A new tracer technique for monitoring groundwater fluxes: the Finite Volume Point Dilution Method

Quantification of pollutant mass fluxes is essential for assessing the impact of contaminated sites on their surrounding environment, particularly on adjacent surface water bodies. In this context, it is essential to quantify but also to be able to monitor the variations with time of Darcy fluxes in relation with changes in hydrogeological conditions and groundwater - surface water interactions. A new tracer technique is proposed that generalizes the single-well point dilution method to the case of finite volumes of tracer fluid and water flush. It is called the Finite Volume Point Dilution Method (FVPDM). It is based on an analytical solution derived from a mathematical model proposed recently to accurately model tracer injection into a well. Using a non-dimensional formulation of the analytical solution, a sensitivity analysis is performed on the concentration evolution in the injection well, according to tracer injection conditions and well-aquifer interactions. Based on this analysis, optimised field techniques and interpretation methods are proposed. The new tracer technique is easier to implement in the field than the classical point dilution method while it further allows monitoring temporal changes of the magnitude of estimated Darcy fluxes, which is not the case for the former technique. The new technique was applied to two experimental sites with contrasting objectives, geological and hydrogeological conditions, and field equipment facilities. In both cases, field tracer concentrations monitored in the injection wells were used to fit the calculated modelled concentrations by adjusting the apparent Darcy flux crossing the well screens. Modelling results are very satisfactory and indicate that the methodology is efficient and accurate, with a wide range of potential applications in different environments and experimental conditions, including the monitoring with time of changes in Darcy fluxes.     

Nitrate-enhanced bioremediation of BTEX-contaminated groundwater: parameter estimation from natural-gradient tracer experiments

Two natural-gradient pulse tracer tests were conducted in a petroleum-contaminated aquifer to evaluate the potential for benzene, toluene, ethylbenzene, and xylenes (BTEX) biodegradation under enhanced nitrate-reducing conditions. Addition of nitrate resulted in loss of toluene, ethylbenzene, and m,p-xylenes (TEX) after an initial lag period of approximately 9 days. Losses of benzene were not observed over the 60-day monitoring period. Tracer breakthrough curves (BTCs) were analyzed to derive transport and biodegradation parameters, including advective velocities, retardation factors, dispersion coefficients, biodegradation rate constants, and nitrate utilization ratios. Using the parameters derived from the BTC analysis, numerical simulations of one of the tracer experiments were conducted using BIONAPL/3D [Molson, J., BIONAPL/3D User Guide, A 3D Coupled Flow and Multi-Component Reactive transport model. University of Waterloo, Waterloo, Ontario, Canada]. Simulations using the BTC-derived transport and biodegradation parameters successfully reproduced benzene, TEX, and nitrate concentrations measured during the tracer experiment. Comparisons of observed and simulated nitrate concentrations indicate that the mass ratio of nitrate-N utilized to TEX degraded increased over time during the experiment, reaching values many times that expected based on stoichiometry of TEX oxidation coupled to nitrate reduction. Excess nitrate loss is likely due to oxidation of other organics in addition to TEX.     

Evaluation of groundwater dynamic regime with groundwater depth evaluation indexes

An accurate quantitative evaluation of anthropogenic effects on regional groundwater dynamics is critical to the rational planning, management, and use of such resources and in maintaining the sustainability of groundwater-dependent ecosystems. Based on groundwater dynamics, a series of groundwater depth evaluation indexes were created to quantitatively evaluate the effects of anthropogenic activities on the groundwater system. These indexes were based on mathematical relationships relating groundwater depth to surface runoff (gammat), precipitation (rhot), and extraction (deltat). The anthropogenic effects on these relationships were evaluated statistically, with respect to both temporal and spatial variation. The anthropogenic effects on groundwater dynamics within the arid Zhangye Basin, located in the middle reaches of northwest China's Heihe River, were investigated. River valley plains in the western portion of the basin excepted, anthropogenic activities have, since 1995, dramatically altered the basin's groundwater dynamics; in particular, in the mid-upper and lower portions of alluvial-diluvial fans and in localized northerly fine-soil plains regions, the relationship of groundwater to surface runoff and atmospheric precipitation has shifted. This and other changes indicate that anthropogenic effects on groundwater systems in this region show clear spatiotemporal variation.     

