Downgradient Side Research Articles

Signal amplification in growth cone gradient sensing by a double negative feedback loop among PTEN, PI(3,4,5)P3 and actomyosin

Axon guidance during neural wiring involves a series of precisely controlled chemotactic events by the motile axonal tip, the growth cone. A fundamental question is how neuronal growth cones make directional decisions in response to extremely shallow gradients of guidance cues with exquisite sensitivity. Here we report that nerve growth cones possess a signal amplification mechanism during gradient sensing process. In neuronal growth cones of Xenopus spinal neurons, phosphatidylinositol-3,4,5-trisphosphate (PIP3), an important signaling molecule in chemotaxis, was actively recruited to the up-gradient side in response to an external gradient of brain-derived neurotrophic factor (BDNF), resulting in an intracellular gradient with approximate 30-fold amplification of the input. Furthermore, a reverse gradient of phosphatase and tensin homolog (PTEN) was induced by BDNF within the growth cone and the increased PTEN activity at the down-gradient side is required for the amplification of PIP3 signals. Mechanistically, the establishment of both positive PIP3 and reverse PTEN gradients depends on the filamentous actin network. Together with computational modeling, our results revealed a double negative feedback loop among PTEN, PIP3 and actomyosin for signal amplification, which is essential for gradient sensing of neuronal growth cones in response to diffusible cues.

Reactive transport modeling of U and Ra mobility in roll-front uranium deposits: Parameters influencing 226Ra/238U disequilibria

Uranium reserve estimates in ore deposits can be significantly impacted by 226Ra/238U disequilibria arising from the differential mobility of uranium and radium during groundwater transport. 1D reactive transport models were developed to investigate the long-term effects of retention processes (UO2(am) precipitation, U(VI) and Ra sorption on smectite, Ra co-precipitation with barite) on the repartitioning of 238U and 226Ra during formation of roll-front type deposits. Analytical solutions to radioactive decay chains were used in complement to examine the influence of geochemical parameters, including fluid 234U/238U activity ratios and α-recoil loss, on 226Ra/238U disequilibria in uranium ores. Model results demonstrate that smectite and barite can produce 226Ra/238U ratios >1 at low uranium contents and may explain 226Ra/238U disequilibria occurring in altered rock up- and downstream of roll-front deposits. The capacity of these phases to take up Ra and generate 226Ra/238U disequilibria depends on both mineral contents and groundwater compositions, and is thus expected to be site-specific. Simulations of ore deposits that advance downstream with time demonstrate the formation of stronger 226Ra/238U disequilibria, as expected, in the downgradient side or nose of the ore, reflecting both younger mineralization ages and the presence of active uranium precipitation. Whether disequilibria are positive or negative with respect to secular equilibrium, however, depends on the 234U/238U activity ratio in the fluid from which uranium minerals precipitate. Smaller hydraulic conductivities are shown to generate a narrower range in 226Ra/238U activity ratios with distance, and may explain the occurrence of disequilibria in the limb ore that are less pronounced than those in the nose. Furthermore, the ability of α-recoil loss to decrease 226Ra/238U activity ratios at secular equilibrium may account for negative disequilibria in high grade ores. The South Tortkuduk uranium deposits (Kazakhstan) are subsequently used as a case study to identify the processes and parameters that may contribute to 226Ra/238U disequilibria at this site. Variations in multiple parameters, including clay contents, barite contents, and mineralization ages, are found to reproduce measured 226Ra/238U activity ratios in the roll-front ore. Prioritization of these parameters will necessitate field measurements targeting both groundwater fluids and the host rock. Results from this study will ultimately aid geologists in building appropriate hydrogeochemical data sets to more efficiently locate and exploit uranium ore deposits.

