Estimates of groundwater seepage flux in lake bottom sediments require knowledge of the hydraulic gradient at the sediment-surface water interface and the hydraulic conductivity of the lake-bottom materials. In deep waters, in situ measurement of these parameters can be accomplished through the use of piezometer probes lowered and monitored remotely from a surface vessel. In this research work a new tethered piezometer probe was developed and tested for use in collecting hydraulic property data in deep-lake bottom sediments. The probe uses a variable-reluctance transducer to measure the differential sediment pore pressure between two ports spaced 100 cm apart. The dissipation of pore pressure transients that develop during rapid emplacement of the probe were extrapolated in time to estimate equilibrium hydraulic gradients. In addition, various data analysis techniques were evaluated for determining sediment hydraulic conductivity and specific storage through interpretation of the pore-pressure dissipation data. The probe was used to estimate groundwater seepage in the bottom sediments of the Hamilton Harbour, at the western end of Lake Ontario. Upward gradients were measured at nine locations within the harbor ranging from 0.010 to 0.425 and a downward gradient of -0.015 was recorded at one site along the harbor's eastern boundary. Hydraulic conductivities determined from pore-pressure dissipation over time ranged from 6.9¿10-9 to 4.8¿10-7 m/s. Specific storage values ranged from 0.08 to 0.19 m-1. Calculated average linear seepage velocities ranged from 4.3¿10-8 to -8.5¿10-9 m/s. The groundwater contribution to the harbor through the deeper, fine-grained sediments was estimated to be 9.1¿10-2 m3/s, or 2.9¿106 m3/yr. This represents approximately 1.0% of the harbor basin's total volume, 15% of precipitation's contribution, 1.2% of the contribution of surface inflows (excluding the Burlington ship canal) and 0.22% of the total surface outflow passing through the Burlington shipping canal, which connects the harbor to Lake Ontario.¿ 1997 American Geophysical Union