Title page for ETD etd-06212008-094319

Type of Document

Master's Thesis

Author

Akah, Wilfred Kwasi,

Author's Email Address

wilfred.akah@gmail.com

URN

etd-06212008-094319

Title

Nitrate transport studies at the Neuse River Waste Water Treatment Plant: Surface Geophysics and Groundwater flow modeling

Degree

Master of Science

Graduate Program

Marine, Earth and Atmospheric Sciences

Advisory Committee

Advisor Name	Title
Dr. John Fountain	Committee Chair
Dr. DelWayne Bohnenstiehl	Committee Member
Dr. William Showers	Committee Member

Keywords

Groundwater flow modeling
Surface Geophysics

Date of Defense

2008-06-09

Availability

unrestricted

Abstract

Groundwater flow models have suffered the lack of adequate geological input data such as bedrock elevations. This can result in poor modeling predictions, especially where there is a significant variation in the topography of the bedrock surface.

Most groundwater flow models have assumed a uniform elevation of the bedrock surface and model results based on this assumption can over-estimate or under-estimate the total groundwater inflow and outflow.

Several groundwater flow models: three uniform elevation bedrock models which assume uniform bedrock elevations of 50 ft (15.24 m), 100 ft (30.5 m) and 146 ft (44.5 m) (similar to the existing groundwater model at the site) and a fourth model which incorporates realistic bedrock elevations were developed and compared to determine whether there are any significant differences between their outputs and also to see which of these uniform elevation bedrock models fits the site’s conceptual and realistic bedrock models.

In order to obtain subsurface features and bedrock data at the Waste Application Fields (WAFs) surrounding the Neuse River Waste Water Treatment Plant (NRWWTP) in Raleigh, two geophysical methods: the Electromagnetic (EM) and Seismic Refraction (SR) were employed. The EM method detected the resistive near surface porous dikes and the conductive overburden. The SR method on the other hand detected two main contrasting layers made up of the weathered top material and the consolidated bedrock.

Lateral variations in seismic P-wave velocity in the top layer coupled with dipping bedrock were also detected by the SR method. Significant depressions in the bedrock were noticed in the SR profiles in the eastern and the north western parts of the study area. These depressions could act as traps for groundwater contaminants in the subsurface. The bedrock data also correlated very well with the available borehole logs at the site with an error estimate of 2.26 m. The NS profiles showed a gentle slope in the bedrock surface toward the north and this happens to be the direction of groundwater flow at the site. The thickness of the top weathered layer also tends to increase in this same direction. The EW profiles on the other hand revealed the subsurface channels and depressions in the bedrock surface.

Amongst the three uniform elevation bedrock models which were simulated, the flow patterns and output from the 146 ft (44.5 m) uniform elevation bedrock model were similar to the site’s conceptual and realistic bedrock models with a difference of only 1.5%. With an estimated average ground surface elevation of 200 ft (60.96 m) at the site, this uniform bedrock elevation of 146 ft (44.5 m) puts the bedrock to an average depth of 54 ft (16.5 m) which agrees with the available well logs. The flow patterns and output from the positive and negative estimated seismic errors were also similar with differences of 4.5% and 2.6% respectively as compared to the realistic bedrock flow model.

This study therefore stresses the importance of incorporating bedrock data, especially when they have significant topography. In areas where limited or no seismic refraction data exists, one way to incorporate the bedrock elevation data is to use an average elevation of the bedrock obtained from available well logs instead of using any arbitrary uniform bedrock elevation. This can significantly improve the groundwater model results. Although hydraulic parameters such as heads, recharge and boundary conditions are vital inputs in groundwater flow models, the importance of bedrock topography in accurately predicting flow and the transport of contaminants in the subsurface can not be underestimated.

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