Thesis Defense - Fall 2008 - Steven Stuart

Groundwater and Surface Water Interactions at the Tijuana Estuary, San Diego County, California

Steven Stuart
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Kathy Thorbjarnarson

Monday, October 15th, 2008

ABSTRACT
The Tijuana Estuary is a unique coastal plain and wetland-dominated estuary influenced by tidal fluctuations and saltwater inflow from the Pacific Ocean and by intermittent freshwater inflow from the Tijuana River. The lower Tijuana River valley, the alluvial river valley that leads into the estuary, collects 96% of runoff in the 4,500 km2 Tijuana River watershed. Stream flow in the Tijuana River is ephemeral and regulated by precipitation events and dam releases upstream in the watershed. The quality of surface water is impacted by sewage discharges in Mexico and storm water runoff. Groundwater in the river valley flows west to the estuary and is impacted by local agricultural and sod farming activities. The purpose of this study was to evaluate the influence of groundwater and surface water from the lower Tijuana River valley on salinity levels in the estuary and to characterize the interaction between surface water and groundwater in the estuary in response to diurnal tidal fluctuations.
This study incorporated simultaneous monitoring of groundwater and surface water elevations and salinity in the lower Tijuana River valley and Tijuana Estuary over a period of 1.5 years. Seasonal variations in water levels and salinity were observed in the river valley and estuary; however, the estuary was influenced more by diurnal tidal fluctuations, high spring tides, major precipitation events, and surface water runoff. Groundwater and surface water salinity in the estuary was brackish to hypersaline. Hypersaline groundwater was measured in areas with low hydraulic conductivities that dictated slow infiltration rates and in areas that were inundated with ocean water during high spring tides. Hypersaline surface water was measured in estuarine channels that were temporarily disconnected to tidal flushing. Precipitation events dramatically affected salinity in surface water where salinity temporarily fell below brackish levels. In groundwater, precipitation events typically caused a short-term spike in salinity as the initial wetting front of infiltrating rainwater carried a higher concentration of dissolved salts from the shallow soil to the water table. Continued recharge by infiltrating rainwater gradually decreased salinity over a few days after the precipitation event.
These observations demonstrated that the Tijuana Estuary is a highly variable and dynamic environment that is greatly influenced by diurnal tidal fluctuations, particularly during high spring tides, and major precipitation events and subsequent storm water runoff. It appeared that seasonal variations observed in the lower Tijuana River valley did not influence the hydrologic nature of the estuary.

SEMINAR - Shuo Ma

A Physical Model for Widespread Near-Surface and Fault Zone Damage Induced by Earthquakes

Shuo Ma
Department of Geological Sciences
San Diego State University

Wednesday, October 15th, 2008

Seismic observations indicate that material velocities at shallow depths decrease over a large area after large earthquakes. The reductions are widespread, and occur at distances of up to several source dimensions. A persistent low-velocity fault zone has also been documented extensively from seismic and geodetic observations, in which the velocity drops further after large earthquakes. Dynamic stresses carried by seismic waves in the near surface or accompanying rupture at depth in the fault zone, are thought to create these velocity reductions by causing material damage. However, a rigorous physical interpretation as to why modest dynamic stresses can cause widespread near-surface damage, and why fault damage zones form, is lacking. By using a Drucker-Prager yielding criterion to simulate dynamic rupture propagation on a vertical strike-slip fault, I show that the widespread near-surface damage is caused by material yielding induced by seismic waves under the low confining pressure. Because the confining pressure increases with depth, materials yield more easily near the surface. The yielding zone at depth is narrowly confined near the fault, but its thickness broadens dramatically near the surface, forming a ‘flower-like’ damage zone, which is commonly observed in the geologic record. The fault zone damage at depth is induced by the large dynamic stress associated with the rupture front, while is induced by strong seismic waves ahead of the rupture front near the Earth’s surface. These results have important implications for the formation and evolution of fault zones, and possibly for the dynamic triggering of earthquakes as well.

SEMINAR - Chun-Ta Lai

Use of stable isotopes in studies of forest-atmosphere H2O and CO2 exchange

Chun-Ta Lai
Department of Biology
San Diego State University

Wednesday, October 8th, 2008

Stable oxygen (18O/16O) and hydrogen (2H/1H) isotopes at natural abundance levels are useful tracers for studying water and carbon cycles in terrestrial ecosystems. Atmospheric water vapor contains fewer amounts of heavier water isotopes (2H and 18O) relative to the source water. In the case of surface water loss via vegetation (transpiration), this preferential loss of lighter water isotopologues result in enriched 2H and 18O contents in leaf water. Meanwhile, CO2 that diffuses into leaf intercellular space and later out of stomata become labeled by this enriched 18O signature. Scientists have used this labile 18O signal in studies of global carbon cycles and paleoclimatic reconstruction. Improving our understanding of leaf water 18O enrichment allows for the development of better process-based models to investigate biosphere-atmosphere water and carbon dioxide exchange processes spanning from ecosystem to global scales. Discussions will be emphasized on how biotic and abiotic factors influence diurnal and vertical variations of 2H and 18O in leaf water and water vapor in forest air.