Geology topics

J.N. Densmore

Publications and source records attributed to J.N. Densmore.

4 recordsLinked to original sources

USGS2000

Use of InSAR to identify land-surface displacements caused by aquifer-system compaction in the Paso Robles area, San Luis Obispo County, California, March to August 1997

The population in San Luis Obispo County has grown steadily during the 1990s, and some land use has been converted from dry farming to grazing to irrigated vineyards and urban areas. Because insufficient surface-water supplies are available to meet the growing demand, ground-water pumpage has increased and the resulting water-level declines have raised concern that this water resource may become overstressed. One particular concern is whether the larger ground-water basins within the county function as large individual basins or whether subsurface structures divide these large basins into smaller subbasins, as differences in ground-water-level data suggest. Interferometric Synthetic Aperture Radar (InSAR) is used in locating land-surface displacement, which may indicate subsurface structures in ground-water basins by determining seasonal and historical land-surface changes. Owing to the high spatial detail of InSAR imagery, the InSAR-derived displacement maps can be used with ground-water-level data to reveal differential aquifer-system compaction related to the presence of geological structures or the distribution of compressible sediments that may define subbasin boundaries. This report describes InSAR displacement maps of the Paso Robles area of San Luis Obispo County and compares them to maps of seasonal changes in ground-water levels to detect the presence of aquifer-system compaction.

Open-File Report↗

USGS1999

Use of nitrogen isotopes to determine sources of nitrate contamination in two desert basins in California

Nitrogen-15/14 isotope ratios (δ 15 N) were measured in groundwater, infiltrating wastewater, and natural uncontaminated soil to determine the source of high NO 3 - concentrations in two desert basins at Fort Irwin National Training Center in the Mojave Desert. The abundance and isotopic composition of dissolved nitrogen gas were measured in some of the groundwater samples to determine the effects of denitrification on the abundance and isotopic composition of the NO 3 - . Delta 15 N-NO 3 values in groundwaters most likely to include sewage effluent ranged from about 11 to 23‰. Delta 15 N values in groundwater not near sources of sewage effluent ranged from 6 to 11‰, a range similar to that found in the overlying soils, which contain large concentrations of leachable NO 3 - presumed to be of natural origin. Some of the samples with relatively low NO 3 - concentrations and high δ 15 N values contained excess non-atmospheric nitrogen gas attributed to denitrification. These data indicate that high NO 3 - concentrations in groundwater unaffected by sewage effluent may result from leaching of naturally occurring nitrogen in desert soils, and that some of the NO 3 - may be removed by denitrification at the water table. Nitrogen-15/14 isotope ratios (δ 15 N) were measured in groundwater, infiltrating wastewater, and natural uncontaminated soil to determine the source of high NO 3 - concentrations in two desert basins at Fort Irwin National Training Center in the Mojave Desert. The abundance and isotopic composition of dissolved nitrogen gas were measured in some of the groundwater samples to determine the effects of denitrification on the abundance and isotopic composition of the NO 3 - . Delta 15 N-NO 3 values in groundwaters most likely to include sewage effluent ranged from about 11 to 23 per mil. Delta 15 N values in groundwater not near sources of sewage effluent ranged from 6 to 11 per mil, a range similar to that found in the overlying soils, which contain large concentrations of leachable NO 3 - presumed to be of natural origin. Some of the samples with relatively low NO 3 - concentrations and high δ 15 N values contained excess non-atmospheric nitrogen gas attributed to denitrification. These data indicate that high NO 3 - concentrations in groundwater unaffected by sewage effluent may result from leaching of naturally occurring nitrogen in desert soils, and that some of the NO 3 - may be removed by denitrification at the water table.

California↗

USGS1996

Lithologic and ground-water data for monitoring wells in the Santa Clara-Calleguas ground-water basin, Ventura County, California, 1989-95

Twenty-three multiple-well (nested) monitoring sites were constructed in the western part of the Santa Clara-Calleguas Hydrologic Unit and ground-water basin between October 1989 and May 1994. This report is a compilation of the data collected at these sites from October 1989 through April 1995 and includes location and design of the monitoring sites, lithologic data, geophysical logs, ground-water-level measurements, and water-quality analyses. Grain-size distribution and mineralogical analyses also are included. Ninety-nine piezometers were installed at the 23 sites to enable collection of depth-dependent data. Lithologic logs were compiled from descriptions of drill cuttings for each site and from observations recorded during logging. Generalized stratigraphic columns were compiled by grouping similar lithologic units. Grain-size and mineralogical analyses were used to identify and characterize the various lithologic units and to distinguish changes among them. Geophysical logs provided information on the character of the lithologic units and on the presence and chemical characteristics of ground water. Water levels were measured periodically during 1989-94. Water-quality data were collected periodically during 1989-95.

Open-File Report↗

USGS1995

Water-quality data for the Santa Clara-Calleguas hydrologic unit, Ventura County, California, October 1989 through December 1993

More than 700 water samples were collected from 232 wells and 34 surface-water sites in the Santa Clara-Calleguas Hydrologic Unit, Ventura County, California, from October 1989 through December 1993 as part of the U.S. Geological Survey's Southern California Regional Aquifer-System Analysis study. Most samples were analyzed for major ions, nutrients, selected trace elements, oxygen-18, and deuterium. Selected samples were analyzed for one or more of the following isotopes: carbon-13/12, carbon-14, strontium-87/86, sulfur-34/32, and tritium. Other samples were analyzed for one or more of the following dissolved gases: hydrogen, methane, oxygen, and freon-11. Location of sampling sites is shown on maps and the results are presented in tables.

Open-File Report↗