2008 Thesis Abstracts

Maryam Taiedi, 2008, Selenium and Water Quality Patterns at the Estuarine Interface of San Diego Creek, Orange County, California ; Committee Members: Dr. Barry Hibbs, Dr. Kim Bishop, Dr. Mohammad Hassan Rezie_Boroon   

     San Diego Creek is at the estuarine interface of Upper Newport Bay. Upper Newport Bay Watershed is located in south central Orange County, California. The north boundary of the study area is San Diego Creek, and the south boundary is Upper Newport Bay. There are three upstream in-channel sedimentation basins located within this area which are about 3.3 Km long, and also a mixing basin adjacent to Upper Newport Bay which is 0.64 Km long. San Diego Creek contains terrestrial waters that flow through the three in-channel sedimentation basins and the mixing basin (pre-basin) and ultimately into Upper Newport Bay. The transformation and circulation of water from San Diego Creek to Upper Newport Bay can affect the quality of the water and therefore the ecosystem and life of the species in this area.

     One of the important aspects of this study is to investigate the spatial extent of the tidal flows and the fresh water and the salinity patterns at the mixing interface. The measured specific conductance for Upper Newport Bay is 40 to 48 mS, compared to the specific conductance of

     San Diego Creek, which is 1.3 to 3.0 mS. The measured specific conductance for the basins (all four of them) lies between the measured values of the Upper Newport Bay and San Diego Creek, which indicates that there is a degree of mixing occurring within the basins at the estuarine interface. There is also a sheet flow scenario occurring within the downstream mixing basin and the adjacent sedimentation basins; where a lens of fresh water up to 1.1 m thick flows on top of a dense layer of salt water of variable thickness. During the dry seasons the water quality analysis for selenium and nitrate in San Diego Creek indicates high concentrations of selenium (15 to 20 ug/L) and nitrate (4 to 8 mg/L NO3-N) in terrestrial flows versus low concentrations of selenium (<2 ug/L) and nitrate (<1.5 mg/L) in Upper Newport Bay. The water quality data explains the fact that the lens of fresh water acts as a pipeline for transferring these contaminants directly into Upper Newport Bay.

     Other water quality parameters were also measured in the water column such as pH, temperature, and dissolved oxygen.  Water was sampled from the top and the bottom of the basins once a month for six months. The water samples were also analyzed for the ions; Cl, F, Br, NO3, PO4, SO4, As, and Se.

     This study determines the effects of the mixing basin (pre-basin) on the selenium concentration.  The surface flow with high concentrations of selenium (20 to 25 ug/L), which exceeds the USEPA standard, flows into the pre-basin and mixes with the sea water of low selenium concentration (<2 ug/L).

Rachel E.Andrus, 2008, Variable Distribution of Selenium and Arsenic in the San Diego Creek Watershed, Orange County, CA; Committee Members: Dr. Barry Hibbs, Dr. Kim Bishop, Dr. Hengchun Ye

     Historically, an extensive wetland known as the “Swamp of the Frogs” occupied the central, low-lying part of the San Diego Creek Watershed that was drained toward the end of the nineteenth century to make way for the expansion of ranching and agriculture.  High levels of selenium in the shallow groundwaters of the watershed coincide with deposits in this historic marsh.  Studies have shown that nitrate can act to oxidize reduced forms of selenium (Wright, 1999).  This has important ramifications for wildlife that depend on water bodies in agricultural watersheds with a geologic source of selenium, and is, in part, the motivation for this study.

     Seasonal variability of nitrate and selenium were monitored in time series over a wet-year (Walker, 2006) and a drought-year.  Arsenic is a common co-contaminant with selenium.  Three shallow groundwater flowpaths were sampled in order to test the feasibility of using stable isotopes to show the link between denitrification and the oxidation of sulfur and selenium in a field setting and to examine the redox controls on the mobility of these two co-contaminants.  Soil leaching experiments were conducted in order to quantify the amount of leachable selenium contained in the soils across the catchment.  Additionally, wet-weather flows were collected in two upland tributaries in order to characterize the upstream sources of selenium. 

