Fate Of Spilled Oil Research Articles

Role of light microplastics in the dispersion process of spilled crude oil in the marine environment

Oil spill and microplastic (MP) pollution are the main problems in the marine environment. After an oil spill, the oil film may be dispersed into the water column in the form of droplets under the action of ocean waves. In this study, the sea condition was simulated through the batch conical flask oscillation experiment. Merey crude oil was selected as experimental oil, and polyethylene (PE) and polystyrene (PS) were used as experimental MP. The effects of MP properties (type, concentration and size) on the dispersion of spilled oil were investigated. It is found that for each MP, the oil dispersion efficiency (ODE) increased rapidly at first and then tended to be stable, which all reached the maximum at 360 min. When the concentrations of PE and PS increased from 0 to 100 mg/L, the maximum ODE decreased from 32.64 % to 13.72 % and 10.75 %, respectively, indicating that the presence of MP inhibits the oil dispersion. At the same oscillation time, the volumetric mean diameter (VMD) of dispersed oil increased with the MP concentration. When the particle size of PE and PS increased from 13 to 1000 μm, the maximum ODE increased from 24.74 % to 31.49 % and 28.60 %, respectively. However, the VMD decreased with the size of MP. In addition, the time series of the oil adsorption rate by the MP were well fitted by the kinetic models. The results of this research deepen the understanding of the migration law of spilled oil to the marine environment in the presence of MP, and may further improve the ability of marine environmental scientists to predict the fate of oil spill.

Managing deepsea oil spills through a systematic modeling approach

Offshore oil exploration and production in deepwater are associated with environmental risks to marine ecosystems. This research introduces DWOSM (Deep Water Oil Spill Model), a three-dimensional Lagrangian model, which is developed to simulate the transport and fate of oil spills resulting from subsea blowouts. DWOSM comprises three interconnected modules: DWOSM-DSD, which predicts the oil droplet size distribution from a blowout release; DWOSM-NearField, simulating plume dynamics and tracking oil droplets within the plume region; and DWOSM-FarField, modeling the evolution of dispersed oil beyond the near-field. Compared to other oil spill models, this integrated approach improves the transition between near and far fields using a near-field particle tracking algorithm. It also employs the thermodynamic models to enable the prediction of oil properties under varying deep water pressure and temperature. To gauge the reliability and efficacy of DWOSM, a hypothetical case situated within a North American context is employed for model testing. The DWOSM and its each module are juxtaposed with other established oil spill models. The outcomes indicate that DWOSM yields comparable results to these models by providing reasonable predictions of a deepwater blowout. The model's verification through case scenario testing and comparison underscores its potential as a decision tool for assessing and managing the potential environmental impacts of offshore petroleum activities.

Modeling weathering processes of spilled oil on the sea surface based on dynamic Bayesian network

Oil spills pose a serious threat to the ecological environment and human health. When the spilled oil enters seawater, oil weathering processes like evaporation, emulsification, biodegradation, photooxidation and dissolution will occur. These processes have high uncertainty and directly affect the spatial distribution and fate of oil spills. This paper presents a stochastic modeling method for weathering process of spilled oil on the sea surface based on Dynamic Bayesian Network (DBN). The physical equations of evaporation, emulsification, biodegradation, photooxidation and dissolution are converted into DBN, and then all the individual DBNs are integrated to obtain the entire DBN of the oil weathering process. The developed DBN could intuitively and graphically express the logical relations and conditional probabilities among physical variables. This method is helpful to find out the influencing mechanism of multi-random variables on the oil weathering process. The proposed method is applied to one type of light oil that is spilled on the sea surface of the Bohai Sea. Based on the developed DBN model, the oil weathering process in the first 18 h are studied by quantitative analysis. Through sensitivity analysis, it shows that the photooxidation and dissolution have greater impact on the heavy oil fraction than biodegradation. For oil-water mixture, it is greatly affected by evaporation in the first few hours, but emulsification becomes the leading weathering process in the 18th hour. The effects of dissolution are low, but higher than biodegradation and photooxidation. The key parameters affecting each oil weathering process are determined. Uncertainty analysis is carried out to reveal the possible upper and lower limit values of some physical quantities caused by the weathering process.