Evaluating the hydrological response of a boreal fen following the removal of a temporary access road

Peatlands dominate the landscape (~50% of total area) in the Athabasca Oil Sands Region (AOSR) in northern Alberta, and as such, are susceptible to hydrological disturbance from oil sands mining and exploration. In this study, we explore the hydrochemical response of a disturbed fen (Firebag Fen) in the AOSR following the removal of a temporary access road that was operational from 2007 to 2013. Prior to road removal flow was impeded at Firebag Fen, evidenced by inundation on the up-gradient side, and a lower water table on the down-gradient side of the road. Reclamation included the road removal of the ~0.5 m thick road, including 0.15 m of clay, 0.3 m of mud and organics, and 0.05 m of gravel, which left a surficial depression following removal. Peat samples (n = 139) collected immediately following road removal demonstrated significantly greater bulk densities on the down-gradient side relative to the up-gradient side of the fen, and in the road-removed area relative to up- and down-gradient areas. These combined changes decreased the average hydraulic conductivity of the fen by ~79%, which reduced water discharge across the site. Another peat sampling campaign in 2016 (n = 48), three years after removal, showed a significant decrease in bulk density where the road was removed, to within the range of values observed at areas up and down-gradient of the road. These changes increased the average hydraulic conductivity of the entire system by 64%, compared to that which was measured in 2013, with water discharge 9 and 12% higher than in 2013, during characteristically dry and wet periods, respectively. Some changes to the water chemistry (n = 24–28 for each sampling date) of the fen were also detected but patterns were different among parameters measured. For example, median concentrations on the down-gradient side of the road were 1651% (NO3–), 280% (SO42−) and 135% (Cl−) higher relative to the up-gradient side of the road. Over time following road removal, differences were reduced to 100% for nitrate, 192% for sulphate, 108% for chloride, while other parameters remained relatively unchanged (Ca2+, Mg2+, K−). Continued peat rebound (thus increasing hydraulic conductivity) may occur over time; however, elucidating this would require long-term monitoring. It is concluded that despite the impacts that were caused to the site, the natural hydrologic regime of the fen was able to operate following removal. Removal and reclamation should be considered as a worthwhile venture for roads that extend through fens and are no longer in use.

Waste Foundry Sand as Permeable and Low Permeable Barrier for Restriction of the Propagation of Lead and Nickel Ions in Groundwater

This work aims to investigate the ability of using waste foundry sand (WFS) resulting as inexpensive by-product from steel industry in the low permeability barrier (LPB) and permeable reactive barrier (PRB) technologies for restriction of the movement of lead and nickel ions in the groundwater. Outputs of flask and tank tests certified that this material could capture these ions with sorption efficiency greater than 95% at time, pH, sorbent dosage, and speed equal to 60 min, 4 for lead and 6 for nickel, 2.5 g/100 mL, and 250 rpm, respectively. Sorption isotherm measurements were represented in a good manner by Langmuir model in comparison with Freundlich model with coefficient of determination (R2) greater than 0.99. So, the chemisorption was the predominant mechanism which could be supported by O-H, H-O-H, C-O, O-Si-O, and Si-O functional groups based on the Fourier transform infrared analysis. The maximum sorption capacity of WFS was 13.966 and 4.227 mg/g for lead and nickel ions, respectively, with corresponding affinities equal to 0.647 and 0.099 L/mg. Measurements signified that the hydraulic conductivity of WFS was 3.8 × 10−7 cm/s which satisfies the requirements of LPB. To obtain the acceptable values of permeability and reactivity, PRB was prepared from mixing 18% WFS with 82% filter sand. COMSOL software was able to simulate the measurements of two-dimensional tank packed with Iraqi soil aquifer in combination with WFS-LPB and WFS-filter sand PRB. Thicker barriers have a high ability in the protection of locations in the down-gradient side because their longevity increased dramatically with increase of barrier thickness.

