Shallow Subsurface Flow Research Articles

Triggering threshold precipitation and soil hydrological characteristics of shallow landslides in granitic landscapes

Sierra de Gredos, a mountain range in Central Spain, is prone to the occurrence of shallow landslides, which can transform into sediment laden flows as demonstrated by geomorphologic evidence. The analyses of these complex processes require reliable design tools for predicting thresholds of triggering precipitation in order to assess the susceptibility characteristics. In the Sierra de Gredos there is a lack of knowledge concerning how weathering mantle properties vary spatially, and what effects these spatial patterns have on infiltration, surface runoff and shallow subsurface flow. In this context, this paper is focused on the hydrogeomorphic characterization of a shallow landslide that occurred on December 17, 1997. The Generalized Quasi-Dynamic Model (GQDM) was implemented to approach the critical rainfall intensity set for different durations and to establish the intensity–duration (ID) threshold. To this purpose, field and laboratory methods, such as: Seismic Refraction Tomography (SRT), Time Domain Reflectometry (TDR), and geotechnical tests were applied. The use of these field investigations and their combination with the GQDM have allowed us to provide a detailed description of the behavior of the catchment, and reproduce the pattern of instability generated by an intense storm which triggered the 1997 shallow landslide. Furthermore, this threshold can be used as a predictive tool for hazard prevention.

Streamflow generation in a large, alpine watershed in the southern Rocky Mountains of Colorado: Is streamflow generation simply the aggregation of hillslope runoff responses?

Spatial and temporal trends in stream chemistry were investigated in a large (1600 km2) alpine watershed in the southern Rocky Mountains of Colorado to help understand mechanisms of streamflow generation. We observed linear increases of concentrations of chemical constituents in streamflow as accumulated drainage area increased along the main channel of Saguache Creek. We tested two conceptual models of streamflow generation against our stream chemistry observations. One model is essentially two‐dimensional and treats streamflow generation at the large watershed scale as the aggregation of runoff responses from individual hillslopes, primarily surface and shallow subsurface flow paths. Alternatively, a fully three‐dimensional conceptual model treats streamflow generation as being controlled by a distribution of large‐scale groundwater flow paths as well as surface and shallow subsurface flow paths. The structure and magnitude of groundwater contributions in streamflow as a function of increasing scale provided a key distinction between these two conceptual models. End‐member mixing analysis and measurements of hydraulic head gradients in streambeds were used to quantify basin‐scale groundwater contributions to streamflow with increasing spatial scale in the Saguache Creek watershed. Our data show that groundwater contributions are important in streamflow generation at all scales and, more importantly, that groundwater contributions to streamflow do increase with increasing watershed scale. These results favor the three‐dimensional conceptual model in which long groundwater flow paths provide a streamflow generation process at large scales that is not operative at smaller scales. This finding indicates that large watersheds may be more than simply the aggregation of hillslopes and small catchments.

Solute behaviour and export rates in neotropical montane catchments under different land-uses

Abstract:To improve our knowledge of the influence of land-use on solute behaviour and export rates in neotropical montane catchments we investigated total organic carbon (TOC), Ca, Mg, Na, K, NO3and SO4concentrations during April 2007–May 2008 at different flow conditions and over time in six forested and pasture-dominated headwaters (0.7–76 km2) in Ecuador. NO3and SO4concentrations decreased during the study period, with a continual decrease in NO3and an abrupt decrease in February 2008 for SO4. We attribute this to changing weather regimes connected to a weakening La Niña event. Stream Na concentration decreased in all catchments, and Mg and Ca concentration decreased in all but the forested catchments during storm flow. Under all land-uses TOC increased at high flows. The differences in solute behaviour during storm flow might be attributed to largely shallow subsurface and surface flow paths in pasture streams on the one hand, and a predominant origin of storm flow from the organic layer in the forested streams on the other hand. Nutrient export rates in the forested streams were comparable to the values found in literature for tropical streams. They amounted to 6–8 kg ha−1y−1for Ca, 7–8 kg ha−1y−1for K, 4–5 kg ha−1y−1for Mg, 11–14 kg ha−1y−1for Na, 19–22 kg ha−1y−1for NO3(i.e. 4.3–5.0 kg ha−1y−1NO3-N) and 17 kg ha−1y−1for SO4. Our data contradict the assumption that nutrient export increases with the loss of forest cover. For NO3we observed a positive correlation of export value and percentage forest cover.

