Distance and flow effects on microsphere transport in a large gravel column

Consumption of microbially contaminated ground water can cause adverse health effects and the processes involved in pathogen transport in aquifers need to be understood. The influences of distance, flow velocity, and colloid size on colloid transport were examined in homogenous pea-gravel media using an 8-m column and three sizes (1, 5, and 10 microm) of microspheres. Experiments were conducted at three flow rates by simultaneously injecting microspheres with a conservative tracer, bromide. Observed concentrations were simulated with CXTFIT and analyzed with filtration theory. The results demonstrate that colloid concentration is strongly log-linearly related to transport distance (as suggested by filtration theory) in coarse gravels, similar to our previous field studies. In contrast, the log-linear relationship is often reported to be invalid in fine porous media. The observed log-linear relationship is possibly because straining is negligible in the coarse gravels investigated. This has implications in predicting setback distances for land disposal of effluent, and suggests that setback distances in gravel aquifers can be estimated using constant spatial removal rates (f). There was an inverse relationship between transport distance and colloidal concentration, but not with temporal attachment rate (katt) and collision coefficient (alpha). Increases in flow velocity result in increasing colloidal recovery, katt and alpha but decreasing f. Increases in sphere size result in decreasing colloidal recovery with increasing katt, f, alpha, and velocity enhancement. Diffusion is the dominant collision mechanism for 1-microm spheres (81-88%), while settling dominates for 5- and 10-microm spheres (> 87%), and interception is very small for all spheres investigated.     

A laboratory study on risk assessment of microcystin-RR in cropland

The persistence time and risk of microcystin-RR (MC-RR) in cropland via irrigation were investigated under laboratory conditions. In order to evaluate the efficiency of the potential adsorption and biodegradation of MC-RR in cropland and the persistence time of MC-RR for crop irrigation, high performance liquid chromatography (HPLC) was used to quantify the amount of MC-RR in solutions. Our study indicated that MC-RR could be adsorbed and biodegraded in cropland soils. MC-RR at 6.5mg/L could be completely degraded within 6 days with a lag phase of 1-2 days. In the presence of humic acid, the same amount of MC-RR could be degraded within 4 days without a lag phase. Accordingly, the persistence time of MC-RR in cropland soils should be about 6 days. This result also suggested the beneficial effects of the organic fertilizer utilization for the biodegradation of MC-RR in cropland soils. Our studies also demonstrated that MC-RR at low concentration (<10microg/L) could accelerate the growth of plants, while high concentration of MC-RR (>100microg/L) significantly inhibited the growth of plants. High sensitivity of the sprouting stage plants to MC-RR treatments as well as the strong inhibitory effects resulting from prolonged irrigation further indicated that this MC-RR growth-inhibition may vary with the duration of irrigation and life stage of the plants.     

Aging effects on cadmium transport in undisturbed contaminated sandy soil columns

Limited information is available on the effects of contaminant aging (i.e., the contact time of Cd with the soil) on Cd transport in soils. We conducted displacement experiments in which indigenous Cd and freshly applied Cd were leached simultaneously from undisturbed samples of three Spodosol horizons. Sorption of Cd was described using Freundlich isotherms, whereas transport was described as a convection-dispersion process. Parameter optimization analysis using a mobile-immobile transport model applied to nonsorbing tracer displacement data showed that 16 to 22% of the water in the columns was immobile. The low dimensionless mass transfer coefficients in the mobile-immobile model were indicative of diffusion-limited transfer between mobile and immobile water, and hence physical nonequilibrium. A two-site kinetic sorption model could be fitted closely to breakthrough curves of the non-aged Cd for three soil horizons. No conclusive evidence was found that contaminant aging in soil affects cadmium transport. On the one hand, predictions of aged Cd leaching, using parameters estimated from displacement experiments with nonaged Cd, differed from those for the aged Cd in the E horizon. On the other hand, no meaningful differences in transport behavior between aged and non-aged Cd were found for the humus Bh and Bh/C horizons. The two-site kinetic rate coefficient alphac was found to depend on water flux, further indicating that mass transfer between sorption sites and the liquid is limited by diffusion rather than by kinetic sorption.     