Complete degradation of chlorinated ethenes and its intermediates through sequential anaerobic/aerobic biodegradation in simulated groundwater columns (complete degradation of chlorinated ethenes)

This study evaluated the effectiveness of sequential anaerobic/aerobic biodegradation of tetrachloroethene (PCE) and its intermediates, cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). Two sand columns were operated in series. The first column simulated the up-gradient side of a groundwater system, was operated under anaerobic conditions, and was continuously fed the target contaminant, PCE (42 µM). The second column simulated the down-gradient side of the groundwater system and was operated under aerobic conditions, using low concentrations of hydrogen peroxide as the dissolved oxygen source. After 15 days of operation, cDCE was detected at the end of the first, anaerobic column, at concentrations of 7.02–15.57 μM. After 36 days of operation, VC (7.32 μM) was also detected at the end of the first column. cDCE and VC then migrated into the second, aerobic column. Results showed that cDCE and VC were almost completely aerobically biodegraded in the second column, with removal efficiencies of up to 97% and 95%, respectively. This study also used batch experiments to compare cDCE removal efficiencies between aerobic metabolism using cDCE as the only substrate, and aerobic cometabolism using methane and cDCE as primary and secondary substrates. Results showed that aerobic cometabolism of cDCE was inhibited at cDCE concentrations greater than 50 mg/L. This inhibition effect was not obvious under aerobic metabolism using cDCE as the only substrate. Results of a Michaelis–Menten/Monod kinetics analysis showed that when cDCE concentrations were greater than 20 mg/L, cDCE could be biodegraded more effectively under aerobic metabolism than under aerobic cometabolism.

Quantifying nitrogen cycling beneath a meander of a low gradient, N‐impacted, agricultural stream using tracers and numerical modelling

AbstractIn watersheds impacted by nitrate from agricultural fertilizers, nitrification and denitrification may be decoupled as denitrification in the hyporheic zone is not limited to naturally produced nitrate. While most hyporheic research focuses on the 1–2 m of sediment beneath the stream bed, there are a limited number of studies that quantify nitrogen (N) cycling at larger hyporheic scales (10s of metres to kms). We conducted an investigation to quantify N cycling through a single meander of a low gradient, meandering stream, draining an agricultural watershed. Chemistry (major ions and N species) and hydrologic data were collected from the stream and groundwater beneath the meander. Evidence indicates that nearly all the shallow groundwater flowing beneath the meander originates as stream water on the upgradient side of the meander, and returns to the stream on the downgradient side. We quantified the flux of water beneath the meander using a numerical model. The flux of N into and out of the meander was quantified by multiplying the concentration of the important N species (nitrate, ammonium, dissolved organic nitrogen (DON)) by the modelled water fluxes. The flux of N into the meander is dominated by nitrate, and the flux of N out of the meander is dominated by ammonium and DON. While stream nitrate varied seasonally, ammonium and DON beneath the meander were relatively constant throughout the year. When stream nitrate concentrations are high (>2 mg litre−1), flow beneath the meander is a net sink for N as more N from nitrate in stream water is consumed than is produced as ammonium and DON. When stream nitrate concentrations are low (<2 mg litre−1), the flux of N entering is less than exiting the meander. On an annual basis, the meander hyporheic flow serves as a net sink for N. Copyright © 2007 John Wiley & Sons, Ltd.

Capillary influences on the operation and effectiveness of LNAPL interceptor trenches

Abstract Interceptor trenches are commonly used to recover light non-aqueous-phase liquids (LNAPL) from the subsurface and are particularly effective in the recovery of floating hydrocarbons. Interceptor trenches function by creating a zone of high permeability within the aquifer either as an open excavation (where the formation permits) or filled with a suitable backfill (typically gravel) that retains the high permeability and effective porosity required of a collection system. Numerous texts suggest that placement of an impermeable membrane is required on the down-gradient side of the trench to prevent captured LNAPL from exiting the trench on its downgradient side. Installation of such a liner can be difficult if not impossible under certain soil conditions. Simple calculations show that the membrane may not be necessary due to the hydraulic conditions created by installing the trench. Using these calculations in the preliminary design will result in an effective trench installation and correspondingly reduce installation and operation costs. LNAPL has been shown to migrate primarily near the top of the capillary fringe in porous media. Since the high permeability and effective porosity of the trench backfill do not support a capillary fringe of similar magnitude, LNAPL rests essentially on the water table in the trench. The zone of capillary saturation on the downgradient side inhibits LNAPL migration from the trench into the native soil. Provided that recovery of the LNAPL is maintained and the thickness of LNAPL in the trench is less than the capillary fringe height in the native soil, the LNAPL will remain hydraulically trapped. To demonstrate this condition, a review of some of the methods used to calculate the capillary fringe height was performed. To verify the magnitude of the calculated capillary fringe height and its effects on LNAPL retention within a trench, a simple laboratory-scale simulation was undertaken.

Estimating Movement of Ground Water from a Pond or Large-Radius Conduit

Evaluation of ground-water exchange between ponds or large-radius conduits and surrounding aquifers is a common problem in contaminant migration studies. Analytic solutions are useful tools for this evaluation, particularly when the system is incompletely characterized. Solutions are derived below for application to ponds, lakes, and natural or artificial conduits with a known discharge/recharge flux. General features of migration in this setting are determined by the ratio of pond flux (source/sink) to regional flux crossing the pond boundary. When this ratio is large (pond dominates) the effect of the pond is indistinguishable from a well. At intermediate values, movement from the pond against the regional flow is inhibited, and discharge plumes are shifted downgradient relative to those for a well. At small ratios only a narrow plume will emerge from the downgradient side of a discharging pond, considerably narrower but longer than predicted for a well of the same strength.

An Analysis of Nitrate‐Nitrogen in Ground Water Beneath Unsewered Subdivisions

AbstractWater samples from private water supply wells in five unsewered subdivisions were tested for nitrate‐nitrogen to determine the possible impact of septic systems on ground water quality. Three subdivisions are located in Eau Claire County and two in LaCrosse County, Wisconsin.The nitrate‐nitrogen concentrations in the wells were analyzed in relation to ground water flow direction, the location of septic systems within the subdivision, and the hydrogeologic and physical characteristics of the subdivisions. A comparison of three nitrogen mass balance models helped to identify the possible sources of nitrate‐nitrogen in the wells.The results indicate that nitrogen from septic systems and lawn fertilizer cause nitrate‐nitrogen to increase in the ground water beneath the downgradient side of the subdivisions. In three of the five subdivisions the highest nitrate‐nitrogen value exceeds the drinking water standard of 10 mg/L.

Estimating groundwater exchange with lakes: 2. Calibration of a three‐dimensional, solute transport model to a stable isotope plume

A three‐dimensional groundwater flow and solute transport model was calibrated to a plume of water described by measurements of δ18O and used to calculate groundwater inflow and outflow rates at a lake in northern Wisconsin. The flow model was calibrated to observed hydraulic gradients and estimated recharge rates. Calibration of the solute transport submodel to the configuration of a stable isotope (18O) plume in the contiguous aquifer on the downgradient side of the lake provides additional data to constrain the model. A good match between observed and simulated temporal variations in plume configuration indicates that the model closely simulated the dynamics of the real system. The model provides information on natural variations of rates of groundwater inflow, lake water outflow, and recharge to the water table. Inflow and outflow estimates compare favorably with estimates derived by the isotope mass balance method (Krabbenhoft et al., this issue). Model simulations agree with field observations that show groundwater inflow rates are more sensitive to seasonal variations in recharge than outflow.

