USGS Science⌕ Search

Geology topics

L. F. Duell

Publications and source records attributed to L. F. Duell.

5 recordsLinked to original sources
USGS1995

Results of a shallow seismic-refraction survey in the Little Valley area near Hemet, Riverside County, California

Little Valley, a small locally named valley southeast of the city of Hemet in Riverside County, California, is being evaluated for development of a constructed wetland and infiltration area as part of a water-resources management program in the area. The valley is a granitic basin filled with unconsolidated material. In August 1993 and June and July 1994, the U.S. Geological Survey conducted a seismic-refraction survey consisting of four lines northwest of the valley, eight lines in the valley, and six lines northeast of the valley. Two interpretations were made for the lines: a two-layer model yielded an estimate of the minimum depths to bedrock and a three-layer model yielded the most likely depths to bedrock. Results of the interpretation of the three-layer model indicate that the unsaturated unconsolidated surface layer ranges in thickness from 12 to 83 feet in the valley and 24 to 131 feet northeast of the valley. The mean compressional velocity for this layer was about 1,660 feet per second. A saturated middle layer was detected in some parts of the study area, but not in others--probably because of insufficient thickness in some places; however, in order to determine the "most likely" depths to bedrock, it was assumed that the layer was present throughout the valley. Depths to this layer were verified on three seismic lines using the water level from the only well in the valley. Data for additional verification were not available for wells near Little Valley. The bedrock slope from most of Little Valley is down toward the northeast. Bedrock profiles show that the bedrock surface is very uneven in the study area. The interpreted most likely depth to bedrock in the valley ranged from land surface (exposed) to a depth of 176 feet below land surface, and northeast of the valley it ranged from 118 to 331 feet below land surface. Bedrock depths were verified using lithologic logs from test holes drilled previously in the area. On the basis of a measured mean compressional velocity of about 12,000 feet per second, the bedrock was interpreted to be weathered granite.

Water-Resources Investigations Report↗
USGS1993

Potential for ground-water contamination from movement of wastewater through the unsaturated zone, upper Mojave River Basin, California

Septic-tank wastewater disposed in 30-foot-deep seepage pits (dry wells) at 46,000 residences is estimated to equal 18 percent of the natural recharge to the sole-source aquifer in the rapidly developing upper Mojave River Basin (Victor Valley) in the high desert northeast of Los Angeles. Vertical rates of movement of the wastewater wetting front through the unsaturated zone at three newly occupied residences ranged from 0.07 to 1.0 foot per day. These rates translate to traveltimes of several months to several years for the wastewater wetting front to reach the water table and imply that wastewater from many disposal systems already has reached the water table, which averages about 150 feet below land surface in the Victor Valley. As wastewater percolates from seepage pits into the adjacent unsaturated zone, the nitrogen present in reduced form is rapidly converted to nitrate. Analyses on soil-core extracts and soil moisturefrom suction lysimeters installed beneath the seepage pits at eight residences showed that nitrate concentrations and nitrate/ chloride ratios generally become lower with increasing depth. The intervals of greatest decline seemed to coincide with finer soil texture or were near the water table. Nitrate-reducing bacteria were tested for and found to be present in soil cores from two residences. Sparse nitrogen-15 data from suction lysimeters at one of these residences, where thenitrate concentration decreased by about one-half at a depth of 200 feet, indicate that the nitrate decline was accompanied by nitrogen-15 enrichment in the residual nitrate with an isotope-separation factor of about -10 permil. Despite the potential input of abundant nitrogen with the domestic wastewater recharge, nitrate concentrations in the area's ground water are generally low. The absence of high nitrate concentrations in the ground water is consistent with the existence of denitrification, a microbial nitrogen-removal mechanism, as wastewater moves through the thick unsaturated zone and mixes with the ground water. The observed low nitrate concentrations also could be explained by a dilution by vertical mixing in the saturated zone and retention of the wastewater in the unsaturated zone. Results of a single-cell mixing model that allows nitrate from wastewater to be mixedinstantaneously with the underlying ground water suggest that measurable increases in nitrate concentration should be expected within 5 to 10 years after wastewater reaches the water table if the mixing depth is less than 100 feet. Although high fecal-coliform densities were measured in wastewater from septic tanks and seepage pits, removal of these enteric bacteria in the unsaturated zone is very effective, as was indicated by their absence in soil only a few feet from the seepage pits. In testing for organic priority pollutants in wastewater, 17 of 85 compounds were detected. Most compounds detected were present in low concentrations, except at one residence where the concentration of three compounds exceeded 100 micrograms per liter. These high concentrations may be a consequence of disposal practices unique to this residence. Extractable organic priority pollutants were not found in any soil cores taken adjacent to seepage pits and, therefore, are not of concern.

Water-Resources Investigations Report↗
USGS1990

Water resources of the Descanso Area, San Diego County, California

Hydrologic information was collected during water year 1988 (October 1987 to September 1988) to evaluate the effects of current pumping on groundwater levels in the Descanso area in south-central San Diego County. Water year 1988 was a period of near-normal precipitation and runoff. The groundwater system in the area consists of aquifers in the metamorphic and granitic bedrock and in the overlying regolith (weathered bedrock). Most wells penetrate both aquifers, but the regolith is the source of most water pumped from wells. Groundwater storage in 1988 was estimated to be 800 to 2,000 acre-ft in the regolith and 300 to 3,000 acre-ft in bedrock. Recharge to the groundwater system from infiltration of precipitation and streamflow was estimated to be about 1,000 acre-ft. Pumpage, which was estimated to be 170 acre-ft, had little effect on groundwater storage. Water levels in wells were nearly the same at the end of the water year as at the beginning. Groundwater quality generally was suitable for domestic uses. Concentrations of iron and manganese , although nontoxic, exceeded California maximum contaminant levels for domestic drinking water in some wells. (USGS)

Water-Resources Investigations Report↗
USGS1989

Appraisal of ground-water quality in the Bunker Hill Basin of San Bernardino Valley, California

Water samples were collected from 47 wells and analyzed for concentration of major inorganic ions, nitrogen species, and volatile (purgeable) organic priority pollutants to assess groundwater quality in the Bunker Hill basin, California. Data were supplemented with additional analysis of nitrate, tetrachloroethylene, and trichloroethylene made by other agencies. The organic quality of groundwater in the basin generally is suitable for most uses, although fluoride concentration exceeded the California public drinking water standard of 1.4 mg/L in water from 5 of 47 wells. Nitrate (as nitrogen) concentration equaled or exceeded the public drinking water standard of 10 mg/L in water from 13 of 47 wells sampled for this study and in an additional 19 of 120 samples analyzed by other agencies. Concentration generally decreased with increasing depth below land surface. Twenty-four of the 33 volatile organic priority pollutants were detected in water from wells sampled during this study. When supplemental data from other agencies are included, tetrachloroethylene concentration exceeded the standard of 5 micrograms/L in water from 49 of 128 wells. No basinwide relation between contamination by these two chemicals and well depth or land use was discerned. A network of 11 observation wells that could be sampled twice a year would enhance the monitoring of changes groundwater quality in the Bunker Hill basin. (USGS)

Water-Resources Investigations Report↗