We've Moved!

Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu

Collection Description

Collection of student theses and dissertations from as early as 1939, but mainly from 2010 to present.

Back to top

Pages

Collinear dipole-dipole resistivity technique applied to the siting of crystalline rock water wells
Twenty-three resistivity pseudosections and electromagnetic lines were conducted about twelve crystalline rock water wells located in San Diego County, California. The electromagnetic survey data was used in the interpretation of the pseudosections. The interpreted resistivity sections were compared with the hydraulic conductivities calculated from pump tests, with mapped lineaments, and with well logs. The dipole-dipole method appears capable of detecting lineaments and the general rock weathering condition. The ground water table appears to be detectable only in relatively unweathered rocks. Well site hydraulic conductivity can be significantly improved by locating the well off interpreted conductive anomalies. These anomalies are likely due to clay zones of low permeability., San Diego State University
Combined remote sensing and field investigations of major lithospheric compression in the Turkmenistan-Iran region of central Asia
Plate 1 Landsat TM (741) RGB Southern Turkmenistan Northern Iran Path 160 Row 34 acquired August 12, 1987. Base map: Landsat-7 satellite imagery, circa 12 August 1987, Path 160, Row 34 [Landsat TM (741) RGB]., The Kopet Dag Mountains are a northwest-trending range that formed during the closure of the Tethys Sea and represent the boundary of the Turan and Iranian plates. The Central Kopet Dag is an example of world-class, fault-bend folds that are beautifully displayed on Landsat, Corona and space shuttle imagery. The imagery can be used to determine exactly how strain is being transferred from strike-slip faults to imbricate thrust sheets and fold systems. Understanding this complex array of fault and fold mechanisms offers a suitable platform for extending subsurface geometries out into the topographically flat Karakum Desert to the north. The ''rigid indentor" style interaction of the Lut block with the 3-D, wedge-shaped fold and thrust belt into which it impinges provides an array of regionally plunging structures that expose an inclined structural view through the orogenic belt. Field studies reveal a three-dimensional, heterogeneously distributed amount of overall horizontal shortening. The Kopet Dag represents a three-dimensional model of the oblique convergence of two continental plates. The Kopet Dag fault as normally viewed in a two-dimensional map representation has been thought by most workers to be purely a strike-slip fault in the western Nebit Dag area. When viewed in a three-dimensional or four-dimensional model, however, an alternative model involving strike-slip faults as sidewall ramps in an imbricate thrust stack being differentially deformed by the Lut block appears more attractive. The incredible exposure of plunging structures allows a more sophisticated reconstruction than is traditionally done with most two-dimensional studies of orogenic belts. The Kopet Dag fault thus appears to represent the frontal boundary of oblique fault-bend fold tectonics between the Iranian plate and Central Asia. Major implications of this study apply to petroleum exploration and overall seismic safety for the densely populated city of Ashgabat, Turkmenistan., Substantial scholarship contributions came from the ARCS Foundation, Chevron and ExxonMobil. Funding was also made available through grants from the Central Asia Project and Project Alert., San Diego State University
Comparison of brittle vs. ductile surface deformation in an Alquist-Priolo earthquake fault zone: Examples from the Helendale Fault, San Bernardino County, California
Plate 1: Study Area Fault Map Plate 2 : Trench Log T-1 Rabbit Springs Plate 3: Trench Log T-2 Waverly Road Base map not specified. Plate 1 is an irregular polygon. Bounding latitude and longitude coordinates for Plate 1 taken from Google Earth satellite imagery January 1, 2015. 