Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K s ) with depth, but differences between lateral K s of a concretion layer and the vertical K s of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low-flow (LF) seeps (flow rates typically less than 0·05 L min − − − − −1 ), persistent high-flow (HF) seeps (average flow rate of 0·39 L min − − − − −1 ) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than − − − − −5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min − − − − −1 and sediment concentrations commonly approached 100 g L − − − − −1 . Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min − − − − −1 ) and sediment concentrations (989 g L − − − − −1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant.

Recent research indicates immediate breakthrough of surface‐applied contaminants in subsurface drainage by transport through macropores directly connected to the surface. This “direct connectivity” phenomenon was verified and investigated by conducting infiltration experiments (1‐cm ponded water at the soil surface) in a laboratory soil column (sandy loam soil with bulk density of 1.6 g cm−3) with a vertical artificial macropore placed directly above or shifted away from a lateral subsurface drain. The experimental setup allowed surface‐connected and buried macropore lengths to be varied from the surface to the subsurface drain depth without unpacking or disturbing the soil column between experiments. It was observed that the longer the buried macropore length (i.e., as the macropore approached the soil surface), the more rapid the response at the drain outlet in addition to an increased percentage of total drain flow through the macropore (35–40%). Breakthrough with surface‐connected macropores was significantly faster than with buried macropores, suggesting that breaking surface connectivity of macropores by tillage may be an important management strategy. For surface‐connected macropore experiments, the average ratio of steady‐state total (macropore and matrix) to matrix flow rates decreased as the distance from the drain increased: 2.4, 2.1, and 1.6 for distances of 0, 6.25, and 12.5 cm, respectively. Extrapolating this data to distances beyond 12.5 cm suggested that macropores located within 20 to 25 cm of the drain act as though directly connected in this sandy loam soil. This research verifies the “contributing area” concept hypothesized in previous field and numerical modeling studies.

Macropores, such as those created by deep‐burrowing earthworms, have the potential to be hydraulically connected not only to the soil surface but also to subsurface drains. This hydraulic connection may lead to rapid movement of surface‐applied chemicals to receiving waters as they bypass the bulk of the soil matrix. In this study, a numerical model (HYDRUS) that solves the three‐dimensional Richards equation for both matrix and macropore domains was used to analyze previously conducted experiments that contained a single, surface‐connected or buried, artificial macropore and a subsurface drain installed in a laboratory soil column. Both matrix and macropore domains were parameterized using continuous soil hydraulic functions. Simulations confirmed that surface‐connected macropores were highly efficient preferential flow paths that substantially reduced arrival times to the subsurface drainage outlet, with this reduction being directly related to the length of the macropore. Surface‐connected macropores need to extend at least halfway to the drain to have a noticeable effect (>50% reduction) on the arrival time. No significant changes were observed in total drain outflows for columns with laterally shifted macropores (away from a drain) compared with centered macropores unless the macropore depth extended significantly (>75%) into the profile. The model predicted that buried macropores became active and contributed to the total outflow only when pressure heads in the soil profile became positive. The effect of buried macropores on drain flow was investigated for a case where an initially partially saturated profile was drained. Under these conditions, the numerical simulations suggested that buried macropores could contribute up to 40% of the total outflow, which confirms laboratory observations with subsurface‐drained soil columns with macropores.

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