Natural Gradient Experiment Research Articles

Nitrogen availability regulates topsoil carbon dynamics after permafrost thaw by altering microbial metabolic efficiency

Input of labile carbon may accelerate the decomposition of existing soil organic matter (priming effect), with the priming intensity depending on changes in soil nitrogen availability after permafrost thaw. However, experimental evidence for the linkage between the priming effect and post-thaw nitrogen availability is unavailable. Here we test the hypothesis that elevated nitrogen availability after permafrost collapse inhibits the priming effect by increasing microbial metabolic efficiency based on a combination of thermokarst-induced natural nitrogen gradient and nitrogen addition experiment. We find a negative correlation between the priming intensity and soil total dissolved nitrogen concentration along the thaw sequence. The negative effect is confirmed by the reduced priming effect after nitrogen addition. In contrast to the prevailing view, this nitrogen-regulated priming intensity is independent of extracellular enzyme activities but associated with microbial metabolic efficiency. These findings demonstrate that post-thaw nitrogen availability regulates topsoil carbon dynamics through its modification of microbial metabolic efficiency.

Natural gradient experiment on transport of jet fuel derived hydrocarbons in an unconfined sandy aquifer

This paper deals with a field experiment, combining the push–pull and tracer tests, conducted under natural gradient conditions at the international Oslo airport. The studied aquifer, showing very complex hydrogeological settings, has been contaminated by a jet fuel spill. The tracer solutes—bromide, toluene, o-xylene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene and naphthalene, have been injected into the plume. Their migration and changes in concentration of the electron acceptors and metabolic by-products have been monitored. Fast removal of both the non-reactive tracer as well as the aromatic organics has been observed. The tracer pulse could only be detected 2 m downgradient from the injection points. At this point, toluene and o-xylene have been completely removed, however, trimethylbenzenes and naphthalene have been detected. Their depletion, based on calculations of available electron acceptors, can, to a large extent, be accounted for intrinsic biodegradation, with Fe(III) and sulphate reductions as the major controlling processes.

Laboratory evidence of MTBE biodegradation in Borden aquifer material

Mainly due to intrinsic biodegradation, monitored natural attenuation can be an effective and inexpensive remediation strategy at petroleum release sites. However, gasoline additives such as methyl tert-butyl ether (MTBE) can jeopardize this strategy because these compounds often degrade, if at all, at a slower rate than the collectively benzene, toluene, ethylbenzene and the xylene (BTEX) compounds. Investigation of whether a compound degrades under certain conditions, and at what rate, is therefore important to the assessment of the intrinsic remediation potential of aquifers. A natural gradient experiment with dissolved MTBE-containing gasoline in the shallow, aerobic sand aquifer at Canadian Forces Base (CFB) Borden (Ontario, Canada) from 1988 to 1996 suggested that biodegradation was the main cause of attenuation for MTBE within the aquifer. This laboratory study demonstrates biologically catalyzed MTBE degradation in Borden aquifer-like environments, and so supports the idea that attenuation due to biodegradation may have occurred in the natural gradient experiment. In an experiment with batch microcosms of aquifer material, three of the microcosms ultimately degraded MTBE to below detection, although this required more than 189 days (or >300 days in one case). Failure to detect the daughter product tert-butyl alcohol (TBA) in the field and the batch experiments could be because TBA was more readily degradable than MTBE under Borden conditions.

Field study of dispersion in a heterogeneous aquifer: 4. Investigation of adsorption and sampling bias

The declining mass balance trend observed for bromide and three fluorobenzoate tracers during a natural gradient experiment at a site near Columbus, Mississippi, prompted investigations related to the reactivity of the tracers and the representativeness of the tracer samples obtained from multilevel samplers. A laboratory soil column study indicated that adsorption of bromide during the field experiment was of the order of 20% and that up to 10% of the three fluorobenzoates was adsorbed. The presence of iron oxides and kaolinite in the alluvial aquifer combined with the low ground water pH of 4.8 produced geochemical conditions conducive to adsorption of the anionic tracers. Multilevel samplers (MLS) used in the field experiment were evaluated by comparing bromide concentrations from the MLS with water samples extracted from adjacent soil cores. Vertically averaged bromide concentrations for the MLS were 21% lower than those for the cores. A matrix diffusion process in conjunction with a natural tendency for preferential sampling from permeable regions in the heterogeneous alluvial sediments is proposed as an explanation for the apparent bias in the MLS sample concentrations. This process is shown to be qualitatively consistent with the tracer mass balance observed during the natural gradient experiment.