1:24,000, The Helendale fault extends from the Mojave River area southeastward through Lucerne Valley into the San Bernardino Mountains. The fault is the westernmost of a series of sub-parallel, northwest-striking, right-lateral faults in the central Mojave Desert which have been zoned by the State of California as requiring further study under the Alquist-Priolo Earthquake Fault Zoning Act. I characterize the fault in the Lucerne Valley region using seismicity, aerial photography, and by exposing the fault in several exploratory trenches. The surface trace of the fault is expressed as a zone of discontinuous scarps, tonal and vegetation lineaments, right-laterally-offset drainages and ridges, linear drainages, sidehill benches, breaks-in-slope, and shutter ridges. The fault displaces alluvium of Pleistocene and Holocene age. Several seasonally active artesian springs delineate portions of the fault in this area. I studied the subsurface character of the fault in detail at two locations. A trench was excavated near Rabbit Springs Ranch across a 0. 5 m high scarp and exposed lacustrian and alluvial sediments which were plastically deformed. Two zones of open fractures associated with the formation of the surface scarp are present in the lacustrian units directly below the scarp. A charcoal sample collected within deformed sediments above the fractures yielded a calibrated age of 2330+28/-189 yrs BP, indicating rupture during the late Holocene. Evidence for several surface rupturing events is apparent in trenches excavated near Waverly Road. At this site the fault is observed to be nearly vertical and offsets or truncates alluvial sediments in a brittle fashion. Fissures lined with carbonate and filled with sediment are resheared to within 0.2 m of the surface, higher than the level of carbonate cementation. This observation suggests two events with sufficient time to allow for the accumulation of the carbonate lining. I interpret the reshearing to be the same event dated at Rabbit Springs, as well as in an adjacent trench (T-3) at Waverly Road. The faulting in T-3 ruptures nearly to the surface in a brittle manner; charcoal collected from 1.5 m depth is dated at 4415+445/-494 yrs BP, again indicating late Holocene rupture. One sample from the innermost lining of one fissure fill yielded a 14C date of 9230 ± 100 yrs BP, suggesting an event at about this time assuming the carbonate was in equilibrium with the atmosphere. Alternatively, the actual age of the event could be 1-2 ka younger, depending on the amount of inherited age for the carbonate. In any case, these data suggest two Holocene surface ruptures in the Lucerne Valley area with evidence of both plastic and brittle deformation in the near surface., San Diego State University
Composition of Holocene Colorado River sand: An example of mixed-provenance sand derived from multiple tectonic elements of the Cordilleran continental margin
Base map: US Geological Survey 30-minute Quadrangle, 1903 Yuma, AZ. Geographic coordinates from Google Earth. 1:253,400, The Colorado River drainage basin is bounded to the northwest by the Sevier-Laramide thrust belt and to the east by Laramide-aged basement uplifts. It also includes: (1) remnants of Precambrian continental North American crust, (2) remnants of Paleozoic miogeoclinal/platform rocks that were deposited along the North American continental margin, (3) remnants of Paleozoic clastic rocks that were deposited in a foredeep during the Late Devonian-Early Mississippian Antlerorogeny, (4) deformed Paleozoic and Mesozoic sedimentary successions that make up the Sevier-Laramide fold-and-thrust belt, (5) Mesozoic and Cenozoic sedimentary successions that represent the fill of the Sevier foreland basin, (6) Cenozoic basement-cored uplifts that formed during the Laramide orogeny, (7) Mesozoic and Cenozoic plutonic and volcanic rocks associated with the Cordilleran-wide continental-margin magmatic arc system, and (8) Cenozoic sedimentary and volcanic rocks associated with strike-slip faults and continental-margin extension during the development of the San Andreas fault system. Hence, sand derived from the Colorado River drainage basin is an example of mixed-provenance sand. Twenty-five samples. of Holocene sand were collected along the lower Colorado River between Bard, California and San Luis, Arizona for detailed petrologic analysis. Samples were artificially consolidated with epoxy, thin-sectioned, and stained for plagioclase and K-feldspar. A. L. Armitage and G. H. Girty, each counted 400 points per sample utilizing the Gazzi-Dickinson point-count method. Their results were subsequently averaged. The average composition of samples examined during this investigation is 69.5% Q, 19.1% F, and 11.4% L when Lc is excluded from L. The mean plagioclase-to-total-feldspar ratio is 0.63, and the mean quartz population is composed of 87% monocrystalline and 14% polycrystalline qrains. On average, aphanitic rock fragments consist of 52% sedimentary, 42% volcanic, 6% metamorphic, and 1% alterite fragments. QFL data plot within