Laboratory studies of steam stripping of LNAPL‐contaminated soils

Bench‐scale laboratory experiments were conducted to evaluate the effectiveness of steam injection for in situ remediation of soils contaminated by light nonaqueous‐phase liquids (LNAPLs). Several parametric studies were performed with various combinations of soils, LNAPLs, and steam injection conditions.

An increase in steam injection pressure produced a significant increase in LNAPL recovery efficiency. An increase in steam injection pressure from 12.4 to 44.8 kPa resulted in increased LNAPL recovery efficiency from 86 to 95% after one pore volume of steam injection. Higher steam injection pressure yielded maximum LNAPL recovery efficiency in significantly less time and required a smaller amount of steam than at low pressure.

An increase in soil grain size or an increase in grain‐size‐distribution slope resulted in increased LNAPL recovery efficiency. The final LNAPL residual saturation was approximately 0.5% for coarse‐grained soils and 1.8% for soils with finer grain sizes. Soils with finer grains required more time for treatment than soils with coarser grains.

Steam injection experiments with No. 2 heating oil and with jet fuel showed no significant variation in steam front propagation, temperature profile, and maximum LNAPL recovery efficiency. The LNAPL residual saturation after steam injection was essentially independent of the starting LNAPL saturation.     

The effect of soil‐particle size on hydrocarbon entrapment near a dynamic water table

The entrapment of residual hydrocarbon globules by water table fluctuations can produce a long‐term contamination threat to groundwater supplies that is difficult to remove. The mobilization of entrapped hydrocarbon globules depends on the balance between capillary and gravitational forces represented by the Bond number. It is important to estimate the potential for hydrocarbon entrapment at a spill site due to its influence on the effectiveness of remediation efforts. The present work focuses on the influence of particle diameter on hydrocarbon entrapment for a typical LNAPL (light nonaqueous‐phase liquid). Laboratory column tests have been conducted using a dual‐beam gamma densitometer to measure saturations of the three phases (water, air, and hydrocarbon). Soltrol 170^®, a solvent manufactured by Phillips 66 Co., is used as the hydrocarbon. Residual saturation of the Soltrol is measured after fluctuations in water table level to establish the distribution and consistency of hydrocarbon entrapment below the water table. Glass particles of nearly uniform size were used to represent a sandy soil. In the experiments, average particle sizes ranged from 210 to 6000 μm. Data were also taken using the synthetic soil matrix approved by the U.S. Environmental Protection Agency (EPA) for contamination studies. Results show that the distribution of trapped LNAPL is quite uniform and that the average residual saturation is about 13% up to a particle diameter of 710 μm. Above this diameter, residual saturation decreases with particle size. The corresponding critical Bond number, determined experimentally, agrees well with the predicted value of 1.6.     

Acknowledgment

In a water wet porous medium, the antecedent moisture content controls the entrapment of LNAPL at low capillary numbers. A two‐phase retention cell study of entrapment was conducted for LNAPL water systems in a carbonate sand. For two different LNAPL water systems, it was found that a linear expression related the residual LNAPL saturation to the antecedent water saturation. The prediction of trapped LNAPL saturation compares favorably to long‐column test data obtained for the same sand. A trapping model was developed, and its predictions were compared to field measurements of trapped LNAPL saturation. Deviations between predicted and measured LNAPL saturations arose when there was a change in the median pore size distribution. In addition, a series of three‐phase (air‐LNAPL‐water) retention cell tests measured the amount of LNAPL that became trapped when the water phase was maintained at a constant saturation during LNAPL withdrawal under a negative pressure head. Under these conditions, the amount of trapped LNAPL was higher than that measured by two‐phase tests at the same water saturation.     

Numerical Simulations and Long-Column Tests of LNAPL Displacement and Trapping by a Fluctuating Water Table

Long-column laboratory tests were performed to validate improvements to the MOFAT program for simulating LNAPL displacement and entrapment in response to a fluctuating water table. The long-column tests consisted of a fluctuating water table and its subsequent displacement and entrapment of an LNAPL. The modifications of MOFAT include a linear LNAPL trapping estimate and a new scaling technique for the inhibition portion of the fluctuation (water table rise). Improved prediction of the LNAPL trapping was obtained by assuming the amount of LNAPL that is trapped by a rising water table is proportional to the antecedent water content of the porous medium. The pressure-saturation relationship for the air-water drainage system was scaled to estimate the LNAPL-water and air-LNAPL drainage relationships. Scaled inhibition pressure-saturation relationships are improved by incorporating a correction for contact angle hysteresis and surface roughness. The incorporation of these changes into MOFAT led to noticable improvements in the numerical simulation of the experimental data.     

