A COMPARISON OF METHODS TO DETERMINE K IN SHALLOW COASTAL AQUIFER

Five methods (tidal damping, tracer, slug, permeameter, and grain-size analysis) for the determination of hydraulic conductivity (K) were compared in both upland and shoreline regions of a sandy outwash aquifer adjacent to a shallow coastal embayment. The 7 m tracer test and tidal damping methods appeared to determine in situ K over sufficient horizontal scales to integrate the spatial variability of K within the aquifer, with the tracer test giving the most reproducible K estimates for the upland, 109 m/d. The tidal K measurements were similar to the tracer results, 106 m/d, integrating K over greater distances (30 to 85 m), but were highly variable due to the large aquifer storativity, small tidal amplitude, and irregular tides. The smaller scale methods yielded lower upland K estimates with the lowest value from permeameters, 29 m/d, only about one-third of the other four methods. Estimates of upland K by both grain-size and slug tests were comparable, 86 m/d and 71 m/d, respectively. However, the K derived from grain-size methods was dependent not only upon the sediment but on the method selected, with consistently higher values of K generated by the Hazen equation than the Krumbein and Monk equation. Verification by in situ tests may be useful in removing this bias. Significant differences in K were found between shoreline and upland regions, though the relative magnitude of the difference was not constant: among the four methods where comparative measurements were possible, shoreline K ranged from 42% to 75% of upland values. Slug tests yielded the largest differences, 30 m/d and 71 m/d, with permeameter, 18 m/d and 24 m/d, the lowest. The lower K of shoreline sediments appears to be the result of processes associated with coastal embayments and should be addressed in the measurement and modeling of coastal ground-water discharge.