the recycled-orogen field on the QFL provenance-discrimination diagram developed by Dickinson and others (1979, 1983, 1985). Recycled-orogenic provenance, include subduction complexes, collision orogens, and foreland uplifts associated with fold-and-thrust belts. Significantly, the rocks in the Colorado River drainage basin include a variety of tectonically-dependent provenance types, and as a result, Colorado River sands are not truly representative of the recycled-orogen model. Hence, the QFL diagram yields an erroneous interpretation for lithofeldspathic sands derived from the Colorado River drainage basin. In contrast, on the QmFLt provenance-discrimination diagram, nine of the Colorado River sand samples plot within the quartzose-recycled field and sixteen samples plot within the mixed-provenance field. On the LmLvLs diagram, Colorado River sands plot midway between Lv and Ls near the Lv - Ls join, and between the magmatic arc, rifted-continental-margin, and suture-belt fields. The complex multiple tectonic elements in the Colorado River drainage basin are therefore, accurately reflected in the QmFLt and the LmLvLs provenance-discrimination diagrams., San Diego State University
Compositional Variation of Holocene Sand Deposited In a Transtensional Rift System, Salton Basin, California
Twenty-five Holocene sand samples were collected from the mouths of drainage basins or from washes that led to homogeneous or mixed sources in the mountains adjacent to Salton Basin for detailed petrologic study. Bulk sand-sized and medium-grained fractions separated from each sample were consolidated with epoxy, thin-sectioned, and stained for K-feldspar and plagioclase. Thin-sections made from bulk fractions were point counted following the Gazzi-Dickinson method. Thin-sections made from medium-grained fractions were point counted for Qmnu, Qmu, Qp (2-3) and Qp (>3). In addition, during the bulk fraction count, a running tabulation of aphanitic rock fragments was kept. The results of this study suggest that the average Holocene sand derived from mountains marginal to Salton Basin contain 44.6% ± 11.3% Q, 49.6% ± 9.5% F, and 5.7% ± 7.9% L. On the QFL provenance-discrimination diagram developed by W. R. Dickinson and associates, 72% of the samples plot in the basement-uplift field and, therefore, accurately reflect their derivation from the coarse-grained metamorphic/plutonic roots of a segment of the Cordilleran magmatic arc. As might be expected, two samples derived from unmetamorphosed volcanic sources plot about the boundary between the transitional-arc and dissected-arc subfields. The remaining five samples, however, extend into the transitional-continental and recycled subfields, probably as a result of mixing plutonic and/or metamorphic detritus with sand derived from quartz-rich metamorphic rocks or from older Neogene sedimentary material. Medium-grained quartz populations are complex, and plot in the middle-to upper-rank and low-rank metamorphic fields on the QmnuQmuQp discrimination diagram developed by A. Basu and colleagues. This result accurately reflects the deep crustal origin of the bedrock in the mountains adjacent to Salton Basin, and its subsequent uplift and deformation in large scale dextral shear zones associated with the San Andreas and related fault systems. Data obtained in this study generally yield internally consistent results which accurately reflect the known provenances and tectonic settings of the samples studied. Therefore, I recommended that petrologists utilize the QmnuQmuQp and QFL provenance-discrimination models when attempting to unravel the provenance of ancient feldspathic sandstones. However, I also caution them that local source areas can produce sands whose compositions, when plotted on the QmnuQmuQp or QFL provenance-discrimination diagrams, may lead to an erroneous or overly simplistic interpretation of source rock characteristics or tectonic setting. In addition, data described here also indicate that the QFL, medium-grained quartz, and aphanitic-rock-fragment populations of sand derived from the mountains adjacent to Salton Basin are unlike those of Holocene Colorado River sand. Distinct compositional differences between the two groups of sand should enable future workers to better recognize and map ancestral Colorado River sand in the Salton Basin. Recognition of displaced Colorado River sand would place constraints on the movement history along the San Andreas and associated strike-slip fault system and, hence, on the origin of the Salton Basin-Gulf of California rift system., San Diego State University