Dynamics of phreatophyte root growth relative to a seasonally fluctuating water table in a Mediterranean-type environment

While seasonal redistribution of fine root biomass in response to fluctuations in groundwater level is often inferred in phreatophytic plants, few studies have observed the in situ growth dynamics of deep roots relative to those near the surface. We investigated the root growth dynamics of two Banksia species accessing a seasonally dynamic water table and hypothesized that root growth phenology varied with depth, i.e. root growth closest to the water table would be influenced by water table dynamics rather than surface micro-climate. Root in-growth bags were used to observe the dynamics of root growth at different soil depths and above-ground growth was also assessed to identify whole-plant growth phenology. Root growth at shallow depths was found to be in synchrony with above-ground growth phenophases, following increases in ambient temperature and soil water content. In contrast, root growth at depth was either constant or suppressed by saturation. Root growth above the water table and within the capillary fringe occurred in all seasons, corresponding with consistent water availability and aerobic conditions. However, at the water table, a seasonal cycle of root elongation with drawdown in summer followed by trimming in response to water table rise and saturation in winter, was observed. The ability to grow roots year-round at the capillary fringe and redistribute fine root biomass in response to groundwater drawdown is considered critical in allowing phreatophytes, in seasonally water-limited environments, to maintain access to groundwater throughout the year.     

Effect of Soil Fabric on Transport of a LNAPL through Unsaturated Fine-Grained Soils: A Centrifugal Model Study

Centrifugal model tests were performed to study the impact of the fabric of a fine-grained soil on transport of a light non-aqueous phase liquid (LNAPL). An image processing technique was developed to extract contaminant transport and fate data from the centrifugal model. Two unconsolidated sites with different moisture contents and a saturated site consolidated due to self-weight were simulated using the centrifuge. The LNAPL migrated in the vertical direction as a narrow plume and formed a free product pool above the saturated zone in unsaturated and unconsolidated soils. However, the LNAPL migrated in the horizontal direction before moving in the vertical direction as a broad plume in the consolidated site. The test results showed that the final width of the plume in the unsaturated zone of the consolidated site was nearly two times as large as that for the unconsolidated sites. In addition, the rate of leak from the underground storage tanks (USTs) on consolidated soils was substantially higher when compared with those on the unconsolidated state. The comparison of LNAPL saturation profiles at the centerline of the centrifugal models during leakage showed that, depending on the soil fabric at a given time and depth, the LNAPL phase would be different; i.e., mobile or immobile (residual) in the same soil type. The test results provided additional insight into contribution of soil fabric on transport and fate of contaminants. The soil fabric controls the geological and hydro-geological properties of fine-grained soils and hence the contamination plume.     

PHYTOREMEDIATION OF BTEX HYDROCARBONS: POTENTIAL IMPACTS OF DIURNAL GROUNDWATER FLUCTUATION ON MICROBIAL DEGRADATION

Volatile hydrocarbons have multiple potential fates in phytoremediation. This research investigated the relationship between biodegradation and plant uptake of BTEX compounds in laboratory and field settings. At a phytoremediation site, preliminary studies revealed minimal uptake into trees and enhanced degradation potential in the rhizosphere and in the bulk soil. Increased oxygen transport to the vadose zone caused by diurnal rise and fall of the water table was hypothesized to enhance degradation in the bulk soil. A detailed greenhouse study was then conducted to investigate potential bioremediation impacts using field-site soil and DN34 hybrid poplar trees.

In rhizosphere soils, the contaminated-planted reactor had significantly higher BTEX degrader populations versus the uncontaminated-planted reactor, as was anticipated. The bulk soil in the planted-contaminated reactor had increased degrader populations than the unplanted-contaminated soil or planted-uncontaminated soil, and planting increased degradation throughout the soil profile, not just in the limited volume of rhizosphere soils. Oxygen diffusive and advective transport into reactors was modeled and calculated. Oxygen input in planted reactors was at least 3 to 5 times higher than in unplanted reactors, and increasing oxygen input lead to increased degrader populations in a linear manner. These results combined with the knowledge that high-transpiration trees draw the contaminated groundwater to the capillary fringe and the rhizosphere indicate that phytoremediation can aid microbial degradation via multiple mechanisms: increasing degrader populations, increasing oxygen input via groundwater diurnal fluctuations, and transporting contaminants to the biologically-enriched soil profile.     

