Field Tracer Experiments Research Articles

Influence of Tree Species on Forest Nitrogen Retention in the Catskill Mountains, New York, USA

This study examines the effect of four tree species on nitrogen (N) retention within forested catchments of the Catskill Mountains, New York (NY). We conducted a 300-day 15N field tracer experiment to determine how N moves through soil, microbial, and plant pools under different tree species and fertilization regimes. Samples were collected from single-species plots of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga canadensis L.), red oak (Quercus rubra L.), and sugar maple (Acer saccharum Marsh). Using paired plots we compared the effects of ambient levels of N inputs (11 kg N/ha/y) to additions of 50 kg N/ha/y that began 1.5 years prior to and continued throughout this experiment. Total plot 15N recovery (litter layer, organic and mineral soil to 12 cm, fine roots, and aboveground biomass) did not vary significantly among tree species, but the distribution of sinks for 15N within the forest ecosystem did vary. Recovery in the forest floor was significantly lower in sugar maple stands compared to the other species. 15Nitrogen recovery was 22% lower in the fertilized plots compared to the ambient plots and red oak stands had the largest drop in 15N recovery as a result of N fertilization. Aboveground biomass became a significantly greater 15N sink with fertilization, although it retained less than 1% of the tracer addition. These results indicate that different forest types vary in the amount of N retention in the forest floor, and that forest N retention may change depending upon N inputs.

Simultaneous analyses and applications of multiple fluorobenzoate and halide tracers in hydrologic studies

An analytical method that employs ion chromatography has been developed to exploit the use of fluorobenzoic acids (FBAs) and halides more fully as hydrologic tracers. In a single run, this reliable, sensitive, and robust method can simultaneously separate and quantify halides (fluoride, chloride, bromide, and iodide) and up to seven FBAs from other common groundwater constituents (e.g. nitrate and sulphate). The usefulness of this analytical method is demonstrated in both field and laboratory tracer experiments. The field study examines the hydrologic response of fractures and the matrix to different flow rates and the contribution of matrix diffusion in chemical transport. Laboratory tracer experiments with eight geologic media from across the USA—mostly from Department of Energy facilities where groundwater contamination is prevalent and where subsurface characterization employing tracers has been ongoing or is in need—reveal several insights about tracer transport behaviour: (1) bromide and FBAs are not always transported conservatively; (2) the delayed transport of these anionic tracers is likely related to geologic media characteristics, such as organic matter, pH, iron oxide content, and clay mineralogy; (3) use of iodine as a hydrologic tracer should take into account the different sorption behaviours of iodide and iodate and the possible conversion of iodine's initial chemical form; (4) the transport behaviour of potential FBA and halide tracers under relevant geochemical conditions should be evaluated before beginning ambitious, large-scale field tracer experiments. Copyright © 2005 John Wiley & Sons, Ltd.

A new modelling approach to simulate preferential flow and transport in water repellent porous media: Model structure and validation

Water repellent soil and surface layers exhibit a complex flow and transport mechanism. Knowledge of the underlying principles is essential, for instance, to simulate water availability for crops and to estimate leaching potentials of agrichemicals. The present study aims to introduce and apply a new modelling approach to simulate preferential flow and transport in water repellent porous media, and to test the model on basis of an extensive field tracer experiment. The process of preferential flow and transport has been incorporated in the well-known SWAP model and applied to field data of tracer transport through a water repellent sandy soil in the Netherlands. Flow concept and model structure have been outlined, and simulation results presented. Results indicated early arrival times of bromide tracer in the subsoil in the case where preferential flow has been taken into account in the model. Comparison of measured and computed bromide concentration profiles and bromide recovery rates show close resemblances. Several strategies to alleviate soil water repellency and prevent the occurrence of preferential flow and transport processes are highlighted, such as irrigation scheduling, clay additions, and application of surfactants.

