Philip B. Bedient

Biorestoration of aquifers contaminated with organic compounds

Several forms of biological treatment can be used to treat contaminated aquifers. In situ treatment increases the activity of the indigenous organisms by the addition of nutrients and electron acceptor. Withdrawal and treatment technologies rely on removal of the ground water and any of several wastewater treatment processes to biodegrade the organics. Addition of acclimated or genetically engineered organisms may overcome the time necessary for acclimation to the contaminants, but requires that the added population establish itself in the environment and be able to locate and continue to degrade the compounds of interest at what are often very low concentrations. Most of these techniques utilize aerobic organisms, but the potential for treatment with anaerobic organisms exists. Mathematical models that include biological decay terms are useful tools in the design and evaluation of biological treatment options. Biological treatment techniques are a valuable tool for the restoration of contaminated aquifers.

Estimation of Rainfall for Flood Prediction from WSR-88D Reflectivity: A Case Study, 17–18 October 1994*

Abstract Flood forecasting requires accurate estimates of the spatial and temporal rainfall rates over a catchment. Depending on the Z–R relationship and threshold values, processing of WSR-88D reflectivity data affects the rainfall estimates used in flood prediction. How much rainfall the catchment receives during an extreme event and the resulting hydrograph response depends on catchment characteristics that transform rainfall into runoff. A case study of an extreme meteorological and hydrologic event occurring 17–18 October 1994 in south Texas is examined. After the event, a review of the National Oceanic and Atmospheric Administration, National Weather Service (NWS) operations raised concerns over low estimates of accumulated rainfall by the WSR-88D radar compared to gauge accumulations. At the time of the review, the Z–R relationship in use during the event, Z = 300R1.4, and a 53-dBZ reflectivity threshold were suspected of having caused poor performance of the radar accumulation estimates. Because o...

Improving the ability of a water market to efficiently manage drought

Some water markets maintain institutional elements that provide allocative advantages to specified water users. In the Lower Rio Grande Valley, water rights are designated as either municipal or agricultural (irrigation), with certain prioritization advantages afforded to municipal accounts. While sales of rights between municipalities and irrigators are allowed, the priority disparity results in a prohibition on leasing between sectors. Concern over meeting future urban demand has led municipalities to purchase rights well in excess of current needs. The inability to lease municipal water to irrigators removes a significant and growing fraction of available water from the market. The additional flexibility provided by leasing provides a valuable tool for managing seasonal drought. In this analysis the justification for prioritized municipal water is investigated. Results indicate that the added security municipalities may derive from higher prioritization during drought is accompanied by economic inefficiencies in regional water allocation. It is argued that eliminating municipal protection and the consequent allowance of intersectoralleasing would contribute to regional well-being at small cost to municipal water users.

Fate of trace organics during land application of municipal wastewater

Recent developments in analytical techniques have provided a better characterization of trace organics in municipal wastewater and have increased public awareness concerning potential health risks of renovated waters. Although renovation of wastewater by land application is an economical alternative to conventional treatment processes and can provide comparable treatment with respect to standard water quality parameters, there have been few quantitative studies on the fate of trace organics in these systems. This review evaluates the potential for trace organic removal by abiotic and biotic processes during rapid infiltration, irrigation, and overland flow of municipal wastewater. Special emphasis is directed towards rapid infiltration because this method of land application has the greatest potential for affecting associated groundwaters. Many trace organics which are present in municipal wastewater are transported to groundwater during rapid infiltration, although concentrations of most compounds are at...

Numerical stochastic analysis of groundwater contaminant transport and plume containment

First- and second-order reliability methods (FORM and SORM) are applied as alternatives to the Monte Carlo simulation method in the probabilistic analysis of groundwater contaminant transport and remediation. A two-dimensional finite-element model is interfaced with a reliability analysis program to account for uncertainty in aquifer media. Hydraulic conductivity is modeled as a spatial random field with prescribed marginal probability distribution and correlation structure. FORM and SORM provide the probability that a contaminant exceeds a target level at a well, termed the probability of failure. Sensitivity of the probability of failure to basic uncertainty in grid block conductivities is also obtained, at no additional computational effort. Component reliability is used to analyze failure in a single well. Results indicate that, at the most likely failure scenario, grid block conductivities attain their maximum value near the source, the receptor well, and along the stream tubes connecting the two. System reliability is used to analyze the joint probability of failure at several wells in the aquifer. Results indicate that system failure probability is greater than the largest component failure probability. Correlation between component failure events is greater when the individual wells are closer. Sensitivity of the upper bound on system probability with respect to grid block conductivities is highest along the path the contaminant follows to reach the receptor wells. Furthermore, the probability of failure to contain a plume from escaping site boundaries is analyzed, along with the corresponding sensitivity information. Probability of failure to contain the plume decreases as the well pumping rate increases. The presence of regions of lower conductivity dramatically increases the probability of remediation failure. A careful analysis of aquifer material uncertainty and heterogeneity is vital to the success of groundwater remediation systems.