Conodont biostratigraphy and depositional environments of the Hidden Valley Dolomite (Early Silurian through Early Devonian), Death Valley, California
Plate 1 Funeral Mountains section, NE 1/4 section 3, T. 26 N., R. 3E., Ryan Quadrangle, Inyo County, California. No base map was identified., Early Silurian through Early Devonian rocks crop out widely in the southern Great Basin in southeastern California. A total of 1,544 feet of dolomite and argillaceous dolomite was measured and systematically sampled at a single section in the southern Funeral Mountains, Death Valley, California. The lower 120 feet correlate with the upper part of the Ely Springs Dolomite, while the remaining 1,424 feet are assigned to the Hidden Valley Dolomite. Conodonts recovered from the Ely Springs Dolomite and lower and upper members of the Hidden Valley Dolomite indicate an age spanning earliest Silurian (early or middle Llandoverian) to middle or late Early Devonian (Siegenian or Emsian). Conodonts from the Ely Springs Dolomite are tentatively assigned to the Bereich I Zone (early and middle Llandoverian). The overlying lower member of the Hidden Valley Dolomite contains diagnostic conodonts of the Spathognathodus celloni and Pterospathodus amorphognathoides Zones (late Llandoverian to early Wenlockian). Conodonts in the upper portion of the lower member are tentatively correlated with the Kockelella patula Zone (middle Wenlockian). The lower 38 feet of the upper member contains numerous specimens of Icriodus latericrescens and a few questionably identified as I. l. huddlei. These suggest a middle to late Early Devonian age (Siegenian or Emsian). Lithologic and paleontologic evidence suggests that upper Ely Springs and Hidden Valley strata were deposited in a variety of shelf environments ranging from moderately deep subtidal to very shallow intertidal or possibly supratidal. An earliest Silurian regression and minor erosional episode is recorded in the Ely Springs. Then moderately deep and restricted water conditions were established during deposition of the basal Hidden Valley Dolomite. Shallowing upward sequences characterize the remaining portion of the lower member. Upbuilding of organic mounds and basinward mound progradation characterize the middle member. Initial deposition may have occurred in water as deep as shallow subtidal but the bulk of the member was deposited in an intertidal, possibly very shallow, mound and/or "back-mound" environment. Lithologic evidence suggests a transgressive event slowly increased water depth during latest Silurian or earliest Devonian time, culminating in the deposition of the shallow subtidal lower part of the upper member. Shallowing followed and continued through the end of the Hidden Valley deposition. Lithologic, paleontologic, and temporal features of the inner and outer continental shelf couplet (Hidden Valley Dolomite-Vaughn Gulch Limestone) in southeastern California are similar to the couplet (Lone Mountain Dolomite-Roberts Mountains Formation) in central Nevada. The similarity of geologic features between the regions suggest deposition of the respective sequences in a similar depositional and paleogeographic framework., San Diego State University
Conodont biostratigraphy of member A (Lower to Lower Middle Ordovician) Mountain Springs Formation, southern Great Basin
Systematic paleontology of conodonts recovered from seven measured sections has refined the conodont biostratigraphy of the basal member A of the Mountain Springs Formation in the Spring Mountains of the southern Great Basin. In ascending order the Early Ordovician conodont biostratigraphic division of member A are: Cordylodus proavus Zone, Cordylodus intermedius Interval, Loxodus bransoni Interval, "Scolopodus" quadraplicatusaff. Scolopodus rex and Acodus deltatus-Macerodus dianae Intervals, and Oepikodus communis Zone. The Middle Ordovician and uppermost strata of member A consist of taxa within the Protoprioniodus aranda-Juanognathus jaanussoni and Jumudontus ganada-? Reutterodus andinus Intervals. The time lines that these divisions make across the Spring Mountains, combined with the depositional analysis of the detailed lithostratigraphy of each section enables a more exact paleo-reconstruction of the Lower Ordovician System in this area of the southern Great Basin. The following conclusions are drawn from this study: (1) seaward increase of abundance and diversity of conodonts; (2) restricted stromatolitic areas of the lagoon and the uppermost mudflat environment were uninhabitable for conodonts and any found in these rocks are