Milford, Nebraska is located approximately 20 miles west of Lincoln, Nebraska. The former USDA CCC site is located 2 miles south of town. Carbon tetrachloride is found in water samples from wells and test borings in the vicinity of the site and the plume is moving eastward toward the Big Blue River. High-resolution shear wave seismic reflection surveys and were proposed to aid in the understanding of the subsurface geology and determine contaminant flow pathways.

 Geology

Approximately of 70 feet of Pleistocene age sediments overlie Cretaceous age bedrock. The Pleistocene overburden consists of a cover of topsoil and loess up to 20 feet thick and three distinct intervals of fluvial sand and clay deposits designated A, B, and C. The top of bedrock unconformity is on the Upper Cretaceous Greenhorn Limestone. Groundwater is pumped from both the unconfined Pleistocene aquifer (> 50 ft) and the confined Dakota Sandstone aquifer (>150+ft). The Big Blue River drains the area and has incised into the Cretaceous bedrock. Carbon tetrachloride contamination is found within the Pleistocene deposits.

Objective:

The objectives of the seismic survey were to:

• Determine if Pleistocene age channels have incised the bedrock surface.
• Identify any possible channels and other textural changes that occur within the Pleistocene section that might suggest possible preferential pathways for contaminant migration.
• Determine the vertical and lateral extent of units A, B, and C within the Pleistocene section and differentiate between the Pleistocene loess and Pleistocene outwash deposits. 

Shear Wave Seismic Data

Four high-resolution shear wave seismic reflection profiles, 7158 CDPs (48 fold), were collected in addition to 5 VSPs. The recording parameters for the seismic data were determined on the basis of a short test program and were optimized to image the Pleistocene section. Seismic data acquisition began in June 1999 by Bay Geophysical Associates and was completed in seven days. The data were processed in the field as acquisition was progressing. Contemporaneous with the seismic profiling, a cone penetrometer (CPT) truck and a drill rig were in the field. Five VSPs were collected using the CPT. As features were discovered on the seismic data, the CPT tested them. At the conclusion of the field effort, all the VSPs had been processed and 90 percent of the seismic profile data had been stacked. Preliminary interpretations identified several channel features that were subsequently verified. Some of these channels were found to contain contaminants.

Milford Line-4 showing some of the features interpreted. Line direction West to East; valley of the Big Blue River on right. Note incisement of the bedrock surface by the river. Pale green indicates channels interpreted in the Pleistocene section.