Transformation of the Herbicides MCPP and Atrazine under Natural Aquifer Conditions

AbstractThe potential for aerobic biotransformation of the herbicides MCPP [(+/ ‐)‐2‐(4‐chloro‐2‐methylphenoxy)propionic acid] and Atrazine [2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine] under natural aquifer conditions was investigated. A natural gradient experiment was carried out in an aerobic ground‐water zone at the Canadian Forces Base Borden, Borden, Ontario, Canada. Over 90 days, a continuous plume was created by weekly injections of the herbicides (average concentrations of 400 jug/1 each), together with a conservative tracer, chloride. The fate of the herbicides was monitored at multilevel piezometers located downgradient from the injection well. In parallel, experiments with in situ testers (Gillham et al., 1990) were set up in the aquifer and microcosms were set up in the laboratory.For MCPP, the natural gradient experiment showed an adaptation period of 42‐56 days followed by transformation from 400 to 30 μg/1. As predicted, MCPP was not retarded compared to the conservative tracer. No transformation was observed in the in situ testers over 47 days. In the laboratory experiments no transformation was observed over 74 days.Atrazine was recalcitrant within the experimental period of 96 days in the natural gradient experiment. A retardation factor R = 1.2, within the predicted range, was found for Atrazine in the Borden aquifer. No transformation was observed either in the in situ testers or in the laboratory experiments after a period of 96 and 74 days, respectively.These results suggest that both MCPP and Atrazine are mobile in sand aquifers. Atrazine appears persistent, at least for 96 days, while MCPP undergoes considerable transformation.

Simulations of physical nonequilibrium solute transport models: Application to a large-scale field experiment

Equations expressing the spatial moments of solute concentration distributions simulated by various models, in terms of model parameters, have recently been presented. Using independently obtained parameter values, these equations are used to compare simulations of physical non-equilibrium models with spatial moment data collected in a large-scale natural gradient experiment on solute transport. The physical nonequilibrium models examined postulate the existence of layered zones of immobile water through which solute is transported by a diffusion mechanism. It is found that the qualitative aspects of the measured moment behavior are simulated by the physical nonequilibrium models if the independently obtained parameters are modified somewhat on the basis of reasonable corrective assumptions. It is further demonstrated that the physical nonequilibrium models, using parameter values obtained from spatial data, can qualitatively simulate temporal behavior at individual well points in this relatively homogeneous aquifer.

Comment on “A natural gradient experiment on solute transport in a sand aquifer: 2. Spatial moments and the advection and dispersion of nonreactive tracers” by D. L. Freyberg

Water Resources ResearchVolume 24, Issue 7 p. 1221-1222 CommentariesFree Access Comment on “A natural gradient experiment on solute transport in a sand aquifer: 2. Spatial moments and the advection and dispersion of nonreactive tracers” by D. L. Freyberg Hugo A. Loaiciga, Hugo A. LoaicigaSearch for more papers by this author Hugo A. Loaiciga, Hugo A. LoaicigaSearch for more papers by this author First published: July 1988 https://doi.org/10.1029/WR024i007p01221Citations: 6AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Aris, R., On the dispersion of a solute in a fluid flowing through a tube, Proc. R. Soc. London, Ser. A., 235, 67– 78, 1956. Bear, J., Dynamics of Fluids in Porous Media, American Elsevier, New York, 1972. Chandrasekar, S., Stochastic problems in physics and astronomy, Rev. Modern Phys., 15, 1– 89, 1943. Curtis, E. P., P. V. Roberts, M. Reinhard, A natural gradient experiment on solute transport in a sand aquifer, 4, Sorption of organic solutes and its influence on mobility, Water Resour. Res., 22(13), 2059– 2067, 1986. Dagan, G., Solute transport in hetereogenous porous formations, J. Fluid Mech., 145, 151– 177, 1984. Dagan, G., Statistical theory of groundwater flow and transport: Pore to laboratory, laboratory to formation, and formation to regional scale, Water Resour. Res., 22(9), 120S– 134S, 1986. Einstein, A., On the movement of suspended particles in stationary fluids deduced from molecular-kinetic theory of heat (in German), Ann. Phys., 17, 539– 560, 1905. Freyberg, D. L., A natural gradient experiment on solute transport in a sand aquifer, 2, Spatial moments and the advection and dispersion of nonreactive tracers, Water Resour. Res., 22(13), 2031– 2046, 1986. Mackay, D. M., D. L. Freyberg, P. V. Roberts, A natural gradient experiment on solute transport in a sand aquifer, 1, Approach overview of plume movement, Water Resour. Res., 22(13), 2017– 2029, 1986. Roberts, P. V., M. N. Goltz, D. M. Mackay, A natural gradient experiment on solute transport in a sand aquifer, 3, Retardation estimates and mass balances for organic solutes, Water Resour. Res., 22(13), 2047– 2058, 1986. Ross, S. M., Introduction to Probability Models, Academic Press, Orlando, Fla., 1985. Sudicky, E. A., A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process, Water Resour. Res., 22(13), 2069– 2082, 1986. Taylor, G., Dispersion of soluble matter in solvent flowing slowly through a tube, Proc. R. Soc. London, Ser. A, 219, 186– 203, 1953. Taylor, G., The dispersion of matter in turbulent flow through a pipe, Proc. R. Soc. London, Ser. A, 223, 446– 468, 1954. Citing Literature Volume24, Issue7July 1988Pages 1221-1222 ReferencesRelatedInformation

Comment on “A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process” by E. A. Sudicky

Water Resources ResearchVolume 24, Issue 7 p. 1209-1210 CommentariesFree Access Comment on “A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process” by E. A. Sudicky F. J. Molz, F. J. MolzSearch for more papers by this authorO. Güven, O. GüvenSearch for more papers by this author F. J. Molz, F. J. MolzSearch for more papers by this authorO. Güven, O. GüvenSearch for more papers by this author First published: July 1988 https://doi.org/10.1029/WR024i007p01209Citations: 10AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Dagan, G., Stochastic modeling of groundwater flow by unconditional and conditional probabilities, 2, The solute transport, Water Resour. Res., 18(4), 835– 848, 1982. Dagan, G., Solute transport in heterogeneous porous formations, J. Fluid Mech., 145, 151– 177, 1984. Freyberg, D. L., A natural gradient experiment on solute transpon in a sand aquifer, 2, Spatial moments and the advection and dispersion of nonreactive tracers, Water Resour. Res., 22(13), 2031– 2046, 1986. Gelhar, L. W., C. L. Axness, Three-dimensional stochastic analysis of macrodispersion in aquifers, Water Resour. Res., 19(1), 161– 180, 1983. Güven, O., F. J. Molz, Deterministic and stochastic analyses of dispersion in an unbounded stratified porous medium, Water Resour. Res., 22(11), 1565– 1574, 1986. Mackay, D. J., D. L. Freyberg, P. V. Roberts, J. A. Cherry, A natural gradient experiment on solute transport in a sand aquifer, 1, Approach and overview of plume movement, Water Resour. Res., 22(13), 2017– 2029, 1986. Sposito, G. W., A. Jury, V. K. Gupta, Fundamental problems in the stochastic convection-dispersion model of solute transport in aquifers and field soils, Water Resour. Res., 22(1), 77– 88, 1986. Sudicky, E. A., A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process, Water Resour. Res., 22(13), 2069– 2082, 1986. Sudicky, E. A., J. A. Cherry, E. O. Frind, Migration of contami. nants in groundwater at a landfill: A case study, 4, A natural-gradient dispersion test, J. Hydrol., 63, 81– 108, 1983. Citing Literature Volume24, Issue7July 1988Pages 1209-1210 ReferencesRelatedInformation

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sand aquifer 2. Spatial moments and the advection and dispersion of nonreactive tracers’ by D. L. Freyberg”

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sand aquifer 2. Spatial moments and the advection and dispersion of nonreactive tracers’ by D. L. Freyberg”

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process’ by E. A. Sudicky”

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process’ by E. A. Sudicky”

Comment on “A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process” by E. A. Sudicky

Comment on “A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process” by E. A. Sudicky

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sand aquifer: spatial variability of hydraulic conductivity and its role in the dispersion process’ by E. A. Sudicky”

