Geology topics

D.W. Hyndman

Publications and source records attributed to D.W. Hyndman.

5 recordsLinked to original sources

USGS2011

Hydrostratigraphic analysis of the MADE site with full-resolution GPR and direct-push hydraulic profiling

Full‐resolution 3D Ground‐Penetrating Radar (GPR) data were combined with high‐resolution hydraulic conductivity ( K ) data from vertical Direct‐Push (DP) profiles to characterize a portion of the highly heterogeneous MAcro Dispersion Experiment (MADE) site. This is an important first step to better understand the influence of aquifer heterogeneities on observed anomalous transport. Statistical evaluation of DP data indicates non‐normal distributions that have much higher similarity within each GPR facies than between facies. The analysis of GPR and DP data provides high‐resolution estimates of the 3D geometry of hydrostratigraphic zones, which can then be populated with stochastic K fields. The lack of such estimates has been a significant limitation for testing and parameterizing a range of novel transport theories at sites where the traditional advection‐dispersion model has proven inadequate.

Mississippi↗

USGS2009

A new method for high-resolution characterization of hydraulic conductivity

A new probe has been developed for high-resolution characterization of hydraulic conductivity (K) in shallow unconsolidated formations. The probe was recently applied at the Macrodispersion Experiment (MADE) site in Mississippi where K was rapidly characterized at a resolution as fine as 0.015 m, which has not previously been possible. Eleven profiles were obtained with K varying up to 7 orders of magnitude in individual profiles. Currently, high-resolution (0.015-m) profiling has an upper K limit of 10 m/d; lower-resolution (???0.4-m) mode is used in more permeable zones pending modifications. The probe presents a new means to help address unresolved issues of solute transport in heterogeneous systems. Copyright 2009 by the American Geophysical Union.

Water Resources Research↗

USGS2009

Examining the influence of heterogeneous porosity fields on conservative solute transport

It is widely recognized that groundwater flow and solute transport in natural media are largely controlled by heterogeneities. In the last three decades, many studies have examined the effects of heterogeneous hydraulic conductivity fields on flow and transport processes, but there has been much less attention to the influence of heterogeneous porosity fields. In this study, we use porosity and particle size measurements from boreholes at the Boise Hydrogeophysical Research Site (BHRS) to evaluate the importance of characterizing the spatial structure of porosity and grain size data for solute transport modeling. Then we develop synthetic hydraulic conductivity fields based on relatively simple measurements of porosity from borehole logs and grain size distributions from core samples to examine and compare the characteristics of tracer transport through these fields with and without inclusion of porosity heterogeneity. In particular, we develop horizontal 2D realizations based on data from one of the less heterogeneous units at the BHRS to examine effects where spatial variations in hydraulic parameters are not large. The results indicate that the distributions of porosity and the derived hydraulic conductivity in the study unit resemble fractal normal and lognormal fields respectively. We numerically simulate solute transport in stochastic fields and find that spatial variations in porosity have significant effects on the spread of an injected tracer plume including a significant delay in simulated tracer concentration histories.

Journal of Contaminant Hydrology↗

USGS2006

Regional-scale assessment of a sequence-bounding paleosol on fluvial fans using ground-penetrating radar, eastern San Joaquin Valley, California

Recently developed sequence stratigraphic models for fluvial fans suggest that sequence boundaries in these deposits are marked by laterally extensive paleosols; however, these models were based on paleosol correlations inferred between wells. To test this, we collected ???190 km of ground-penetrating radar (GPR) profiles on three fluvial fans from the eastern San Joaquin Valley, California, to determine the lateral extent and character of a buried near-surface sequence-bounding-paleosol. This paleosol, recognized on GPR by rapid shallow signal attenuation, extends across large areas on all three fluvial fans. Limited areas of significantly increased signal penetration were also identified, and these zones are interpreted to indicate the absence of the paleosol. The zones where the paleosol is missing likely correspond to paleooutwash channel activity on the fan surfaces that, when active, was able to partially or fully scour through the paleosol and deposit coarse-grained channel sediments in place of the sequence boundary. Erosional breaks are most common on the Kings River fan, while few breaks on the Tuolumne and Merced River fans may indicate less paleochannel activity on these fan surfaces during the last outwash event. Differences in channel activity between fans indicate that the Kings River migrated across its fan during the last outwash event, as evidenced by the large number of areas with increased GPR signal penetration and the presence of numerous channel deposits recorded on the soil surveys, while the Tuolumne and Merced Rivers only deposited floodplain fines, with the channels remaining inside a shallow incised valley, as evidenced by the relatively low number of areas with increased GPR signal penetration and the presence of primarily fine-grained material recorded on the soil surveys. Factors controlling these differences may include variable valley subsidence rates and differences in the San Joaquin Basin overall width at each fan location. ?? 2006 Geological Society of America.

Geological Society of America Bulletin↗

USGS2004

Spatial and temporal changes in microbial community structure associated with recharge-influenced chemical gradients in a contaminated aquifer

In a contaminated water‐table aquifer, we related microbial community structure on aquifer sediments to gradients in 24 geochemical and contaminant variables at five depths, under three recharge conditions. Community amplified ribsosomal DNA restriction analysis (ARDRA) using universal 16S rDNA primers and denaturing gradient gel electrophoresis (DGGE) using bacterial 16S rDNA primers indicated: (i) communities in the anoxic, contaminated central zone were similar regardless of recharge; (ii) after recharge, communities at greatest depth were similar to those in uncontaminated zones; and (iii) after extended lack of recharge, communities at upper and lower aquifer margins differed from communities at the same depths on other dates. General aquifer geochemistry was as important as contaminant or terminal electron accepting process (TEAP) chemistry in discriminant analysis of community groups. The Shannon index of diversity ( H ) and the evenness index ( E ), based on DGGE operational taxonomic units (OTUs), were statistically different across community groups and aquifer depths. Archaea or sulphate‐reducing bacteria 16S rRNA abundance was not clearly correlated with TEAP chemistry indicative of methanogenesis or sulphate reduction. Eukarya rRNA abundance varied by depth and date from 0 to 13% of the microbial community. This contaminated aquifer is a dynamic ecosystem, with complex interactions between physical, chemical and biotic components, which should be considered in the interpretation of aquifer geochemistry and in the development of conceptual or predictive models for natural attenuation or remediation.

Environmental Microbiology↗