Impact of Surfactant on Configuration of Petroleum Hydrocarbon Lens

A laboratory-scale physical model was constructed for visual observation of the basic 2-D flow characteristics of a gasoline spill through an unconfined aquifer and the subsequent treatment with a surfactant. The model consists of a parallel-plate glass tank (1?m×1?m×5?cm) packed with Ottawa sand. Gasoline was released from a point source in the vadose zone. As the specific gravity of gasoline is less than one (LNAPL), it pooled above the water saturated pores of the tension saturated region of water. Beyond the lens of gasoline, the height of the capillary fringe was reduced due to capillary pollution. The gasoline lens was then treated with an aqueous phase surfactant solution of 2% dodecyl benzene sulfonate (anionic) and 2% polyethoxylate nonyl phenol (nonionic). This surfactant solution reduced the interfacial tension between the gasoline and the aqueous phase by an order of magnitude. The surfactant solution was released from the same point source in the vadose zone as the gasoline. As a result, the location and geometry of the gasoline lens and the polluted capillary fringe were significantly altered. These changes were investigated using vertical equilibrium models, the capillary number, the buoyancy number and the total trapping number to evaluate the approach of pretreatment as a potential remediation strategy.     

Feasibility of in situ implementation of vibrations to mobilize NAPL ganglia

A growing number of incidents of nonaqueous phase liquid (NAPL) spills in the recent past have warranted development of innovative and cost‐effective remediation technologies. Of particular concern is the entrapment of LNAPL (NAPL lighter than water) in the form of ganglia or blobs near the water table by virtue of strong capillary forces. The residual ganglia are the leftover component after pumping of free product and typically occupy 20 to 60% of the pore space. Mobilization of these ganglia would require unrealistically high hydraulic gradients and is often beyond the scope of pump‐and‐treat processes. This paper deals with the feasibility of in situ implementation of localized vibrations for controlled mobilization and collection of LNAPL ganglia. Specifically, the paper covers three components. First, the principles involved in soil‐water‐NAPL interactions under the influence of vibrations are discussed. The effects of vibrations on a soil‐NAPL‐water medium are postulated in terms of pore structure and relative density changes, changes in the permeability of the medium as a result of the changes in pore structure, and development of cyclic pore pressures. Second, results from bench‐scale experiments are presented that involved vibrating contaminated soils under the simultaneous influence of hydraulic gradients. A bench‐scale model consisting of a vibrator integrated with an injection and pumping system was found to be successful in these experiments. The results from the tests showed that up to 85% removal of ganglia can be achieved using this process. Third, the principles involved in the vibratory mobilization were applied to in situ conditions to develop a methodology to estimate the zone of influence of the process. The analogy between this process and an existing geotechnical process known as vibroflotation is exploited to demonstrate the methodology.     

Root growth of soybean (Glycine max L. Merr.) and cowpea (Vigna unguiculata Walp.) on a hydromorphic toposequence in Western Nigeria

Summary The effects of variations in edaphic and hydrologic factors over a short distance on the root growth of soybean cv. Hernon 237 and cowpea cv. IT 82 E-60 were studied in a hydromorphic toposequence. During the growing season the water table (WT) fluctuated from 0.43 to 0.94 m (high), 0.60 to 1.12 m (medium) and 0.72 to 1.51 m (low), respectively in 1983 and from 0.47 to 0.84 m (high), 0.63 to 1.13 m (medium) and 0.83 to 1 20m (low). respectively, in 1984. Poor soil aeration did not limit growth, even for high WT.Root penetration into the deeper soil was prevented at the low and medium water table sites by the presence of a naturally occurring compacted gravel layer at the 0.30–0.40 m depth. The absence of this layer at the high water table site resulted in root growth and proliferation of soybean roots even within the capillary fringe zone immediately above the water table. Cowpea roots, however, were not observed in this saturated soil zone. Cowpea roots penetrated deeper in high than in medium and low WT. Evapotranspiration (Et) and E_t/E_o values of both crops were significantly greater at the high than at medium or low water table.     

