SIMULATING IMPACT OF FUNNEL FLOW ON CONTAMINANT SAMPLING

Soil-solution sampling methods developed more than 50 yr ago are still routinely used to monitor the fate of contaminants in unsaturated soi ls. The measured results, expressed as a function of depth or time, ar e called breakthrough curves (BTC). In this paper, ETC from limited sa mples are called SBTC, while ABTC are the ETC from the entire soil pro file and MBTC are mass-flux-based BTC. Conventionally, assumptions hav e been that: (i) ABTC and MBTC are identical; and (ii) SBTC would accu rately indicate MBTC when enough samples are collected. In soil with p referential flow paths, such assumptions may be invalid. The objective s of this study were to: (i) determine whether increasing the total nu mber of samples would make SBTC an accurate indicator of contaminant l oading; (ii) determine whether ABTC and MBTC are identical; and (iii) explore how to obtain representative MBTC with only a limited number o f samples, Numerical simulations were conducted in three sets of four, two-dimensional hypothetical profiles with inclined layers that could trigger funnel-type preferential Bow paths. Soil-solution samplers at four different spacings were placed at four depths in each profile. S imulated results indicate that ABTC always significantly underestimate MBTC. By increasing the total number of randomly located samples, SBT C will never accurately indicate MBTC. The SBTC could be modified to b etter estimate MBTC when preferential weight is assigned according to the local water flux.