Large‐scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 1. Experimental design and observed tracer movement

TLDR

A large‑scale natural gradient tracer experiment was conducted on Cape Cod, Massachusetts, to examine the transport and dispersion of solutes in a sand and gravel aquifer. Bromide, lithium, and molybdate tracers were pulsed into the aquifer in July 1985 and tracked in three dimensions over 280 m down‑gradient using multilevel samplers. Bromide migrated 0.42 m day⁻¹ horizontally, sank ~4 m, and after 200 m had spread >80 m downstream but remained narrow (≈14 m wide, 4–6 m thick); lithium and molybdate followed the same path but moved ~50 % slower because of sorption.

Abstract

A large‐scale natural gradient tracer experiment was conducted on Cape Cod, Massachusetts, to examine the transport and dispersion of solutes in a sand and gravel aquifer. The nonreactive tracer, bromide, and the reactive tracers, lithium and molybdate, were injected as a pulse in July 1985 and monitored in three dimensions as they moved as far as 280 m down‐gradient through an array of multilevel samplers. The bromide cloud moved horizontally at a rate of 0.42 m per day. It also moved downward about 4 m because of density‐induced sinking early in the test and accretion of areal recharge from precipitation. After 200 m of transport, the bromide cloud had spread more than 80 m in the direction of flow, but was only 14 m wide and 4–6 m thick. The lithium and molybdate clouds followed the same path as the bromide cloud, but their rates of movement were retarded about 50% relative to bromide movement because of sorption onto the sediments.