Environmental fate of polycyclic aromatic hydrocarbons (PAHs) in different layers of tar balls in the Bohai Sea, China

Several tar balls were transversely cut to investigate the fate of polycyclic aromatic hydrocarbons (PAHs) in different layers of tar balls under natural conditions. The highly recalcitrant C30 αβ-hopane was used as an internal biomarker to quantify the weathering levels of different PAHs compounds. The data showed that the values of 16 parent PAHs and their alkylated PAHs in the surface layer were lower than that in the center layer of tar balls, which indicated that the surface layer of tar balls were suffered more serious degradation. In addition, the loss rates of different isomers of the same series were almost the same, indicated that the degradation rate of different isomers in the same series was the same. Despite some degradation, the relative diagnostic ratios of alkylated PAHs in the different layers of the tar balls remained stable. Compare the loss rate of parent and alkylated PAHs, it revealed that the loss rates increased with the increase of alkylation degree. This is the first report on the fate of PAHs in different layers of tar balls. This study provides a quantitative analysis of the PAHs in different layers of tar balls under natural conditions, which is very important for understanding the fate of oil spills.

Development and testing of a 2D offshore oil spill modeling tool (OSMT) supported by an effective calibration method

An Oil Spill Modeling Tool (OSMT) has been developed in this study to predict the transport and fate of oil spills resulting from surface releases. Particularly, the Kullback-Leibler (KL) divergence method is adopted as a performance metric for the first time to formulate a calibration framework for spill trajectory prediction (STP) from the Lagrangian transport model (LTM). By finding the candidate with minimal KL divergences from modeling scenarios using designed parameter combinations, the prediction discrepancy between simulated trajectories of the LTM and oil slicks detected from satellite images is reduced. The developed approach has been evaluated through a comparison analysis between OSMT and Operational Oil Modeling Environment (GNOME) model. Subsequently, a real case study is conducted to examine the applicability and effectiveness of the OSMT. The study results indicate that OSMT can provide reliable spill trajectory simulations, and the KL divergence-based calibration method is effective in calibrating the oil spill LTM.

Development of an integrated tool responding to accidental oil spills in riverine and shoreline areas of Ho Chi Minh City, Vietnam

Setting up contingency planning strategies for spilled oil is always a challenging and difficult task for environmental management and decision-making. A large number of riverine systems and large ports (approximately 42) make Ho Chi Minh City, the most important economic hub of Vietnam, vulnerable to oil spills. There is, however, no reliable tool to identify the vulnerability of biological and human-use resources to oil spills along Ho Chi Minh City's riverine systems and coastal areas. The objective of this study is to develop an integrated tool combining the Environmental Sensitivity Index with a numerical model. Wherein, the Environmental Sensitivity Index (ESI) is developed according to the guidelines of the National Oceanic and Atmospheric Administration (NOAA); and the MIKE 21 model is used as a simulation tool after careful calibration and validation against observation data. Based on the knowledge of accidental spillage that occurred in the past, four typical scenarios of the oil spill are proposed to estimate the spread of spilled oil in regards to volume, locations, oil types, and time of occurrence. This study revealed that the oil spread and dispersion are strongly affected by the flow regimes; and the fate of oil spills has severe impacts on the environmental system and biological resources along the river systems and coastal areas; particularly on the rich biodiversity of the Mangrove Biosphere Reserve established by UNESCO of Ho Chi Minh City. The results obtained from this study can provide the underlying knowledge to prepare an effective contingency plan to mitigate and respond to the adverse effects of spilled oil on the local natural environment and socioeconomics of the study region.

Numerical simulation of multiphase oil behaviors in ice-covered nearshore water

There has been an increase in marine transportation in cold regions, which in turn has led to an increasing risk of oil spills in these areas. To better support risk assessment and pollution control of oil spills, it is important to have a good understanding of oil transport in the environment. This information is essential to manage response priorities and help prepare contingency and mitigating measures. This study aims to simulate 3D wave propagation in shallow water with different broken-ice aerial coverage percentages to assess the fate and transport of oil spill in a nearshore area under different conditions. Based on the Reynolds-averaged Navier-Stokes momentum equations for an incompressible viscous fluid and the Volume of Fluid (VOF) method that is coupled with Six Degree of Freedom (6-DOF) model, a 3D numerical model of three-phase transient flow was developed. It was found that the presence of ice makes the spreading of spilled oil slower in the horizontal direction since the ice can build natural barriers to oil movement. The higher the ice concentration, the slower spilled oil migrates in all directions. The maximum oil volume fraction varies with increasing ice coverage on the water surface area. The wave frequency, the averaged flow velocity, and oil properties affect the oil spread extent and the oil volume fraction. The dumping effect of the wave due to the presence of ice makes the impact of this factor less critical than those in open water.

