A Wooster geologist’s summer research experience in southeast Wisconsin: Sarah Frederick (’15) and the sourcing of molybdenum in groundwater
Mark Wilson August 19th, 2013
This summer, Sarah Bender (’15) and Sarah Frederick (’15) had the opportunity to complete National Science Foundation funded Research Experiences for Undergraduates (REUs). Each spent a good part of their summer completing a research project under the mentorship of accomplished and enthusiastic geologists. Sarah Frederick worked under the mentorship of Dr. Timothy Grundl, a professor within the school of Freshwater Sciences and Department of Geosciences at the University of Wisconsin – Milwaukee. This is her summer research story in her words and images.
My project focused on the elevated levels of molybdenum that have been found in the groundwater of portions of southeast Wisconsin. Molybdenum, for those of you who may not be familiar with this metallic element, is an essential nutrient naturally present at low levels within our environment. High concentrations, however, which are often linked to the improper disposal of such waste products as sewage sludge and fly ash, can create agricultural and health complications. Therefore, the Wisconsin Department of Natural Resources is very interested in this excess and recently completed a two-year study of this problem. The DNR found molybdenum concentrations greater than the recommended quality standards in the water of private wells throughout southeastern Wisconsin, however; this study was largely inconclusive as to a source of contamination. This is where my project began.
I spent ten weeks investigating the Emerald Park Landfill in Muskego, Wisconsin. The image at the top of this post is an aerial view of Emerald Park. Water samples were collected from the wells marked in blue while the yellow denotes the boring from which soil samples were collected. The red line marks the cross-section I analyzed.
Fieldwork in a landfill was a completely new experience for me. There were two extremes involved. First, near the landfill itself, sampling was a dirty, dusty job and you had to be careful not to get run over by trucks! However, sampling on the outskirts presented its own challenges. As part of this landfill’s wildlife remediation it has created extensive wetland environments. Thus, sampling involved swimming through grass two feet taller than myself in search of wells that one feared may be nonexistent, all while lugging sampling equipment over damp, pitted terrain. In the end we did manage to find all of the wells that we were looking for and successfully collect data and water samples from each.
As I was running my samples through the ion chromatography system and the atomic absorption spectrometer back in the laboratory, it quickly became clear that there was a significant difference between the surface water and the water collected from the deeper wells. Piper diagrams clearly illustrated this difference in relative water chemistry. The Piper diagram below displays the relative chemistry of all thirteen of the Emerald Park monitoring wells analyzed. The two A wells, which are shown to be distinct from the rest of the wells, are the wells screened within the surface water.
This hypothesized disconnect between the surface water and the aquifer below was corroborated by the analysis of these water samples using a modeling system called PHREEQC, which was unable to accurately model the transition between the two waters. Further confirmation was provided by the molybdenum concentrations detected within the wells from which a vertical suite of samples was collected. The graph below shows that the surface water contains very little molybdenum while the greatest concentrations appear in the nest deepest wells.
The presence of an aquitard, a dense, impermeable clay hardpan that effectively separates the surface water from the water below, was finally confirmed by the boring logs for this site, which record increased blow counts and a description of this layer. Through the use of well logs and the conclusions of my own research, I thus was able to illustrate the stratigraphy of this site.
The existence of this aquitard and the absence of elevated molybdenum levels within the surface water eliminated the Emerald Park Landfill as a source of contamination since the lowest extent of the landfill storage does not penetrate this clay barrier.
My study therefore concluded that the molybdenum is sourced in the clay. Since almost all the molybdenum detected was dissolved, it was not being transported into the site on colloids. Instead, molybdenum sorbed to hydrous ferric oxides and the cation exchange sites of the clay is being released through reductive dissolution or increased sorption competition.
While this project was only a case study, it is my hope that the Wisconsin DNR will further investigate the possibility that the molybdenum is naturally from the clay of southeast Wisconsin, and that elevated groundwater molybdenum concentrations are a result of the water chemistry.