Using geochemical indicators to distinguish high biogeochemical activity in the subsurface

A better understanding of how microbial communities interact with their surroundings in physically and chemically heterogeneous subsurface environments will lead to improved quantification of biogeochemical reactions and associated nutrient cycling. This study develops a methodology to predict elevated rates of biogeochemical activity (microbial "hotspots") in subsurface environments by correlating microbial community structure with the spatial distribution of geochemical indicators in subsurface sediments. Statistical hierarchical cluster analyses (HCA) of simulated precipitation leachate, HCl and hydroxylamine extractable iron, total organic carbon (TOC), and microbial community structure were used to identify sample characteristics indicative of biogeochemical hotspots within fluvially-derived aquifer sediments. The method has previously been applied to alluvial materials collected at a former uranium mill site near Rifle, Colorado and this study introduces a new field site of relatively undisturbed floodplain deposits (soils and sediments) collected along the East River near Crested Butte, Colorado. At the East River 46 soil/sediment samples were collected across and perpendicular to 3 active meanders and an oxbow meander. Results indicate a strong relationship between TOC and microbial DNA whereas the influence of metals as terminal electron acceptors is specific to the dominant terminal electron accepting process. Linear regression of log-transformed TOC and bulk microbial DNA for 34 East River shallow meander and 14 alluvial Rifle samples produces a preliminary empirical relationship. Applying the method to identify hotspots in both contaminated and natural floodplain deposits and their associated alluvial aquifers demonstrates the broad applicability of a geochemical indicator based approach.