Comparison of Escherichia coli and Campylobacter jejuni transport in saturated porous media

Due to the difficulties in testing for specific pathogens, water samples are tested for the presence of nonpathogenic indicator organisms to determine whether a water supply has been contaminated by fecal material. An implicit assumption in this approach is that where pathogenic microorganisms are present fecal indicator organisms are present as well; yet surprisingly few studies have been conducted that directly compare the transport of indicator organisms with pathogenic organisms in ground water environments. In this study we compared the cell properties and transport of Escherichia coli, a commonly used indicator organism, and Campylobacter jejuni, an important enteropathogen commonly found in agricultural wastes, through saturated porous media. Differences in cell properties were determined by measuring cell geometry, hydrophobicity, and electrophoretic mobility. Transport differences were determined by conducting miscible displacement experiments in laboratory columns. Under the experimental conditions tested, C. jejuni was much more negatively charged and more hydrophobic than E. coli. In addition, C. jejuni cells were slightly longer, narrower, and less spherical than E. coli. The variations in cell properties, primarily surface charge, resulted in significant differences in transport between these two microorganisms, with the transport of C. jejuni exceeding that of E. coli when conditions favored low attachment rates, thus calling into question the usefulness of using E. coli as an indicator organism for this important pathogen.     

Nuclear magnetic resonance imaging for studies of flow and transport in porous media

Nuclear magnetic resonance imaging (NMRI) methods for visualization of fluid flow and transport in porous media are reviewed in this paper. They are illustrated with experiments showing applications of velocity imaging, NMRI measurements of multiphase flow, and NMRI measurements of density flow. The latter two are compared with numerical simulations. The examples show the capacity of NMRI to give structural information both of the medium and the fluid distributions as well as their temporal development. The resulting data can be used in a black box-white box comparison and as benchmarks for numerical models.     

Implications of cation exchange on clay release and colloid-facilitated transport in porous media

Column experiments were conducted to study chemical factors that influence the release of clay (kaolinite and quartz minerals) from saturated Ottawa sand of different sizes (710,360, and 240 microm). A relatively minor enhancement of clay release occurred when the pH was increased (5.8 to 10) or the ionic strength (IS) was decreased to deionized (DI) water. In contrast, clay release was dramatically enhanced when monovalent Na+ was exchanged for multivalent cations (e.g., Ca2+ and Mg2+) on the clay and sand and then the solution IS was reduced to DI water. This solution chemistry sequence decreased the adhesive force acting on the clay as a result of an increase in the magnitude of the clay and sand zeta potential with cation exchange, and expansion of the double layer thickness with a decrease in IS to DI water. The amount of clay release was directly dependent on the Na+ concentration of the exchanging solution and on the initial clay content of the sand (0.026-0.054% of the total mass). These results clearly demonstrated the importance of the order and magnitude of the solution chemistry sequence on clay release. Column results and scanning electron microscope (SEM) images also indicated that the clay was reversibly retained on the sand, despite predictions of irreversible interaction in the primary minimum. One plausible explanation is that adsorbed cations increased the separation distance between the clay-solid interfaces as a result of repulsive hydration forces. A cleaning procedure was subsequently developed to remove clay via cation exchange and IS reduction; SEM images demonstrated the effectiveness of this approach. The transport of Cu2+ was then shown to be dramatically enhanced by an order of magnitude in peak concentration by adsorption on clays that were released following cation exchange and IS reduction.     

The geoenvironmental cycle of cadmium: a case study

Amidst the hue and cry about anthropogenic sources of pollution, there are regions in the world facing silent metal discharges purely due to geochemical factors. The present investigation relates to such a situation and deals with the non-anthropogenic discharge of cadmium into the local environment. The phosphorite rocks of the Roorkee area of India contain cadmium and, particularly as these are used in the manufacture of superphosphate, the rocks could lead to the large-scale disbursement of cadmium into the open environment. The area under study has soil rich in cadmium-containing minerals, as a result of which water is likely to leach out mobile cadmium and the ground water and crops could become contaminated. This is evident from the high values of cadmium in the tap water and food samples in the area. The Ganga and the Yamuna, the two major rivers traversing the area that originate in the central crystalline rocks of the Himalayas, show high cadmium values at Rishikesh and Dakpathar. Moreover, the sediments of these rivers have cadmium in easily bioavailable form. Food samples and milk analysed from the area show, in some cases, cadmium levels above the safety limits. This high exposure to cadmium is probably causing ailments so far unreported.     