民勤绿洲边缘地下水位变化对植物种群生态位的影响

在民勤绿洲边缘,利用空间区域不同地下水位(湖区8～12m,泉山区15～17m,坝区20～23m)和时间序列(1984～1992年)民勤沙井子地区地下水位下降(7.45～11.65m)梯度,研究地下水位下降对荒漠植物种群生态位的影响。结果表明:空间区域地下水位下降,植物种群生态位宽度均减小,种群退化;时间序列地下水位下降,白刺种群在扩展,其它植物种群在退化。白刺种群生态位宽度在民勤绿洲边缘荒漠植物群落中最大,是该区的建群种。由于白刺种群在地下水位7.45～11.65m范围内扩展,民勤绿洲生态环境治理中地下水位达到该范围是一个主要目标。     

The effect of interfacial tension on hydrocarbon entrapment and mobilization near a dynamic water table

The entrapment of residual hydrocarbon ganglia during water table fluctuations can produce a long‐term contamination threat to groundwater supplies that is difficult to remove. The mobilization of entrapped hydrocarbon ganglia depends on the balance between capillary and gravitational forces represented by the Bond number. The present work focuses on the influence of the interfacial tension between the hydrocarbon and the surrounding water on the entrapment and mobilization of the residual ganglia. Laboratory column tests using glass beads as the porous medium have been conducted to determine the residual saturation of a hydrocarbon (Soltrol 170) trapped during vertical displacements due to a rising water table and the necessary decrease in interfacial tension to mobilize these trapped ganglia. The interfacial tension was decreased by the addition of isopropyl alcohol to the water phase. Saturations of the three phases (water, hydrocarbon, and air) were measured with a dual‐beam y‐densitometer. The results for residual hydrocarbon saturation at various interfacial tensions were combined with previous results for different particle diameters to provide a general relationship between residual saturation and Bond number. The relationship is expressed in an empirical correlation valid for Bond numbers between 0.001 and 1.2.     

Fringe mode reflectance laser Doppler microscope system

A reflectance fringe mode Laser Doppler microscope anemometer (LDMA) has been designed and tested. A novel beam splitter permits adjustment of the direction and separation of the two laser beams which form the fringe sensing volume, and furnishes the control needed to compensate for chromatic aberration and variation in optical quality. Tests conducted in a narrow 28:1 aspect rectangular flow channel filled with water showed close agreement between the predicted and measured velocity profiles. Velocity measurements, made alternately in the reflectance and transmittance modes using the same sensing volume in the channel, revealed deviations between the modes of less than 1.1% for water and 2.3% for whole blood. The deviation for the in vivo experiment was better than 2.8%. Velocity profiles were measured alternately in the same sensing volume, (dimensions were 10 × 12 μm) by reflectance and transmittance LDMA in 40 hamster cheek-pouch arterioles, ranging from 65 to 140 μm in diameter. These showed close agreement throughout the range of measured vessels. Reflectance LDMA makes non-invasive, high resolution velocity measurements in the range of 0.03 to 40 cm/s possible in small blood vessels near the surface of opaque organs and tissues. The accuracy and reproducibility of the readings were calculated to be better than 1.1%. Reflectance fringe mode LDMA greatly extends the applicability of LDMA because sites where transmittance LDMA can be applied are limited.     

Limited dispersal and geographic barriers cause population differentiation and structuring in Begonia maxwelliana at both large and small scales

Background: Genetic divergence is one of the key processes in speciation. In the Begoniaceae, genetic divergence caused by limited gene flow may explain its high species diversity and endemicity. This hypothesis has been supported by past genetic work but there is a lack of empirical studies on the causes of limited gene flow.

Aim: To identify the causes of limited gene flow in Begonia.

Methods: We examined the genetic structure among the populations of Begonia maxwelliana at the macro- and micro-spatial scales using microsatellites, measured seed dispersal range and observed flowering phenology.

Results: Population differentiation and structuring were detected at both the macro- and micro-scales. Dispersal range was short, and all populations showed similar reproductive behaviour.

Conclusions: The strong population differentiation and structuring among the populations studied imply that they are evolutionarily significant units and possible candidates for speciation. Geographical barriers and limited seed dispersal restrict gene flow in the populations, and these factors may be responsible for the rapid speciation and large diversity in the family.     

