Three-dimensional deformation and strain induced by municipal pumping, part 1 : analysis of field data

Abstract

A 62-day controlled aquifer test was performed at the site of a new municipal well that penetrates a thick sequence of compressible basin-fill deposits in Mesquite, Nevada. Observation data were acquired from a high precision Global Positioning System (GPS) network to accompany the water-level data from the pumped well and a nearby observation well. The purpose of the investigation was to quantify and characterize three-dimensional surface deformation and strain associated with cyclic pumping and to evaluate aquifer properties on the basis of these data. Results indicate that both horizontal (<8 mm total displacement with 0.2 mm precision) and vertical deformation (<12 mm total displacement with 2 mm precision) were measured within the first 22 days of pumping with a slowly migrating outward wave of compressional strain in spite of the fact that an 83-m brittle unsaturated zone resided over the dynamically active aquifer. The hydraulic head and strain data indicate that steady-state pumping conditions were likely reached after 35–40 days. Results also indicate that vertical mechanical rebound occurs after about 35 days even though the rate of pumping remains unchanged. Strain analyses indicate that this result is due to the changing shape in the pore structure of the pumped aquifer as an elongation occurs in the horizontal direction to more than accommodate a small compressional strain in the vertical direction. Uniaxial specific storage was estimated from time-subsidence data to be 2.8X10-6 /m. GPS-observed deformations reveal a mechanical and hydraulic anisotropy field that increases in rotation toward the northwest and in magnitude from west to east at the study site. This represents the first investigation where the magnitude and direction of the hydraulic conductivity tensor of the aquifer is estimated without using well data.