Contaminant removal efficiency depending on primary treatment and operational strategy in horizontal subsurface flow treatment wetlands

This study aimed to evaluate the contaminant removal efficiency of shallow horizontal subsurface flow treatment wetlands (SSF TWs) as a function of (1) primary treatment (hydrolytic upflow sludge blanket (HUSB) reactor vs. conventional settling) and (2) operation strategy (alternation of saturated/unsaturated phases vs. permanently saturated). An experimental plant was constructed, operated and surveyed for the main water quality parameters over a period of 2.5 years. The plant had 3 treatment lines: a control line (settler-wetland permanently saturated), a batch line (settler-wetland operated with saturated/unsaturated phases) and an anaerobic line (HUSB reactor-wetland permanently saturated). In each line wetlands had a surface area of 2.80m2, a water depth of 25cm and a granular medium D60=7.3mm, and were planted with common reed. During the study period the wetlands were operated at a hydraulic and organic load of 28.5mm/d and about 4.7g BOD/m2d, respectively. Effluent average redox potential was lower for the anaerobic line (−45±78mV) than for the other two lines (3±92.7 and −5±71mV for control and batch, respectively). Overall, chemical oxygen demand (COD), biochemical oxygen demand (BOD5) and ammonium mass removal efficiencies were slightly greater for the batch line (88%, 96% and 87%, respectively) than for the control line (83%, 94% and 80%) and the anaerobic line (80%, 87% and 73%). During cold seasons, COD and ammonium removal in the batch line was around 30% and 50% higher than in the control line, respectively. The results of this study indicate that the implementation of a HUSB reactor as primary treatment did not enhance the treatment capacity of the system (in comparison with a conventional settler). The efficiency of treatment wetland systems with horizontal subsurface flow can be improved using a batch operation strategy.

Agricultural Herbicide Transport in a First-Order Intermittent Stream, Nebraska, USA

The behavior of herbicides in surface waters is a function of many variables, including scale of the watershed, physical and chemical properties of the herbicide, physical and chemical properties of the soil, rainfall intensity, and time of year. In this study, the transport of 6 herbicides and 12 herbicide degradates was examined during the 2004 growing season in an intermediate-scale agricultural watershed (146 ha) that is drained by a first-order intermittent stream, and the mass load for each herbicide in the stream was estimated. The herbicide load during the first week of storm events after application ranged from 17% of annual load for trifluralin to 84% of annual load for acetochlor. The maximum weekly herbicide load in the stream was generally within the first 3 weeks after application for those compounds that were applied within the watershed during 2004, and later for herbicides not applied within the watershed during 2004 but still detected in the stream. The apparent dominant mode of herbicide transport in the stream--determined by analysis amongst herbicide and conservative ion concentrations at different points in the hydrograph and in base flow samples--was either overland runoff or shallow subsurface flow, depending on the elapsed time after application and type of herbicide. The load as a percentage of use (LAPU) for the parent compounds in this study was similar to literature values for those compounds applied by the farmer within the watershed, but smaller for those herbicides that had rainfall as their only source within the watershed.

Translation of field tracer-test results into bounding predictions of matrix diffusion in the shallow subsurface at Idaho National Laboratory, USA

Matrix-diffusion parameters deduced from an infiltration tracer test at Idaho National Laboratory (INL), USA, are combined with other site information in an analysis involving two dimensionless lumped parameters to assess the effects of matrix diffusion on contaminant transport at the INL over longer distance and time scales than were evaluated in the test. Matrix diffusion was interrogated in the test by comparing, in three different observation wells, the breakthrough curves of two simultaneously injected nonsorbing solutes that have different diffusion coefficients. The matrix-diffusion parameters deduced from the different breakthrough curves were in good agreement, suggesting that the parameters may be broadly applicable at the INL. With this in mind, the uncertainties in the individual parameters that make up the two lumped parameters were estimated, and the resulting ranges of parameter values were used to assess matrix diffusion over larger scales. Assessments of the effects of flow transients, spatial heterogeneity in transport parameters, and sorption on solute transport in the shallow subsurface flow system were also conducted. The methods presented here should be generally applicable to other settings for making bounding assessments of the effects of matrix diffusion while honoring the information obtained from tracer tests and other supporting data.

