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Paper No. 296-12
Presentation Time: 9:00 AM-6:30 PM
HYDROGEOLOGICAL CHARACTERIZATION OF COMPLEX QUATERNARY ICE MARGINAL SEDIMENTS AT A CONTAMINATED SITE IN SOUTH CENTRAL, WISCONSIN
Groundwater at an industrial site in south central, Wisconsin was found to be contaminated from the release of dense non-aqueous phase liquids (DNAPL) prior to 1970. The DNAPL migrated through the unconsolidated sediment to accumulate within the Lone Rock Formation of the Tunnel City Group. Contamination persists within the overlying Quaternary unconsolidated sediments. The purpose of this study is to better understand the nature and distribution of Quaternary sediments at the site in order to determine their influence on the groundwater flow system and contaminant attenuation, distribution, and transport. Five rotosonic boreholes were continuously cored 10 feet into bedrock in 3 geological settings: drumlinized till plain, outwash, and lacustrine. Core was logged (every 0.1 foot) in the field to capture changes in texture, sorting, sedimentary structures, and clast characteristics with depth. Samples were taken (~every ft.) for further lab analyses (volatile organic compounds, moisture content, grain size distribution, permeameter testing, and fraction of organic carbon). Westbay multilevel groundwater monitoring systems with numerous, depth-discrete monitoring zones were installed to monitor head and contaminant distribution and obtain information on the hydraulic properties of several units. The position and lengths of screens and seals were carefully chosen based on detailed geologic logs and natural gamma in order to minimize blending. Most cores have a basal diamict package overlying bedrock, an interval of variable sand layers, and an uppermost diamict package at surface. Diamict packages have stacked units defined by changes in moisture, colour, grain size, and clast content. The diamict package above bedrock is typically interbedded with gravelly sand, whereas the diamict package at surface can be absent due to glaciofluvial erosion. Data collected to date provides evidence of variable depositional conditions in an ice marginal setting over both time and space. Detailed vertical profiles obtained at this site, together with pre-existing hydrogeological and surface geophysical data sets, will be used to develop a high-resolution site conceptual model that will provide insight into subsurface heterogeneity, groundwater recharge, and contaminant transport to the bedrock aquifer.