Probabilistic modeling of aquifer heterogeneity using reliability methods

A probabilistic model of groundwater contaminant transport is presented. The model is based on coupling first- and second-order reliability methods (FORM and SORM) with a two-dimensional finite element solution of groundwater transport equations. Uncertainty in aquifer media is considered by modeling hydraulic conductivity as a spatial random field with a prescribed correlation structure. FORM and SORM provide the probability that a contaminant exceeds a target level at a well, termed the probability of failure. Sensitivity of the probability of failure to basic variabilities in grid block conductivity is also obtained, at no additional computational effort. The effect of the choice of the predetermined target level at the observation well is provided, along with its effect on the relevant sensitivity information. Considerable saving in computational time was achieved by superimposing a coarse random variables mesh on a finer numerical mesh. The presence of regions of lower conductivity on the probabilistic event is analyzed, and the regions in which conductivity most affects the results are identified.

Environmental Impact of Fipronil to the Louisiana Crawfish Industry

The Mermentau River Basin, located in southwestern Louisiana, is an agriculturally vital region. The 3,800-square-mile basin is a chief producer of rice and crawfish, which are grown in rotation on the same fields. The rice farmers face a constant problem in controlling rice water weevils (Lissorhoptrus oryzophilus), which feed on their rice crops. In 1998, the EPA banned the use of Furadan, which had been used to combat the rice water weevil nervous system. Furadan, also known by its chemical name carbofuran, weakens the nervous system by inhibiting cholinesterase and restricting the degradation of acetylcholine. The replacement for Furadan, issued in 1999, was Fipronil, sold under the brand name of IconTM. Fipronil is an arthropod-selective gamma-aminobutyric acid receptor antagonist. A low crawfish production in 2000 appeared to be correlated with the use of Fipronil on rice crops during the previous year. Fipronil and its degradation products strongly sorb to soils during the rice planting process, causing runoff from fields that have been seeded with Fipronil-laden rice to contaminate the surface water system. Fipronil-laden water that is pumped onto previously uncontaminated fields will kill crawfish within days of application. Fipronil degradates will persist in rice and crawfish-producing fields, and will continue to worsen commercial crawfish-production for several proceeding years. This article details the findings and conclusions of studies performed on behalf of the affected farmers, as well as a review of studies on Fipronil conducted by Louisiana State University (LSU) Agricultural Center and their recommendations.

Using a Distributed Hydrologic Model to Evaluate the Location of Urban Development and Flood Control Storage

A physics-based, distributed hydrologic model (Vflo) was used to evaluate the effects of urbanization and detention storage on watershed stream flows with grid cells as small as 0.09  km2 (22 acres). The analysis was performed for the Little Cypress Creek watershed in Houston, Texas. Hypothetical development and flood mitigation scenarios in the upstream, midstream, and downstream portions of the creek were analyzed for their impacts on peak flows for the 100-year design storm. Local ponds and regional reservoirs were compared for their ability to bring downstream flows back to existing conditions at all points along the creek. Analysis showed that downstream flows would be most effectively mitigated when storm water detention volumes and release rates are imposed with an awareness of locations relative to watershed headwaters or a referenced outlet point. This is in contrast to many existing policies that are uniformly applied across a watershed.

Examining the impact of land use/land cover characteristics on flood losses

Characteristics of the built environment and overall local-level land use patterns are increasingly being attributed to greater surface runoff, flooding and resulting economic losses from flood events. Specific configurations of impervious surfaces and land cover may be as important to determining a communitys flood risk as baseline environmental conditions. This study addresses this issue by statistically examining the impacts of adjacent land use and land cover (LULC) on flood damage recorded on parcels within a coastal watershed in southeast Texas. We analyse empirical models to identify the influence of different LULCs surrounding over 7900 properties claiming insured flood losses from 1999–2009. Results indicate that specific types of surrounding LULCs impact observed flood losses and provide guidance on how neighbourhoods can be developed more resiliently over the long term.

Delineating the Reality of Flood Risk and Loss in Southeast Texas

AbstractAlthough the 100-year floodplain is the traditional indicator of risk from flooding and a catalyst for mitigation decisions in the United States, increasing evidence indicates that this boundary is not sufficient in representing actual economic losses caused by floods. Although studies have demonstrated that up to 50% of losses occur outside floodplain boundaries, as of this writing it is believed little or no research has been conducted on the precise spatial characteristics of these losses or offers an alternative approach for depicting flood exposure at the local level. This perceived lack of inquiry is addressed by spatially examining the pattern of insured flood loss within the Clear Creek watershed near Houston as a first step in better understanding the relationship between floodplain boundaries and actual loss. First, property damage claims are mapped under the National Flood Insurance Program over an 11-year period from 1999 to 2009 and then these points of loss are analyzed in relation t...