Flow and transport in wetland deposits

Abstract

At present, very little is known about the mechanisms that control flow and transport through wetland deposits. Therefore, the objective of this research was to investigate the factors that influenced flow in wetland soils. To accomplish this, Column Experiments, run on a specialized permeameter, were conducted on both re-sedimented and undisturbed wetland soil specimens. During these experiments, sodium chloride (NaCl) tracer was injected into the soil specimen, and its breakthrough was monitored concurrently with other parameters, such as flow velocity and hydraulic gradient. Subsequently, the breakthrough data collected during the Column Experiments were fit using both the One-Region and Two-Region transport models, and the fit results were analyzed and compared to the geotechnical data collected for the soil. The data collected during the experimental program indicate enormous complexity in the mechanisms controlling flow and transport through wetland soils. From their analysis the following observations were made: First, even though wetland soils are considered to be two-region soils, having both an effective and an immobile porosity, the One-Region model was able to describe Sodium Chloride breakthrough in the soil. This indicates that the NaCl tracer was not interacting with the immobile region of the specimen. Second, the results demonstrated that wetland soil hydraulic conductivity is highly variable and sensitive to volume of flow. In fact, hydraulic conductivity was seen to decrease irreversibly by up to 6% per pore volume of flow. It was also found that hydraulic conductivity was sensitive to increases in pore water salt concentration, and to the flushing out of salts from wetland specimens. Finally, it was observed that, for the most part, large changes in hydraulic conductivity did not correspond to changes in the specimen's effective pore size or pore distribution. In fact, unless salt concentrations were increased drastically, the effective pore space remained invariant over an order of magnitude change in soil hydraulic conductivity. This suggests that changes in soil hydraulic conductivity might be due to increases or decreases in the number of flow channel constrictions in a specimen. From the results of this research it is hypothesized that the number of flow channel constrictions increased when flow and a decrease in salt concentration mobilized organic and mineral particles, which collected and clogged narrow pore throats along the flow channels. It is also hypothesized that the number of flow channel constrictions decreased when increases in pore water salt concentration causect organic fibers along the flow channel walls to coil.