COLLOIDAL TRANSPORT OF NANOSCALE TO MICROSCALE GRAINS IN THE CENTRAL OKLAHOMA AQUIFER

The Central Oklahoma Aquifer (COA) provides a source of water for many of the most populated regions of Oklahoma. Elevated levels of arsenic, chromium, and uranium above EPA limits prevent usage of some public supply groundwater wells screened within the predominately red sand and siltstone units of Permian age. Despite significant hydrogeochemical modeling efforts, groundwater well locations yielding elevated trace element concentrations are spatially difficult to predict. We hypothesize that the presence of horizons containing abundant diagenetic iron oxides and post-diagenetic clays elevate available surface areas, serving as sinks for trace metals when groundwater geochemical conditions favor adsorption and sources when conditions favor desorption. Additionally, these nano- to micro-scale grains likely contribute to colloidal transport of adsorbed trace elements. Two forms of investigation explored trace metal distributions and aquifer mineralogy at the nano- to microscales, in hopes of developing more robust explanations for trace element distributions in sediment and groundwater.

The first investigation involved the collection of 50 sediment samples from the USGS/EPA Norman Test Hole Core spanning the productive zones of the aquifer. These samples were analyzed for BET surface area, micromorphology with SEM, color, texture, and grain size with optical microscopy, and whole-rock trace element geochemistry. Initial results demonstrated anomalously high surface areas in many samples, ranging up to ~60 m^­2/g. High surface area values were obtained in both clay-rich and clay-poor intervals. SEM imaging of samples from these intervals revealed the presence of abundant microscale hematite rosettes assembled from nanoscale platelets, in addition to clay minerals and other nanoscale iron oxides. Trace element concentrations followed a general power-law relationship with surface area, with much scatter.

In the second investigation TEM grids were deployed in monitoring wells associated with a City of Norman municipal drinking water supply well using an in-house designed subsurface nanoparticle collector. After exposure to groundwater for one week, TEM analysis of the grids revealed abundant iron oxides, quartz with or without surface-bound iron oxides, gypsum, and other trace phases.