A GIS-based DRASTIC vulnerability and net recharge reassessment in an aquifer of a semi-arid region (Metline-Ras Jebel-Raf Raf aquifer, Northern Tunisia)

This paper aims to elaborate new generic DRASTIC aquifer vulnerability maps of the coastal aquifer of Metline-Ras Jebel-Raf Raf (Northeast of Tunisia) using the GIS technique, making the data analyses easier to handle and providing better capabilities of dealing with large spatial data. A similar study was carried out in 1999 in the same aquifer using a method based on the soil water balance equation to determine the net recharge parameter. Unfortunately, the lack of data in the study area made the results unsatisfactory. By applying the Williams and Kissel equation and the Rao relationship, we intend to demonstrate that we could correctly evaluate the net recharge parameter. Moreover, new data related to the aquifer hydraulic conductivity, the soil cover and the vadose zone lithology have become available, and allowed us to elaborate suitable DRASTIC maps.     

Uptake and transport of radioactive nickel and cadmium into three vegetables after wet aerial contamination

Knowledge of radionuclide or trace element retention and translocation to plants following an aerial contamination event, for example, sprinkling with contaminated water, is necessary for the evaluation of human exposure through consumption of contaminated vegetables. The fate of 63Ni and 109Cd in all plant parts of three different vegetables after wet deposition on leaves or on fruits was studied. Lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), and bean (Phaseolus vulgaris L.) grown under controlled conditions in a growth chamber were contaminated with 63Ni and 109Cd either on leaves, by means of two different contamination methods (a single early contamination and a repetitive one), or on bean husks (third contamination method: a single contamination at a late stage). Spiked and nonspiked organs were harvested at maturity and radionuclide contents were measured. The fraction retained was on average 56% of the initially administered doses of 63Ni and 87% of 109Cd. The leaf-to-other organ translocation factor was considerably higher for 63Ni (on average 43% of retained radioactivity) than for 109Cd (8%). Nickel-63 migrated throughout the whole plant following foliar contamination, and mainly toward young leaves, seeds in formation, and sink organs, whereas 109Cd migrated to a much lesser extent and only to the organs that were closest to the spiked one, and not at all into fruit. After a fruit contamination event, both radionuclides were translocated into the seeds of spiked fruits. Radionuclide retention and translocation were not affected by plant species, but principally by the type of organ contaminated.     

A regional-scale study on the crop uptake of cadmium from sandy soils: measurement and modeling

Plant uptake is one of the major pathways by which cadmium (Cd) in soils enters the human food chain. This study was conducted to investigate the uptake of Cd by crops from soils within the wastewater irrigation area (WIA) of Braunschweig (Germany) and to develop a simple process-oriented model that is suited to predict Cd uptake at the regional scale. The sandy soils within the WIA (4300 ha) have received considerable loads of heavy metals by irrigation using municipal wastewater for up to 40 years. In 1998 and 1999, we sampled soil and plant material at 40 potato (Solanum tuberosum L.), 40 sugar beet (Beta vulgaris L.), and 32 winter wheat (Triticum aestivum L.) fields. In both years and for all three crops, we found close linear relationships between the Cd content of plant material and the Cd concentration in soil solution. For all three crops, we observed a trend of relatively increased Cd uptake in the year with the higher saturation deficit of the atmosphere. We interpret this to indicate that transpiration plays an important role in the Cd uptake of crops under the conditions of the WIA. In modeling the uptake of Cd by crops, we assume that uptake is proportional to mass flow, that is, the product of water transpired, Cd concentration in soil solution, and a plant-specific empirical parameter. The simulations agreed well with the observed Cd contents in crops. Our model explained between 66 and 87% of the observed variance.     