Fluorescence-based monitoring of tracer and substrate distribution in an UASB reactor

In this work, rhodamine-related fluorescence was measured on-line at four reactor heights in order to study hydrodynamics within an upflow anaerobic sludge bed reactor. A linear dependence of the dispersion coefficient (D) on the upflow velocity was observed, while the influence of the organic loading rate (OLR) was insignificant. Furthermore, the Bodenstein number of the reactor loaded with granulated sludge was found to be position-dependent with the largest values measured at the bottom of the sludge bed. This trend was not observed in the reactor without sludge. Chemical oxygen demand (COD) and volatile fatty acid (VFA) concentrations were measured at the same reactor heights as in rhodamine tests using conventional off-line analytical methods and on-line multiwavelength fluorometry. Significant spatial COD and VFA gradients were observed at organic loading rates above 6g COD l(R)(-1)d(-1) and linear upflow velocities below 0.8m h(-1).     

Use of tandem circulation wells to measure hydraulic conductivity without groundwater extraction

Conventional methods to measure the hydraulic conductivity of an aquifer on a relatively large scale (10-100 m) require extraction of significant quantities of groundwater. This can be expensive, and otherwise problematic, when investigating a contaminated aquifer. In this study, innovative approaches that make use of tandem circulation wells to measure hydraulic conductivity are proposed. These approaches measure conductivity on a relatively large scale, but do not require extraction of groundwater. Two basic approaches for using circulation wells to measure hydraulic conductivity are presented; one approach is based upon the dipole-flow test method, while the other approach relies on a tracer test to measure the flow of water between two recirculating wells. The approaches are tested in a relatively homogeneous and isotropic artificial aquifer, where the conductivities measured by both approaches are compared to each other and to the previously measured hydraulic conductivity of the aquifer. It was shown that both approaches have the potential to accurately measure horizontal and vertical hydraulic conductivity for a relatively large subsurface volume without the need to pump groundwater to the surface. Future work is recommended to evaluate the ability of these tandem circulation wells to accurately measure hydraulic conductivity when anisotropy and heterogeneity are greater than in the artificial aquifer used for these studies.     

Hydraulic constraints on the performance of a groundwater denitrification wall for nitrate removal from shallow groundwater

Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.     

基于两步优劣解距离法的地下水污染监测网优化

地下水污染监测网的优化能让监测网以合理监测成本获取精确的地下水水质信息.为兼顾监测网空间特征与统计特征,综合考虑区域地下水水质状况和水文地质状况,提出了两步优劣解距离法优化监测网.首先以地下水典型污染物加权综合污染指数和防污性能指数为指标,采用优劣解距离法对现有监测网各监测井评分,并筛选得到不同监测井数量条件下的备选优...     

Evaluation of environmental monitoring strategies

In environmental monitoring, it is important that the monitoring system should emit early warnings when undesired events occur. These events may be sudden or of a more subtle nature. In the design of such monitoring systems, a proper balance between cost and risk must be achieved. There are 2 classic types of risk connected with early warning systems, namely the risk of not detecting significant changes and the risk of false alarms. The purpose of this paper is to describe a method for comparing the performance of different monitoring systems, considering the classic types of risk and cost. The method is applied to the monitoring of the lichen cover as a test case. The expected utility has been used as a measure of performance. When estimating the probabilities of the events, spatial microsimulation and Monte-Carlo simulation techniques have been used. The monitoring programs studied are based on satellite images, aerial photos, field samples, and land-cover maps. The major conclusions of this study are that standardized quality measures are extremely useful for evaluating the usability of environmental monitoring methods. In addition, when estimating gains and costs, spatial microsimulation techniques are useful. To improve the method, however, macroconstraints should also be used for aligning the simulation model.     