presumed to have been redeposited during storm events or laterally channel filling (3) the transgression of the Sauk Sea is illustrated by the lack of conondonts in the landward section until the Loxodus bransoni Interval. (4) the lateral variability of the carbonate tidal flat environment is illustrated by comparing temporally equivalent areas of each section with the conodont biostratigraphic time lines, enhancing paleo-geographic reconstruction of the southern Great Basin (5) the Lange Range Eustatic Event made environmental conditions adverse for conodonts during the deposition of the lower strata of member A of the Mountain Springs Formation in Early Ordovician time. Regional temporal correlations to sections in Utah, Nevada, California, Idaho and northern Baja California, Mexico can be made with other measured sections which have also been dated with conodonts. The conodonts of member A of Mountain Springs Formation support the recent biofacies and province models suggested for the western United States during Early Ordovician time., San Diego State University
Continuous magnetotelluric profiling across the La Bajada fault, Rio Grande drift, New Mexico
During the summers of 2000-2003, 79 magnetotelluric soundings were recorded during the Summer of Applied Geophysical Experience (SAGE) program in tbe Rio Grande rift spanning approximately 9.5 km in an east-west transect across the La Bajada Fault within the Santo Domingo Basin. The main purpose of studying this region of the Rio Grande rift was to obtain a better understanding of the shallow (upper ~3 km) geological structure along the eastern margin of the Santo Domingo Basin. The overall geoelectric strike direction obtained from magnetotellurics is 45 degrees east of north and is coincident with the trends of the Santa Anna accommodation zone and the large complete Bouguer anomaly gravity low prevalent throughout the Santo Domingo Basin. Mapped intrabasinal faults are mostly orthogonal to the northeastern trend seen in the magnetotellurics and gravity results. Faults not detected by surface mapping were inferred from aeromagnetic data as small linear anomalies. Two Tertiary monzonite intrusives appear as prominent resistive zones in the magnetotelluric results. The intrusives also correlate with gravity and magnetic anomalies, proprietary seismic data and borehole information. Detailed mapped sections within the Santa Fe River canyon reveal approximately 1.3 km of the stratigraphic sequence of Tertiary sediments while three regional exploratory water wells estimate less than 1.5 km of Paleozoic and Mesozoic sediments in the vicinity of the Santa Fe River canyon. The basin resolved by the magnetotellurics between the two monzonite intrusives shows definite thickening westward. By utilizing the point at which the rate of inverted resistivity change is greatest with depth, a trio of differing resistive boundaries is apparent delineating the offset of the La Bajada Fault and an unnamed intrabasinal fault of 200 and 1400 m, respectively. The distribution of resistivities along the MT profile within the basin reveals: (1) an uppermost, thickening westward wedge of relatively high resistivity (~ 10 – 40 Ω-m), (2) a deeper, more conductive zone of <5 – 10 Ω-m, and (3) an underlying resistive basement where values exceed 1000 Ω-m. The preponderance of geological and geophysical evidence supports the interpretation that these three zones represent: (1) a surficial wedge of Tertiary sands and gravels containing the freshest water and/or least amount of conductive clay (the maximum thickness of this aquifer is ~ 0.5 km at the western end of the MT profile), (2) a relatively poorer quality, water-saturated zone with salty water and/or high clay content composed of faulted Mesozoic and Paleozoic sedimentary rocks, and (3) the faulted, crystalline Precambrian basement complex with a maximum depth of ~ 2.5 km on the western end of the profile., San Diego State University
Controlled-source electromagnetic studies of the Southern California continental shelf
