Identification of reducing conditions and correlated hydrological and biogeochemical properties in a heterogeneous floodplain aquifer

Abstract

Previous work has shown that subsurface heterogeneity plays a major role in the fate and transport of nutrients. Although many studies indicate the significance of river-groundwater interactions, subsurface heterogeneity has generally been neglected. Furthermore, it has been shown that heterogeneous redox conditions, geology, water residence times, and microbial activity influence the speciation of carbon. Here we evaluate the correlation between physical and geochemical heterogeneity in a floodplain on the banks of the Colorado River as part of the U.S. Department of Energy's Integrated Field Research Challenge (IFRC). Microbial activity and heterogeneous redox conditions have been shown to influence carbon cycling at the site which made it suitable for this study along with the availability of wells for sampling. The geochemical data along with hydraulic conductivity (K[subscript h]) from the samples were used to evaluate any that can influence carbon distribution, specifically with the use of cluster analysis and hypothesis testing. Analysis of this dataset indicates the presence of two distinct groups within the alluvium material, one of the groups with parameters, such as high total organic carbon (TOC), high bioavailable iron, and dark colored sediments, which indicate the presence of a naturally reduced zone (NRZ). However, K[subscript h] within some of these sediments was high (ex. 276.48 ft./day) which is in disagreement with the more common theme of reduced zones occurring within finer grained material. This suggests that K[subscript h] may not be the dominant factor in the distribution of these NRZs at the IFRC site. Knowing these correlations between geochemical and physical parameters will aid in future numerical models of nutrient cycling.