Sand‐rich injectites in the context of short‐lived and long‐lived fluid flow

AbstractSand‐rich injectites are a common attribute of clastic sedimentary successions, and they have received increased attention in the last decade by geoscientists and engineers dealing with shallow subsurface flow and retention of aqueous and hydrocarbon fluids. Injectites form due to fluidized flow of sediment‐entraining fluids through high‐permeability strata. Given that sediment entrainment is confined to high‐permeability sediments with low horizontal effective stress and negligible cementation, the formation of sand‐rich injectites is restricted to the first kilometre of burial, where sand‐rich sediments are prone to fluidization and clay‐rich sediments are generally more cohesive and may hydraulically fracture to allow the creation of injectites. When reviewing conditions that may lead to aqueous and hydrocarbon flow within this shallow section at velocities that may cause sand fluidization and injection, we can rule out geologic time‐scale processes (disequilibrium compaction, hydrocarbon migration, and lateral pressure transfer) as plausible causes. This emphasizes the need for high‐strain‐rate processes to cause rapid fluid overpressuring and flow associated with sand injection. Earthquake‐induced shaking, instantaneous loading and release of overpressured fluids along moving fault planes are the most likely causes of sand remobilization and injection and these processes may enforce each other during tectonic stress‐release events. An additional mechanism that may trigger sand‐rich injections is the mechanical failure of shallow oil accumulations, particularly as the fluidization velocity of sand entrained in oil can be several orders of magnitude lower than for sand entrained in aqueous fluids, particularly if the oil is biodegraded (and thus has a high dynamic viscosity). Flow of hydrocarbon gas is unlikely to cause sand injection, although gas dissolved in upward‐flowing aqueous fluids may evolve at or near the surface as pressures in the aqueous phase drop.

Development and application of a catchment similarity index for subsurface flow

In this study, we develop a similarity parameter to describe shallow subsurface hydrological response of small catchments on the basis of the hillslope‐Péclet number. This new similarity parameter, named the catchment‐Péclet (caPe) number, provides a theoretical framework to compare the relative hydrologic response derived from shallow subsurface flow of small catchments on the basis of geomorphic properties. Using 400,000 synthetically derived catchments to model catchment‐scale characteristic response functions (CRFs), we see good agreement between the synthetic and theoretical relationships relating the caPe number to the first two dimensionless moments of the CRF of small catchments. Working with real‐world data, however, requires the estimation of hydrologic parameters and delineation of hillslopes to apply the caPe number. Allowing for uncertainty in the estimation of hydrologic parameters and in the definition of the extent of the channel network, the caPe number is able to recreate the observed moments of an approximate catchment‐scale CRF for four small catchments ranging in size from 0.025 to 880 ha in two distinct climatic and geologic settings. By using physics to underpin the link between landscape and hydrological response, the caPe number creates a functional relationship between hydraulic theory and a catchment's pedogeomorphological structure. While this study is limited to small headwater catchments, it lays the groundwork for a catchment‐scale similarity parameter that could be expanded to larger scales where channel network structure and storage become more important.

Hydrological processes in the subsurface investigated by water isotopes and silica

 The hillslope rainfall-outflow interactions, groundwater fluxes, and hydrological balance were examined in the small mountainous headwater catchment Uhlířská, the Jizera Mountains, the Czech Republic. The hillslope soil profile is formed by paleozoic crystalline bedrock overlaid by shallow highly permeable shallow Cambisol, and by thick saturated glacial deposits in the valley, overlaid by Histosol. A quick communication of the vadose zone with the granitic bedrock via preferential subsurface flowpaths is hypothesised, in agreement with the observation of storm-caused instant water transformation to outflow through the permeable Cambisol. A quick response of a high magnitude outflow occurs regularly, although the surface runoff is very rare. Standard climatic and hydrological monitoring in the Uhlířská catchment is supplemented by the measurements of the soil moisture, soil pore water suction, subsurface hillslope stormflow in the vadose zone, and water table fluctuation in the saturated subsurface, and is accompanied by water sampling for the analyses of the contents of the isotope <SUP>18</SUP>O and <SUP>2</SUP>H and geochemical tracer silica in the form of SiO<SUB>2</SUB>. The episode based isotopic data serve for the separation of the outflow hydrograph to determine the contributions of the event and pre-event water in the hypodermic hillslope outflow and in the catchment outflow. The variation of silica content in the water cycle components was examined to assess the contributions from the soil profile and the aquifer. Up to 75% of the event catchment runoff was assigned to pre-event water, of which about 50% had been stored in the shallow soil subsurface on the hillslopes. The hypothesis was confirmed that the hillslope soil layers control the distribution of the flow into the groundwater recharge and/or the shallow subsurface flow during the rainfallrunoff episode.