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sand aquifer: spatial variability of hydraulic conductivity and its role in the dispersion process’ by E. A. Sudicky”

Comment on “A natural gradient experiment on solute transport in a sandy aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process” by E. A. Sudicky

Comment on “A natural gradient experiment on solute transport in a sandy aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process” by E. A. Sudicky

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sandy aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process’ by E. A. Sudicky”

Reply [to “Comment on ‘A natural gradient experiment on solute transport in a sandy aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process’ by E. A. Sudicky”

A natural gradient experiment on solute transport in a sand aquifer: Spatial variability of hydraulic conductivity and its role in the dispersion process

The spatial variability of hydraulic conductivity at the site of a long‐term tracer test performed in the Borden aquifer was examined in great detail by conducting permeability measurements on a series of cores taken along two cross sections, one along and the other transverse to the mean flow direction. Along the two cross sections, a regular‐spaced grid of hydraulic conductivity data with 0.05 m vertical and 1.0 m horizontal spatial discretization revealed that the aquifer is comprised of numerous thin, discontinuous lenses of contrasting hydraulic conductivity. Estimation of the three‐dimensional covariance structure of the aquifer from the log‐transformed data indicates that an exponential covariance model with a variance equal to 0.29, an isotropic horizontal correlation length equal to about 2.8 m, and a vertical correlation length equal to 0.12 m is representative. A value for the longitudinal macrodispersivity calculated from these statistical parameters using three‐dimensional stochastic transport theory developed by L. W. Gelhar and C. L. Axness (1983) is about 0.6 m. For the vertically averaged case, the two‐dimensional theory developed by G. Dagan (1982, 1984) yields a longitudinal djspersivity equal to 0.45 m. Use of the estimated statistical parameters describing the ln (K) variability in Dagan's transient equations closely predicted the observed longitudinal and horizontal transverse spread of the tracer with time. Weak vertical and horizontal dispersion that is controlled essentially by local‐scale dispersion was obtained from the analysis. Because the dispersion predicted independently from the statistical description of the Borden aquifer is consistent with the spread of the injected tracer, it is felt that the theory holds promise for providing meaningful estimates of effective transport parameters in other complex‐structured aquifers.

A natural gradient experiment on solute transport in a sand aquifer: 3. Retardation estimates and mass balances for organic solutes

The long‐term behavior of five organic solutes during transport over a period of 2 years in ground water under natural gradient conditions was characterized quantitatively by means of moment estimates. Total mass was conserved for two of the organic compounds, carbon tetrachloride and tetrachloroethylene, while the total mass declined for three other compounds, bromoform, 1,2‐dichlorobenzene, and hexachloroethane. The declines in mass for the latter three compounds are interpreted as evidence of transformation of the compounds. Retardation factors for the organic solutes, relative to chloride, ranged from 1.5 to 9.0, being generally greater for the more strongly hydrophobic compounds. The retardation is attributed to sorption. The apparent retardation factor increased markedly for all compounds over the duration of the experiment, by as much as 150%. Results from temporal and spatial sampling were in good agreement when compared at the same scale of time and distance.

A natural gradient experiment on solute transport in a sand aquifer: 4. Sorption of organic solutes and its influence on mobility

Laboratory investigations were conducted to determine whether the observed field retardation of bromoform, carbon tetrachloride, tetrachloroethylene, 1,2‐dichlorobenzene, and hexachloroethane at the Borden field site could be explained by the linear, reversible, equilibrium sorption model. The five halogenated organic solutes, which have octanol‐water partition coefficients ranging from 200 to 4000, were the same as those used in the field study. The sorbent, a medium sand containing 0.02% organic carbon, was excavated 11.5 m from the experimental well field at the Borden site. Sorption isotherms were linear in the aqueous concentration range from 1 to 50 μg/L and could be described by a single distribution coefficient Kd. The experimentally determined Kd exceed those predicted by the hydrophobic sorption model that accounts only for partitioning into organic matter, by factors ranging from 1.7 for hexachloroethane to 10 for tetrachloroethylene. Retardation factors inferred from the laboratory determined distribution coefficients fell within the range estimated from spatial sampling data in the field experiment.