Fractional calculus applied to the analysis of spectral electrical conductivity of clay-water system

The analysis of the low-frequency conductivity spectra of the clay-water mixtures is presented. The frequency dependence of the conductivity is shown to follow the power-law with the exponent n=0.67 before reaching the frequency-independent part. When scaled with the value of the frequency-independent part of the spectrum the conductivity spectra for samples at different water content values are shown to fit to a single master curve. It is argued that the observed conductivity dispersion is a consequence of the anomalously diffusing ions in the clay-water system. The fractional Langevin equation is then used to describe the stochastic dynamics of the single ion. The results indicate that the experimentally observed dielectric properties originate in anomalous ion transport in clay-water system characterized with time-dependent diffusion coefficient.     

Applicability of polar organic compound integrative samplers for monitoring pesticides in groundwater

Polar organic chemical integrative samplers (POCISs) for the monitoring of polar pesticides in groundwater were tested on two sites in order to evaluate their applicability by comparison with the spot-sampling approach. This preliminary study shows that, as in surface water, POCIS is a useful tool, especially for the screening of substances at low concentration levels that are not detected by laboratory analysis of spot samples. For quantitative results, a rough estimation is obtained. The challenge is now to define the required water-flow conditions for a relevant quantification of pesticides in groundwater and to establish more representative sampling rates for groundwater.     

Evaluation of groundwater contamination in a coastal area of south-eastern Sicily

This investigation was under taken to evaluate the groundwater resources contamination due to intensive agricultural practices (particularly greenhouses). The study-area is located in the coastal area of the Ragusa province (South-East Sicily), where numerous existing greenhouses may cause the contamination of groundwater systems (unconfined and confined aquifers) beneath the cropped land. The pollution risk is mainly related with the seepage process of macro-elements nitrogen (N), phosphorus (P), potassium (K), held in the irrigation water and the massive use of fertilizers and pesticides, that may pass through the unsaturated zone of the soil profile. Moreover, the area is characterized by the presence of several wells (about 15 wells/km2) for agricultural use that cause the aquifer overexploitation and the consequent risk of seawater intrusion. The agriculture practices adopted in the study area (irrigation volumes, fertilizer concentrations, use of pesticides…) were monitored since February 2009; moreover, the pollution risk of the aquifers was evaluated through the analysis of groundwater water samples collected (monthly) in the monitoring wells; in particular, nitrogen compounds, soluble phosphorous (PO?2?), potassium, as well as the main pesticides commonly used in the study area, were measured.The results show that electrical conductivity and chloride concentration values can cause reduction of production and leaf damage problems, respectively, for most of the monitored farm systems. The high nitrogen compounds concentrations observed in the monitored wells can cause health and environmental problems. Moreover high pesticide contamination of groundwater was found in two of the five monitored wells.     

Comparison of constant volume balloons, model trajectories and tracer transport during ETEX

In the field phases of the European Tracer EXperiment (ETEX), an inert tracer was released for 12 h into the atmosphere and samples taken at several locations downwind. During the same time, several Constant Volume Balloons (CVB) (10 and 6 for ETEX first and second release, respectively) were launched into different altitudes and followed as far as 21–188 km, to indicate the initial dispersion directions of the tracer puff. A model simulating the CVB behaviour in hydrostatic meso-scale model forecasts is applied to ETEX data to demonstrate its capability to predict the tracer puff mean axis over long distances (−2000 km). CVB model results are first compared to air parcels trajectories and 2D (i.e. isentropic, isobaric and isodensity) trajectories. Then they are compared to the measured CVB trajectories and finally to the tracer puff trajectories. As expected, the CVB model and isodensity model trajectories are found to be identical. The 16 CVBs calculated trajectories nearly overlap the real ones over 21–188 km with mean absolute horizontal transport deviations less than 20 km (average value of 8.2 km). The corresponding relative transport deviations are less than 45% with an average value of 20.6%. Better predictions are obtained for the ETEX second release. During the 60 h following ETEX’s first release start, the simulated CVBs are mainly found in the area of the maximum surface concentrations of the released tracer, up to 2000 km. Up to 36 h after ETEX second tracer release start, the simulated CVB trajectories predict well the mean axis of the tracer puff, but failed later.     