Numerical Study on the Influence of Model Uncertainties on the Transport of Underwater Spilled Oil.

Oil pollution influences marine biology, ecology, and regional sustainable development capacity, but model uncertainties limit the ability of the numerical model to accurately predict the transport and fate of the underwater oil spill. Based on a three-dimensional underwater oil spill model validated by satellite images of the oil slick at the sea surface, the Penglai 19-3 oil spill accident in the Bohai Sea was simulated; in addition, several sensitivity experiments were set up to investigate the influence of model uncertainties in the background wind, current, start time of the oil spill, and spill site on the transport of underwater spilled oil in the Penglai 19-3 oil spill accident. The experimental results indicate that the uncertainty in the background wind has a certain impact on the simulated centroid position at the sea surface, and little effect on the simulated underwater results, while the uncertainty in the background current has a significant influence on the transport of the underwater spilled oil both at the sea surface and underwater. An uncertainty of 24 h in the start time of the oil spill can cause more than 1 time larger than the benchmark case displacement of the oil spill centroid point and sweeping area at the sea surface, as the periodic tidal current is the main constituent of the ocean current in the Bohai Sea. The uncertainty in the spill site has a large influence on the final position of the oil spill centroid point, but the oil spill trajectories do not intersect with each other within 48 h, which makes it possible to identify the oil spill platform from the actual observations. The influence of uncertainties in the important model inputs and key model parameters on the transport of underwater spilled oil in the Penglai 19-3 oil spill accident is evaluated for the first time, which is of substantial significance for improving the prediction accuracy of the transport and fate of underwater oil spills.

Effect of spilled diluted bitumen on chemical air-water exchange in boreal lake limnocorrals

Following spills into water, petroleum oils can spread widely and produce surface slicks. Resulting slicks may impede volatilization and possibly increase chemical persistence in water. While the influence of oil films on chemical air-water exchange has been examined through theoretical and laboratory studies, field studies have not been conducted to assess the relevance of these effects following actual oil spill events. Here we evaluated the effect of diluted bitumen (dilbit) experimentally spilled in limnocorrals installed in a boreal lake on the volatilization of sulfur hexafluoride (SF6), a non-reactive volatile tracer gas. Dilbit spills were monitored over 70 days and SF6 was introduced twice (after 7 and 48 days) to evaluate the influence of spilled dilbit on the loss of SF6 from water. Volatilization rate constants of SF6 (kVOL) significantly decreased by up to 80% with increasing total dilbit spill cover. Using a theoretical equation, decreases in kVOL were largely explained by a reduction in open water area where chemical exchange across the air-water interface occurs. Apparent effects of the slick on SF6 mass transfer were estimated to be smaller by comparison (20%).To account for this reduction in volatilization, oil spill fate models should include a correction to consider the impact of spill cover on the air-water exchange of organic chemicals.

A numerical model for the simulation of oil–ice interaction

Accurate modeling of the interaction between oil and sea ice is essential for predicting oil spill fate and transport in ice-infested waters. A three-dimensional numerical model based on the smoothed particle hydrodynamics (SPH) method is incorporated to model such interactions. The effects from air and water are well captured using suitable force components and without explicit inclusion of air and water phases. This reduces the four-phase SPH model into a two-phase model, significantly reducing computational costs and potentially enabling the use of this model for large-scale simulations. We validate the model against experimental data recently available in the literature on oil–ice interactions. The experiments studied the interaction in a flume between an ice floe and oil slick for different types of crude oils. The current velocities were varied and the thicknesses of the oil slicks were measured. The validation results show that our SPH model can adequately simulate the interaction between oil slicks and ice floes. The simulated average thicknesses fit well with the measured thicknesses despite the considerable difference in the viscosity of the tested crude oil. Moreover, the effects of oil density, surface tension, viscosity, and current velocity on oil slick accumulation in front of the ice floe are studied. The higher current velocities and higher oil density lead to thicker oil slick thickness next to the ice floe. The surface tension effect on oil slick thickness is not significant. Finally, we provide estimates for the minimum oil slick thickness for a finite range of oil viscosities.