This dissertation presents research exploring the application of marine electromagnetic methods toward studying the nearshore continental shelf. The shallow-water controlled-source electromagnetic (CSEM) method has been shown to be a useful tool to study continental shelves. This marine CSEM method uses a man-made source of EM energy that passes through seawater and propagates into the seafloor and to towed-receivers which measure the resulting electric fields. These fields are processed into amplitude and phase data and then inverted to image subseafloor electrical resistivity. Electrical resistivity, while not a unique identifier, can be indicative of porosity and pore fluids, mineral chemistry, melt, and temperature. Thus, the CSEM method is well-suited to identify and characterize a variety of features and systems, both anthropogenic and naturally occurring, within continental shelves. Offshore San Diego, surface-towed CSEM data were collected to detect the possible offshore extent of the county’s onshore aquifer. Little was known of the offshore character of the aquifer, making it vulnerable to over-extraction and saltwater intrusion. Thus, this survey mapped pore-fluid salinity and groundwater pathways offshore to better constrain the freshwater-bearing formation. The results mapped a previously unidentified aquifer extending offshore San Diego which contains considerable volumes of fresh-to-brackish water, doubling the known groundwater volume of the county, in both continuous lenses and isolated pockets that appear influenced by fault systems and shallow stratigraphy. Near Santa Barbara, California, a surface-towed CSEM survey was used to target marine hydrocarbon seeps (MHS) within Coal Oil Point seep field (COP) at intermediate depths (<400 m). The results show significant spatial variability of MHS within COP and indicate at least two previously unidentified subseafloor accumulation sites. The depth and lateral extent of these accumulation sites could constrain overall seep-emission models for COP. From these studies, it became evident that the resolution and sensitivity of marine CSEM systems should be formally tested. Thus, rigorous and practical resolution and sensitivity studies were conducted to better constrain the depth of inference for several CSEM systems. The results from these tests indicate that the depth of inference for CSEM systems is deeper than previously thought. Finally, motivated by the search for archeological sites submerged offshore, a new CSEM system capable of detecting subtle and small targets in culturally and biologically sensitive regions was developed. Initial inversions from first deployments of the new system offshore the northern Channel Islands, California show significant improvement in resolution when compared to surface-towed CSEM systems., San Diego State University; University of California, San Diego
Controls on recovery efficiency of aquifer storage and recovery projects in brackish water aquifers
The use of brackish aquifers for aquifer storage and recovery (ASR) projects requires careful examination of the recoverability of potable water since mixing with the native water causes the injected water quality to degrade, resulting in a reduced volume of potable water which can be recovered. The success of ASR injection/extraction wells in brackish aquifers is determined by evaluating the recovery efficiency of the system, the volume of water recovered (before an established threshold concentration is surpassed) divided by the volume of water injected. In conjunction with traditional aquifer tests and borehole geophysics, computer models have become an essential tool for evaluating the logistical feasibility and potential for adequate recovery efficiency of ASR programs. ASR modelers have often characterized aquifers by a single averaged hydraulic conductivity for the entire storage zone in which the water was to be injected. Since real aquifer systems are often heterogeneous in nature, it is reasonable to conclude that computer models generated using the averaged hydraulic conductivity (homogeneous) approach are much less likely to accurately predict the extent injected water will travel and the amount of mixing which will occur, and thus the recovery efficiency of the system. To understand the effects of aquifer heterogeneity and other controlling parameters on ASR recovery efficiency, two models were constructed. One model was designed with a homogeneous permeability distribution and the other with a heterogeneous permeability distribution for the flow zone. The models are based on the San Diego Formation, a vertically and laterally heterogeneous aquifer in southwest San Diego County with a regional hydraulic gradient of 0.002 and typical total dissolved solids (TDS) concentrations ranging from 1,200 milligrams per liter (mg/l) to 3,300 mg/l. A hypothetical ASR project was modeled to simulate cyclic injection and extraction, each lasting for 180 days, for a 5 year period. Numerous simulations were run for both the homogeneous and heterogeneous models to test the effect on recovery efficiency of various aquifer