Groundwater Transport of a Salt Tracer Through a Sandy Lakebed

In an investigation of groundwater advection and dispersion in a lakebed, a vertical zone of salt solution was injected between 1.7 and 3 m beneath the shoreline and entered the lake through a large area of lakebed at a reduced concentration. Migration of the tracer pulse and flux to the lake were monitored over a 7- × 8-m area of lakebed by measuring the electrical conductance of water collected from sampling points beneath the lakebed and from seepage meters on the lakebed. The tracer moved horizontally, curved gently upward, and entered the lake through an area of 17 m2-an area 5.7 times larger than the initial cross-sectional area of the tracer zone. Seepage flux through the sediment-water interface declined exponentially with offshore distance and averaged 240 m3· yr-1 per meter of shoreline. Results showed that prediction of solute flux from onshore zones of groundwater contamination requires consideration of dispersion (mixing) and the ratio of horizontal to vertical permeability (Kn:Kv). Numerical simulations showed that with increasing Kn:Kv, tracer-or contaminant-entry areas on the lakebed extend over larger areas and are displaced farther offshore. A Kn:Kvof 10 matched the field observations. An analytical model indicated that the observed reduction in peak concentration (a decline to 31% of the initial value over a 6-m flow path) would result from values of longitudinal dispersivity as small as 1 cm.