T128 The Hydrogeological Effects of Urbanization I

Monday, 1 November 2010
1:30 PM - 5:30 PM
Room 402

POTENTIAL EFFECTS OF CONSTRUCTION DEWATERING on A MIGRATING GROUNDWATER CONTAMINANT PLUME IN ANN ARBOR, MICHIGAN, USA

Paper number: T128-4
Program: Technical Sessions
Time: 2:05 PM – 2:20 PM

Authors

Abstract

Urbanization increasingly affects surface and subsurface hydrological conditions as cities expand outward and downward. In areas with a legacy of subsurface contamination, groundwater pumping associated with new municipal or industrial developments can have unintended consequences on the transport and fate of preexisting contaminant plumes. A case in point is an ongoing construction project in Ann Arbor, Michigan, USA, where excavation for an underground parking structure to depths up to 50 ft (15 m) below ground surface requires significant dewatering to facilitate installation of the foundation. The site is located one half city block (approximately 100 m) south of a groundwater use Prohibition Zone established by court order in 2005 to prevent public exposure to groundwater contaminated with 1,4-dioxane as it migrates beneath the city toward the Huron River. Pumping rates of approximately 500 GPM (1,900 LPM) over a 10 month period were estimated to be necessary to achieve local drawdown of the water table by as much as 11 ft (3.3 m) during construction.

A regional groundwater flow and contaminant transport model was adapted to evaluate drawdown and investigate the effects of pumping at the parking structure site. Advective transport (particle tracking) under steady state and transient flow conditions was used to predict the influence of proposed and potential prolonged pumping on 1,4-dioxane migration through the glacial aquifer system in the adjacent Prohibition Zone. Sensitivities to aquifer quality and hydrostratigraphic architecture, including the influence of a hypothesized silt layer 30 ft (10 m) below the water table, were explored. Model results indicate that in the absence of engineered cut-off walls to isolate the construction site from the surrounding aquifer, short term pumping at high rates required for dewatering could accelerate or redirect plume migration. Long term pumping, should it become necessary to maintain a lower water table during operation of the parking structure, may lead to partial plume capture outside the current boundaries of the groundwater use Prohibition Zone.