A natural gradient experiment on solute transport in a sand aquifer: 1. Approach and overview of plume movement

A large‐scale field experiment on natural gradient transport of solutes in groundwater has been conducted at a site in Borden, Ontario. Well‐defined initial conditions were achieved by the pulse injection of 12 m3 of a uniform solution containing known masses of two inorganic tracers (chloride and bromide) and five halogenated organic chemicals (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2‐dichlorobenzene, and hexachloroethane). A dense, three‐dimensional array of over 5000 sampling points was installed throughout the zone traversed by the solutes. Over 19,900 samples have been collected over a 3‐year period. The tracers followed a linear horizontal trajectory at an approximately constant velocity, both of which compare well with expectations based on water table contours and estimates of hydraulic head gradient, porosity, and hydraulic conductivity. The vertical displacement over the duration of the experiment was small. Spreading was much more pronounced in the horizontal longitudinal than in the horizontal transverse direction; vertical spreading was very small. The organic solutes were retarded in mobility, as expected.

A natural gradient experiment on solute transport in a sand aquifer: 2. Spatial moments and the advection and dispersion of nonreactive tracers

The three‐dimensional movement of a tracer plume containing bromide and chloride is investigated using the data base from a large‐scale natural gradient field experiment on groundwater solute transport. The analysis focuses on the zeroth‐, first‐, and second‐order spatial moments of the concentration distribution. These moments define integrated measures of the dissolved mass, mean solute velocity, and dispersion of the plume. Moments are estimated from the point observations using quadrature approximations tailored to the density of the sampling network. The estimators appear to be robust, with acceptable sampling variability. Estimates of the mass in solution for both bromide and chloride demonstrate that the tracers behaved conservatively, as expected. Analysis of the first‐order moment estimates indicates that the experimental tracer plumes traveled along identical trajectories. The horizontal trajectory is linear and aligned with the hydraulic gradient. The vertical trajectory is curvilinear, concave upward. The total vertical displacement is small, however, so that the vertical component of the mean solute velocity vector is negligible. The estimated mean solute velocity is identical for both tracers (0.091 m/day) and is spatially and temporally uniform for the first 647 days of travel time. After 647 days of transport, the plume apparently encountered a relatively large‐scale heterogeneity in the velocity field, leading to a distinct vertical layering, and slowing the rate of advance of the center of mass of the plume as a whole. The estimated horizontal components of the covariance tensor evolve over time in a manner consistent with the qualitative shape changes observed from plots of the concentration data. The major principal axis, initially aligned roughly perpendicular to the hydraulic gradient, rotates smoothly over time until it is nearly aligned with the mean solute velocity vector, as the plume itself elongates and orients its long axis with the direction of movement. Plots of the components of the covariance tensor as functions of time show evidence of what is commonly called “scale‐dependent” dispersion: the rate of growth of the covariance over time is not linear. The theoretical results of G. Dagan (1984) calibrate well to the estimated covariance data for the first 647 days of transport. The calibrated values of the parameters of the hydrualic conductivity distribution closely match independently measured values from the site. The asymptotic longitudinal dispersivity obtained from the calibration is 0.49 m, although asymptotic conditions were apparently not reached. The estimated covariance terms for the last sampling session, 1038 days after injection, are inconsistent with the earlier data and with the Dagan model, particularly for the transverse and off‐diagonal components. This behavior is probably attributable to the observed large‐scale heterogeneity in the velocity field.

Migration and Attenuation of Selected Organics in a Sandy Aquifer – A Natural Gradient Experiment

ABSTRACTA natural gradient tracer test was conducted in an unconfined sandy aquifer at Borden, Ontario. N‐butyric acid, phenol, p‐chlorophenol, and dimethyl phthalate (DMP) were used as organic tracers at concentrations ranging from 70 to 100 ppm and chloride was used as a nonreactive tracer at a concentration of 243 ppm. Approximately 2,100 1 were injected below the water table and were monitored with a three‐dimensional network of bundle‐type, multilevel piezometers over a period of 60 days and once again after 204 days.The plume gradually split into two zones due to local heterogeneity. The centers of mass for the organic compounds migrated at essentially the same rate as chloride, but attenuated at different rates. Butyric acid and phenol attenuated the most rapidly. Chlorophenol attenuated only slightly faster than chloride and DMP attenuated at roughly the same rate as chloride. Sorption does not appear to be important in the system studied. Dispersion and biodegradation probably produced the observed attenuation of organics.