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 soils. The measured results, expressed as a function of depth or time, are called breakthrough curves (BTC). In this paper, BTC from limited samples are called SBTC, while ABTC are the BTC from the entire soft profile and MBTC are mass-flux-based BTC. Conventionally, assumptions have been that: (i) ABTC and MBTC are identical; and (ii) SBTC would accurately indicate MBTC when enough samples are collected. In soil with preferential flow paths, such assumptions may be invalid. The objectives of this study were to: (i) determine whether increasing the total number of samples would make SBTC an accurate indicator of contaminant loading; (ii) determine whether ABTC and MBTC are identical; and (iii) explore how to obtain representative MBTC with only a limited number of samples. Numerical simulations were conducted in three sets of four, two-dimensional hypothetical profiles with inclined layers that could trigger funnel-type preferential flow paths. Soil-solution samplers at four different spacings were placed at four depths in each profile. Simulated results indicate that ABTC always significantly underestimate MBTC. By increasing the total number of randomly located samples, SBTC will never accurately indicate MBTC. The SBTC could be modified to better estimate MBTC when preferential weight is assigned according to the local water flux.