Characterizing Pb Mobilization from Upland Soils to Streams Using 206Pb/207Pb Isotopic Ratios

Anthropogenically deposited lead (Pb) binds efficiently to soil organic matter, which can be mobilized through hydrologically mediated mechanisms, with implications for ecological and potable quality of receiving waters. Lead isotopic ((206)Pb/(207)Pb) ratios change down peat profiles as a consequence of long-term temporal variation in depositional sources, each with distinctive isotopic signatures. This study characterizes differential Pb transport mechanisms from deposition to streams at two small catchments with contrasting soil types in upland Wales, U.K., by determining Pb concentrations and (206)Pb/(207)Pb ratios from soil core profiles, interstitial pore waters, and stream water. Hydrological characteristics of soils are instrumental in determining the location in soil profiles of exported Pb and hence concentration and (206)Pb/(207)Pb ratios in surface waters. The highest Pb concentrations from near-surface soils are mobilized, concomitant with high dissolved organic carbon (DOC) exports, from hydrologically responsive peat soils with preferential shallow subsurface flows, leading to increased Pb concentrations in stream water and isotopic signatures more closely resembling recently deposited Pb. In more minerogenic soils, percolation of water allows Pb, bound to DOC, to be retained in mineral horizons and combined with other groundwater sources, resulting in Pb being transported from throughout the profile with a more geogenic isotopic signature. This study shows that (206)Pb/(207)Pb ratios can enhance our understanding of the provenances and transport mechanisms of Pb and potentially organic matter within upland soils.

Hydropedological processes and their implications for nitrogen availability to corn

Hydropedological processes affect soil water and nutrient transport and cycling. This study evaluated the impact of hydropedological properties on soil N availability and corn ( Zea mays L.) growth in three areas within the same field representing distinguishing but typical mid-Atlantic (USA) landforms. These areas included: a depressional area (Site A), a steep (14%) slope area (Site B), and a flood plain area with 1% slope (Site C); different soil types (Hagerstown, Opequon, and Melvin series, respectively); and varying hydrological features (soil water content, matric potential, and subsurface flow) in a Ridge and Valley agricultural landscape. A small-plot replicated study was conducted in each of these three areas, including four blocks of two N treatments (NH 4NO 3) applied to corn at planting (0 and 150 kg N ha − 1 ). Soil and above-ground plant biomass samples were collected during the growing season and grain yield was determined at harvest. Site A had the greatest mean grain yield (fertilized: 8.5 Mg ha − 1 ; control: 4.5 Mg ha − 1 ) and above-ground plant biomass (at physiological maturity or PM, fertilized: 11.7 Mg ha − 1 ; control: 7.1 Mg ha − 1 ), probably due to the fine-textured (> 19% clay) and low saturated hydraulic conductivity (Ksat < 0.3 cm min − 1 ) soil with good water and N holding capacity. In contrast, Site B had the smallest grain yield (fertilized: 6.9 Mg ha − 1 ; control: 3.5 Mg ha − 1 ), which corresponded to the smallest inorganic soil N content due to its thin Ap1 (10 cm) and Ap2 (21 cm) horizons, greater Ap1 horizon Ksat (1.61 cm min − 1 ), and shallow lateral subsurface flow at the steep soil–bedrock interface (0.4 m below the surface). Despite the greater inorganic soil N content at Site C, the above-ground plant biomass (at PM, fertilized: 7.0 Mg ha − 1 ; control: 6.2 Mg ha − 1 ) and grain yield (fertilized: 7.1 Mg ha − 1 ; control: 4.5 Mg ha − 1 ) were less than observed at Site A and probably resulted from the low soil water content during the growing season, which also corresponded to greater solar radiation (564 kw h m − 2 ), coarser texture (> 20% sand), and deep subsurface lateral convergent flow (0.7 m below the surface). Understanding these types of relationships between corn response to N fertilizer and hydropedological features will improve N management decisions for corn production.