     Data show that there are higher levels of leachable selenium in the soils collected from within the boundary of the historic swamp (average 3.7 mg/kg) than within soils collected from approximately five miles outside the boundary of the historic swamp (average 0.01 mg/kg).  Nevertheless, appreciable quantities of selenium were found in surface run-off collected from upstream tributaries (average 5.1 ug/L), signifying that there is an upland geologic source of selenium in the catchment, most likely marine shales.  Weathering and erosion of these upland marine shales delivered selenium to the reducing waters of the historic marsh where it was subsequently reduced and immobilized in the sediments of the historic marsh.  After the marsh was drained, orchards were planted in the vicinity of the historic marsh.  Highly alkaline soils and shallow depth to water table prevented the lands previously occupied by the marsh from ever being put into cultivation.  Fertilizer application associated with historic agriculture continues to contribute high levels of nitrate to the shallow aquifer.  Time series data show that infiltrating precipitation results in increased nitrate loads in groundwater inside the area of the historic orchards, whereas infiltrating rainwater has a diluting effect on groundwaters that flow within the boundaries of the historic swamp.  Infiltrating precipitation also results in leaching of selenium from the vadose zone both within and outside the historic marsh boundary.  This anthropogenic source of nitrate has important ramifications for the continued mobilization of selenium from the soils of the catchment.  Isotopic data along two of the three groundwater flowpaths support a model of sulfide oxidation linked to denitrification.  The chemical similarity between sulfur and selenium permits the inference that nitrate is also acting to oxidize reduced forms of selenium.  Streams and channels of the catchment are fed primarily by groundwater baseflow.  High levels of nitrate and selenium in the shallow aquifer has important ramifications for wildlife that depend on surface water bodies in the San Diego Creek watershed.

Jack Tung, 2008, Examining the Style and Uplift Along the Claremont Strand of the San Jacinto Fault Zone; Committee Members: Dr.Nate Onderdonk (Chair), Dr. Kim Bishop, Dr. Pedro Ramirez 

     Potrero Valley is a small basin in the northwestern-most San Jacinto Mountains and is currently undergoing uplift as a result of slip along the San Jacinto fault zone. The valley is characterized by Mio-Pliocene to late Pleistocene sedimentary rocks and a sequence of alluvial terraces while the surrounding ridges are composed of Triassic metasedimentary rock and Cretaceous plutonic rock. Potrero Creek flows through the length of Potrero Valley and is currently in an incisional stage as it attempts to reach base level. Potrero Creek passes through Massacre Canyon at the southwestern end of Potrero Valley and feeds into the San Jacinto River in San Jacinto Valley.

      The San Jacinto fault is a right-lateral fault, yet has apparently produced a significant amount of topographic development. The mode of uplift and topographic development of Potrero Valley and the adjacent San Jacinto Mountains are not completely clear. Relatively little research has been conducted in Potrero Valley due to the lack of access into the area in the past. The presence of preserved fluvial terraces within the valley provides a record of fluvial deposition and uplift. From the preserved terraces, the rate of deposition, incision and uplift can be inferred. In order to understand the topographic development of Potrero Valley and the San Jacinto Mountains, research consisted of geologic field mapping, aerial photo interpretation, structural data collection, and radiometric dating of detrital charcoal found within terrace units.

      A series of preserved alluvial fill and strath terraces, representing at least four distinct periods of aggradation, deposited in the valley record a history of deposition, uplift, and incision. Using radiometric dating of recovered detrital charcoal from two terraces and the height above the active channel where the samples were collected, the data suggest deposition rates of 2.0 to >5.4 mm/year during the Holocene, an incision rate of 2.9 mm/year since late Pleistocene, and uplift rates of 1.1 to 1.2 mm/year since late Pleistocene. Uplift is accomplished by right-lateral reverse oblique slip along segments of the Claremont fault and the Soboba-Campbell Ranch fault.