Experimental verification for real-time environmental emergency response system: WSPEEDI by European tracer experiment

Abstract Japan Atomic Energy Research Institute has developed an emergency response system WSPEEDI to forecast long-range atmospheric dispersions of radionuclides discharged into the atmosphere. The latest version of WSPEEDI consists of an atmospheric dynamic model MM5 for calculating meteorological fields and a particle random-walk model for atmospheric dispersion. The performance of WSPEEDI was evaluated by data obtained from a field tracer experiment over Europe (ETEX) in this paper. The model validation was done with respect to the following points: (1) the dependence of model accuracy on the temporal and spatial resolutions of the meteorological fields and (2) the superiority of an atmospheric dynamic model over a mass-consistent wind model. Regarding (1), it was shown that the calculation accuracy of the new version with high temporal resolution was improved, especially at the edge of the plume. Moreover, although the increase in horizontal spatial resolution of the old version had no substantial effect on the model performance, increase in horizontal resolution of the new version contributed to the significant improvement of the calculation accuracy. These results showed that the dynamically calculated meteorological field with the spatial resolution of the meso- β – γ scale greatly improved calculation accuracy.

Transport of conservative and reactive tracers through a naturally structured upland podzol field lysimeter

Compared to agricultural soils, descriptions of solute transport in upland soils at scales smaller than catchments are scarce. This study evaluates data from three consecutive field tracer experiments in which Cl, Na and Ca (stepped rates of 4.5, 9 and 4.5 mm h −1) were irrigated onto a ferric podzol monolith (3×2 m 2 and 0.6 m depth) isolated from the surrounding soil except at the base. For each individual experiment a one hour pulse of tracer was applied and subsequently washed through with water for approximately 1 pore volume and monitored using suction cup samplers located in the O and B horizons. Breakthrough curves showed considerable heterogeneity in the response of both horizons and inferred that downward leaching would result in considerable temporal spreading of solute mass across wide time scales. Patterns of Cl and Na breakthrough were similar and were characterised by low peak concentrations (maximum C/C 0 of 0.05 Cl and 0.03 Na). Minimal Ca breakthrough was observed in the O horizon, although a higher breakthrough was observed in the B horizon. These factors suggest that isolated highly active pathways were controlling the O horizon response, although these were not captured by the sampler network. This was supported by more rapid mean times to peak in the B horizon. Lateral drainage, intercepted at the base of the O horizon on the downslope face, accounted for approximately 45% of solute volume and 25, 21 and 14% of Cl, Na and Ca tracer mass, respectively. This rapid drainage pathway comprised surface flow and flow along the organic mineral interface, likely fed from preferential flow pathways in the O horizon. The breakthroughs observed in the mineral soil, although heterogeneous, indicated a response for this horizon of sustained leaching to shallow ground waters.

Comparisons of Transport and Dispersion Model Predictions of the URBAN 2000 Field Experiment

The tracer releases of the “URBAN 2000” urban tracer and meteorological field experiment conducted in Salt Lake City, Utah, in October 2000 provided a wealth of data for comparison with the predictions of transport and dispersion models. Comparisons of several sets of predictions created with the U.S. Defense Threat Reduction Agency's urban Hazard Prediction and Assessment Capability (HPAC) model are compared with the observations of the URBAN 2000 field study. This analysis focuses on paired-in-space-and-time comparisons of predictions and observations and uses conventional statistics as well as a previously described transport and dispersion model user-oriented measure of effectiveness. Four urban HPAC model configurations and five weather input options were examined, and, thus, 20 sets of URBAN 2000 predictions were created. Novel features of this analysis include 1) an inclusive data protocol that allowed for the use of 94% of the observed URBAN 2000 sulfur hexafluoride (SF6) 30-min average concentrations, 2) the application of the previously developed user-oriented measure of effectiveness (MOE) to urban predictions and observations, and 3) the use of nonparametric hypothesis test procedures to guide objectively the search for significant performance differences between urban HPAC model configurations through over 25 000 comparisons of metrics. Robust findings are reported that include the statistical discernment of general trends associated with urban HPAC predictive performance as well as the identification of differences in performance among various model configurations.

Salt tracer experiments in constructed wetland ponds with emergent vegetation: laboratory study on the formation of density layers and its influence on breakthrough curve analysis

Constructed wetlands are a rapidly expanding and intensively studied wastewater treatment system. One of the main types in use is the free water surface (FWS) wetland or wetland pond. In studies on these ponds, salt tracer experiments are a convenient tool to determine travel time distributions, which are, in turn, related to hydraulic and sedimentation (trapping) as well as nutrient removal efficiencies. Typically, flows encountered in constructed wetland ponds are characterized by low Reynolds numbers, at times even within the laminar flow regime. In such conditions the injection of salt may cause strong density effects, thereby threatening the usefulness of the recorded breakthrough curves. The processes and mechanisms governing the formation of density stratification due to salt tracer injections into wetland ponds with emergent vegetation were studied in the laboratory. The results reported are expected to be useful in the planning of future field tracer experiments.