Land use and topography as predictors of nutrient levels in a tropical catchment

The influence of landscape on nutrient concentration and yield was analyzed in a tropical catchment, the Guare River in northern Venezuela. Spatial and temporal variation in nitrate, SRP and total P were determined in 15 sites located along the river mainstem and tributaries. Higher nitrate concentrations and yields were reported from upper sites and both decreased in the downstream direction along the river mainstem. These trends appear to be related to more pronounced slopes and larger proportions of agricultural and forest lands in subcatchments located in the upper part of the basin, and dense algal mats in the lower reaches. Nitrate values were higher during periods of high discharge, suggesting that nitrate is primarily transported by shallow subsurface flow. SRP represented between 60 and 80% of total P. Phosphorus concentrations were homogeneous along the river mainstem and showed little seasonal variation, while yields were higher in the upper basin. Multiple regression identified slope, runoff and agriculture as primary predictors of nitrate and phosphorus across the watershed, which suggests that both natural and anthropogenic landscape characteristics have a strong influence on nutrient levels in the Guare catchment.

Dynamics of stream nitrate sources and flow pathways during stormflows on urban, forest and agricultural watersheds in central Pennsylvania, USA

AbstractUnderstanding the influence of storm events on nitrate (NO3−) dynamics is important for efficiently managing NO3− pollution. In this study, five sites representing a downstream progression of forested uplands underlain by resistant sandstone to karst lowlands with agricultural, urban and mixed land‐use were established in Spring Creek, a 201 km2 mixed land‐use watershed in central Pennsylvania, USA. At each site, stream water was monitored during six storm events in 2005 to assess changes in stable isotopes of NO3− (δ15N‐NO3− and δ18O‐NO3−) and water (δ18O‐H2O) from baseflow to peakflow. Peakflow fractions of event NO3− and event water were then computed using two‐component mixing models to elucidate NO3− flow pathway differences among the five sites. For the forested upland site, storm size appeared to affect NO3− sources and flow pathways. During small storms (&lt;35 mm rainfall), greater event NO3− fractions than event water fractions indicated the prevalence of atmospheric NO3− source contributions at peakflow. During larger storms (&gt;35 mm rainfall), event NO3− fractions were less than event water fractions at peakflow suggesting that NO3− was flushed from stored sources via shallow subsurface flow pathways. For the urbanized site, wash‐off of atmospheric NO3− was an important NO3− source at peakflow, especially during short‐duration storms where event water contributions indicated the prevalence of overland flow. In the karst lowlands, very low fractions of event water and even lower fractions of event NO3− at peakflow suggested the dominance of ground water flow pathways during storms. These ground water flow pathways likely flushed stored NO3− sources into the stream, while deep soils in the karst lowlands also may have promoted NO3− assimilation. The results of this study illustrated how NO3− isotopes and δ18O‐H2O could be combined to show key differences in water and NO3− delivery between forested uplands, karst valleys and fully urbanized watersheds. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Influence of bed media characteristics on ammonia and nitrate removal in shallow horizontal subsurface flow constructed wetlands

Two bed media were tested (gravel and Filtralite) in shallow horizontal subsurface flow (HSSF) constructed wetlands in order to evaluate the removal of ammonia and nitrate for different types of wastewater (acetate-based and domestic wastewater) and different COD/N ratios. The use of Filtralite allowed both higher mass removal rates (1.1 g NH 4–N m −2 d −1 and 3 g NO 3–N m −2 d −1) and removal efficiencies (>62% for ammonia, 90–100% for nitrate), in less than 2 weeks, when compared to the ones observed with gravel. The COD/N ratio seems to have no significant influence on nitrate removal and the removal of both ammonia and nitrate seems to have involved not only the conventional pathways of nitrification–denitrification. The nitrogen loading rate of both ammonia (0.8–2.4 g NH 4–N m −2 d −1) and nitrate (0.6–3.2 g NO 3–N m −2 d −1) seem to have influenced the respective removal rates.

The northwestern margin of the Basin and Range province: Part 2: Structural setting of a developing basin from seismic and potential field data

Surprise Valley in northeastern California offers an ideal opportunity to examine the structural setting of a developing extensional basin due to its late Miocene to recent activity in isolation from other major normal fault-bound basins. Seismic velocity and potential field modeling help determine the nature of basin fill and identify intra-basin faults. Based on a detailed gravity and magnetic profile, we identify shallow subsurface basalt flows and several faults within the valley that may accommodate hundreds of meters of vertical offset, possibly cutting and offsetting the ~ 30° east-dipping Surprise Valley fault that rotated during footwall tilting of the adjacent Warner Mountains. Some of these intra-basin faults correspond with mapped Quaternary fault scarps, but others have no surface expression. These faults may represent the currently active fault system within the basin. If so, they would indicate that basin development is transitioning away from the main range-front normal fault to a new set of steep intra-basin faults that are more favorable for accommodating regional transtensional strain.

Deep Processes and Dynamic Responses Associated with the Wenchuan Ms8.0 Earthquake of 2008

AbstractThe Wenchuan Ms8.0 earthquake of 2008 occurred on the Longmen Shan tectonic zone which seems not very active from available data. And it is reported that no noticeable precursors have been observed before this event. It might imply that the crustal deformation at depth associated with this gigantic shock had no expression on the surface or shallow subsurface which was detectable. In this article, we attempt to analyze the processes of brewing and generation of this event at depth that can account for the reason of this devastating earthquake. Tectonically, it is obvious that the India‐Eurasia plate collision serves as the most important dynamic setting of major earthquakes in southwestern China. The Eastern Himalayan Syntaxis is moving rapidly in NNE direction into the Tibetan plateau, forcing deep material to flow toward east. Due to the obstruction of the NE trending Longmen Shan tectonic zone in the northwestern margin of the Sichuan basin, the partial flow turns to southeast. Longmen Shan is a conspicuous boundary of topography and crust structure, across which the elevation difference is as large as 3500±500 m, with crustal thickness 60±5 km in its west and 40±2 km in its east, respectively. Thus it must experience intense deformation and stress accumulation, though very slow without appreciable manifestation. Based on available data, it is inferred that material of lower crust and covering strata of upper mantle is flowing eastward along two deep slide surfaces, of which one is the layer of low‐velocity and low‐resistivity (20~25 km depth), and the other is the top of mantle asthenosphere. Hindered by rigid matter beneath the Sichuan basin, the flow of lower crust and covering strata of upper mantle material turns overthrusting at high angles at the Longmen Shan. On the other hand, three thrust faults visible west of the Longmen Shan extend downward to depth 20 km, where they converge into one active zone. Thus the Longmen Shan tectonic zone is a place where occurs the coupling between slippage on faults of shallow subsurface and material flow of lower crust and covering strata of upper mantle. Under the intense exchange of matter and energy, the source medium ruptures suddenly at a large scale to release a huge amount of elastic strain, resulting in the Wenchuan Ms8.0 earthquake.

Evaluation of extremely shallow vertical subsurface flow constructed wetland for nutrient removal

Mesocosm-scale vertical subsurface flow constructed wetlands (SSF, 0.5 m length, 0.3 m width) with different reed-bed thickness, including standard SSF (SD, 0.6 m deep), shallow SSF (S, 0.3 m deep) and extremely shallow SSF (ES, 0.075 m deep) were set up at sewage treatment plant and their nutrient removal efficiencies from the sewage plant effluent were compared under three hydraulic loading rate (HLR) conditions of 0.15, 0.45 and 0.75 m(3) m(-2) d(-1). A very interesting characteristics was found for the extremely shallow SSF, in which a high nitrogen removal efficiency was obtained despite the effective hydraulic retention time was only 1/8 times as long as the standard SSF. The results of kinetic analysis confirmed that the high volumetric nitrogen removal efficiency observed in the extremely shallow SSF did not depend on high response against the water temperature but on much higher basic nitrogen removal activity compared with other SSF. The phosphorus removal depending on the adsorption to sand in the reed-bed filter was, however, the lowest in the extremely shallow SSF although the volumetric removal efficiency was much higher compared with other SSF. Results of morphological analysis of rhizosphere collected from respective reed-bed suggested that the extremely shallow SSF lead to a very high-density rhizosphere, resulting in a high basic nitrogen removal activity and volumetric phosphorus removal efficiency.

