Visualization of TCE recovery mechanisms using surfactant-polymer solutions in a two-dimensional heterogeneous sand model

This research focused on the optimization of TCE dissolution in a physical two-dimensional model providing a realistic representation of a heterogeneous granular aquifer. TCE was infiltrated in the sand pack where it resided both in pools and in zones of residual saturation. Surfactant was initially injected at low concentration to minimize TCE remobilization at first contact but was incrementally increased later during the experiment. Xanthan gum was added to the injected surfactant solution to optimize the sweep efficiency through the heterogeneous medium. Photographs and digital image analysis illustrated the interactions between TCE and the injected fluids. During the polymer flood, the effects of heterogeneities inside the sand pack were greatly reduced by the increased fluid viscosity and the shear-thinning effects of the polymer. The polymer also improved the contact between the TCE ganglia and the surfactant-polymer solution, thereby promoting dissolution. Surfactants interacted with the polymer reducing the overall viscosity of the solution. At first contact with a 0.5%(mass) surfactant solution, the TCE pools drained and some remobilization occurred. However, no TCE bank was formed and TCE did not penetrate into any previously uncontaminated areas. As a result, TCE surface area was increased. Subsequent surfactant floods at higher surfactant concentrations did not trigger more remobilization. TCE was mainly dissolved by the solution with the highest surfactant concentration. Plugging from bacterial growth or microgel formation associated to the polymer at the inflow screen prevented the full completion of the experiment. However, more than 90% of TCE was recovered with the circulation of less than 6 pore volumes of surfactant-polymer solution.     

Combination of photoacoustic detector with gas diffusion probes for the measurement of methane concentration gradients in submerged paddy soil

Dissolved methane was monitored by means of a diffusion probe in combination with a photoacoustic (PA) detector cell placed in the cavity of a liquid nitrogen-cooled CO laser. The detection limit of the photoacoustic detector was 1 ppbv methane (≈ 2 μM in aqueous solution), the time response was 60 s, the spatial resolution was 1.36 mm. These limits were determined by the acoustic noise and the configuration of the diffusion probe. The combination of PA detector with gas diffusion probes was found to be useful for monitoring gaseous compounds. However, the membrane material of the diffusion probe was critical. Silicone as membrane material was useful only for measurement of CH₄. Goretex as membrane material was applicable to measurement of dimethylsulfide (DMS), but did not give a stable signal for trimethylamine (TMA).

Vertical concentration profiles of CH₄ in anoxic paddy soil agreed well with earlier results obtained with a gas chromatograph as detector. Methane was produced in anoxic soil layers below 8–10 mm depth and diffused upwards to the surface through a layer of CH₄-consuming bacteria situated at about 2 mm depth. In the oxic upper 2 mm soil layer the concentration of CH₄ decreased below the detection limit of our system. Methane-containing gas bubbles that were embedded in the soil were detected by a steep increase of the CH₄ signal. The combination of PA detector and gas diffusion probe was found to be a useful tool to measure CH₄ gradients in submerged soil or sediment with high temporal and spatial resolution, thus allowing the localization and quantification of CH₄ production and CH₄ oxidation rates within the soil profile.     

Living with a large reduction in permited loading by using a hydrograph-controlled release scheme

The Total Maximum Daily Load (TMDL) for ammonia and biochemical oxygen demand for the Pee Dee, Waccamaw, and Atlantic Intracoastal Waterway system near Myrtle Beach, South Carolina, mandated a 60-percent reduction in point-source loading. For waters with a naturally low background dissolved-oxygen concentrations, South Carolina anti-degradation rules in the water-quality regulations allows a permitted discharger a reduction of dissolved oxygen of 0.1 milligrams per liter (mg/L). This is known as the 0.1 rule. Permitted dischargers within this region of the State operate under the 0.1 rule and cannot cause a cumulative impact greater than 0.1 mg/L on dissolved-oxygen concentrations. For municipal water-reclamation facilities to serve the rapidly growing resort and retirement community near Myrtle Beach, a variable loading scheme was developed to allow dischargers to utilize increased assimilative capacity during higher streamflow conditions while still meeting the requirements of a recently established TMDL.As part of the TMDL development, an extensive real-time data-collection network was established in the lower Waccamaw and Pee Dee River watershed where continuous measurements of streamflow, water level, dissolved oxygen, temperature, and specific conductance are collected. In addition, the dynamic BRANCH/BLTM models were calibrated and validated to simulate the water quality and tidal dynamics of the system. The assimilative capacities for various streamflows were also analyzed.The variable-loading scheme established total loadings for three streamflow levels. Model simulations show the results from the additional loading to be less than a 0.1mg/L reduction in dissolved oxygen. As part of the loading scheme, the real-time network was redesigned to monitor streamflow entering the study area and water-quality conditions in the location of dissolved-oxygen sags. The study reveals how one group of permit holders used a variable-loading scheme to implement restrictive permit limits without experiencing prohibitive capital expenditures or initiating a lengthy appeals process.     

