Teresa B. Culver

Dynamic optimal control for groundwater remediation with flexible management periods

A successive approximation linear quadratic regulator (SALQR) method with management periods is combined with a finite element groundwater flow and transport simulation model to determine optimal time-varying groundwater pump-and-treat reclamation policies. Management periods are groups of simulation time steps during which the pumping policy remains constant. In an example problem, management periods reduced the total computational demand, as measured by the CPU time, by as much as 85% compared to the time needed for the SALQR solution without management periods. Conversely, the optimal costs increased as the number of times that the control can change is reduced. With two simulation periods per management period, the optimal cost increased by less than 1% compared to the optimal cost with no management periods, yet the computational work was reduced by a third. The optimal policies, including the number and locations of wells, changed significantly with the number of management periods. Complexity analysis revealed that the SALQR algorithm with management periods can significantly reduce the computational requirements for nonsteady optimization of groundwater reclamation and other management applications.

OPTIMAL CONTROL FOR GROUNDWATER REMEDIATION BY DIFFERENTIAL DYNAMIC PROGRAMMING WITH QUASI-NEWTON APPROXIMATIONS

Differential dynamic programming with quasi-Newton approximations (QNDDP) is combined with a finite element groundwater quality simulation model to determine optimal time-varying pumping policies for reclamation of a contaminated aquifer. The purpose of the QNDDP model is to significantly reduce the large computational effort associated with calculation of optimal time-varying policies. A Broyden rank-one quasi-Newton technique is developed to approximate the second derivatives of the groundwater quality model; these second derivatives are difficult to calculate directly. The performance of the QNDDP algorithm is compared to the successive approximation linear quadratic regulator (SALQR) technique, which sets the complicated second derivatives to 0. QNDDP converged to the optimal pumping policy in approximately half the time that the SALQR technique required. The QNDDP algorithm thus shows great promise for the management of complex, time-varying systems.

Interrelationships between prey body size and growth of age-0 yellow perch

Abstract We conducted laboratory experiments in the summers of 1986 and 1987 to examine the effects of Daphnia pulex body size and ration level on growth of age-0 yellow perch Perca flavescens. Daphnia pulex rations were set at 25 and 40% of yellow perch dry weight. Specific growth rate tended to be higher for smaller fish and decreased with fish size at both ration levels. For the 25% ration, the highest growth rates, 0.023 to 0.024 g/d, were observed when yellow perch were fed meals of D. pulex between 1.4 and 1.8 mm long; growth declined for prey sizes outside this size range. For the 40% ration, no relationship was found between body length of D. pulex and specific growth rate; the mean specific growth rate for the 40% ration was the same as the peak rate for the 25% ration. Multiple-regression analysis was used to examine the dependence of specific growth rate of yellow perch on initial fish length, prey size, and temperature. For the 25% ration, prey size and the square of prey size were the only va...

Impact of Vapor Sorption on the Subsurface Transport of Volatile Organic Compounds: A Numerical Model and Analysis

A flexible finite element transport model, which includes the impact of vapor sorption, is developed to simulate the movement of volatile organic compounds (VOCs) in variably saturated porous media. The two-dimensional numerical model predicts contaminant transport by aqueous advection, aqueous dispersion, aqueous and vapor diffusion, and surface volatilization. For a soil with increasing water contents with depth, one-dimensional simulations demonstrate that strong vapor sorption may reduce the total amount of VOC that volatilizes in 100 days from 84.6% of the initial mass volatilized without vapor sorption to 73.2%. Yet, for a soil with low water contents at depth, such as in an area with limited recharge, vapor sorption enhanced the rate of volatilization in 100 days from 72.4% without vapor sorption to a peak of 90.3%. When low soil moisture was combined with a soil type that has strong vapor sorption characteristics, VOC transport was significantly retarded.

Effects of soil moisture and physical-chemical properties of organic pollutants on vapor-phase transport in the vadose zone

Abstract Vapor-phase transport of organic pollutants is one of the important pathways in the distribution and attenuation of volatile organic compounds in the vadose zone. In this study, the impact of vapor-phase partitioning and of the physical-chemical properties of organic pollutants on vapor-phase transport was assessed. An experimentally derived relationship to predict vapor sorption for a variety of soil types under varying soil moisture conditions was incorporated into the two-dimensional finite-element model, Vocwaste . The revised model was then used to simulate the transport of volatile organics. Vapor-phase partitioning in the model accounted for vapor uptake by sorption onto moist mineral surfaces as well as sorption at the liquid-solid interface and dissolution into soil water. Under dry conditions, vapor-phase sorption of volatile organic pollutants was shown to have a retarding effect on transport of organic vapors. However, for shallow, contaminated soils, volatilization was controlled by vapor diffusion, even under dry conditions where vapor-phase sorption was high. The influence of Henrys law constant and of the aqueous-phase (solid-liquid) partition coefficient for volatile organic pollutants was considered in the simulations. Volatilization of organic vapors was shown to be favored for contaminants with high Henrys law constants and low aqueous-phase partitioning coefficients. Because of the interdependence of these two physical-chemical properties, individual properties of the contaminant should not be considered in isolation in the evaluation of vapor transport.