Passive flux meter measurement of water and nutrient flux in saturated porous media: bench-scale laboratory tests

The passive nutrient flux meter (PNFM) is introduced for simultaneous measurement of both water and nutrient flux through saturated porous media. The PNFM comprises a porous sorbent pre-equilibrated with a suite of alcohol tracers, which have different partitioning coefficients. Water flux was estimated based on the loss of loaded resident tracers during deployment, while nutrient flux was quantified based on the nutrient solute mass captured on the sorbent. An anionic resin, Lewatit 6328 A, was used as a permeable sorbent and phosphate (PO4(3-)) was the nutrient studied. The phosphate sorption capacity of the resin was measured in batch equilibration tests as 56 mg PO4(3-) g(-1), which was determined to be adequate capacity to retain PO4(3-) loads intercepted over typical PNFM deployment periods in most natural systems. The PNFM design was validated with bench-scale laboratory tests for a range of 9.8 to 28.3 cm d(-1) Darcy velocities and 6 to 43 h deployment durations. Nutrient and water fluxes measured by the PNFM averaged within 6 and 12% of the applied values, respectively, indicating that the PNFM shows promise as a tool for simultaneous measurement of water and nutrient fluxes.     

The reclamation concept in regulation of gravel mining in California

Mining of gravel for construction aggregate in California is regulated under the concept of ‘reclamation’, borrowed from open‐pit coal mines, which are recontoured and returned to productive use (such as agriculture) after mine closure. The concept works for dry terrace pits and can be adapted to creation of wildlife habitat in wet terrace pits. However, the state's regulatory structure also applies the concept to gravel extraction in the active channel, a dynamic system where the extraction site cannot be isolated from the surrounding environment. In this paper, I review reclamation in terrace and floodplain deposits but question the appropriateness of extending the traditional notion of reclamation to instream mining operations.     

A morphological comparison of narrow,low-gradient streams traversing wetland environments to alluvial streams

Twelve morphological features from research on alluvial streams are compared in four narrow, low-gradient wetland streams located in different geographic regions (Connecticut, Indiana, and Wisconsin, USA). All four reaches differed in morphological characteristics in five of the features compared (consistent bend width, bend cross-sectional shape, riffle width compared to pool width, greatest width directly downstream of riffles, and thalweg location), while three reaches differed in two comparisons (mean radius of curvature to width ratio and axial wavelength to width ratio). The remaining five features compared had at least one reach where different characteristics existed. This indicates the possibility of varying morphology for streams traversing wetland areas further supporting the concept that the unique qualities of wetland environments might also influence the controls on fluvial dynamics and the development of streams. If certain morphological features found in streams traversing wetland areas differ from current fluvial principles, then these varying features should be incorporated into future wetland stream design and creation projects. The results warrant further research on other streams traversing wetlands to determine if streams in these environments contain unique morphology and further investigation of the impact of low-energy fluvial processes on morphological development. Possible explanations for the morphology deviations in the study streams and some suggestions for stream design in wetland areas based upon the results and field observations are also presented.     

Anaerobic degradation of vinyl chloride in aquifer microcosms

The anaerobic degradation potential at a chloroethene-contaminated site was investigated by operating two anoxic column aquifer microcosms enriched in iron(III). One column was fed with vinyl chloride (VC) only (column A) and one with VC and acetate (column B). In column A, after about 600 pore volume exchanges (PVEs), VC started to disappear and reached almost zero VC recovery in the effluent after 1000 PVEs. No formation of ethene was observed. In column B, effluent VC was almost always only a fraction of influent VC. Formation of ethene was observed after 800 PVEs and started to become an important degradation product after 1550 PVEs. However, ethene was never observed in stoichiometric amounts compared with disappeared VC. The average stable isotope enrichment factor for VC disappearance in column A was determined to be -4.3‰. In column B, the isotope enrichment factor shifted from -10.7 to -18.5‰ concurrent with an increase in ethene production. Batch microcosms inoculated with column material showed similar isotope enrichment factors as the column microcosms. These results indicated that two degradation processes occurred, one in column A and two in parallel in column B with increasing importance of reductive dechlorination with time. This study suggests that in addition to reductive dechlorination, other degradation processes such as anaerobic oxidation should be taken into account when evaluating natural attenuation of VC and that isotope analysis can help to differentiate between different pathways of VC removal.     