Simulation of the Dispersion of Radioactive Effluents over the Kori site using Field Tracer Experiment

Field tracer experiments have been conducted at the Kori nuclear site for the purpose of analyzing the site-specific atmospheric dispersion characteristics and validating a real-time radiological dose assessment system FAD AS. Field tracer experiments were performed for three years from 2000 to 2002. Tracer gas SF6 was released at the top of the meteorological tower of the site and sampled by 140 automatic sequential gas samplers. The samplers were disposed at sampling points along the traffic roads around the site. The coordinates of all the sampling points were investigated by GPS. The concentration of the sampled gas was analyzed by gas chromatography. And the measured concentration distribution was compared with the distribution simulated by FAD AS. During the experiments, meteorological data was measured at several sampling points including the release point. The measured meteorological data were used for the numerical simulation by FAD AS. The comparative study between the measured and the simulated concentration was conducted. Due to the effects of the complex terrain of the Kori site, the degree of agreement between the measure and the simulated concentration is somewhat lower than that of the site of a flat terrain. The analysis of the site-specific field tracer experiment provides the useful information for the realistic application of an atmospheric dispersion model for a nuclear site.

Estimation of the Concentration Distribution according to the Variations in a Wind Field over a Complex Terrain

A three-dimensional wind field and an atmospheric dispersion model have been developed for real-time accident generation of a wind field over the whole domain using several measured data. A Lagrangian particle model was used for the calculation of the concentration distribution of the radionuclides in the atmosphere. The field tracer experiment near the Younggwang nuclear power plant was carried out for the purpose of analyzing the site-specific environmental characteristics and validating the atmospheric dispersion model in May 1996. The calculated concentration the most important factors for calculating the concentration of the released particles in the dispersion model. Several numerical experiments using the measured wind data were performed to get more accurate concentration distributions compared with the results of the field tracer experiment of SF6. The developed numerical models are being used as a basic tool for emergency preparedness in Korea.

Dual-permeability modeling of preferential bromide leaching from a tile-drained glacial till agricultural field

A tile-drained agricultural field can be regarded as a ‘field-scale’ lysimeter that may be used to study soil water and chemical transport under relatively natural conditions. Tile discharge and effluent bromide concentrations measured in a previous field tracer experiment for a structured clayey loam at Bokhorst, Northern Germany, indicated strong preferential flow. Simulation using single domain HYDRUS numerical flow and transport model could nevertheless explain water outflow, however, completely failed to describe tile-drain leaching patterns of the conservative tracer. The objective of this paper was to analyze whether the nonequilibrium-type dual-permeability model concept could better capture soil structure related principle mechanisms of preferential leaching in the unsaturated soil at that study site. The dual-permeability model (DUAL) describes for soil matrix and fracture pore systems Darcian flow with coupled Richards' equations and convective–dispersive (CD) solute transport with coupled CD equations. The hydraulic parameters of the dual-permeability model were obtained from standard soil hydraulic measurements by adopting a bimodal fitting procedure, whereas transport parameters were inferred from soil column tracer experiments and geometrical transfer term parameters were derived using qualitative soil structure descriptions. The hydraulic conductivity K a in the inter-domain water transfer term and the effective diffusion coefficient D a in the solute mass transfer term were calibrated by comparing simulated with measured tile flow and effluent concentrations. The DUAL approach described water flow similarly well as the single-domain model. Bromide concentrations in the tile effluent could be approximated with DUAL when decreasing the K a and D a values by three orders of magnitude compared with the values of the soil matrix domain. The dual-permeability approach seems to reflect nonequilibrium transport mechanisms at this structured soil since it not only predicts concentration peaks in initial leaching phases during early time events of the experiment but also the observed reverse reaction of decreased effluent concentrations in initial phases of leaching events during later times of the experiment. The latter occurs when bromide-free infiltrating water is diluting bromide concentrations in the fracture domain such that less concentrated water is preferentially draining from tiles in initial phases of infiltration. Bromide effluent concentrations gradually increase in later phases since vertical flow velocities decrease while ‘diffusive’ solute transfer from the soil matrix continues. Mass transfer restrictions (e.g. due to soil aggregate coatings) may effectively be controlling preferential leaching at this site.