parameters including native water quality, dispersivity, vertical hydraulic conductivity, and permeability contrast, as well as to examine design parameters such as pumping rate and the inclusion of a storage period between injection and extraction cycles. Results of the three-dimensional flow and solute-transport models, obtained using MODFLOW-SURFACT, were used to produce cross-sectional figures of the predicted distributions of injected water and residual injected water following extraction. Examination of these figures showed that the primary source of the extracted water was the up-gradient supply of low concentration water, indicating the strong effect of the regional hydraulic gradient. Also apparent from these figures, is that a significant amount of injected water moved vertically into the leaky confining layers of the aquifer, and that this water was less easily swept down-gradient by the regional hydraulic gradient, suggesting these low permeability layers serve as additional source areas for the extracted low concentration water. Low TDS concentration water injected into the model with a heterogeneous distribution of permeability was concentrated in the lower portion of the screened section of the aquifer where the most permeable layer was located. Increasing the hydraulic gradient of the heterogeneous model resulted in much of the injected water effectively escaping down-gradient beyond the capture zone of the well. Similarly, increasing the hydraulic conductivity of the most permeable layer of the heterogeneous model resulted in loss of injected water down-gradient due to its preferential movement into the high permeability layer. The addition of storage periods resulted in less of the injected water stored up-gradient due in part to the smaller volume of water injected but also due to the time available for the water to move down-gradient of the well before commencement of the extraction period. Increasing the pumping rate resulted in a significantly larger plume of low TDS water both vertically and laterally. Results of the models showed that recovery efficiencies are sensitive to several parameters. Recovery efficiencies are overwhelmingly dependent upon the native water quality of the aquifer. A change in the native water quality from a TDS concentration of 1,200 mg/l to 3,000 mg/l resulted in approximately half the recovery efficiency. Regional hydraulic gradients also significantly control recovery efficiencies of ASR projects. Increasing the hydraulic gradient from 0.002 to 0.005 resulted in approximately one-quarter less recovery efficiency. The addition of vertically heterogeneous layers within the flow zone, with hydraulic conductivities ranging from 1 ft/day to 205 ft/day, resulted in a minor decrease in the recovery efficiency of approximately 8 percent after the final cycle. To further evaluate the possible implications of heterogeneity the heterogeneous model was altered by increasing the hydraulic conductivity of the most permeable layer from 205 ft/day to 500 ft/day. This change resulted in a more significant decrease in the recovery efficiency of 16 percent, relative to a homogeneous model with an equivalent transmissivity, suggesting that the hydraulic conductivity contrast between layers is also a controlling factor on ASR recovery efficiency. The addition of storage periods between the injection and extraction periods only slightly decreased the recovery efficiency. Conversely, doubling the pumping rate only slightly increased the recovery efficiency. Changes in the dispersivity values and vertical hydraulic conductivities had little to no affect. The results presented here do not entirely agree with results presented in two previous studies which similarly examined the effect of heterogeneity on recovery efficiency, but used the solute-transport model MT3D. The models in this study were modified to allow comparison with the results of those studies, and in both cases the recovery efficiencies predicted in the MT3D models were approximately 30 percent below those predicted by MODFLOW-SURFACT. It has previously been documented that early versions of the MT3D code are not mass conservative which may be one explanation for the disparity. The results of this study should encourage ASR project designers to carefully assess native groundwater quality, regional hydraulic gradient and vertical heterogeneity of a prospective brackish aquifer through fieldwork before attempting to model an ASR project upon which the project’s feasibility will be judged., San Diego State University

Pages

Bookmark

Bookmarks: