Previous Research Projects


Source of organic carbon fueling groundwater arsenic contamination

Field work, Bangladesh

Laboratory experiments

In parts of Asia, groundwater is severely contaminated with arsenic, a carcinogenic element that occurs naturally in soil and sediment. The contamination reduces life expectancy for millions of people that use groundwater for drinking and irrigation. Our research has identified the sources of organic carbon that fuel the microbial processes responsible for mobilizing arsenic off of soil and sediment and into groundwater at a site in Bangladesh. A decade-long debate has centered on the relative importance of ancient, sedimentary organic carbon versus modern, surface-derived organic carbon transported into aquifers with recharging water for fueling arsenic mobilization. Identifying the source of organic carbon is important because it clarifies how human activities can affect the contamination problem. While our initial work established the involvement of surface-derived organic carbon, our later work identified a role for sedimentary organic carbon as well, conceptually bridging the apparently conflicting lines of evidence driving the carbon debate.


Neumann, R. B., L. E. Pracht, M. L. Polizzotto, A. B. M. Badruzzaman, and M. A. Ali (2014), Biodegradable Organic Carbon in Sediments of an Arsenic-Contaminated Aquifer in Bangladesh, Environmental Science & Technology Letters, 1(4), 221–225, doi:10.1021/ez5000644.

Neumann, R. B., A. P. St. Vincent, L. C. Roberts, A. B. M. Badruzzaman, M. A. Ali, and C. F. Harvey (2011), Rice Field Geochemistry and Hydrology: An Explanation for Why Groundwater Irrigated Fields in Bangladesh are Net Sinks of Arsenic from Groundwater, Environmental Science & Technology, 45(6), 2072–2078, doi:10.1021/es102635d.

Neumann, R. B., K. N. Ashfaque, A. B. M. Badruzzaman, M. Ashraf Ali, J. K. Shoemaker, and C. F. Harvey (2010), Anthropogenic influences on groundwater arsenic concentrations in Bangladesh, Nature Geoscience, 3(1), 46–52, doi:10.1038/ngeo685.

Neumann, R. B., M. L. Polizzotto, A. B. M. Badruzzaman, M. A. Ali, Z. Zhang, and C. F. Harvey (2009), Hydrology of a groundwater-irrigated rice field in Bangladesh: Seasonal and daily mechanisms of infiltration, Water Resources Research, 45(9), doi:10.1029/2008WR007542.

Measuring and modeling plant-mediated water flow processes

plant growth experiments

reactive transport modeling

Plant roots connect the atmosphere with the subsurface and affect terrestrial water, carbon and nutrient cycles.  However, the influence of plant-controlled below-ground processes on water and carbon balances is poorly quantified. At present, treatment of roots in ecosystem models is simplistic, reflecting our incomplete understanding of root-water uptake behavior and limiting our ability to improve predictions of climate change. Our research has improved understanding of two globally relevant, plant-mediated water flow processes: hydraulic redistribution (HR; movement of water from moist to dry soil layers through plant roots) and nighttime transpiration. By synthesizing data from published studies, we quantified how much water moves through the environment by HR. We conducted greenhouse experiments that demonstrated that modeled rates of HR could only match observed data by including nighttime transpiration. Using a reactive transport model, we clarified the impact both processes have on nutrient availability to roots and to microbes living in the rhizosphere, the soil surrounding roots.

Funding: DOE Office of Science, Biological and Environmental Research Program; NOAA Climate and Global Change Postdoc Fellowship


Espeleta, J. F., Z. G. Cardon, K. U. Mayer, and R. B. Neumann (2017), Diel plant water use and competitive soil cation exchange interact to enhance NH4 + and K+ availability in the rhizosphere, Plant and Soil, 414(1–2), 33–51, doi:10.1007/s11104-016-3089-5.

Neumann, R. B., Z. G. Cardon, J. Teshera-Levye, F. E. Rockwell, M. A. Zwieniecki, and N. M. Holbrook (2014), Modelled hydraulic redistribution by sunflower ( Helianthus annuus L.) matches observed data only after including night-time transpiration: HR model with night-time transpiration fits data, Plant, Cell & Environment, 37(4), 899–910, doi:10.1111/pce.12206.

Neumann, R. B., and Z. G. Cardon (2012), The magnitude of hydraulic redistribution by plant roots: a review and synthesis of empirical and modeling studies: Tansley review, New Phytologist, 194(2), 337–352, doi:10.1111/j.1469-8137.2012.04088.x.


Neumann, Rebecca B; Espeleta, Javier F; Cardon, Zoe G; Mayer, K Ulrich (2017): Modeled profiles of NH4+ and K+ in the rhizosphere resulting from diel plant water use and competitive soil cation exchange, Links to model results. doi:10.1594/PANGAEA.876349