Occurrence of arsenic in ground water in the Choushui River alluvial fan, Taiwan

An investigation of shallow ground water quality revealed that high arsenic (As) concentrations were found in both aquifers and aquitards in the southern Choushui River alluvial fan of Taiwan. A total of 655 geological core samples from 13 drilling wells were collected and analyzed. High As contents were found primarily in aquitards, to a maximum of 590 mg/kg. The contents were correlated with the locations of the marine sequences. Additionally, strong correlations among the As concentrations of core samples, the clay, and the geological age of the Holocene transgression were identified. Most of the As in ground water originated from the aquitard of the marine sequence. The high As content in marine formations with high clay contents may be attributable to the bioaccumulation of As in the sea organisms, which accrued and were deposited in the formation. A preliminary geogenic model of the origin of the high As concentration in the shallow sedimentary basin of the Choushui River alluvial fan of Taiwan is proposed.     

Small-scale spatial variability of atrazine and dinoseb adsorption parameters in an alluvial soil

Soil sorption processes largely control the environmental fate of herbicides. Therefore, accuracy of sorption parameters is crucial for accurate prediction of herbicide mobility in agricultural soils. A combined experimental and statistical study was performed to investigate the small-scale spatial variability of sorption parameters for atrazine and dinoseb in soils and to establish the number of samples needed to provide a value of the distribution coefficient (K(d)) next to the mean, with a given precision. The study explored sorption properties of the two herbicides in subsurface samples collected from four pits distributed along a transect of an alluvial soil; two to four samples were taken at about 30 cm apart at each sampling location. When considering all the data, the distribution coefficients were found to be normally and log-normally distributed for atrazine and dinoseb, respectively; the CVs were relatively high (close to 50% for dinoseb and 40% for atrazine). When analyzed horizon by horizon, the data revealed distribution coefficients normally distributed for both herbicides, whatever the soil layer, with lower CVs. The K(d) values were shown to vary considerably between samples collected at very short distance (a few centimeters), suggesting that taking a single soil sample to determine sorption properties through batch experiments can lead to highly unrepresentative results and to poor sorption/mobility predictions.     

Biodegradation during contaminant transport in porous media: V. The influence of growth and cell elution on microbial distribution

This study investigated the interaction between microbial growth and cell elution, and their influence on resultant microbial distribution between the aqueous and solid phases during solute transport in a sandy, low-organic-carbon-content porous medium. Miscible displacement experiments were conducted with salicylate as the model compound, and with different initial conditions (e.g., substrate concentrations and cell densities) to attain various degrees of microbial growth. For each experiment, salicylate and dissolved oxygen concentrations as well as cell densities were monitored in the column effluent. Cell densities were also measured in the porous medium at the beginning and end of each experiment. Total microbial growth was determined in two ways, one based on a cell mass balance for the system and the other based on total amount of salicylate degraded. For conditions yielding a considerable amount of microbial growth, the majority of the biomass was associated with the aqueous phase (68-90%). Conversely, under minimal-growth conditions, most cells (approximately 60-70%) were attached to particle surfaces. Significant cell elution was observed for most conditions, the rate of which increased in the presence of the substrate. The results suggest that the increase in aqueous-phase cells observed for the experiments exhibiting the greatest growth is associated with the production of new cells, and that under appropriate conditions aqueous-phase biomass can contribute significantly to contaminant biodegradation.     

Impact of dissolved organic matter on copper mobility in aquifer material

Naturally occurring dissolved organic matter (DOM) and biosolids-derived DOM have been implicated in the mobility of metals in soils and aquifer materials. To investigate the effect of DOM on copper mobility in aquifer material, DOM derived from sewage biosolids was separated into two apparent molecular-weight (MW) fractions, 500 to 3500 Da (LMW) and > 14 000 Da (HMW). In each MW fraction, the DOM was further fractionated into hydrophilic, hydrophobic acid, and hydrophobic neutral compounds by an XAD-8 chromatography technique. The mobility of these DOM components and their influences on copper transport in a sesquioxide-coated, sandy aquifer material were examined with column transport experiments. The LMW DOM was found to be highly mobile, whereas the HMW DOM had a greater tendency to be retained by the aquifer material. Within the same MW fraction, the mobility of DOM followed the order of hydrophilic DOM > hydrophobic acid DOM > hydrophobic neutral DOM. Copper breakthrough curves in the presence of various DOM components showed that, except for the HMW hydrophilic fraction, DOM components enhanced Cu transport through the aquifer columns at early stages of transport (the first 75 pore volumes). In the later stages, however, all the DOM components substantially inhibited Cu mobility. We hypothesize that several mechanisms could account for retardation of Cu movement in the presence of the DOM fractions, including the formation of ternary complexes between the aquifer material, Cu, and DOM; changes in the electrostatic potential at the solid-phase surface; and pH buffering by DOM.     

Degradation of potassium formate in the unsaturated zone of a sandy aquifer

This paper presents results from a lysimeter experiment on the fate of potassium formate, an alternative deicing agent. The experiment was performed through the winter and spring to identify any thermal sensitivity in the transport and biodegradation of formate in the lysimeter. Ninety-eight percent of the total quantity of formate applied was degraded while percolating through the 1.7-m-thick unsaturated sand layer within the lysimeter. Concomitantly, the bicarbonate concentration of the percolating water increased. The low concentrations of nitrogen (0.02 mg L(-1)) and phosphorous (<0.002 mg L(-1)) in the percolated water, however, potentially limited microbial activity. During the study period, 99% of the applied potassium was retained in the lysimeter, and the ion exchange between the potassium and a variety of monovalent and divalent ions was assumed to be responsible for the leaching of barium, calcium, magnesium, and sodium from the soil material. Except for manganese, the concentrations of the studied metals in the percolated water did not exceed the threshold values set for drinking water by the Council of the European Union. By contrast, the application of potassium formate had a detrimental effect on the vegetation on the lysimeter. To conclude, formate was effectively degraded in the sandy lysimeter and its application did not cause major undesirable changes in the quality of the percolating water. Further research at field scale is, however, needed for instance on the biodegradation of potassium formate and on its impacts on roadside vegetation.     

A comparison of reliability of soil cadmium determination by standard spectrometric methods

Inductively coupled plasma emission spectrometry (ICP-OES) is the most common method for determination of soil Cd, yet spectral and matrix interferences affect measurements at the available analytical wavelengths for this metal. This study evaluated the severity of the interference over a range of total soil Cd by comparing ICP-OES and inductively coupled plasma mass spectrometry (ICP-MS) measurements of Cd in acid digests. Using the emission at 226.5 nm, ICP-OES was generally unable to quantify soil Cd at low (near-background) levels and gave unreliable values compared with ICP-MS. Using the line at 228.8 nm, a marked positive bias in Cd measurement (relative to the 226.5 nm measurement) was attributable to arsenic (As) interference even at soil As concentrations below 10 mg kg. This spectral interference in ICP-OES was severe in As-contaminated orchard soils, giving a false value for soil total Cd near 2 mg kg when soil As was 100 to 150 mg kg. In attempting to avoid these ICP emission-specific interferences, this study evaluated a method to estimate total soil Cd using 1 M HNO extraction followed by determination of Cd by flame atomic absorption (FAA), either with or without preconcentration of Cd using an Aliquat-heptanone extractant. The 1 M HNO extracted an average of 82% of total soil Cd. The FAA method had no significant interferences and estimated the total Cd concentrations in all soils tested with acceptable accuracy. For Cd-contaminated soils, the Aliquat-heptanone preconcentration step was not necessary, as FAA sensitivity was adequate for quantification of extractable soil Cd and reliable estimation of total soil Cd.