The use of an available SO2 tracer during the 1983 captex experiment

Considerable effort has been spent in recent years to develop sophisticated inert perfluorocarbon tracers for use in long-range transport of air pollution studies. During favourable transport situations, existing pollution plumes can still be utilized to give similar information regarding regional scale dispersion. Identification of a smelter plume from Sudbury, Canada, during the CAPTEX study shows that complementary techniques exist which should be considered for future major tracer releases.     

Continuous monitoring of particle emissions during showering

Particle formation from showering may be attributed to dissolved mineral aerosols remaining after evaporation of micron-sized satellite droplets produced by the showerhead or from splashing of larger shower water droplets on surfaces. Duplicate continuous particle monitors measured particle size distributions in a ventilated residential bathroom under various showering conditions, using a full-size mannequin in the shower to simulate splashing effects during showering. Particle mass concentrations were estimated from measured shower particle number densities and used to develop emission factors for inhalable particles. Emission source strengths of 2.7-41.3 microg/ m3/min were estimated under the various test conditions using residential tap water in Columbus, OH. Calculated fine particulate matter (PM2.5) concentrations in the bathroom reached several hundred micrograms per cubic meter; calculated coarse particulate matter (PM10) levels approached 1000 microg/m3. Rates of particle formation tended to be highest for coarse shower spray settings with direct impact on the mannequin. No consistent effects of water temperature, water pressure, or spray setting on overall emission rates were apparent, although water temperature and spray setting did have an effect when varied within a single shower sampling run. Salt solutions were injected into the source water during some tests to assess the effects of total dissolved solids on particle emission rates. Injection of salts was shown to increase the PM2.5 particle formation rate by approximately one third, on average, for a doubling in tap water-dissolved solids content; PM10 source strengths approximately doubled under these conditions, because very few particles >10 microm were formed.     

Hydrogeochemical investigations to assess groundwater and saline water interaction in coastal aquifers of the southeast coast,Tamil Nadu,India

Environmental Science and Pollution Research - Groundwater and saline water interaction is the most common processes in the coastal aquifers that alters the quality of aquifer waters. The...     

Long-term ground penetrating radar monitoring of a small volume DNAPL release in a natural groundwater flow field

An earlier field experiment at Canadian Forces Base Borden by Brewster and Annan [Geophysics 59 (1994) 1211] clearly demonstrated the capability of ground penetrating radar (GPR) reflection profiling to detect and monitor the formation of DNAPL layers in the subsurface. Their experiment involved a large volume release (770 L) of tetrachloroethylene into a portion of the sand aquifer that was hydraulically isolated from groundwater flow by sheet pile walls. In this study, we evaluated the ability of GPR profiling to detect and monitor much smaller volume releases (50 L). No subsurface confining structure was used in this experiment; hence, the DNAPL impacted zone was subjected to the natural groundwater flow regime. This condition allowed us to geophysically monitor the DNAPL mass loss over a 66 month period. Reflectivity variations on the GPR profiles were used to infer the presence and evolution of the solvent layers. GPR imaging found significant reflectivity increases due to solvent layer formation during the two week period immediately after the release. These results demonstrated the capacity of GPR profiling for the detection and monitoring of lesser volume DNAPL releases that are more representative of small-scale industrial spills. The GPR imaged solvent layers subsequently reduced in both areal extent and reflectivity after 29 months and almost completely disappeared by the end of the 66 month monitoring period. Total DNAPL mass estimates based on GPR profiling data indicated that the solvent mass was reduced to 34%-36% of its maximum value after 29 months; only 4%-9% of the solvent mass remained in the study area after 66 months. These results are consistent with independent hydrogeological estimates of remaining DNAPL mass based on the downgradient monitoring of the dissolved solvent phase. Hence, we have concluded that the long-term GPR reflectivity changes of the DNAPL layers are likely the result from the dissolution of chlorinated solvents residing in those layers. The long-term monitoring results demonstrated that GPR profiling is a promising non-invasive method for use at DNAPL contaminated sites in sandy aquifers where temporal information about immiscible contaminant mass depletion due to either natural flow or remediation is needed. However, our results also indicated that the GPR signature of older DNAPL impacted zones may not differ greatly from the uncontaminated background if significant mass reduction due to dissolution has occurred.     

Evaluation and application of biomagnetic monitoring of traffic-derived particulate pollution

Inhalation of particulate pollutants below 10 μm in size (PM₁₀) is associated with adverse health effects. Here we use magnetic remanence measurements of roadside tree leaves to examine levels of vehicle-derived PM around Lancaster, UK. Leaf saturation remanence (SIRM) values exhibit strong correlation with both the SIRM and particulate mass of co-located, pumped-air samples, indicating that these leaf magnetic values are an effective proxy for ambient PM₁₀ concentrations. Biomagnetic monitoring using tree leaves can thus provide high spatial resolution data sets for assessment of particulate pollution levels at pedestrian-relevant heights. Leaf SIRM values not only increase with proximity to roads with higher traffic volumes, but are also ～100% higher at 0.3 m than at ～1.5–2 m height. Magnetic and SEM data indicate that the particle populations are dominated by spherical, iron-rich particles ～0.1–1 μm in diameter, with fewer larger, more angular, silica-rich particles. Comparison of the roadside leaf-calculated PM₁₀ concentrations with PM₁₀ concentrations predicted by a widely-used atmospheric dispersion model indicates some agreement between them. However, the model under-predicts PM₁₀ concentrations at ‘urban hotspots’ such as major–minor road junctions and traffic lights. Conversely, the model over-predicts PM₁₀ concentrations with distance from the road wherever one tree is screened by another, indicating the filtering/protective effect of roadside trees in leaf.     

Evaluating the ability of a numerical weather prediction model to forecast tracer concentrations during ETEX 2

In this paper the meteorological processes responsible for transporting tracer during the second ETEX (European Tracer EXperiment) release are determined using the UK Met Office Unified Model (UM). The UM predicted distribution of tracer is also compared with observations from the ETEX campaign. The dominant meteorological process is a warm conveyor belt which transports large amounts of tracer away from the surface up to a height of 4 km over a 36 h period. Convection is also an important process, transporting tracer to heights of up to 8 km. Potential sources of error when using an operational numerical weather prediction model to forecast air quality are also investigated. These potential sources of error include model dynamics, model resolution and model physics. In the UM a semi-Lagrangian monotonic advection scheme is used with cubic polynomial interpolation. This can predict unrealistic negative values of tracer which are subsequently set to zero, and hence results in an overprediction of tracer concentrations. In order to conserve mass in the UM tracer simulations it was necessary to include a flux corrected transport method. Model resolution can also affect the accuracy of predicted tracer distributions. Low resolution simulations (50 km grid length) were unable to resolve a change in wind direction observed during ETEX 2, this led to an error in the transport direction and hence an error in tracer distribution. High resolution simulations (12 km grid length) captured the change in wind direction and hence produced a tracer distribution that compared better with the observations. The representation of convective mixing was found to have a large effect on the vertical transport of tracer. Turning off the convective mixing parameterisation in the UM significantly reduced the vertical transport of tracer. Finally, air quality forecasts were found to be sensitive to the timing of synoptic scale features. Errors in the position of the cold front relative to the tracer release location of only 1 h resulted in changes in the predicted tracer concentrations that were of the same order of magnitude as the absolute tracer concentrations.     

Production of dibenzofuran fumes during splicing of PCB contaminated electrical cable

In the course of evaluating worker exposure during the hot splicing of underground electrical cable, it was discovered that the insulating tape used to make the original joints contained a high percentage (up to 10%) of PCB in the surface varnish. The heating of this PCB by a propane torch flame and molten solder as part of the normal operation of joint breakdown and remake resulted in a composition of airborne fume with a significant fraction (up to 1.3%) of the PCB that was converted to polychlorinated dibenzofurans (PCDF) with the tetra- and penta- chlorinated compounds as the predominant cogeners. No polychlorinated dibenzodioxins were detected.