Bromide Tracer Tests Research Articles

3D surface–subsurface modeling of a bromide tracer test in a macroporous tile‐drained field: Improvements and limitations

AbstractThe assessment and forecasting of nutrient loss by tile drains in agricultural areas often rely on physically based models that have adequate representations of macropores and tile drains. Macroporosity has been adequately represented in hydrological models using a dual continuum approach. However, its implementation in hydrological and solute transport models is limited to plot‐scale or to one‐ and two‐dimensional models due to the large number of parameters that are rarely available and the long computational times. The purpose of this study is to simulate a tracer test using a 3D coupled surface–subsurface model to improve the representation of the tracer concentration at the drainage discharge. A three‐dimensional HydroGeoSphere model was developed and calibrated to simulate tile drainage discharge, Br mass discharge, and hydraulic heads from a Br tracer test in a densely tile‐drained field. The conductivity of the drain was one of the most important parameters for drain discharge and solute transport simulations. The model accurately simulated drainage discharge and Br transport to tile drains. However, most of the Br peaks and the late‐time Br mass in the drain outflow were underestimated. Our simulation results indicate that explicitly representing tile drains with seepage nodes allows for a physically based, yet computationally efficient representation of Br transport behavior surrounding tile drains at field scale. However, we cannot confirm that the single‐porosity model with immobile zone is suitable for simulating the Br peaks at the drain outlet and the late‐time Br mass. Improvements to the model include the implementation of heterogeneous soil layers and the inclusion of more measured data to reduce uncertainty during calibration.

High-Frequency Bromide Sensor Performance during Denitrifying Bioreactor Tracer Testing

Abstract. High-frequency bromide (Br-) sensors could resolve challenges during denitrifying bioreactor tracer tests. Deployment of a Br- sensor during a tracer test resulted in both under- and over-reported concentrations. Collecting high-resolution Br- concentrations generally did not add value to calculated tracer metrics. However, high-frequency Br- sensors were overall considered a convenient tool, in part, due to ease of deployment. Bromide (Br-) tracer tests are an important environmental monitoring method for assessing internal hydraulics in denitrifying woodchip bioreactors. The objective was to assess performance of relatively new high-frequency Br- sensing technology to help resolve challenges associated with sampling uncertainty and failure during bioreactor tracer tests. A Seametrics TempHion™ Bromide Logger was used to: (1) continuously log Br- concentrations at least every 30 s during three tracer tests in the field and (2) determine Br- concentrations in sample bottles post-hoc following eight tracer tests (sampling frequency in the field: every 2 to 300 min). Both were compared to lab-analyzed Br- concentrations. The sensor underreported Br- concentrations at two real-time tracer tests and overreported during one test, but this mainly impacted the tracer metric of percent recovery. While Br- sensor deployment during a tracer test is the ideal use of this convenient tool, use of the sensor post-hoc in collected bottles matched the lab-analyzed concentrations more closely based on the Root Mean Square Errors and the Mean Absolute Errors. The sensor’s ability to collect large amounts of high-resolution concentrations did not necessarily add value because most tracer metrics were interpolated with reasonable accuracy from the discrete lab-analyzed samples. The relatively continuous nature of the sensed concentrations, however, along with ease of deployment were important sensor benefits. Keywords: Conservative tracer test, Continuous sensor, Double peak, Subsurface drainage, Woodchip bioreactor.

Subsurface mobility of land applied greenhouse nutrient feed water and environmental implications

Greenhouse nutrient feedwater (GNF) discharge is considered a potential contributor to eutrophication issues in Lake Erie, Ontario. Land application of GNF is an accepted legislated management response to mitigate the impact of such nutrient loads. To assess the potential environmental impacts of this management practice, field infiltration experiments were conducted at four different greenhouse operations near Leamington, Ontario. Over a three-year study, GNF was applied on agricultural land adjacent to the greenhouse operations in the fall during the first year, and along with a bromide tracer in the summer and fall in Years 2 and 3, respectively. The GNF was applied at the maximum allowable rates as defined in legislation. Chemical constituents (nutrients, metals and the conservative tracer bromide) were monitored within the soil profile matrix and pore water above the water table. The results showed that, even with the GNF being applied at the highest permissible rates, the species of interest remained within the unsaturated soil zone at low concentrations over three to six months sampling intervals. The bromide tracer test demonstrated that highly mobile species could move through permeable soils to the water table depth in a potential worst-case application scenario. However, considering the low initial concentrations, long vadose zone residence time and the low mass flux, it would appear that land application of GNF, when applied in accordance with Ontario's Regulations, is a feasible and environmentally reasonable treatment option for managing GNF.

Nitrate removal and environmental side-effects controlled by hydraulic residence time in woodchip bioreactors treating cold agricultural drainage water

ABSTRACT Denitrifying woodchip bioreactors (WBRs) remove nitrate () from agricultural drainage water at field-scale, but their efficacy at cold temperatures remains uncertain. This study shows how hydraulic residence time (HRT) controls removal and environmental side-effects of WBRs at low water temperature under pilot-scale conditions with controlled operation of nine WBRs (94 dm3). Hydraulic properties were assessed by a bromide tracer test, and removal, emissions of nitrous oxide (N2O) and methane (CH4), and losses of dissolved organic carbon (DOC) were measured at HRTs of 5–30 h. Inlet concentrations were increasingly reduced at higher HRTs. The relationship between HRT and the efficiency (%) of removal was linear ( = 0.94), while the relationship between HRT and reduction rates (NRR) was logistic ( = 0.88). Gaseous emissions of N2O were equally low at HRTs of 10–30 h, but higher at 5 h (P < 0.05). Methane fluxes were small, but with consistent emissions at HRTs of 20–30 h and uptake at 5–15 h. HRT had limited effect on effluent DOC concentrations, but strong effect on mass losses that were five-fold higher (320 mg L−1) at the HRT of 5 h than at 30 h. In summary, at cold temperatures HRTs of ≤ 20 h resulted in suboptimal NRR, accelerating DOC losses, and increased risk of N2O losses at least below a threshold HRT of 5–10 h. HRTs of 20–30 h gave maximal NRR, smallest losses of DOC and N2O, but an increased risk of CH4 emissions.

Modeling transport, fate, and removal kinetics of nitrate and orthophosphate using recycled adsorbents for high and low-flow stormwater runoff treatment

Excessive nitrate and orthophosphate carried by the stormwater runoff potentially lead to eutrophication in surface water bodies. Various green infrastructures are used that commonly consider the biological treatment of nutrients from the runoff. Due to the leaching and clogging complexities in biological mechanisms, the selection of high-flow, eco-friendly, and recycled adsorbents has been advocated to promote the physiochemical treatment of nutrients as an alternative. In this study, column experiments were conducted to investigate the transport, fate, adsorption equilibria, and reaction kinetics of nitrate (NO3–N) and orthophosphate (PO4–P) onto three recycled adsorbents - recycled concrete aggregate (RCA), recycled crushed glass (RCG), rice husks (RH), and a layered media (LM), under high and low-flow conditions. The non-reactive solute transport in columns was investigated through the bromide tracer test. The HYDRUS-1D model was used to estimate adsorption coefficients and reaction kinetics of pollutants in unsaturated media columns. Our results indicated the maximum superficial pore velocity (v = 4.40 cm/s) and dispersion (α = 2.50 cm) in RCA at the low-flow condition. Overall, NO3–N removal at the exhaustion was low in all columns, ranging between 1 and 25%. Conversely, orthophosphate removal was significant (p < 0.05) in RCA (≤94%) under low flow conditions with increased reaction kinetics (kr,d = 3.45 min−1, kr,s = 0.55 min−1) and enhanced adsorption capacity at saturation (qmax = 1.87E+05–2.33E+05 mg/kg). In conclusion, the dissolved-phase reaction kinetics (kr,d) played a significant role apart from the physisorption for the satisfactory removal of orthophosphate in RCA.

Using bromide data tracer and HYDRUS‐1D to estimate groundwater recharge and evapotranspiration under film‐mulched drip irrigation in an arid Inland Basin, Northwest China

AbstractAccurate estimation of groundwater recharge (GR) and evapotranspiration (ET) are essential for sustainable management of groundwater resources, especially in arid and semi‐arid regions. In the Manas River Basin (MRB), water shortage is the main factor restricting sustainable development of irrigated agriculture, which relies heavily on groundwater. Film‐mulched drip irrigation significantly changes the pattern and dominant processes of water flow in the unsaturated zone, which increases the difficulty of GR and ET estimation. To better estimate GR and ET under film‐mulched drip irrigation in the MRB, bromide tracer tests and soil lithologic investigation were conducted at 12 representative sites. A one‐dimensional variably saturated flow model (HYDRUS‐1D) was calibrated at each site using soil evaporation data inferred from the bromide tracer tests. The results showed that average annual soil evaporation in uncultivated lands calculated from bromide trace tests was 25.55 mm. The annual GR ranged from 5.5 to 37.0 mm under film‐mulched drip irrigation. The annual ET ranged from 507.0 to 747.1 mm, with soil evaporation between 35.7 and 117.0 mm and transpiration between 460.9 and 642.3 mm. Soil evaporation represented 7% to 16% of the total ET and more than 70% of precipitation and irrigation water was used by cotton plants. Spatial variations of soil lithology, water table depth and initial soil water content led to the spatial differences of GR and ET in the MRB. Our study indicated that bromide tracer tests are useful for inferring ET in the arid and semi‐arid oases. The combination of bromide tracer tests and HYDRUS‐1D enhances reliability for estimation of GR and ET under film‐mulched drip irrigation in the MRB and shows promise for other similar arid inland basins around the world.

Diffusion of perfluoroalkyl acids through clay-rich soil

Diffusion through a water saturated silty clay soil column was measured for six perfluoroalkyl acids (PFAAs), including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS). An aqueous pore diffusion model, which incorporated linear adsorption parameters measured independently in batch tests and a tortuosity factor determined independently using a bromide tracer test, was used to describe the experimental diffusion data. The diffusion model substantially underpredicted PFAA diffusion through the soil column for the more strongly sorbing PFAAs (most notably PFOS). Instead, application of a diffusion model that included a surface diffusion-like process provided substantially improved prediction of PFAA diffusion through the soil. The ratio of the observed pore diffusion coefficient to the observed surface diffusion coefficient ranged from 13 (for perfluorohexane sulfonate) to 0.88 for PFOS. These results suggest that surface diffusion serves a potentially important role for strongly sorbing PFAAs in clay-rich soils, and highlights the need for additional studies into the coupled adsorption and diffusion of PFAAs in low permeability media.

Performance of a Woodchip Bioreactor for the Treatment of Nitrate-Laden Agricultural Drainage Water in Northeastern Germany

Reactive barriers, such as denitrifying bioreactors, have been identified as a clean-up option for nutrient-laden agriculture runoff. Here we tested a 20 m long, 3.75 m wide and 2.2 m deep woodchip bioreactor receiving tile drainage water from a 5.2 ha field site, aiming at testing the hydraulic functioning of a dual-inlet system and quantifying its impact on nutrient loads (nitrogen, reactive phosphorus, organic carbon) in a region with a drainage season taking place in the hydrological winter (November to April). The hydraulic conditions in the dual-inlet bioreactor system developed differently than expected; asymmetric flow rates led to long average hydraulic retention times and a highly dispersed residence time distribution, which was revealed by a bromide tracer test. With a nitrate load reduction of 51 to 90% over three drainage seasons, the woodchip bioreactor proved at the same time to be very effective under the winter conditions of northeastern Germany. The bioreactor turned from an orthophosphate source in the first year of operation into an orthophosphate sink in the second and third year, which was not expected because of anoxic conditions (favorable for denitrification) prevailing within the woodchips. Besides an efficient nutrient retention, the woodchip bioreactor contributed to the total organic carbon load of receiving waters, which impairs the overall positive role of bioreactors within intensively agriculturally used landscapes. We consider this promising low-maintenance biotechnology particularly suitable for single drainage pipes with high discharge and high nitrate concentrations.

Estimating impacts of water-table depth on groundwater evaporation and recharge using lysimeter measurement data and bromide tracer

The presented study evaluates the effect of water-table depth on groundwater evaporation (Eg) and recharge (Rg) when they occurred alternately. A lysimeter experiment incorporating a 1-year-long bromide tracer test was conducted under conditions with a range of maintained water-table depths. The results revealed that both the Rg and Eg decreased as the water table fell, until it was down to the extinction depth of groundwater evaporation (EDGE, 2.4 m). The annual quantity of Rg started to be stable at 100 mm when the water table was below the EDGE, since the maximum soil-water deficit no longer increased. When the water table was above the EDGE, Rg and Eg restricted each other and thus occurred alternately; in the wet season, >68% of the annual Rg occurred, with only <10% of the annual Eg. The fast response of the soil-water potential to irrigation and soil evaporation tended to make the gradient of the whole potential profile unidirectional when the water table was shallow, which promoted both Rg and Eg. Taking soil evaporation, Rg and Eg into account, the inversely calculated position of the bromide concentration peak was close to the actual position, suggesting that bromide tracer is effective for tracing the complicated processes of the unsaturated zone flow when Rg and Eg occur alternately. The study improved understanding of the way that the water table affects Rg and Eg in the shallow groundwater area and proved that bromide tracer would be an innovative technique to estimate Eg.

Efficacy of bromide tracers for evaluating the hydraulics of denitrification beds treating agricultural drainage water

One biotechnology that can reduce nitrate concentration in subsurface drainage water is called a denitrification bed. Bromide tracer testing has been used to determine the hydraulic properties of these systems to be used in design and modeling, but efficacy of bromide tracers has not been investigated. One important hydraulic property of woodchip beds is its in-situ effective porosity. The objectives of this study were to (1) determine if bromide is a suitable tracer for evaluating the hydraulics of woodchip beds based on laboratory sorption experiments and field tracer testing, and (2) to determine the in-situ effective porosity of denitrification beds. To achieve this goal, we conducted bromide sorption experiments in the laboratory, and conducted bromide tracer testing in seven denitrification beds near Willmar, Minnesota, USA. We did not find sorption (neither adsorption nor absorption) of bromide to woodchips in our laboratory sorption experiments. In contrast, bromide tracer tests showed an average bromide recovery of 82%, revealing that bromide was retarded in the woodchip denitrification beds. To the best of our knowledge, our experiment is the first study to estimate the in-situ effective porosity (average of 0.61) of a typical field-scale denitrification bed using a tracer test. A more accurate estimate of the actual hydraulic retention time (HRT) can be obtained by using the in-situ effective porosity rather than total porosity. Our laboratory experiment did not show any sorption of bromide to woodchips, but in contrast and for unclear reasons, our field tracer testing showed bromide retardation.

Characterization of a hydraulically induced crystalline‐rock fracture

AbstractHydraulic fracturing has become an important technique for enhancing the permeability of hydrocarbon source rocks and increasing aquifer transmissivity in many hard rock environments where natural fractures are limited, yet little is known about the nature or behaviour of these hydraulically induced fractures as conduits to flow and transport. We propose that these fractures tend to be smooth based on observed hydraulic and transport behaviour. In this investigation a multi‐faceted approach was used to quantify the properties and characteristics of an isolated hydraulically induced fracture in crystalline rocks. Packers were used to isolate the fracture that is penetrated by two separate observation wells located approximately 33 m apart. A series of aquifer tests and an induced gradient tracer test were performed to better understand the nature of this fracture. Aquifer test results indicate that full recovery is slow because of the overall low permeability of the crystalline rocks. Drawdown tests indicate that the fracture has a transmissivity of 1–2 m2/day and a specific storage on the order of 2–9 × 10−7/m. Analysis of a potassium–bromide tracer test break through curve shows classic Fickian behaviour with minimal tailing analogous to parallel plate flow. Virtually all of the tracer was recovered, and the breakthrough curve dilution indicates that the swept area is only about 11% of a radial flow field and the estimated aperture is ≤0.5 mm, which implies a narrow linear flow region. These outcomes suggest that transport within these hydraulically induced ‘smooth’ fractures in crystalline rocks is rapid with minimal mixing, small local velocity fluctuations and no apparent diffusion into the host rock or secondary fractures. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Electrokinetic Migration of Nitrate Through Heterogeneous Granular Porous Media

AbstractThis study investigates and quantifies the influence of physical heterogeneity in granular porous media, represented by materials with different hydraulic conductivity, on the migration of nitrate, used as an amendment to enhance bioremediation, under an electric field. Laboratory experiments were conducted in a bench‐scale test cell under a low applied direct current using glass bead and clay mixes and synthetic groundwater to represent ideal conditions. The experiments included bromide tracer tests in homogeneous settings to deduce controls on electrokinetic transport of inorganic solutes in the different materials, and comparison of nitrate migration under homogeneous and heterogeneous scenarios. The results indicate that physical heterogeneity of subsurface materials, represented by a contrast between a higher‐hydraulic conductivity and lower‐hydraulic conductivity material normal to the direction of the applied electric field exerts the following controls on nitrate migration: (1) a spatial change in nitrate migration rate due to changes in effective ionic mobility and subsequent accumulation of nitrate at the interface between these materials; and (2) a spatial change in the voltage gradient distribution across the hydraulic conductivity contrast, due to the inverse relationship with effective ionic mobility. These factors will contribute to higher mass transport of nitrate through low hydraulic conductivity zones in heterogeneous porous media, relative to homogeneous host materials. Overall electrokinetic migration of amendments such as nitrate can be increased in heterogeneous granular porous media to enhance the in situ bioremediation of organic contaminants present in low hydraulic conductivity zones.

Evaluating two infiltration gallery designs for managed aquifer recharge using secondary treated wastewater

As managed aquifer recharge (MAR) becomes increasingly considered for augmenting water-sensitive urban areas, fundamental knowledge of the achievable scale, longevity and maintenance requirements of different options will become paramount. This paper reports on a 39 month pilot scale MAR scheme that infiltrated secondary treated wastewater through unsaturated sand into a limestone and sand aquifer. Two types of infiltration gallery were constructed to compare their hydraulic performance, one using crushed, graded gravel, the other using an engineered leach drain system (Atlantis Leach System®). Both galleries received 25 kL of nutrient-rich, secondary treated wastewater per day. The Atlantis gallery successfully infiltrated 17 ML of treated wastewater over three years. The slotted distribution pipe in the gravel gallery became clogged with plant roots after operating for one year. The infiltration capacity of the gravel gallery could not be restored despite high pressure cleaning, thus it was replaced with an Atlantis system. Reduction in the infiltration capacity of the Atlantis system was only observed when inflow was increased by about 3 fold for two months. The performance of the Atlantis system suggests it is superior to the gravel gallery, requiring less maintenance within at least the time frame of this study. The results from a bromide tracer test revealed a minimum transport time of 3.7 days for the recharged water to reach the water table below 9 m of sand and limestone. This set a limit on the time available for attenuation by natural treatment within the unsaturated zone before it recharged groundwater.

Simplified behaviors from increased heterogeneity: II. 3-D Uranium transport at the decimeter scale and intertank comparisons

Upscaling from bench scale systems to field scale systems incorporates physical and chemical heterogeneities from atomistic up to field scales. Heterogeneities of intermediate scale (~10(-1) m) are impossible to incorporate in a bench scale experiment. To transcend these scale discrepancies, this second in a pair of papers presents results from an intermediate scale, 3-D tank experiment completed using five different particle sizes of uranium contaminated sediment from a former uranium mill field site. The external dimensions of the tank were 2.44 m×0.61 m×0.61 m (L×H×W). The five particle sizes were packed in a heterogeneous manner using roughly 11 cm cubes. Small groundwater wells were installed for spatial characterization of chemical gradients and flow parameters. An approximately six month long bromide tracer test was used for flow field characterization. Within the flow domain, local uranium breakthrough curves exhibited a wide range of behaviors. However, the global effluent breakthrough curve was smooth, and not unlike breakthrough curves observed in column scale experiments. This paper concludes with an inter-tank comparison of all three experimental systems presented in this pair of papers. Although there is a wide range of chemical and physical variability between the three tanks, major chemical constituent behaviors are often quite similar or even identical.

Optimized denitrification bioreactor treatment through simulated drainage containment

Abstract In the design of wood-based, enhanced-denitrification bioreactors to treat nitrate in agricultural drainage, the consideration of the highly variable flow rates and nitrate concentrations inherent to many drainage systems is important. For optimized mitigation of these nitrate loads, it may be best to contain drainage water prior to treatment in order to facilitate longer, more constant retention times rather than to allow cycles of flushing and dry periods in the denitrification bioreactor. Simulated containment prior to bioreactor treatment compared to passing drainage directly through a bioreactor was investigated with the use of six pilot-scale denitrification bioreactors constructed with plywood and filled with Pinus radiata woodchips at Massey University No. 4 Dairy Farm (Palmerston North, New Zealand). Initial bromide tracer tests were followed with a series of five simulated drainage events each at successively declining inflow nitrate concentrations. During each drainage event, three pilot bioreactors received a simulated hydrograph lasting 1.5 days (Non-Containment treatment) and three pilot bioreactors received the same total drainage volume treated over 4 days at a constant flow rate (i.e. constant retention time; Containment treatment). Results showed significantly different total mass removal efficiencies of 14.0% vs. 36.9% and significantly different removal rates of 2.1 g N m −3 day −1 vs. 6.7 g N m −3 day −1 for the Non-Containment and Containment treatments, respectively, which indicated that treating drainage at constant retention times provided more optimized nitrate removal. While this work was done to evaluate treatment under New Zealand drainage conditions, it also provides valuable information for optimizing agricultural drainage denitrification bioreactor performance in general.

Automated Equilibrium Tension Lysimeters for Measuring Water Fluxes through a Layered, Volcanic Vadose Profile in New Zealand

In this technical note we present the design, installation, and evaluation of a field monitoring system to directly measure water fluxes through a vadose zone. The system is based on use of relatively new measurement technology—automated equilibrium tension lysimeters (AETLs). An AETL uses a porous sintered stainless‐steel plate to provide a comparatively large sampling area (0.20 m2) with a continuously controlled vacuum applied under the plate. This vacuum is in “equilibrium” with the surrounding vadose zone tension to ensure measured fluxes represent those under undisturbed conditions. Fifteen of these AETLs have been installed at five depths through a layered volcanic vadose zone to study the impact of land use changes on water quality in Lake Taupo, New Zealand. We describe the development and testing of the AETLs, the methods used for installing these devices, a condensed data set of the measured physical properties of the vadose zone, and the initial results from the in situ operation of the AETLs, including the preliminary results from a bromide tracer test. For an AETL installed at the 0.4‐m depth, where soil pressure heads are most dynamic, the average deviation between the target reference pressure head, as measured in the undisturbed vadose zone and the pressure head measured above the sampling plate was only 5.4 hPa over a 180‐d period. The bromide recovered in an AETL at the same depth was equivalent to 96% of the bromide pulse applied onto the surface area directly above the AETL. We conclude that this measurement technique provides an accurate and robust method of measuring vadose zone fluxes. These measurements can ultimately contribute to better understanding of the water transport and contaminant transformation processes through vadose zones.

Coupling of zero valent iron and biobarriers for remediation of trichloroethylene in groundwater

This study attempted to construct a three series barrier system to treat high concentrations of trichloroethylene (TCE; 500 mg/L) in synthetic groundwater. The system consisted of three reactive barriers using iron fillings as an iron-based barrier in the first column, sugarcane bagasse mixed with anaerobic sludge as an anaerobic barrier in the second column, and a biofilm coated on oxygen carbon inducer releasing material as an aerobic barrier in the third column. In order to evaluate the extent of removal of TCE and its metabolites in the aquifer down gradient of the barrier system, a fourth column filled with sand was applied. Residence time of the system was investigated by a bromide tracer test. The results showed that residence time in the column system of the control set and experimental set were 23.62 and 29.99 days, respectively. The efficiency of the three series barrier system in removing TCE was approximately 84% in which the removal efficiency of TCE by the iron filling barrier, anaerobic barrier and aerobic barrier were 42%, 16% and 25%, respectively. cis-Dichloroethylene ( cis-DCE), vinyl chloride (VC), ethylene and chloride ions were observed as metabolites following TCE degradation. The presence of chloride ions in the effluent from the column system indicated the degradation of TCE. However, cis-DCE and VC were not fully degraded by the proposed barrier system which suggested that another remediation technology after the barrier treatment such as air sparging and adsorption by activated carbon should be conducted.

Relative importance of dispersion and rate‐limited mass transfer in highly heterogeneous porous media: Analysis of a new tracer test at the Macrodispersion Experiment (MADE) site

A single‐well injection‐withdrawal (SWIW) bromide tracer test was conducted to further investigate transport processes at the Macrodispersion Experiment (MADE) site on Columbus Air Force Base in Mississippi. The bromide breakthrough curve is highly asymmetric and exhibits an early time high‐concentration peak followed by an extended period of low‐concentration tailing. Comparisons of results simulated by advection‐dispersion (AD) and dual‐domain mass transfer (DDMT) models with the field data show that the DDMT model more accurately represents the magnitudes of both the early high‐concentration peak and the later low‐concentration tail. For both the AD and DDMT models, the match with field data is enhanced by incorporating hydraulic conductivity information from new direct‐push profiling methods. The Akaike information criterion for the DDMT models is much smaller than that for the AD models in both the homogeneous and heterogeneous cases investigated in this work. The improved match of the DDMT model with the SWIW test data supports the hypothesis of mass transfer processes occurring at this highly heterogeneous site.

Phosphorus reduction from agricultural runoff in a pilot-scale surface-flow constructed wetland

Excess P in surface waters in Quebec is the primary cause of water quality deterioration and the majority of it is coming from agricultural land as non-point source pollution. The objective of this study was to compare how two substrates, a sandy clay loam and a sand soil, influenced P retention in a surface-flow constructed wetland (CW). A secondary objective was to determine if the hydraulic residence time of the wetland differed between soil types. Measurements were taken at a pilot-scale CW site between July 5 and October 1, 2007. Three cylindrical tank replicates filled with sandy clay loam soil, and three with a sandy soil were planted with cattails ( Typha latifolia L.) and reed canary grass ( Phalaris arundinaceae L.). The tanks were flooded continuously with an artificial agricultural runoff solution containing 0.3 mg L −1 dissolved reactive P. The six treatment tanks retained 0.9–1.6 g P m −2, which corresponded to an average removal efficiency of 41%; there was no significant difference in the P retention by the two soil types. A bromide tracer test revealed a mean hydraulic retention time of 2.2 days for all tanks; however, the active volume of the sand tanks was greater. This investigation suggests that a sandy soil may be less prone to reducing conditions in a surface-flow CW and therefore maintain its role as a P sink for longer than the sandy clay loam.

Residence time distribution and oxygen transfer in a novel constructed soil filter

AbstractBACKGROUND: The constructed soil filter (CSF), also known as soil biotechnology is a system for water renovation, which makes use of formulated media, culture of soil micro‐ and macro‐organisms, additives and plantation to purify water and wastewater. The process gives benefits in terms of applicability across very small to large scale, natural aeration, absence of moving parts, no biological sludge generation, odor free green aesthetic ambience.RESULTS: Residence time distribution (RTD) studies were carried out using laboratory scale CSF. Pulse potassium bromide tracer tests were carried out to determine RTD, and the Peclet number found to be 9–13 for a 2 m bed, and 2–3 for a 0.30 m bed with oxygen transfer of 0.08 h−1.CONCLUSION: The two‐channel dispersion model for flow behavior shows a good fit to the experimental data, indicating a reactor Peclet number 9–13 for a 2 m bed and 2–3 for a 0.3m bed. Oxygen transfer studies carried out using various methods gave an oxygen transfer coefficient of about 0.08 h−1. Wastewater purification studies indicate overall COD removal rate of around 50 mg L−1 h−1, suggesting that highly aerobic conditions are prevalent in the CSF system. Copyright © 2009 Society of Chemical Industry