Geology topics

Kenneth D. Ehman

Publications and source records attributed to Kenneth D. Ehman.

4 recordsLinked to original sources

USGS2017

A foundation for future assessment and management of groundwater resources

Sequence stratigraphic models for the Pleistocene to Holocene sediments of the Los Angeles (LA) Basin will provide better understanding of regional groundwater flow and have helped identify seawater intrusion pathways into important groundwater aquifers. Because groundwater provides more than one-third of the municipal water supply for the coastal LA Basin, the aquifer architecture of this system is a high priority for groundwater managers. Seismic and sequence stratigraphy are now incorporated into groundwater resource assessments and environmental investigations. By evaluating subsurface data using sequence stratigraphy, the geometry and distribution of aquifer and aquitard sediments are defined, thus groundwater contaminant plumes and sea water intrusion pathways are better understood. The U. S. Geological Survey (USGS), in cooperation with Los Angeles County Department of Public Works and the Water Replenishment District of Southern California undertook an investigation of the groundwater stratigraphy of the Wilmington – Long Beach area of the LA Basin. Sequence stratigraphic methods were used to integrate preexisting groundwater well data with: (1) new borehole observations, (2) structural and physical properties data derived from geophysical measurements, (3) hi-resolution seismic reflection data obtained offshore of the present shoreline, and (4) vintage oil company exploration seismic reflection data from both onshore and offshore of the shoreline. These data were used to construct a series of Pleistocene to Holocene environment of deposition maps that show the overall progradation of sequences seaward over time. In addition, seawater intrusion pathways into coastal groundwater aquifers were identified and mapped. On a more regional scale, a vintage Texaco seismic reflection data set from the greater LA Basin was interpreted and integrated with a regional network of multi-level ground-water monitoring wells. The sequence stratigraphic correlation shows that many of the defined groundwater aquifers are not correlative. For example, one of the major groundwater supply aquifers, the Silverado Aquifer, when tied to the seismic reflection data has different ages depending on location. This new understanding demonstrates the need for a re-evaluation of the Pleistocene to Holocene stratigraphy using existing seismic reflection data integrated with well data from the LA Basin in order to understand the regional distribution of groundwater aquifers.

Conference Paper↗

USGS2015

Sequence stratigraphic framework of upper pliocene to holocene sediments of the Los Angeles Basin, California: Implications for aquifer architecture

Executive Summary Groundwater provides more than one-third of the municipal water supply for the coastal Los Angeles Basin and defining the aquifer architecture is a high priority for ground-water managers. Sequence stratigraphy, the state-of-the-art method for delineating reservoir geometry and continuity in the petroleum industry, is now being incorporated into ground water resource assessments and environmental investigations. By evaluating subsurface data using sequence stratigraphy, the geometry and distribution of aquifer and aquitard sediments are linked to the original depositional processes that formed the sediments. Skyline Ridge, Inc., the U.S. Geological Survey (USGS), in cooperation with Los Angeles County Department of Public Works (LACDPW) and the Water Replenishment District of Southern California (WRDSC) completed an investigation of the Wilmington – Long Beach area by integrating data from new exploratory research boreholes, marine reflection seismic, vintage land reflection seismic, and high-resolution gravity measurements. Sequence stratigraphy is shown to define pathways for saltwater intrusion into freshwater coastal aquifers by integrating preexisting data with (1) the new borehole observations and (2) structural and physical properties data derived from the geophysical measurements. By constructing a series of seismic reflection and well log cross sections (presented as sheets), this investigation further defines and delineates ten sequences of Late Pliocene to Holocene age in the Wilmington – Long Beach area of the Los Angeles Basin. These sequences were first described by Ponti and others (2007), and the implications for sea water intrusion was discussed by Edwards and others (2009a). In addition, this investigation presents regional seismic facies – environment of deposition maps for the five youngest sequences: 1) the Dominquez Sequence; 2) the Mesa Sequence; 3) the Pacific Sequence; the Harbor Sequence; and 4) the Bent Spring Sequence. The stratigraphic framework established in the Wilmington – Long Beach area is extended into the Central Basin of the greater Los Angeles area by utilizing 1980s and older vintage petroleum exploration land seismic reflection data. Tying this data to the available groundwater monitoring well network in the Central and West Coast Basins demonstrate aquifer correlation problems, and may provide insights into establishing a more robust groundwater model for the greater Los Angeles Basin area.

Book↗

USGS2009

Stratigraphic controls on saltwater intrusion in the Dominguez Gap area of coastal Los Angeles

The Los Angeles Basin is a densely populated coastal area that significantly depends on groundwater. A part of this groundwater supply is at risk from saltwater intrusion—the impetus for this study. High-resolution seismic-reflection data collected from the Los Angeles–Long Beach Harbor Complex have been combined with borehole geophysical and descriptive geological data from four nearby ~400-m-deep continuously cored wells and with borehole geophysical data from adjacent water and oil wells to characterize the Pliocene to Holocene stratigraphy of the Dominguez Gap coastal aquifer system. The new data are shown as a north-south, two- dimensional, sequence-stratigraphic model that is compared to existing lithostratigraphic models of the Los Angeles Basin in an attempt to better understand pathways of saltwater intrusion into coastal aquifers.

California↗

USGS2007

A 3-Dimensional Model of Water-Bearing Sequences in the Dominguez Gap Region, Long Beach, California

A 3-dimensional computer model of the Quaternary sequence stratigraphy in the Dominguez gap region of Long Beach, California has been developed to provide a robust chronostratigraphic framework for hydrologic and tectonic studies. The model consists of 13 layers within a 16.5 by 16.1 km (10.25 by 10 mile) square area and extends downward to an altitude of -900 meters (-2952.76 feet). Ten sequences of late Pliocene to Holocene age are identified and correlated within the model. Primary data to build the model comes from five reference core holes, extensive high-resolution seismic data obtained in San Pedro Bay, and logs from several hundred water and oil wells drilled in the region. The model is best constrained in the vicinity of the Dominguez gap seawater intrusion barrier where a dense network of subsurface data exist. The resultant stratigraphic framework and geologic structure differs significantly from what has been proposed in earlier studies. An important new discovery from this approach is the recognition of ongoing tectonic deformation throughout nearly all of Quaternary time that has impacted the geometry and character of the sequences. Anticlinal folding along a NW-SE trend, probably associated with Quaternary reactivation of the Wilmington anticline, has uplifted and thinned deposits along the fold crest, which intersects the Dominguez gap seawater barrier near Pacific Coast Highway. A W-NW trending fault system that approximately parallels the fold crest has also been identified. This fault progressively displaces all but the youngest sequences down to the north and serves as the southern termination of the classic Silverado aquifer. Uplift and erosion of fining-upward paralic sequences along the crest of the young fold has removed or thinned many of the fine-grained beds that serve to protect the underlying Silverado aquifer from seawater contaminated shallow groundwater. As a result of this process, the potential exists for vertical migration of seawater into the producing aquifers and subsequent landward migration of intruded waters beneath the existing Dominguez barrier. Incipient invasion of the Silverado aquifer by chloride-enriched waters is observed in a recently drilled well located along the crest of the fold seaward of the barrier and at a depth of 440 feet (134 meters). These new observations and interpretations indicate that the new sequence-based approach to defining the stratigraphy of the Dominguez Gap area may have important implications for seawater intrusion management. To test this, it will be useful to overlay existing water-quality and water level data onto the framework model and to incorporate the new stratigraphy into a transport model.

Open-File Report↗