Increase in the thermostability of Bacillus sp. strain TAR-1 xylanase using a site saturation mutagenesis library

Site saturation mutagenesis library is a recently developed technique, in which any one out of all amino acid residues in a target region is substituted into other 19 amino acid residues. In this study, we used this technique to increase the thermostability of a GH10 xylanase, XynR, from Bacillus sp. strain TAR-1. We hypothesized that the substrate binding region of XynR is flexible, and that the thermostability of XynR will increase if the flexibility of the substrate binding region is decreased without impairing the substrate binding ability. Site saturation mutagenesis libraries of amino acid residues Tyr43–Lys115 and Ala300–Asn325 of XynR were constructed. By screening 480 clones, S92E was selected as the most thermostable one, exhibiting the residual activity of 80% after heat treatment at 80°C for 15 min in the hydrolysis of Remazol Brilliant Blue-xylan. Our results suggest that this strategy is effective for stabilization of GH10 xylanase.

Abbreviations: DNS: 3,5-dinitrosalicylic acid; RBB-xylan: Remazol Brilliant Blue-xylan     

Effects of short-term drying and irrigation on CO2 and CH4 production and emission from mesocosms of a northern bog and an alpine fen

Atmospheric CO₂ and CH₄ exchange in peatlands is controlled by water table levels and soil moisture, but impacts of short periods of dryness and rainfall are poorly known. We conducted drying-rewetting experiments with mesocosms from an ombrotrophic northern bog and an alpine, minerotrophic fen. Efflux of CO₂ and CH₄ was measured using static chambers and turnover and diffusion rates were calculated from depth profiles of gas concentrations. Due to a much lower macroporosity in the fen compared to the bog peat, water table fluctuated more strongly when irrigation was stopped and resumed, about 11 cm in the fen and 5 cm in the bog peat. Small changes in air filled porosity caused CO₂ and CH₄ concentrations in the fen peat to be insensitive to changes in water table position. CO₂ emission was by a factor of 5 higher in the fen than in the bog mesocosms and changed little with water table position in both peats. This was probably caused by the importance of the uppermost, permanently unsaturated zone for auto- and heterotrophic CO₂ production, and a decoupling of air filled porosity from water table position. CH₄ emission was <0.4 mmol m^?2 day^?1 in the bog peat, and up to >12.6 mmol m^?2 day^?1 in the fen peat, where it was lowered by water table fluctuations. CH₄ production was limited to the saturated zone in the bog peat but proceeded in the capillary fringe of the fen peat. Water table drawdown partly led to inhibition of methanogenesis in the newly unsaturated zone, but CH₄ production appeared to continue after irrigation without time-lag. The identified effects of irrigation on soil moisture and respiration highlight the importance of peat physical properties for respiratory dynamics; but the atmospheric carbon exchange was fairly insensitive to the small-scale fluctuations induced.     

Improvement of phytoremediation of an aged petroleum hydrocarbon-contaminated soil by Rhodococcus erythropolis CD 106 strain

The aim of this study was to assess the impact of soil inoculation with the Rhodococcus erythropolis CD 106 strain on the effectiveness of the phytoremediation of an aged hydrocarbon-contaminated [approx. 1% total petroleum hydrocarbon (TPH)] soil using ryegrass (Lolium perenne). The introduction of CD 106 into the soil significantly increased the biomass of ryegrass and the removal of hydrocarbons in planted soil. The fresh weight of the shoots and roots of plants inoculated with CD 106 increased by 49% and 30%, respectively. After 210 days of the experiment, the concentration of TPH was reduced by 31.2%, whereas in the planted, non-inoculated soil, it was reduced by 16.8%. By contrast, the concentration of petroleum hydrocarbon decreased by 18.7% in non-planted soil bioaugmented with the CD 106 strain. The rifampicin-resistant CD 106 strain survived after inoculation into soil and was detected in the soil during the entire experimental period, but the number of CD 106 cells decreased constantly during the enhanced phytoremediation and bioaugmentation experiments.

The plant growth-promoting and hydrocarbon-degrading properties of CD 106, which are connected with its long-term survival and limited impact on autochthonous microflora, make this strain a good candidate for improving the phytoremediation efficiency of soil contaminated with hydrocarbons.     