Effects of hydraulic conductivity variability on hillslope‐scale shallow subsurface flow response and storage‐discharge relations

An important problem in scientific hydrology is accounting for the effect of small‐scale heterogeneity in lumped representations of flow through landscapes. In this work we investigate the effect of lateral variations in hydraulic conductivity on saturated subsurface lateral flow through hillslopes using an idealized model based on the Boussinesq equation. The model simulates flow over an impervious planar base through heterogeneous conductivity fields in response to simple recharge inputs and periodic recharge events. The results indicate that the hillslope‐scale response depends on the relative importance of topographic and water table potential gradients driving flow. The relative magnitude of these drivers is related to a dimensionless variable . Where water table gradients dominate, a network of flow paths channel flow through the hillslope toward high‐conductivity areas, and the response is primarily dependent on the boundary condition at the toe of the hillslope. Where the topographic gradients dominate, the flow is delayed by low‐conductivity areas, producing a power law–like response, suggesting that high exponents in observed storage‐discharge relations may be traced to the presence of heterogeneity in the landscape. These observations suggest that the nature of the lumped hillslope‐scale hydrologic models depends on the emergent behaviors that arise at that scale as a result of the interactions between hillslope properties (including their variability in space), the relationship between the hillslopes and their surrounding landscape elements (through the boundary conditions), and the temporal variability of the recharge that enters the system.

A similarity framework to assess controls on shallow subsurface flow dynamics in hillslopes

An understanding of the dominant controls on water fluxes through landscapes at scales smaller than the resolution of a hydrologic model is useful for choosing appropriate parameterizations, and for developing a classification system for landscape hydrology. The links between these controls and the parameters in hillslope‐scale models of subsurface lateral saturated flow have been treated implicitly in both physical (e.g., Characteristic Response Function) and empirical formulations, such as power law storage‐discharge relations. In particular, the controls exerted by the boundary condition at the toe of the hilslope and the temporal variability of the recharge have not been integrated with the understanding of the controls of topography and soil properties. In this work, we develop a dimensionless similarity framework for assessing these controls on the subsurface flow dynamics, based on the Boussinesq equation in an idealized hillslope system. Using this framework we demonstrate the relationship between (1) the exponents of the storage‐discharge relations, (2) the boundary conditions and (3) the characteristic storage thickness used in the Characteristic Response Function parameterization. We use the concept of hydrologic regimes to incorporate information about the population of recharge events that drive the system and set up the initial condition of saturated storage for each storm event. The analysis demonstrates that for planar hillslopes with uniform soil properties, exponents range between 0 (when the flow through the hillslope is dominated by the topographic gradients, and is in the early stages of draining) and 2 (when the flow is driven by water table gradients, and the thickness of the aquifer at the seepage face is small compared to the average thickness). The similarity framework can be used to make first‐order predictions of these exponents, and classify hillslopes using a simple typology of subsurface flows based on two components of the behavior: the relative dominance of the topographic and water‐table gradients, and the responsiveness of the hillslope discharge to temporal variability of the recharge inputs.

PATHWAYS OF ACID MINE DRAINAGE TO CLEAR LAKE: IMPLICATIONS FOR MERCURY CYCLING

Pore fluids from Clear Lake sediments collected near the abandoned Sulphur Bank Mercury Mine have low pH (locally <4) and elevated sulfate (> or =197 mmol/L), aluminum (> or =52 mmol/L), and iron (> or =28 mmol/L) contents derived from oxidation of sulfide minerals at the mine site. Acid mine drainage (AMD) is entering Clear Lake by advective subsurface flow nearest the mine and by diffusion at greater distances. Oxygen and hydrogen isotope ratios, combined with pore fluid compositions, constrain the sources and pathways of contaminated fluids. Sediment cores taken nearest the mine have the highest concentrations of dissolved sulfate, aluminum, and iron, which are contributed by direct subsurface flow of AMD from sulfide-bearing waste rock. Sediment cores as far as 100 m west of the Clear Lake shoreline show the presence of AMD that originated in the acidic lake that occupies the abandoned Herman Pit at the mine site. High sulfate content in the AMD has the potential to promote the activity of sulfate-reducing bacteria in the organic-rich lake sediments, which leads to methylation of Hg+2, making it both more toxic and bioavailable. Quantitative depletion of pore water sulfate at depth and sulfur isotope values of diagenetic pyrite near 0 per thousand indicate that sulfate availability limits the extent of sulfate reduction in the lake sediments away from the mine. Profiles of pore water sulfate in the sediments near the mine show that excess sulfate is available to support the activity of sulfate-reducing bacteria near the mine site. Enriched isotope values of dissolved sulfate (as high as 17.1 per thousand) and highly depleted isotope values for diagenetic pyrite (as low as -22.6 per thousand) indicate active bacterial sulfate reduction in the AMD-contaminated sediments. Sulfate- and iron-rich acid mine drainage entering Clear Lake by shallow subsurface flow likely needs to be controlled in order to lower the environmental impacts of Hg in the Clear Lake ecosystem.