Optimizing Groundwater Remediation Design under Uncertainty

There always exists some degree of uncertainty associated with groundwater problems, often in determining the aquifer parameter values. Therefore finding an optimal remediation strategy based on a deterministic description of the system may not yield an optimal and feasible design. This work develops a genetic algorithm (GA) approach that takes into account the uncertainty of hydraulic conductivity values when determining the best remediation design possible. During the GA optimization, the heterogeneous hydraulic conductivity field realization varies between generations and on-going performance is measured. A policys fitness is based on its performance over multiple generations. Therefore the most fit policy should provide a robust solution since this policy would be a good design over a range of aquifer realizations. Results of this approach applied to a hypothetical contaminated aquifer remediated by a pump-and-treat system indicate that a set of non-dominated policies can be generated by this modified GA. Additionally, this work shows that using a deterministic description of the aquifer, either homogeneous or heterogeneous, can result in significant under-design, with poor reliability.

Design of Outlet Control Structures for Ecological Detention Ponds

AbstractThrough a simulation modeling approach, this paper seeks to modify the design of detention ponds to preserve the natural ecological flows while satisfying the requisite regulatory flow requirements. This paper will utilize an innovative ecological flow paradigm: the ecoflow statistics. The ecoflow statistics consist of nine hydrological flow statistics that have been shown to be particularly relevant to ecological quality. The statistics include annual and seasonal ecodeficits and ecosurplus, calculated using median annual and seasonal flow duration curves, and the total seasonal ecochange. A new metric called the ecodifference is defined as the weighted sum of the nine ecoflow statistics and represents the hydrologic alteration in the stream. The ecodifference in a receiving stream can be calculated using the outflow hydrograph from a detention pond hydrologic simulator. A design approach using a hydrologic model, detention pond model, and the ecodifference metric will be used to design a series ...

Reservoir Operation for Recession Agriculture in Mekong Basin, Laos

AbstractAs hydropower dam construction in rapidly growing economies dislodges communities, rural development experts must help the displaced make their livelihoods in new lacustrine environments. One question is whether the dam infrastructure can directly benefit those who remain within the vicinity of the reservoir. Integrated water resource management seeks to concurrently consider hydrological, socioeconomic, and ecological factors, yet water managers lack the information needed to include livelihoods in their analyses. The objective of this paper is to develop tools and plans for coordinating hydropower reservoir operation and management for rural livelihoods. Specifically, this study investigates how dam management may accommodate vegetable farming on the banks of a reservoir. The intervention investigated is to lower water levels during the cultivation period in order to expose shoreline gardens. Based on the recession agriculture rule, evaluated through simulation of a dam in Lao People’s Democrati...

Constraint-handling methods for optimal groundwater remediation design by genetic algorithms

Typically in optimal groundwater remediation design, the objective is to minimize the cost of remediation while meeting the water quality constraints by the end of the remediation period. Given that many common groundwater contaminants are hazardous at very low concentrations, even a small violation of the water quality may be the difference between reaching a hazardous or nonhazardous end point. Furthermore, the remediation costs increase dramatically as one attempts to remove the last units of concentration. This work compares two methods for constraint-handling, an additive penalty method and a multiplicative penalty, for use in optimal groundwater remediation design with a genetic algorithm. The multiplicative approach was found to be a more robust technique for finding cost-effective designs, while enforcing the water quality constraints.

Environmental Flow Components for Measuring Hydrologic Model Fit during Low Flow Events

AbstractThe indicators of hydrologic alteration (IHA) is a statistical flow methodology for characterizing ecologically important streamflows. Typically, IHA has been used to identify the extent of human effects on a stream’s hydrology and to set management goals to restore the stream ecology. In this work, the use of the seven “extreme low flow” statistics of the IHA is extended to the evaluation of the performance of a hydrologic simulation model under low flow conditions. Specifically, this work uses the IHA framework to evaluate the accuracy of the Chesapeake Bay Program Phase 5 (CBP5) watershed model during low flow events on a regional scale that is relevant to many water supply planners and managers. Because the CBP5 model’s primary focus is predicting the Bay’s water quality, the measures used to calibrate the CBP5 model focused primarily on the calibration of the entire hydrological record and had only a secondary emphasis on specific flow regimes, such as low flows and very low flows, although t...