Timing of the water-to-land transition and metamorphosis in the land hermit crab Coenobita compressus H. Milne Edwards: evidence that settlement and metamorphosis are de-coupled

After metamorphosing from the last larval stage to the transitional megalopal stage in the marine plankton, the hermit crab Coenobita compressus moves ashore where it undergoes a second metamorphosis to the first juvenile instar on land. In two experiments using laboratory-reared crabs, I moved megalopae from water to land after different amounts of time at this stage and investigated the impact of this manipulation on the timing of and survival through the second metamorphosis. In the Involuntary Settlement experiment, megalopae were moved to land when they were 3, 6, 9, 12, or 15 days old. None of those moved between the ages of 3 and 6 days survived through metamorphosis, but the majority of 9-day-old megalopae survived, as did most 12- and 15-day-old megalopae. This suggests that developmental changes early in the megalopal stage prepare C. compressus for terrestrial life. Once on land, megalopae that had been moved to land at 9 days spent about nine additional days there before metamorphosing, while 12- and 15-day-old megalopae metamorphosed after spending about 5 and 4 days, respectively, on land. In the Voluntary Settlement experiment, megalopae were given access to land when they were 1, 5, 10, or 15 days old, but were not forced to make the transition. Those given access to land after 1 day voluntarily left their dishes for the first time after an average of 7 days in water. Those given access when they were 5 days old remained in water about 4 days longer, while those given access when they were 10 and 15 days old left after less than a day. In both experiments, the timing of metamorphosis relative to settlement (i.e., transition to land) showed that these events are dissociated to a degree and revealed the presence of a metamorphic clock. I discuss why the dissociation of settlement and metamorphosis may have been favored in the land hermit crab and in another anomuran crab.     

Effects of microtopography and water table on Sphagnum palustre L. in subtropical high mountains and implications for peatland restoration

Introduction. Human disturbance has recently led to increasingly serious destruction of Sphagnum L. wetlands in subtropical high mountains, resulting in an urgent need for wetland restoration.

Methods. Through a field experiment conducted in western Hubei Province, China, the effects of four different microtopographic types [concave surface, convex surface, concave and convex surface (CC surface), and flat surface] and water table depth (0 to ?30?cm) on three growth indicators (number of capitula, coverage and biomass) of Sphagnum palustre L. were examined. The objective was to identify the optimal hydrological conditions for S. palustre growth and thus facilitate its rapid recolonisation and restoration of these wetlands.

Key results. The results showed that different microtopographic conditions significantly influenced S. palustre growth. Among them, S. palustre in the CC surface showed the worst growth, while no significant differences existed among the other three microtopographic types. Additionally, as the water table increased, the growth of S. palustre increased, but long-term flooding impeded growth. The water table affected S. palustre growth via effects on its tissue water content.

Conclusions. Microtopographic reshaping was not essential for the success of S. palustre recolonisation, and microtopography that maintained the water table to within ?10?cm of the surface without flooding were best, independent of the microtopographic types. In addition, the growth patterns of S. palustre changed with changes in the environment, which may be related to its long-term adaptation to conditions of a lower water table.     

The effect of salmon farming on the benthos of a Scottish sea loch

The effects of waste from a salmon farm on the benthos of a fjordic sea loch on the western coast of Scotland have been studied. Within 3 m of the floating cages the sediment was highly reducing, and dissolved oxygen content of the water overlying the sediment ranged from 35 to 75% saturation. At ⩾ 15 m from the cages the sediment was oxygenated, and the dissolved oxygen content of the water overlying the sediment was 50–85% saturation. Sedimentary redox potential and dissolved oxygen content of bottom water showed a seasonal variation. The benthic fauna showed marked changes in species number, species diversity, faunal abundance, and biomass in the region of the fish farm, with four zones of effect identified. Directly beneath, and up to the edge of the cages, there was an azoic zone. A highly enriched zone, dominated byCapitella capitata (Fabricius) andScolelepis fuliginosa (Claparède), occurred from the edge of the cages out to as 8 m. A slightly enriched ″transitional ″ zone occurred at ⩽ 25 m, and a “clean” zone at distances > 25 m. This study showed that salmon farming had similar effects on the benthos as other forms of organic enrichment, but the effects were limited to a small area in the immediate vicinity of the cages