Geology topics

J. Lee

Publications and source records attributed to J. Lee.

7 recordsLinked to original sources

USGS2008

Nodal failure index approach to groundwater remediation design

Computer simulations often are used to design and to optimize groundwater remediation systems. We present a new computationally efficient approach that calculates the reliability of remedial design at every location in a model domain with a single simulation. The estimated reliability and other model information are used to select a best remedial option for given site conditions, conceptual model, and available data. To evaluate design performance, we introduce the nodal failure index (NFI) to determine the number of nodal locations at which the probability of success is below the design requirement. The strength of the NFI approach is that selected areas of interest can be specified for analysis and the best remedial design determined for this target region. An example application of the NFI approach using a hypothetical model shows how the spatial distribution of reliability can be used for a decision support system in groundwater remediation design. ?? 2008 ASCE.

Journal of Geotechnical and Geoenvironmental Engin↗

USGS2006

Quantitative methods to direct exploration based on hydrogeologic information

Quantitatively Directed Exploration (QDE) approaches based on information such as model sensitivity, input data covariance and model output covariance are presented. Seven approaches for directing exploration are developed, applied, and evaluated on a synthetic hydrogeologic site. The QDE approaches evaluate input information uncertainty, subsurface model sensitivity and, most importantly, output covariance to identify the next location to sample. Spatial input parameter values and covariances are calculated with the multivariate conditional probability calculation from a limited number of samples. A variogram structure is used during data extrapolation to describe the spatial continuity, or correlation, of subsurface information. Model sensitivity can be determined by perturbing input data and evaluating output response or, as in this work, sensitivities can be programmed directly into an analysis model. Output covariance is calculated by the First-Order Second Moment (FOSM) method, which combines the covariance of input information with model sensitivity. A groundwater flow example, modeled in MODFLOW-2000, is chosen to demonstrate the seven QDE approaches. MODFLOW-2000 is used to obtain the piezometric head and the model sensitivity simultaneously. The seven QDE approaches are evaluated based on the accuracy of the modeled piezometric head after information from a QDE sample is added. For the synthetic site used in this study, the QDE approach that identifies the location of hydraulic conductivity that contributes the most to the overall piezometric head variance proved to be the best method to quantitatively direct exploration. ?? IWA Publishing 2006.

Journal of Hydroinformatics↗

USGS2005

MODFLOW 2000 Head Uncertainty, a First-Order Second Moment Method

A computationally efficient method to estimate the variance and covariance in piezometric head results computed through MODFLOW 2000 using a first-order second moment (FOSM) approach is presented. This methodology employs a first-order Taylor series expansion to combine model sensitivity with uncertainty in geologic data. MODFLOW 2000 is used to calculate both the ground water head and the sensitivity of head to changes in input data. From a limited number of samples, geologic data are extrapolated and their associated uncertainties are computed through a conditional probability calculation. Combining the spatially related sensitivity and input uncertainty produces the variance-covariance matrix, the diagonal of which is used to yield the standard deviation in MODFLOW 2000 head. The variance in piezometric head can be used for calibrating the model, estimating confidence intervals, directing exploration, and evaluating the reliability of a design. A case study illustrates the approach, where aquifer transmissivity is the spatially related uncertain geologic input data. The FOSM methodology is shown to be applicable for calculating output uncertainty for (1) spatially related input and output data, and (2) multiple input parameters (transmissivity and recharge).

Ground Water↗

USGS2003

Use of input uncertainty and model sensitivity to guide site exploration

Three Quantitatively Directed Exploration (QDE) methods to identify optimum field sampling locations based on model input covariance and model sensitivity are presented. The first method bases site exploration only on the spatial variation in the uncertainty of input properties. The second method uses only the spatial variation in model sensitivities. The third method uses a first-order second-moment (FOSM) method to estimate the spatial variation in the output covariance. The FOSM method estimates output uncertainty using the product of the input covariance and model sensitivity. The three methods are illustrated by means of a synthetic groundwater site simulated with MODFLOW-2000. The groundwater-flow model computes piezometric head and the sensitivity of head to changes in input values. The QDE methods are evaluated by comparing model results to the "true" head. For the synthetic site used in this study, the most effective QDE method was the FOSM method.

Conference Paper↗

USGS1989

Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite

The adsorption of aromatic compounds by smectite exchanged with tetramethylammonium (TMA) has been studied. Aromatic compounds adsorbed by TMA-smectite are assumed to adopt a tilted orientation in a face-to-face arrangment with the TMA tetrahedra. The sorptive characteristics of TMA-smectite were influenced strongly by the presence of water. The dry TMA-smectite showed little selectivity in the uptake of benzen, toluene and xylene. In the presence of water, TMA-smectite showed a high degree of selectivity based on molecular size/shape, resulting in high uptake of benzene and progressively lower uptake of larger aromatic molecules. This selectivity appeared to result from the shrinkage of interlamellar cavities by water.

Journal of the Chemical Society, Faraday Transacti↗

USGS1985

A new look at deep-sea video

A deep-towed photographic system with completely self-contained recording instrumentation and power can obtain color-video and still-photographic transects along rough terrane without need for a long electrically conducting cable. Both the video- and still-camera systems utilize relatively inexpensive and proven off-the-shelf hardware adapted for deep-water environments. The small instrument frame makes the towed sled an ideal photographic tool for use on ship or small-boat operations. The system includes a temperature probe and altimeter that relay data acoustically from the sled to the surface ship. This relay enables the operator to monitor simultaneously water temperature and the precise height off the bottom.

Deep Sea Research Part A, Oceanographic Research P↗

USGS1984

Seed predation due to the yucca moth symbiosis

All species of Yucca (Agavaceae) require the pollinator services of a species of moth in the genus Tegeticula ( Lepidoptera : Incurvariidae). These moths oviposit in the ovary of the plants and the larvae are entirely dependent upon Yucca seeds for food. The extent and distribution of larval seed predation was examined in nine Yucca species in the southwestern United States. The proportion of seeds destroyed by the yucca moth ranged from 3 % in Y. schidigera from coastal southern California to 45 % in one population of Y. angustissima from southern Utah. This sampling was done in 1979 at which time the Y. schidigera population averaged 0.6 larvae per fruit and the population of Y. angustissima averaged 9.3 larvae per fruit. A second sampling of these populations in 1982 averaged 0.5 for Y. schidigera and 5.6 for Y. angustissima . Several species showed significant differences between populations in the number of larvae per fruit. Contrary to expectation, based on the dogma that fruit production is dependent upon Tegeticula pollination (which is always followed by oviposition), a large number of fruits were found without larvae. The proportion varied greatly between populations but was as high as two thirds of all fruits in some populations. Observations suggested that these flowers had been pollinated by Tegeticula and the moths had oviposited in them but that the eggs failed to hatch.

American Midland Naturalist↗