Rapid Flowpaths Research Articles

Comparison of unsaturated flow and solute transport through waste rock at two experimental scales using temporal moments and numerical modeling

This study analyzed and compared unsaturated flow response and tracer breakthrough curves from a 10-m high constructed pile experiment (CPE) in the field (Antamina, Peru) and two 0.8m high laboratory-based columns. Similar materials were used at both experimental scales, with the exception of a narrower grain size distribution range for the smaller column tests. Observed results indicate that flow and solute transport regimes between experimental scales were comparable and dominated by flow and solute migration through granular matrix materials. These results are supported by analogous breakthrough curves (normalized to cross-sectional area and flow path length) that suggest observation- or smaller-scale heterogeneities within the porous media have been homogenized or smoothed at the transport-scale, long breakthrough tails, and similar recovered tracer mass fractions (i.e., 0.72–0.80) at the end of the experiment. CPE breakthrough curves do indicate a portion of the fluid flow follows rapid flow paths (open void or film flow); however, this portion accounts for a minor (i.e., ~0.1%) component of the overall flow and transport regime. Flow-corrected temporal moment analysis was used to estimate flow and transport parameter values; however large temporal variations in flow indicate that this method is better suited for conceptualizing transport regimes. In addition, a dual-porosity mobile–immobile (MIM), rate-limited mass-transfer approach was able to simulate tracer breakthrough and the dominant transport regimes from both scales. Dispersivity values used in model simulations reflect a scale-dependency, whereby column values were approximately 2× smaller than those values applied in CPE simulations. The mass-transfer coefficient, for solute transport between mobile and immobile regions, was considered as a model calibration factor. Column experiments are characterized by a larger “mobile to immobile” porosity ratio and a shorter experimental duration and flow path, which supports larger mass-transfer coefficient values (relative to the CPE). These results demonstrate that laboratory-based experiments may be able to mimic flow regimes observed in the field; however, the requirement of scale-dependent dispersivities and mass-transfer coefficients indicates that these tests may be more limited in understanding larger-scale solute transport between regions of different mobility. Nevertheless, the results of this study suggest that the reasonably simplistic modeling approaches utilized in this study may be applied at other field sites to estimate parameters and conceptualize dominant transport processes through highly heterogeneous, unsaturated material.

First-order catchment mass balance during the wet season in the Panama Canal Watershed

Summary Tropical hydrology is poorly understood for a number of reasons. Intense biological activity in the tropics introduces complexities to the hydrologic process. Bioturbation, rapid rates of decay, and intensive insect activity all tend to promote rapid flow paths in the upper soil. Aggressive weathering leads to clays depleted of light cations and deep soil profiles. Processes in the seasonal tropics are further complicated by seasonal transitions, and very large changes in catchment storage between seasons. Beginning in 2005, we installed a suite of hydrologic sensors in a 16.7 ha first-order catchment in the Panama Canal Watershed to observe hydrologic variables and identify the dominant streamflow generation processes. The site is located near the village of Gamboa, which is located on the east bank of the Panama Canal at the confluence of Lake Gatun and the Chagres River. The study catchment is located on the north side of a ridge off the eastern flank of a 230 m tall hill known as Cerro Pelado, and is covered by 70–120 year old re-growth triple-canopy forest. Measurements included: rainfall above the canopy, throughfall, stemflow, evapotranspiration, shallow groundwater levels and streamflow. Deep groundwater storage was not measured. This paper describes measurements made, data collected, and the worth of those data in estimating the mass balance closure of a first-order catchment during the wet season. We compare measurements of the different components of the water cycle with observations from other published studies from the tropics. Data analysis results indicate water balance closure errors of approximately 8%.