Fate of MV Wakashio oil spill off Mauritius coast through modelling and remote sensing observations

This study aims at assessing the fate of MV Wakashio oil spill, and the driving forces responsible for possible environmental consequences of polluted coastal region. GNOME simulations were performed, considering various meteo-oceanographic forcings such as (i) winds and currents, (ii) only winds, and (iii) only winds with different diffusion coefficients, and validated with the satellite images. The results revealed that the simulations performed with ‘only winds’ reasonably match with the satellite observations, indicating that winds are the primary driving forces. The conducive stokes drift is an added contribution to the predominant northwestward drift of the spill. The oil budget analysis suggests that beaching and evaporation together accounted for a significant portion of the spilled oil (1000 tons), in which ~60% of the oil was accounted only for beaching. Our results depict that the diffusion coefficient of 100,000 cm2/s and 3% windages are optimal for oil-spill simulations off the southeastern Mauritius coast.

An Improved Particle Number-Based Oil Spill Model Using Implicit Viscosity in Marine Simulator

Improving the physical realism of oil spill scenes in marine simulators can further enhance the emergency response capabilities of officials in charge and crew members and help reduce losses caused by oil spill disasters. In order to uniformly simulate the spreading, drift, breakup, and merging of oil spills at sea, we propose an improved divergence-free position-based fluid (DFPBF) framework based on the particle number density model. In our DFPBF framework, the governing equations for oil spills and ocean are discretized by Lagrangian particles, and the incompressibility of oil spills and ocean is ensured by solving the divergence-free velocity constraint solver and constant density constraint solver. In order to stably simulate the fate and transport of oil spills with higher viscosity, we introduce an implicit viscosity solution scheme for our DFPBF framework. The implicit solver uses a splitting concept to decouple viscosity solve and adopts an implicit scheme to discretize the integration of viscous force. Moreover, our DFPBF framework can ensure a divergence-free velocity field before applying the implicit viscosity scheme, which avoids the undesired bulk viscosity effects. The simulation results show that our DFPBF framework can stably simulate oil spills of various viscosities, especially high-viscosity crude oils.

Potential Role of Marine Snow in the Fate of Spilled Oil in Cook Inlet, Alaska

ABSTRACT The objective of the research is to inform response decision-making and understanding of the potential association of spilled oil with marine snow in Cook Inlet, Alaska. While extensive research has been conducted on minerals aggregating with spilled oil, larger organic aggregates, such as marine snow, have only recently been studied as a transport mechanism. This knowledge gap in understanding the fate of oil was highlighted as part of the Gulf of Mexico Research Initiative (GoMRI) following the Deepwater Horizon (DWH) spill. It was determined that significant percentages of spilled oil reached the seafloor as a result of association with marine snow during both DWH and the Ixtoc 1 blowouts. As development of oil resources continues in Alaska and the Arctic, marine snow is a significant oil exposure pathway that must be considered during oil spill response. In parallel with a corresponding sedimentation study, input from local, federal and industry experts was used to develop laboratory scale oil exposure experiments to evaluate the potential for oil-marine snow aggregate formation in Cook Inlet. Roller-bottle experiments were conducted from May to July 2019 to assess the interactions between a 5 μm sheen of Alaska North Slope crude oil and Cook Inlet surface water. Aggregate formation was documented and sinking flocs were observed and analyzed with fluorescence microscopy to estimate oil content. The total oil volumes estimated in aggregates were between 0.01 to 0.4 μl. Estimates of total oil volume associated with the aggregates ranged from 0.6 to 9.3 % ± 1.4% of the total oil volume (80 μl) that was added to the bottles. The incorporation of spilled oil in surface forming aggregates will contribute to understanding fate and response implications in Cook Inlet and other northern regions at risk of spilled oil entering the benthic food web via association with sinking marine snow.