Bench-scale visualization of DNAPL remediation processes in analog heterogeneous aquifers: surfactant floods and in situ oxidation using permanganate

We have conducted well-controlled DNAPL remediation experiments within a 2-D, glass-walled, sand-filled chamber using surfactants (Aerosol MA and Tween 80) to increase solubility and an oxidant (permanganate) to chemically degrade the DNAPL. Initial conditions for each remediation experiment were created by injecting DNAPL as a point source at the top of the chamber and allowing the DNAPL to migrate downward through a water-filled, heterogeneous, sand-pack designed to be evocative of a fluvial depositional environment. This migration process resulted in the DNAPL residing as a series of descending pools. Lateral advection across the chamber was used to introduce the remedial fluids. Photographs and digital image analysis illustrate interactions between the introduced fluids and the DNAPL. In the surfactant experiments, we found that DNAPL configured in a series of pools was easily mobilized. Extreme reductions in DNAPL/water interfacial tension occurred when using the Aerosol MA surfactant, resulting in mobilization into low permeability regions and thus confounding the remediation process. More modest reductions in interfacial tension occurred when using the Tween 80 surfactant resulting in modest mobilization. In this experiment, capillary forces remained sufficient to exclude DNAPL migration into low permeability regions allowing the excellent solubilizing properties of the surfactant to recover almost 90% of the DNAPL within 8.6 pore volumes. Injection of a potassium permanganate solution resulted in precipitation of MnO2, a reaction product, creating a low-permeability rind surrounding the DNAPL pools. Formation of this rind hindered contact between the permanganate and the DNAPL, limiting the effectiveness of the remediation. From these experiments, we see the value of performing visualization experiments to evaluate the performance of proposed techniques for DNAPL remediation.     

OPTIMIZATION PROCEDURE FOR COST EFFECTIVE BMP PLACEMENT AT A WATERSHED SCALE1

ABSTRACT: A combinatorial optimization procedure for best management practice (BMP) placement at the watershed level facilitates selection of cost effective BMP scenarios to control non point source (NFS) pollution. A genetic algorithm (GA) was selected from among several optimization heuristics. The GA combines an optimization component written in the C++ language with spatially variable NFS pollution prediction and economic analysis components written within the Arc View geographic information system. The procedure is modular in design, allowing for component modifications while maintaining the basic conceptual framework. An objective function was developed to lexicographically optimize pollution reduction followed by cost increase. Scenario cost effectiveness is then calculated for scenario comparisons. The NPS pollutant fitness score allows for evaluation of multiple pollutants, based on prioritization of each pollutant. The economic component considers farm level public and private costs, cost distribution, and land area requirements. Development of a sediment transport function, used with the Universal Soil Loss Equation, allows the optimization procedure to run within a reasonable timeframe. The procedure identifies multiple near optimal solutions, providing an indication of which fields have a more critical impact on overall cost effectiveness and flexibility in the final solution selected for implementation. The procedure was demonstrated for a 1,014‐ha watershed in the Ridge and Valley physiographic region of Virginia.     

LOW-INPUT AGRICULTURE AS A GROUND WATER PROTECTION STRATEGY1

ABSTRACT: Protection of ground water quality is of considerable importance to local, state, and federal governments. This study uses a 15-year mathematical programming model to evaluate the effectiveness of low-input agriculture, under alternative policy scenarios, as a strategy to protect ground water quality in Richmond County, Virginia. The analysis considers eight policy alternatives: cost-sharing for green manures, two restrictions on atrazine applications levels, chemical taxation, a restriction on potential chemical and nitrogen levels in ground water only and in surface and ground water, and two types of land retirement programs. The CREAMS and GLEAMS models were used to estimate nitrate and chemical leaching from the crop root zone. The economic model evaluates production practices, policy constraints, and water quality given a long-term profit maximizing objective. The results indicate that low-input agriculture alone may not be an effective ground water protection strategy. The policy impacts include partial adoption of low-input practices, land retirement, and the substitution of chemicals. Only mandatory land retirement policies reduced all chemical and nutrient loadings of ground water; however, they did not promote the use of low-input agricultural practices.     

A review of citizen science and community-based environmental monitoring: issues and opportunities

Worldwide, decision-makers and nongovernment organizations are increasing their use of citizen volunteers to enhance their ability to monitor and manage natural resources, track species at risk, and conserve protected areas. We reviewed the last 10 years of relevant citizen science literature for areas of consensus, divergence, and knowledge gaps. Different community-based monitoring (CBM) activities and governance structures were examined and contrasted. Literature was examined for evidence of common benefits, challenges, and recommendations for successful citizen science. Two major gaps were identified: (1) a need to compare and contrast the success (and the situations that induce success) of CBM programs which present sound evidence of citizen scientists influencing positive environmental changes in the local ecosystems they monitor and (2) more case studies showing use of CBM data by decision-makers or the barriers to linkages and how these might be overcome. If new research focuses on these gaps, and on the differences of opinions that exist, we will have a much better understanding of the social, economic, and ecological benefits of citizen science.