Macropores are subsurface connected void spaces caused by processes such as fracture of soils, micro‐erosion, and fauna burrows. They are common near streams (e.g. hyporheic and riparian zones) and may act as preferential flow paths between surface and groundwaters, affecting hydrologic and biogeochemical processes. We tested the hydrologic function of macropores by constructing an artificial macropore within the saturated zone of a meander bend (open macropore, ‘OM’) and later filling its upstream end (partially filled macropore, ‘PFM’). For each treatment, we injected saline tracer at an upgradient monitoring well within the meander and monitored downgradient hydraulics and tracer transport. Pressure transducers in monitoring wells indicated hydraulic gradients within the meander were 32% higher perpendicular to and 6% higher parallel to the macropore for the OM than for the PFM. Additionally, hydraulic conductivities measured via falling head tests were 29 to 550 times higher along the macropore than in nearby sediment. We used electrical conductivity probes in wells and electrical resistivity imaging to track solute transport. Transport velocities through the meander were on average 9 and 21% higher (per temporal moment analysis and observed tracer peak, respectively) for the OM than for the PFM. Furthermore, temporal moments of tracer breakthrough analysis indicated downgradient longitudinal dispersion and breakthrough tracer curve tailing were on average 234% and 182% higher for the OM, respectively. This suggests the OM enabled solute transport at overall shorter timescales than the matrix but also increased tailing. Our results demonstrate the importance of macropores to meander bend hydrology and solute transport. Copyright © 2012 John Wiley & Sons, Ltd.

Macropores and soil pipes in stream banks are common geomorphic features. Macropores and soil pipes that are open to the channel (i.e. "bank face-connected" macropores) are inundated when channel stage is elevated (e.g., from precipitation, snowmelt, dam release). However, previous studies have not investigated macropore impact on bi-directional water exchange between the channel and bank/riparian groundwater under variable hydrologic conditions. We monitored two transects of riparian groundwater wells perpendicular to the bank of a 2 nd order stream for a year: one with bank face-connected macropores (M transect) and one without bank face-connected macropores (NM transect). Fluctuations in water level and temperature during storms in those wells closest to the channel were on average 139% and 29% higher, respectively, in the presence of macropores. Rising head tests in the same wells indicated that hydraulic conductivity was 61 to 140 times higher in the presence of macropores. Bank storage, indicated by gradient reversals between channel and riparian zone, occurred on two temporal scales. Bank storage during storms was more frequent in the M transect (occurred all year) than in the NM transect (occurred just in winter and spring). Smaller magnitude gradient reversals at the M transect are consistent with faster head equilibration and greater exchange volume. Bank storage also occurred on an annual basis, with channel water entering storage during summer and fall and returning to the channel during winter and spring. Taken together, these results suggest that macropores act as preferential flow paths that enhance the connectivity between channels and riparian groundwater that enhance bank storage. Where bank macropores are present, conceptual models of hyporheic and groundwater flow should account for their effects.

In-stream structure-induced hyporheic exchange and associated thermal dynamics affect stream ecosystems. Their importance is controlled by spatial variability of sediment hydraulic conductivity (K). We calibrated a computational fluid dynamics (CFD) model of surface and groundwater hydraulics near a channel-spanning weir (represents log dams, boulder weirs) to field data and varied K from 10 27 to 10 The specific values of K where such trend shifts occur is likely variable among streams based on flow conditions, but we expect the presence of such trend shifts to be widespread.

Macropores are connected void spaces in the subsurface and can act as preferential flow paths for groundwater transport, but their dimensions and distribution patterns have not been well characterized in stream banks and hyporheic sediments. We ran field surveys in 5 streams of varying size, bed slope, and watershed land use in the Appalachian Mountains of southwestern Virginia. Natural surface-connected macropores were nearly ubiquitous. Macropores in this region arise by a variety of mechanisms, including soil piping, tree root decay, erosion around hardened structures, macroinvertebrate burrows, and invertebrate burrows. Macropore openings were slightly wider than tall, with median cross-sectional widths and heights of 3.5 cm and 3.0 cm, respectively. The median and maximum macropore lengths to first bend were 15.0 cm and 120.5 cm into the bank, respectively. True (tortuous) macropore lengths probably are often greater, and methods to map macropores are needed. Median interspacing of macropores across all streams was 0.38 m, but mean interspacing was 1.12 m, indicating that macropores were clustered in space. Macropores were inundated at different times because of differences in their heights on the stream bank, channel geometry, and stream stage. HEC-RAS modeling of channel hydraulics indicated that only 1 to 32% of macropores were inundated by channel water at base flow, whereas up to 97% were inundated during the largest storms. We ran conservative-tracer injection experiments at base flow in a 30-m reach of a small tributary to 1 study stream. We calculated transient-storage parameters for 2 treatments: 1 without macropores and 1 with artificially created macropores. Our results suggest that constructed macropores weakly increased transient storage (channel:storage zone cross-sectional area [A s /A] by 15.0% and fraction of median travel time spent in the transient-storage zone [F med ] by 31.0%) and may affect surface-water-groundwater interactions. Many macropore openings were above typical baseflow water levels, so macropores may enhance bank storage, a possibility that bears further research.

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