Masters Theses

Date of Award

12-2002

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

R. Bruce Robinson

Abstract

The University of Tennessee (UT) Civil and Environmental Engineering Department monitored the impact of road and tunnel reconstruction on adjacent streams in the Great Smoky Mountains National Park (GSMNP) for the National Park Service and Best Management Practice (BMP) performance. The potential stream impacts of concern were sediment and acid generating geology. Sediment can adversely affect aquatic life and ecosystems by mechanical abrasion, by obstructive silting, by reducing the penetration of light, by providing a surface for the growth of microorganisms, by absorbing or adsorbing chemicals, and by inhibiting normal temperature fluctuations (Swerdon and Kountz, 1973). Anakeesta exposure can upset aquatic life through depression of stream water pH and increased concentrations of toxic mineral elements. BMPs were monitored because little information was available on the performance of these structures and efficiency results help in evaluating each structure's usefulness. Monitoring took place on a reconstruction project 1.13 km (0.70 mi) in length on US 441 and included two tunnels through Chimney Tops Mountain and Morton Mountain. Reconstruction tasks included repairing the tunneling ( deepening the tunnels, installing rock bolts, placing concrete tunnel sidewalls), roadway excavation, grading, drainage, new road base and paving, constructing guard walls, and other work. This report summarizes the pre-, during and post-construction monitoring from May 2001 through September 2002. During pre- and post-construction periods, grab samples from Walker Camp Prong and the West Prong of the Little Pigeon River (WPLPR) were collected every two weeks at each of four stations: upstream and downstream of the two tunnel reconstruction sites. A passive sampling device was also located at each of the four stations, taking samples at various stage heights during high flows. During construction more frequent sampling occurred at these stations and around BMPs. The BMPs monitored were rock check dams, inlet protection for existing storm drains and disposal areas encapsulating spoil material. The rock check dams and inlet protection were installed to reduce suspended solids totals (by ponding water and allowing heavier particles to settle out). The check dams also reduced erosion by lowering the water's velocity and dissipating energy. The tunnel reconstruction had no statistically identifiable impact on stream water quality. This conclusion is based on the following: • Statistical results did not indicate that construction altered pH or suspended solids concentrations. At a 95% confidence level (p=0.05) a statistically significant difference was not detected in the pH of runoff samples entering and leaving the construction area at the Chimney Tops site (p=0.375) or Morton Mountain (p=0.109). A statistically significant difference was also not detected in the suspended sediment concentrations entering and leaving the construction sites at Chimney Tops (p=0.375) and Morton Mountain (p=0.742). • Median suspended solids concentrations for the stream sampling locations were lower during construction than in the pre- and post-construction periods. The low suspended solids totals during construction (during winter) are likely because the ground was frozen during much of this period. • Stream pH was lower during the construction period than the pre- and postconstruction periods. Paired differences (downstream value less the upstream value for each collection date) were examined to see if these pH trends were a result of seasonal variation. Other streams within the same watershed did show pH differences consistent with the variation observed at the Morton sites (indicating seasonality). • For each tunnel the water quality parameters that varied between the upstream and downstream sampling during the construction periods were the same ones that varied prior to construction. • The area of disturbance was small compared to the watershed. Approximate calculations showed that little impact should be expected because the storm runoff volume from the construction site was very small in comparison to the stream flow from the watershed. BMP performance varied. Success or failure was tied to proper/improper location, design, construction and maintenance. Specific conclusions include: • Many BMPs were installed according to the plans but were not constructed in locations where they would receive runoff from construction activities. • Multiple BMPs were added and/or altered after construction began as unforeseen or newly developed runoff conditions were discovered. • The efficiency of check dams was dependent upon their ability to pond water and the pond volume available for ponding and storage. The rock check dam at the Old Trace Road Disposal site provided area for storage and offered good settleable solids trapping efficiency for runoff events with high suspended solids concentrations (90% and 65% for influents of 23,567 and 42,527 mg/L respectively). The rock check dams installed at the Chimneys site provided very little ponding and therefore negligible trapping efficiency. A statistically significant difference was not detected in the suspended solids concentrations before and after Chimney Tops rock check dams (p=0.813) and inlet protection at Morton Mountain (p=0.195). Despite these findings field observance did reveal trapped sediment behind rock check dams indicating that some material was retained by the structures. • A statistically significant difference was not detected in the pH above and below the rock check dams and inlet protection at Chimney Tops (p=0.813) and Morton Mountain (p=0.250). • Silt fences were not observed ponding water at any location • The disposal practices used for the excavated materials were effective in neutralizing any acidic leachate. The pH of leachate from the disposal mound at Old Trace Road ranged from 6.9 to 7.7. Gnatty Branch disposal mound leachate pH measured 7.2 or greater. Information in addition to construction impact or BMP performance was gained from this project: • Storm runoff lowered stream pH values. The effect was more pronounced at the Morton locations. Median pH at the uppermost stream sampling station decreased by 1.1, 0.7 and 0.9 units for the three monitoring periods respectively when comparing baseflow samples with the highest stage passive collection bottle. Likewise, the Morton sites experienced greater changes during storm events than the Chimney's sites for other water quality parameters. • Higher flows also resulted in higher suspended solids concentrations. Median concentrations were at least three-fold greater for the highest stage passive collection bottles than for the lowest stage (for all but one out of twelve cases). • Regular inspection and maintenance is required for BMPs to function properly. • Excavated material was tracked onto the haul road and contaminated runoff during rain events. Much of this material could have been recovered if a street sweeping truck had been used.

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