Oil Spill Fate and Trajectory Simulation for Sierra Leone's Offshore Exploration Basin, Using the Savanah-1X Well as the Focal Point

ABSTRACTThe demand for crude oil and petroleum products have subsequently led to an increase in the likelihood of occurrence of oil spills. It is therefore imperative to understand the impacts of these spills on humans and the environment. Developing appropriate oil spill response and remediation techniques can be achieved by understanding the fate and likely trajectory of different types of oils, once they come in contact with the sea surface. Though still at the nascent stage of oil and gas exploration, the Sierra Leone basin is known to hold large undeveloped hydrocarbon reserves which are being exploited with the aim of contributing to national development. With the availability of oil spill modelling tools, and owing to the fact that environmental conditions vary over time, there is the need to carry out routine studies on the likely behavior of a spill offshore Sierra Leone. This study aims to develop a prediction model that would aid in understanding the fate, trajectory and uncertainties of oil spilled on Sierra Leone waters in the dry, rainy, and harmattan seasons. In order to analyze the trajectory of a spill in the Sierra Leone basin, the GNOME software was used. Furthermore, ADIOS2 was also employed to analyze the weathering processes of the spill. The results obtained from GNOME showed that during the dry and rainy seasons, approximately 15% of oil would be stranded on the shores of Sierra Leone, within three – five days. Owing to the permanently warm water temperatures in the Gulf of Guinea basin, a high percentage of the oil is expected to evaporate and disperse within few days of the spill. The weathering models from ADIOS2 reveal that 34% of oil would be lost to evaporation in the dry season, and 36% and 38% will be lost in the rainy and harmattan seasons respectively. Furthermore, it can be seen that dispersion accounts for 2.5% of oil lost in the dry season, 7.8% during the rainy season and 6.2% in the harmattan period. Within 5-days, ADIOS2 reveals a stable water-in-oil emulsion, leading to an increase in viscosity and density. Airborne benzene concentration is expected to be high on the first day of the spill, but would decrease as the days go by. Based on these results, it is recommended that oil spill response personnel are professionally trained, and equipment must be available to respond to spills in a timely and efficient manner.

Review of the Science behind Oil Spill Fate Models: Are Updates Needed?

AbstractOil spill models play an important role in the oil spill response decision making and contingency planning processes. The current generation of spill models mostly use Lagrangian based particle tracking random walk methods for oil transport processes combined with individual algorithms for oil fate processes (Spaulding, 2017). The fate of near surface oil movement is modeled using algorithms describing oil spreading, evaporation, emulsification, entrainment, dissolution, and biodegradation. These fate processes are applied to oil in the Lagrangian particle tracking elements to alter the physical and chemical properties of the oil, and subsequently the oil behavior.In this paper, we review the major algorithms used in oil spill models and identify the science and physics underpinning them. For each, we evaluated how far the science has advanced since the algorithms were developed to identify those that could be upgraded based on current understanding. We also identified algorithms where future research is needed because the physical and chemical behaviors are not fully understood. These areas include the spreading behavior of surface slicks, surface-slick emulsification, and the physical transport of small oil droplets near the air-water interface.

Risk Assessment of Hydrocarbons’ Storing in Different Textured Soils in Small-Scale lysimeters

The objectives of this study are to test the penetration depth and determine the soil content in hydrocarbons’ components BTEX, PAHs, and TPH, after vertical movement of crude oil through undisturbed soils of different textures, and to assess their risk. The experiment simulates the fate of oil spills in the cold season. Three soil types were investigated: a sandy-textured eutric Arenosol, AR-eu; a loamy-textured haplic chernozem, CH-ha; and a clayey-textured, swell-shrink, Luvic Chernic Phaeozem, PH-ch-lv. Undisturbed small-scale lysimeters, 0.3 m in diameter and 1 m in length, were used. Crude oil was first applied and then water. TPH, BTEX, and PAH retention in the soils depends on soil texture. PH-ch-lv soil stored most of them in the upper horizons. TPH, BTEX, and PAH were found in different amounts as percentage from the applied ones, TPH as 48% (PH-ch-lv), and > 76% in the other two soils. BTEX was stored as 1.6% from the applied in PH-ch-lv, 5.1–5.1% in CH-ha and AR-eu soils, while PAH ranged from 15.1% in AR-eu soil to 24.8% in PH-ch-lv and 31% in CH-ha. Though subject to volatilization, chemical transformation, and bioremediation, TPH, BTEX, and PAH stored in soils present a health risk if they reach the groundwater. PH-ch-lv appears to be the most resilient soil to oil pollution. The results concern different soil textures and might be used in various countries. Future experimentation should focus on soil behavior in relation to crops, water, and certain crude oil types.

