Hydraulic head in a clayey sand till over multiple timescales

Abstract

This paper presents an analysis of hydraulic head data measured every 2 h for 2 years in the leading edge of a drumlin that consists of weathered, brown clayey sand till and underlying unweathered gray clayey sand till. The brown till responds as an unconfined aquifer and acts as a boundary condition for the confined gray till aquifer. Analytical models are presented for interpretation of the hydraulic head data using annual, monthly, and daily timescales, for estimation of drumlin (landform) hydraulics and till (formation) properties. The annual average head data from open-standpipe piezometers calibrate drumlin hydraulics that feature steady radial flow in the brown till, because of its higher permeability (1.1 × 10–13 m2) and the finite radius of the drumlin. The lower permeability (1.4 × 10–15 m2) gray till has steady cylindrical flow with an appreciable downward gradient. Open-standpipe piezometer data are modeled at the monthly timescale to confirm the landform hydraulics and calibrate a brown till macroporosity of 0.016. The amplitude of periodic monthly head fluctuations increases towards the edge of the drumlin. Hydraulic head response to aperiodic storm events is significantly magnified by this macroporosity as measured by precipitation gages and buried pressure transducers. The storm data are modeled on a daily timescale to confirm the brown till macroporosity value estimated from the monthly data and to further elucidate gray till hydraulics. Compressibility delays and attenuates the groundwater mound, with a calibrated gray till compressibility of 3 × 10–9 Pa–1. The calibrated models accurately recover drumlin flow fields with plausible property values of the clayey sand till, and accordingly should describe other sites with similar landforms and deposits.Key words: drumlins, till, hydraulics, compressibility, permeability, macroporosity, piezometer.