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ARS Home » Research » Publications at this Location » Publication #63681
Title: SIMULATING IMPACT OF FUNNEL FLOW ON CONTAMINANT SAMPLING IN SANDY SOILS

Author
item JU, S.-H. - UNIV WISCONSIN
item KUNG, K.-J. - UNIV WISCONSIN
item Helling, Charles

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Sampling of soil water is useful for determining nutrient or pesticide transport into soil, and possible contamination of groundwater. Existing methods, however, may be invalid when preferential flow (unexpectedly rapid and deep downward movement of water and solutes, in isolated locations) occurs. The present research used numerical simulation in hypothetical sandy soil profiles, with conditions triggering funnel-type preferential flow, in order to determine how contaminant movement could be measured and described accurately. The results showed that currently used sampling and evaluation methods could underestimate substantially contaminant loading into groundwater; sampling methods can be improved greatly if water flow rate (flux) as well as contaminant concentration, at each sampling position, is included. This research will be used by research scientists, environmental engineers, and regulators concerned with measuring and predicting leaching of pesticides and other contaminants through soil and into groundwater.

Technical Abstract: Contaminant transport in unsaturated soils is currently one of the most important research topics in soil science. Soil-solution sampling methods developed more than a half-century ago are routinely used to monitor fate of contaminants. The measured results are called breakthrough curves (BTC). In this paper, BTC from limited samples are called SBTC, while ABTC are from a very large number of samples and MBTC are mass-flux based BTC. Conventionally, the assumptions have been that: (1) ABTC and MBTC are identical; and (2) SBTC would accurately indicate MBTC when enough samples are collected. In soil with preferential flow paths, validity of these assumptions has not been investigated. The objectives of this paper are to: (1) determine whether increasing the total number of samples would make SBTC an accurate indicator of contaminant loading; (2) determine whether ABTC and MBTC are identical; and (3) explore how to obtain representative MBTC with only a limited number of samples. Numerical simulations were conducted in three sets of four two-dimensional (2-D) hypothetical sandy 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 underestimates MBTC. Increasing the total number of samples at random locations does not guarantee that SBTC can accurately indicate MBTC. The SBTC could be modified to better estimate MBTC when preferential weight was assigned according to the local water flux. The SBTC from soil coring methods would always underestimate MBTC.