Geology topics

David R. O’Leary

Publications and source records attributed to David R. O’Leary.

5 recordsLinked to original sources

USGS2016

Selected techniques for monitoring water movement through unsaturated alluvium during managed aquifer recharge

Managed aquifer recharge is used to augment natural recharge to aquifers. It can be used to replenish aquifers depleted by pumping or to store water during wetter years for withdrawal during drier years. Infiltration from ponds is a commonly used, inexpensive approach for managed aquifer recharge. At some managed aquifer-recharge sites, the time when infiltrated water arrives at the water table is not always clearly shown by water-level data. As part of site characterization and operation, it can be desirable to track downward movement of infiltrated water through the unsaturated zone to identify when it arrives at the water table.

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USGS2015

Storage and mobilization of natural and septic nitrate in thick unsaturated zones, California

Mobilization of natural and septic nitrate from the unsaturated zone as a result of managed aquifer recharge has degraded water quality from public-supply wells near Yucca Valley in the western Mojave Desert, California. The effect of nitrate storage and potential for denitrification in the unsaturated zone to mitigate increasing nitrate concentrations were investigated. Storage of water extractable nitrate in unsaturated alluvium up to 160 meters (m) thick, ranged from 420 to 6600 kilograms per hectare (kg/ha) as nitrogen (N) beneath undeveloped sites, from 6100 to 9200 kg/ha as N beneath unsewered sites. Nitrate reducing and denitrifying bacteria were less abundant under undeveloped sites and more abundant under unsewered sites; however, δ 15 N–NO 3 , and δ 18 O–NO 3 data show only about 5–10% denitrification of septic nitrate in most samples—although as much as 40% denitrification occurred in some parts the unsaturated zone and near the top of the water table. Storage of nitrate in thick unsaturated zones and dilution with low-nitrate groundwater are the primary attenuation mechanisms for nitrate from septic discharges in the study area. Numerical simulations of unsaturated flow, using the computer program TOUGH2, showed septic effluent movement through the unsaturated zone increased as the number and density of the septic tanks increased, and decreased with increased layering, and increased slope of layers, within the unsaturated zone. Managing housing density can delay arrival of septic discharges at the water table, especially in layered unsaturated alluvium, allowing time for development of strategies to address future water-quality issues.

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USGS2012

Groundwater data for selected wells within the Eastern San Joaquin Groundwater Subbasin, California, 2003-8

Data were collected by the U.S. Geological Survey from 2003 through 2008 in the Eastern San Joaquin Groundwater Subbasin, 80 miles east of San Francisco, California, as part of a study of the increasing chloride concentrations in groundwater processes. Data collected include geologic, geophysical, chemical, and hydrologic data collected during and after the installation of five multiple-well monitoring sites, from three existing multiple-well sites, and from 79 selected public-supply, irrigation, and domestic wells. Each multiple-well monitoring site installed as part of this study contained three to five 2-inch diameter polyvinyl chloride (PVC)-cased wells ranging in depth from 68 to 880 feet below land surface. Continuous water-level data were collected from the 19 wells installed at these 5 sites and from 10 existing monitoring wells at 3 additional multiple-well sites in the study area. Thirty-one electromagnetic logs were collected seasonally from the deepest PVC-cased monitoring well at seven multiple-well sites. About 200 water samples were collected from 79 wells in the study area. Coupled well-bore flow data and depth-dependent water-quality data were collected from 12 production wells under pumped conditions, and well-bore flow data were collected from 10 additional wells under unpumped conditions.

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USGS2012

Data from a thick unsaturated zone in Joshua Tree, San Bernardino County, California, 2007--09

Data were collected on the physical properties of unsaturated alluvial deposits, the chemical composition of leachate extracted from unsaturated alluvial deposits, the chemical and isotopic composition of groundwater and unsaturated-zone water, and the chemical composition of unsaturated-zone gas at four monitoring sites in the southwestern part of the Mojave Desert in the town of Joshua Tree, San Bernardino County, California. The presence of denitrifying and nitrate-reducing bacteria from unsaturated alluvial deposits was evaluated for two of these monitoring sites that underlie unsewered residential development. Four unsaturated-zone monitoring sites were installed in the Joshua Tree area—two in an unsewered residential development and two adjacent to a proposed artificial-recharge site in an undeveloped area. The two boreholes in residential development areas were installed by using the ODEX air-hammer method. One borehole was drilled through the unsaturated zone to a depth of 541 ft (feet) below land surface; a well screened across the water table was installed. Groundwater was sampled from this well. The second borehole was drilled to a depth of 81 ft below land surface. Drilling procedures, lithologic and geophysical data, construction details, and instrumentation placed in these boreholes are described. Core material was analyzed for water content, bulk density, matric potential, particle size, and water retention. The leachate from over 500 subsamples of cores and cuttings was analyzed for soluble anions, including fluoride, sulfate, bromide, chloride, nitrate, nitrite, and orthophosphate. Groundwater was analyzed for major ions, inorganic compounds, select trace elements, and isotopic composition. Unsaturated-zone water from suction-cup lysimeters was analyzed for major ions, inorganic compounds, select trace elements, and isotopic composition. Unsaturated-zone gas samples were analyzed for argon, oxygen, nitrogen, methane, carbon dioxide, ethane, nitrous oxide, and carbon monoxide. Drill cuttings were analyzed for denitrifying and nitrate-reducing bacteria. One of the boreholes installed adjacent to the Joshua Basin Water District proposed groundwater-recharge facility was installed by using the ODEX air-hammer method and the other was installed by using a 7.875-inch hollow-stem auger. Drilling procedures, lithologic and geophysical data, construction details, and instrumentation placed in these boreholes are described; however, geochemical data were not available at the time of publication.

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USGS2010

Aquifer Storage Recovery (ASR) of chlorinated municipal drinking water in a confined aquifer

About 1.02 × 10 6 m 3 of chlorinated municipal drinking water was injected into a confined aquifer, 94–137 m below Roseville, California, between December 2005 and April 2006. The water was stored in the aquifer for 438 days, and 2.64 × 10 6 m 3 of water were extracted between July 2007 and February 2008. On the basis of Cl − data, 35% of the injected water was recovered and 65% of the injected water and associated disinfection by-products (DBPs) remained in the aquifer at the end of extraction. About 46.3 kg of total trihalomethanes (TTHM) entered the aquifer with the injected water and 37.6 kg of TTHM were extracted. As much as 44 kg of TTHMs remained in the aquifer at the end of extraction because of incomplete recovery of injected water and formation of THMs within the aquifer by reactions with free-chlorine in the injected water. Well-bore velocity log data collected from the Aquifer Storage Recovery (ASR) well show as much as 60% of the injected water entered the aquifer through a 9 m thick, high-permeability layer within the confined aquifer near the top of the screened interval. Model simulations of ground-water flow near the ASR well indicate that (1) aquifer heterogeneity allowed injected water to move rapidly through the aquifer to nearby monitoring wells, (2) aquifer heterogeneity caused injected water to move further than expected assuming uniform aquifer properties, and (3) physical clogging of high-permeability layers is the probable cause for the observed change in the distribution of borehole flow. Aquifer heterogeneity also enhanced mixing of native anoxic ground water with oxic injected water, promoting removal of THMs primarily through sorption. A 3 to 4-fold reduction in TTHM concentrations was observed in the furthest monitoring well 427 m downgradient from the ASR well, and similar magnitude reductions were observed in depth-dependent water samples collected from the upper part of the screened interval in the ASR well near the end of the extraction phase. Haloacetic acids (HAAs) were completely sorbed or degraded within 10 months of injection.

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