Hydrologic flowpaths influence inorganic and organic nutrient leaching in a forest soil

Hydrologic pathways through soil affect element leaching by determining the relative importance of biogeochemical processes such as sorption and decomposition. We used stable hydrogen isotopes of water (δD) to examine the influence of flowpaths on soil solution chemistry in a mature spruce–hemlock forest in coastal Oregon, USA. Soil solutions (50 cm depth, n = 13) were collected monthly for 1 year and analyzed for δD, major ions and dissolved organic carbon (DOC) and nitrogen (DON). We propose that the variability of δD can be used as an index of flowpath length and contact time. Throughfall variability in δD was much greater than soil solution variability, illustrating that soil solution integrates the variation in inputs. Lysimeters with greater variation in δD presumably have a greater proportion of flow through rapid flowpaths such as macropores. The variation in soil solution δD for individual lysimeters explained up to 53% of the variation in soil solution chemistry, and suggests that flowpaths influence leaching of some constituents. Soil solutions from lysimeters with greater δD variation had higher DOC and DON (r 2 = 0.51 and 0.37, respectively), perhaps because transport via macropores reduces interaction of DOM with the soil matrix. In contrast, nitrate concentrations were highest in lysimeters with a small variation in δD, where long contact time and low DOC concentrations may yield higher net nitrification. Our results demonstrate the utility of stable isotopes to link flowpaths and soil solution chemistry, and illustrate how the spatial complexity of soils can influence ecosystem-level nutrient losses.

The influence of grazing on vegetation, soil properties and stream discharge in a small Dartmoor catchment, southwest England, UK

AbstractIntensive grazing has been responsible for environmental degradation of moorland landscapes over the last 50 years. Anecdotal evidence on Dartmoor, UK, for example, has suggested that it is particularly associated with a greater frequency of large floods, lower baseflow and a greater occurrence of erosion. A study was therefore conducted at Holne Moor, a small catchment on east Dartmoor, to investigate the impacts of grazing animals on hillslope hydrology and stream discharge. The vegetation in the study area ranged from heather (Calluna vulgaris), in areas with low grazing intensity, to short grass with occasional erosion scars visible associated with larger sheep numbers. The physical characteristics of the topsoil beneath heather species were very different compared with short grass. This was not due so much to the direct physical impact of grazing animals, but indirectly to the chemical and biological differences brought about by changes in vegetation response and soil structure. The experiment at Holne Moor indicated that soil water content on the hillslope was linked directly to the stream's rainfall–runoff response via the soil water pathways. In relatively dry conditions, the soil water content is mainly determined by local factors such as vegetation. In wet periods, non‐local influences such as slope and contributing area are more important. A soil water content threshold of around field capacity separated the two conditions. A comparison between soil physical properties and soil moisture under different vegetation types indicated that this ‘runoff’ threshold between dry and wet states is lower in heavily grazed areas. It is argued that more intense grazing causes conditions suited to the increased delivery of soil water to rapid flowpaths such as sheep tracks and other rapid routes which connect the source of this hillslope water to the contributing areas during large floods. Any alteration of vegetation and soil properties due to management causes a substantial increase in ‘active’ source areas and therefore discharge during large storm events even before any erosion occurs. Copyright © 2006 John Wiley & Sons, Ltd.

The Role of Rapid Flow Paths for Nitrogen Transformation in a Forest Soil A Field Study with Micro Suction Cups

Preferential flow is a common phenomenon in soils. This study was conducted to investigate the significance of rapid flow paths for N transformation in a forested Humaquept in central Switzerland. Fifty micro suction cups, each with a surface area of 12 mm2, were installed in a regular grid in the uppermost 5 cm. First, the location of each “microcup” relative to main flow paths was estimated based on the response to applications of a dye, SO2−4, and Cl− Then, a N‐addition experiment was carried out to study the N transformation at locations along flow paths and within the soil matrix. Only 23 of 50 microcups responded to the application of the dye within the first 24 h, which indicates that a large portion of the soil volume is not in contact with the infiltrating rainwater. Those microcups which responded to the added dye were regarded to be located along flow paths. At depths below 2 cm, under temporarily reducing conditions, sampling locations in or near flow paths had higher NO−3 concentrations (20–25 μM) than those of the soil matrix (below 12 μM). Within 24 h after a simulated rainfall, the ratio decreased more in the flow paths (between −2.4 and −4.9 mol mol−1) than in the soil matrix (−0.7 to −0.8 mol mol−1), which indicates an enhanced denitrification at these locations. In the subsequent dry period, nitrification started 2 d earlier and was more pronounced along flow paths. The results of this study suggest that flow paths are microhabitats with an increased N transformation compared with the soil matrix.