Borehole cylindrical noise during hole–surface and hole–hole resistivity measurements

Drilled boreholes generally are the only feasible means to access the subsurface for the emplacement of downhole electrodes for most hole–hole and hole–surface resistivity experiments. However, the very existence of the borehole itself creates the potential for significant noise due to the inevitable conductivity contrast that develops between the borehole walls and the formation. Borehole cylindrical noise develops whenever a current source is placed in a drilled borehole. Borehole geometries may range from nearly perfect cylinders to highly, irregular, rugose holes in consolidated rock, to relatively minor, collapsed, disturbed zones in caving sediments. Boreholes in non-caving formations generally are filled with artificial, conductive materials to afford crucial, electrical continuity between downhole electrodes and the borehole walls. Filled boreholes form cylindrically shaped heterogeneities that create significant noise due to preferential current flow up and down the conductive columns. Selected conditions are simulated with a finite difference model to illustrate the significance of borehole cylindrical noise on hole–hole and hole–surface mise-à-la-masse electrical potentials near a current electrode. Mise-à-la-masse electrical potentials measured during a field tracer experiment also are presented. These measurements are used to illustrate significant errors may develop in the interpretation of apparent resistivity estimates out to a distance of several meters from the current source if borehole cylindrical noise is not recognized and accounted for in the analysis of electrical potential data.

Tracer migration experiments in unsaturated fractured tuff

Field tracer experiments were conducted at an underground tunnel—the Exploratory Studies Facility (ESF)—at Yucca Mountain, Nevada, to investigate the localized preferential flow and transport resulting from multiple episodic liquid-release tests in unsaturated fractured tuff. Liquid was released into an isolated test interval (0.30 m long) within a borehole drilled along ESF. Out of 60 liquid-release tests, 36 contained tracers (mostly food and fluorescent dyes) to help us to elucidate the flow paths in unsaturated tuff. After the tracer-migration test, an array of 12 boreholes was drilled into the test area to collect rock samples. These samples were then analyzed to delineate the extent of tracer migration. Complementary laboratory column transport studies using crushed rock showed that the dye tracers were—at most—very weakly sorbed to the tuff matrix. We found that the tracer-migration test, with a limited release volume of 1.5 L, resulted in a localized distribution of tracers, most likely confined to a 1.0 × 1.6-m area directly below the test interval (over a vertical experimental length of about 0.65 m). This conclusion was corroborated by detecting dyes that had been introduced during previous liquid-release seepage tests either before or after niche excavation; the spatial distribution of all dyes was near their individual release intervals. Limited lateral spreading of the dyes occurred, even when several non-dye-containing seepage tests were intermittently conducted following dye release. This work also demonstrates the utility of applying multiple tracers (including food and fluorescent dyes) to help delineate transport characteristics in unsaturated fractured rock.

Modeling tracer injection and well‐aquifer interactions: A new mathematical and numerical approach

A new mathematical and numerical approach is presented to model solute exchange between a well and the surrounding aquifer for the interpretation of field tracer tests. On the basis of water and tracer mass balance equations integrated over the volume of water in the well, the approach allows for finite volumes of tracer fluid and water flush. It deals with tracer mixing and capturing in the well bore, local distortion of the flow field around the well, and possible tracer back‐migration into the well. A numerical solution, implemented in the three‐dimensional finite element groundwater flow and transport simulator SUFT3D, is proposed that allows for modeling nonuniform distributions of tracer mass fluxes along the well screens related to variations in aquifer hydraulic conductivity. Showing its ability to reproduce concentration evolutions monitored in a well during field tracer experiments, considering various injection conditions, validates the approach.

Analysis and numerical simulation of a single-well tracer test in homogeneous, layered and slightly tilted formations

Simulation of a field tracer experiment from an injection well in an axially symmetrical flow field in homogenous, stratified and slightly tilted aquifers is presented. The simulation has been verified by analytical solutions of the evolution of the first and second radial spatial moments of the tracer displacements derived in the current study under pure advective transport in case of layered formation. The study focused also on the discrepancies in the transport mechanisms between uniform (linear) and axially symmetrical radial flow fields in homogenous, layered and slightly tilted formations. Excellent agreement exists between analytical solution and the numerical simulation for the case of pure advection in both first and second moments supporting the validity of the numerical simulations. A subdiffusive dispersion regime in case of transport by advection and dispersion in homogeneous aquifer is observed due to the decline of the velocity field. N K;) is the representative effective medium of the layered aquifer in case of pure advection under axially symmetrical flow field.

Imaging and characterisation of subsurface solute transport using electrical resistivity tomography (ERT) and equivalent transport models

We assess the usefulness of electrical resistivity tomography (ERT) in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers. A field tracer experiment was conducted at the Krauthausen test site, Germany. The spatial and temporal evolution of the injected NaBr solute plume was monitored in a 2D ERT image plane located downstream of the injection well for 90 days. Since ERT maps changes in bulk electrical conductivity, the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation. The concentration maps are then analysed using an equivalent convection–dispersion model (CDM), which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity. As demonstrated by associated synthetic model studies, ERT resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm. Since for heterogeneous media, local concentrations within the plume deviate from those predicted by the equivalent CDM, we also interpret the ERT-derived pixel breakthrough curves in terms of an equivalent stream-tube model (STM). The STM represents transport in the aquifer by a set of 1D convection–dispersion processes, allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified. We believe that the observed tracer plume is satisfactorily described by the equivalent CDM, probably because the tracer plume was small relative to the heterogeneity scale of the aquifer. Even though application of the STM revealed some deviation from the ideal homogeneous case, the equivalent dispersivity in the STM matches the longitudinal dispersivity of the CDM closely, consistent with predominantly homogeneous mixing. However, the STM analysis illustrates how ERT results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers.

Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

Field and laboratory tracer experiments were conducted to investigate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments were carried out in the Exploratory Studies Facility (ESF), an underground tunnel at Yucca Mountain. Water containing dye was released into horizontal boreholes drilled into the wall of the ESF main drift. The region was then mined to observe the flow pathways and to collect dye-stained rock samples for subsequent laboratory quantification. Dye concentration profiles in the rock, measured using a newly developed sampling technique, showed that liquid flowing through the fractures penetrated into the matrix to a depth of several millimeters. Laboratory studies of tracer penetration into the rock matrix were conducted using tracer-free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer-imbibition tests were performed on cores at two different initial water saturations with both sorbing (dyes) and nonsorbing tracers. The travel distance for sorbing dyes was a few millimeters after ∼16 to 20 h, similar to the extent measured in samples from the field test. The nonsorbing bromide front coincided with the wetting front in the rock core at the initial water saturation of 12%, and the imbibition depth agreed very well with the prediction, using independently measured properties. At the high initial water saturation of 76%, the bromide front lagged significantly behind the wetting front. Sorption coefficients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel-distance and mass-distribution calculations, and were found to yield comparable results. Utilization of nonsorbing tracers with different molecular sizes helped to identify the effects of pore-size restriction on tracer transport during imbibition. The results from this work have a direct application to radionuclide transport at Yucca Mountain, and the methods presented are broadly applicable to the investigation of water and solute transport in unsaturated rock.

Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

Field and laboratory tracer experiments were conducted to investigate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments were carried out in the Exploratory Studies Facility (ESF), an underground tunnel at Yucca Mountain. Water containing dye was released into horizontal boreholes drilled into the wall of the ESF main drift. The region was then mined to observe the flow pathways and to collect dye‐stained rock samples for subsequent laboratory quantification. Dye concentration profiles in the rock, measured using a newly developed sampling technique, showed that liquid flowing through the fractures penetrated into the matrix to a depth of several millimeters. Laboratory studies of tracer penetration into the rock matrix were conducted using tracer‐free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer‐imbibition tests were performed on cores at two different initial water saturations with both sorbing (dyes) and nonsorbing tracers. The travel distance for sorbing dyes was a few millimeters after ∼16 to 20 h, similar to the extent measured in samples from the field test. The nonsorbing bromide front coincided with the wetting front in the rock core at the initial water saturation of 12%, and the imbibition depth agreed very well with the prediction, using independently measured properties. At the high initial water saturation of 76%, the bromide front lagged significantly behind the wetting front. Sorption coefficients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel‐distance and mass‐distribution calculations, and were found to yield comparable results. Utilization of nonsorbing tracers with different molecular sizes helped to identify the effects of pore‐size restriction on tracer transport during imbibition. The results from this work have a direct application to radionuclide transport at Yucca Mountain, and the methods presented are broadly applicable to the investigation of water and solute transport in unsaturated rock.

Contribution of nitrification and denitrification sources for seasonal N 2O emissions in an acid German forest soil

A 15N tracer field experiment was carried out in an acid soil of a beech forest gap at the Solling area in Germany to determine the different formation processes of N 2O. This site has been classified as a seasonal N 2O emission type forest. Small amounts of highly enriched 15N tracer were supplied separately as labelled NH 4 + and NO 3 − seven times from May to October 1998. The 15N compounds were injected with syringes into the upper 10 cm of the soil. The N 2O emission showed a seasonal pattern with low emission in spring, high emission in summer, and low emission in autumn and followed the soil temperature. N 2O was evolved only from the 15N-labelled NO 3 − pool in summer under field moist conditions, indicating that its formation was due to denitrification. In spring, early summer and autumn trapped N 2O or heterotrophic nitrification may also have been involved in N 2O emission.

Results from Three Field Tracer Experiments on the Neighbourhood Scale in the City of Birmingham UK

The physical processes governing flow and pollutantdispersion at the neighbourhood scale, a spatial scaleintermediate between the street scale and the city scale, is notwell understood. Furthermore, it is not clear whether a traditional approach using averaged characteristics such as theaerodynamic roughness length is sufficient to predict the concentration field at this scale. To investigate pollutant dispersion in a real urban area, three field experiments were designed within the UK-URGENT programme sponsored by NERC. Theexperiments were performed in the City of Birmingham using a finite duration release of inert, non-toxic and non-depositingtracers, vis. perfluoromethylcyclohexane (PMCH) and perfluoromethylcyclopentane (PMCP). Measurements were taken using air bag samplers placedin an arc at 3.5 km (first experiment) and 1 km (second andthird experiments) from the source; some trap samplers wereplaced outside the main arc in the outskirts of the city. Measurements were analysed in the laboratory using anovel gas-chromatography technique. Data so obtained werecompared with predictions from a simple steady-state modeland a time-dependent model. The concentration-time serieswere very asymmetrical with a sharp rise, a plateau followedby a relatively slow decrease and finally a long-livedplateau above (or possibly very slow decrease to) thebackground level.

A field tracer study of attenuation of atrazine, hexazinone and procymidone in a pumice sand aquifer.

A field tracer experiment, simulating point source contamination, was conducted to investigate attenuation and transport of atrazine, hexazinone and procymidone in a volcanic pumice sand aquifer. Preliminary laboratory incubation tests were also carried out to determine degradation rates. Field transport of the pesticides was observed to the significant under non-equilibrium conditions. Therefore, a two-region/two-site non-equilibrium transport model, N3DADE, was used for analysis of the field data. A lump reduction rate constant was used in this paper to encompass all the irreversible reduction processes (e.g. degradation, irreversible adsorption, complexation and filtration for the pesticides adsorbed into particles and colloids) which are assumed to follow a first-order rate law. Results from the field experiment suggest that (a) hexazinone was the most mobile (retardation factor R = 1.4) and underwent least mass reduction; (b) procymidone was the least mobile (R = 9.26) and underwent the greatest mass reduction; (c) the mobility of atrazine (R = 4.45) was similar to that of rhodamine WT (R = 4.10). Hence, rhodamine WT can be used to delimit the appearance of atrazine in pumice sand groundwater. Results from the incubation tests suggest that (a) hexazinone was degraded only in the mixture of groundwater and aquifer material (degradation rate constant = 4.36 x 10(-3) day-1); (b) procymidone was degraded not only in the mixture of groundwater and aquifer material (rate constant = 1.12 x 10(-2) day-1) but also in the groundwater alone (rate constant = 2.79 x 10(-2) and-1); (c) atrazine was not degraded over 57 days incubation in either the mixture of aquifer material and groundwater or the groundwater alone. Degradation rates measured in the batch tests were much lower than the total reduction rates. This suggests that not only degradation but also other irreversible processes are important in attenuating pesticides under field conditions. Hence, the use of laboratory-determined degradation rates could underestimate reduction of pesticides in field conditions.