Groundwater interaction with a kettle-hole lake: Relation of observations to digital simulations

Observations made on a small, fully penetrating, kettle hole lake are used to test predictions made using digital stimulation about the exchange of water between lakes and groundwater systems. On the upgradient side of Lower Nashotah Lake, groundwater inseepage decreases exponentially away from the shore. Water exchange on the downgradient side is controlled by the presence and strength of a groundwater mound, but that mound and its effects are ephemeral. When the mound increases in strength, it generates a local groundwater flow cell beneath it which contains water of chemical composition distinct from deeper, regional groundwater. The growing mound also causes local inseepage of groundwater to the lake in an expanding zone along the downgradient shore. All of these phenomena occur as they have been predicted in published studies where digital simulation of relatively simple groundwater-lake systems has been carried out. The field observations thus confirm the validity of the simulation studies and their usefulness as a tool for understanding water exchanges in unmonitored systems.

Seasonal reversals of groundwater flow around lakes and the relevance to stagnation points and lake budgets

Several researchers have observed seasonal reversals in the direction of groundwater flow around lakes. If these reversals are prolonged and are accompanied by the formation of a stagnation point, they may have a significant effect on a lake's water and nutrient budgets. The formation of a stagnation point at a flow‐through lake (i.e., a lake that receives groundwater through part of the lake basin and recharges the groundwater system over the rest of the lake basin) is accomplished by the formation of a groundwater mound on the downgradient side of the lake. In this paper the seasonal formation of a stagnation point at Snake Lake, Wisconsin, is investigated with the aid of two‐dimensional transient computer models applied in cross section and areally. The analysis demonstrates the potential for the seasonal formation of a stagnation point at a flow‐through lake and provides some insight into the transient development of the stagnation point.

Numerical simulation of steady state three‐dimensional groundwater flow near lakes

Numerical simulation of three‐dimensional groundwater flow near lakes shows that the continuity of the boundary encompassing the local groundwater flow system associated with a lake is the key to understanding the interaction of a lake with the groundwater system. The continuity of the boundary can be determined by the presence of a stagnation zone coinciding with the side of the lake nearest the downgradient side of the groundwater system. For most settings modeled in this study the stagnation zone underlies the lakeshore, and it generally follows its curvature. The length of the stagnation zone is controlled by the geometry of the lake's drainage basin divide on the side of the lake nearest the downgradient side of the groundwater system. In the case of lakes that lose water to the groundwater system, three‐dimensional modeling also allows for estimating the area of lake bed through which outseepage takes place. Analysis of the effects of size and lateral and vertical distribution of aquifers within the groundwater system on the outseepage from lakes shows that the position of the center point of the aquifer relative to the littoral zone on the side of the lake nearest the downgradient side of the groundwater system is a critical factor. If the center point is downslope from this part of the littoral zone, the local flow system boundary tends to be weak or outseepage occurs. If the center point is upslope from this littoral zone, the stagnation zone tends to be stronger (to have a higher head in relation to lake level), and outseepage is unlikely to occur.

Streamlining of the Carolina Bays

Ground-water studies in the area of the Carolina bays have revealed a singularly consistent set of subsurface conditions when considered collectively. These conditions, apparently related to the origin of the bays, are (1) the presence of distinctly bedded clays in which at least one bed of limestone occurs, the limestone being generally less than 25 feet thick and less than 150 feet below land surface; (2) a remarkably simple homoclinal structure, ideally suited for artesian conditions, especially in the uppermost beds; (3) solution under artesian rather than water-table conditions in the near-surface bed of limestone, resulting in slow but regular removal of mineral constituents during the long periods since deposition of the soluble beds. The hypothesis advanced is that solution in a near-surface artesian aquifer composed of limestone resulted in subsidence of overlying clay into this aquifer, thus forming a normal sink. This condition was followed by streamline ground-water flow around this relatively impervious clay obstruction, leading to increased rate of solution and subsidence in the down-gradient side of the area. The slow, continuous streamline flow of ground water around the impermeable clay enlarged the dissolved area into an ellipsoid or, more generally; into an ovoid, the more pointed end of which is down-gradient. The hypothesis of streamline origin apparently accounts for genetic features of the bays. Sand rims that characterize parts of some bays are shown to be superimposed features, not requiring explanation under this hypothesis.