The influence of coatings and fills on flow in fractured, unsaturated tuff porous media systems

A numerical study of a single fracture embedded in a porous matrix was performed to investigate the role of fracture coatings and fills on water movement in permeable, fractured porous media. The variables considered were conductivity and continuity of fracture coatings; location, length, and conductivity of fracture fills; combinations of fills and coatings; initial matrix saturation; and inflow boundary conditions. Results from the simulations indicate that in low‐saturation, high‐capillarity tuff systems, the conditions under which fractures act as rapid flow paths are limited. These conditions include a continuous coating with conductivity several orders of magnitude lower than that of the neighboring matrix, and large inflow rates. However, as initial matrix saturation increases, the amount of fracture flow also increases. Discontinuities in coatings substantially reduce their effectiveness in preventing matrix imbibition. The presence of any coating, however, does produce increased infiltration depths. Fills appear to be effective barriers to fracture flow.

Migration of dissolved petroleum hydrocarbons, MTBE and chlorinated solvents in a karstified limestone aquifer, Stamford, UK

Abstract Two incidents of hydrocarbon contamination to the Lincolnshire Limestone in east Stamford, UK, have been investigated. No evidence of LNAPL contamination of groundwater was observed, suggesting that the spills may largely have been retained in the unsaturated zone. Some groundwater contamination by dissolved hydrocarbons occurred, apparently especially at times of high recharge. Rapid flow paths were proven to nearby springs in the River Welland (with groundwater flow velocities of up to 240 m day −1 ), and dissolved hydrocarbon and MTBE contamination appears to have been flushed rapidly from these systems. MTBE contamination at Tallington Pumping Station (5 km east of the site) is not clearly linked to these incidents. Of potentially more concern was the discovery of dissolved chlorinated solvent contamination in the groundwater at the spill sites, possibly related to a landfilled quarry and/or a nearby engineering works. No direct evidence of DNAPL was observed. A conceptual model of solvent distribution suggests independent sources of TCE, PCE and TCA.

The character and causes of a pronounced snowmelt-induced ‘acidic episode’ in a stream in a Scottish subarctic catchment

Specific objectives were: (1) to characterize the major ‘episode’ of the year in a catchment in a region which is regarded as being sensitive to pollutant deposition; (2) to examine downstream compositional gradients and to explain them in terms of downstream changes in flowpath regime and differences in soil depth or type; (3) to test the hypothesis that both direct pollution inputs to the stream and the sea-salt effect are reflected in streamwater composition. Streamwater was collected, during two distinct snowcover melt events at two points, 400 m apart, with high time resolution. Snowcover and soilwater samples were also collected. There were distinct differences in the response of the stream at the two sites to the melt events; this was a consequence of spatially variable snowmelt regimes as well as of differences in soil and sub-snowcover hydrology. There were a number of unusual features apparent. There was no evidence of the almost ubiquitous base cation dilution associated with acidic episodes. There was also a most pronounced direct effect of pollution input at the uppermost stream sampling site because of the accumulation of pollution on top of a deep snowcover, the melt from which followed rapid flowpaths to the stream. The direct effect was not apparent at the lower sampling site, although evidence for the sea-salt mechanism was very clear, unlike the case for the top site. The evidence for an ion exchange control on streamwater Al 3+ was strong at the lower site, but weak at the top site. The observations are interpreted via a schematic flowpath model.