A Relocatable Ocean Modeling Platform for Downscaling to Shelf-Coastal Areas to Support Disaster Risk Reduction

High-impact ocean weather events and climate extremes can have devastating effects on coastal zones and small islands. Marine Disaster Risk Reduction (DRR) is a systematic approach to such events, through which the risk of disaster can be identified, assessed and reduced. This can be done by improving ocean and atmosphere prediction models, data assimilation for better initial conditions and developing an efficient and sustainable impact forecasting methodology for Early Warnings Systems. A common user request during disaster remediation actions is for high-resolution information, which can be derived from easily deployable numerical models nested into operational larger-scale ocean models. The Structured and Unstructured Relocatable Ocean Model for Forecasting (SURF) enables users to rapidly deploy a nested high-resolution numerical model into larger-scale ocean forecasts. Rapidly downscaling the currents, sea level, temperature, and salinity fields is critical in supporting emergency responses to extreme events and natural hazards in the world’s oceans. The most important requirement in a relocatable model is to ensure that the interpolation of low-resolution ocean model fields (analyses and reanalyses) and atmospheric forcing is tested for different model domains. The provision of continuous ocean circulation forecasts through the Copernicus Marine Environment Monitoring Service (CMEMS) enables this testing. High-resolution SURF ocean circulation forecasts can be provided to specific application models such as oil spill fate and transport models, search and rescue trajectory models, and ship routing models requiring knowledge of meteo-oceanographic conditions. SURF was used to downscale CMEMS circulation analyses in four world ocean regions, and the high-resolution currents it can simulate for specific applications are examined. The SURF downscaled circulation fields show that the marine current resolutions affect the quality of the application models to be used for assessing disaster risks, particularly near coastal areas where the coastline geometry must be resolved through a numerical grid, and high-frequency coastal currents must be accurately simulated.

Prediction of the Fate of Spent Engine Oil Spill from a Base Transceiver Station in the Unsaturated Zone

Prediction of the Fate of Spent Engine Oil Spill from a Base Transceiver Station in the Unsaturated Zone

The choice of droplet size probability distribution function for oil spill modeling is not trivial

The droplet size distribution (DSD) formed by gas-saturated oil jets is one of the most important characteristics of the flow to understand and model the fate of uncontrolled deep-sea oil spills. The shape of the DSD, generally modeled as a theoretical lognormal, Rosin-Rammler or non-fundamental distribution function, defines the size and the mass volume range of the droplets. Yet, the fundamental DSD shape has received much less attention than the volume median size (d50) and range of the DSD during ten years of research following the Deepwater Horizon (DWH) blowout. To better understand the importance of the distribution function of the droplet size we compare the oil rising time, surface oil mass, and sedimented and beached masses for different DSDs derived from the DWH literature in idealized and applied conditions, while keeping d50 constant. We highlight substantial differences, showing that the probability distribution function of the DSD for far-field modeling is, regardless of the d50, consequential for oil spill response.

The bioavailability of oil droplets trapped in river gravel by hyporheic flows.

Little is known about the fate of oil spills in rivers. Hyporheic flows of water through river sediments exchange surface and groundwater and create upwelling and downwelling zones that are important for fish spawning and embryo development. Risk assessments of oil spills to rivers do not consider the potential for hyporheic flows to carry oil droplets into sediments and the potential for prolonged exposure of fish to trapped oil. This project assessed whether oil droplets in water flowing through gravel will be trapped and whether hydrocarbons partitioning from trapped oil droplets are bioavailable to fish. Columns packed with gravel were injected with oil-in-water dispersions prepared with light crude, medium crude, diluted bitumens, and heavy fuel oil to generate a series of oil droplet loadings. The concentrations of oil trapped in the gravel increased with oil loading and viscosity. When the columns were perfused with clean water, oil concentrations in column effluents decreased to the detection limit within the first week of water flow, with sporadically higher concentrations associated with oil droplet release. Despite the low concentrations of hydrocarbons measured in column effluent, hydrocarbons were bioavailable to juvenile rainbow trout (Oncorhynchus mykiss) for more than three weeks of water flow, as indicated by strong induction of liver ethoxyresorufin-o-deethylase activity. These findings indicate that ecological risk assessments and spill response should identify and protect areas in rivers sensitive to contaminant trapping.