Measurement of advective water velocity in lake sediment using natural helium gradients

Abstract

Natural helium (He) concentration profiles in the sediments of lakes at the Experimental Lakes Area in northwestern Ontario are modelled, using the advection-diffusion equation to calculate groundwater velocities. Two headwater lakes with little excess He in bottom water have sediment He concentration profiles indicating slow (2–7 cm a −1) recharge of lake water to groundwater. Data from three lakes showing He enrichment in bottom water indicate groundwater seepage at some sites. Sediment He gradients in headwater Lake 239 were usually weak, although a strong gradient was found at one site. Probes located within suspected seepage areas in Lake 239 indicated groundwater discharge velocities of 3–22 cm a −1. Fifth order Lake 240 had a He gradient suggesting groundwater seepage at 11–17 cm a −1. Eighth-order Lake 625, a lineament basin with high He concentrations in bottom water, had very strong He gradients in sediment. One probe indicated diffusive He transport through sediment, whereas another indicated seepage at 3–6 cm a −1. Direct He fluxes to the lakes, estimated from its accumulation in bottom water during summer stratification, are in reasonable agreement with fluxes calculated independently from the He gradients in sediment. Measurements of natural He gradients in lake sediment give a sensitive and reliable indication of the direction and rate of groundwater transport at velocities of 1–100 cm a −1.