and determined Fe(II) and Fe(III) concentrations using the Ferrozine assay. The results showed no significant difference in iron concentrations between mineralized and non-mineralized pools, but the iron content in sediments of mineralized pools was significantly higher than in non-mineralized pools. This suggests that the iron source for mineralization was not from the stream water but likely from groundwater. Furthermore, phosphorus quantification revealed that phosphorus concentrations in the stream were higher than in groundwater, indicating potential phosphorus inputs from agricultural runoff.
4. Microbial Community Analysis
The researchers analyzed the microbial communities in mineralized and non-mineralized pools using 16S rRNA amplicon sequencing. The results showed that the abundance of putative iron-oxidizing bacteria (e.g., Gallionellaceae and Comamonadaceae) and iron-reducing bacteria (e.g., Geobacteraceae and Rhodobacteraceae) was significantly higher in mineralized pools than in non-mineralized pools. Additionally, seasonal variations significantly affected microbial community composition, with higher abundances of iron-oxidizing bacteria in winter, suggesting that biological oxidation may outpace chemical oxidation under low temperatures.
5. Characterization of Iron Films
The researchers characterized iron films using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that iron films were mainly composed of iron and oxygen, with a thickness of approximately 150 nm, and exhibited short-range ordered mineral structures. The oxidation state of the iron films was primarily Fe(III), but their crystallinity was poor, possibly due to the presence of phosphate or carbonate ions interfering with mineral growth.
Research Conclusions and Significance
This study revealed the controlling mechanisms of iron mineralization in intermittent streams, with the following key conclusions: 1. Iron Source from Groundwater: The iron source for mineralization was not from the stream water but from groundwater inputs. Fe(II) in groundwater oxidizes to form Fe(III) minerals in the stream. 2. Microbial Activities Drive Iron Mineralization: The abundance of putative iron-oxidizing and iron-reducing bacteria was significantly higher in mineralized pools than in non-mineralized pools, indicating that microbial activities play a key role in the mineralization process. 3. Formation Mechanism of Iron Films: Iron films are primarily composed of Fe(III), and their short-range ordered mineral structures may result from the presence of phosphate or carbonate ions interfering with mineral growth.
The scientific value of this study lies in revealing the complex mechanisms of iron cycling in agricultural streams, particularly the influence of groundwater inputs and microbial activities on iron mineralization. Additionally, the research provides important scientific insights for water pollution control and nutrient cycling in agricultural watersheds.
Research Highlights
- Multidisciplinary Integrated Study: This study combined hydrology, geochemistry, and microbiology to comprehensively uncover the controlling mechanisms of iron mineralization.
- Characterization of Iron Films: Detailed characterization of iron films using SEM and TEM revealed their mineral composition and oxidation state.
- Seasonal Impact: The study found that seasonal variations significantly affect microbial community composition and iron mineralization processes, particularly with higher abundances of iron-oxidizing bacteria in winter.
Other Valuable Information
The study also found that organic debris (e.g., wood) in the stream may form small dams and stagnant pools, prolonging the hydraulic residence time of water and promoting the exchange between groundwater and stream water, further driving the process of iron mineralization. This finding provides a new perspective for understanding iron cycling in agricultural streams.
This study offers important scientific insights into iron cycling and nutrient dynamics in agricultural watersheds, with broad application value.