Author: Nicholas Reed de Sieyes

Publisher: Stanford University

Published: 2011

Total Pages: 186

ISBN-13:

is a numerical investigation of groundwater flow at the land-sea interface forced by precipitation and evapotranspiration typical of the Mediterranean climate of coastal California. A numerical groundwater model was developed using the variable density groundwater flow code SEAWAT-2000 to examine the influence of seasonally variable recharge conditions typical of coastal California on the magnitude and timing of fresh submarine groundwater discharge from a generic coastal aquifer with a constant head (non-tidal) ocean boundary. Model dimensions and hydrogeologic characteristics were chosen based on a combination of observations from field studies at Stinson Beach, California, and published numerical investigations of coastal groundwater flow. Average monthly recharge was calculated from historical precipitation records and potential evapotranspiration rates calculated from climatological observations made near the field site. Calculated recharge was approximately sinusoidal across the year, with positive recharge rates dominated by precipitation during the rainy winter and negative recharge rates dominated by evapotranspiration during the hot, precipitation-free summer. Rates of fresh discharge from the model aquifer to the ocean exhibited similar temporal characteristics for two modeled scenarios, a first including a constant head fresh landward boundary condition and a second including a constant flux fresh landward boundary condition. Discharge in both models peaked in January during the period of maximum precipitation and recharge, and declined until reaching a minimum in September, two months after the minimum recharge period in July. Minimum simulated discharge rates for two simulated scenarios were 17% and 18% lower in September than the maximum simulated discharges in winter. Monthly mean discharge from Lagunitas Creek, a creek near Stinson Beach, reached maximum and minimum values in February and September, respectively. The exponential decline in creek discharge was fast compared to the decline in modeled SGD, however, suggesting that fresh SGD and associated nutrient fluxes may play a particularly important role in coastal ecosystems in early summer when surface water discharge has nearly reached a minimum but discharge of substantial quantities of fresh groundwater is still substantial. The final research chapter "Nitrogen, fecal indicator bacteria, and coliphage attenuation and flux from a septic leach field to the coastal ocean" describes a two-year field study to measure the flux and attenuation of nitrogen, fecal indicator bacteria, and bacteriophage in groundwater adjacent to a large coastal septic system in Central California. The study was carried out at Stinson Beach Park, Golden Gate National Recreation Area, sixteen kilometers northwest of the San Francisco Golden Gate Bridge. Long-term measurements of septic effluent quality and volumetric discharge to the leach field, synoptic DC resistivity profiling of the saltwater/freshwater interface, continuous measurements of hydraulic head in the coastal aquifer, and the installation and subsequent monitoring of a dense array of multi-level monitoring wells adjacent to the leach field for chemical and microbiological constituents were carried out. Our results indicate a nitrogen- and inorganic carbon-rich plume of septic effluent flowing from the leach field through the beach to the subterranean estuary, or the mixing zone of fresh and saline groundwaters. Attenuation of E. coli and coliphage was complete within the vadose zone and the first few meters of transport. Enterococci were detected throughout the well network during one sampling event during which no attenuation was observed, and no attenuation of total nitrogen was observed along the flowpath during the experiment. Median estimates of total nitrogen fluxing toward the ocean downgradient from the leach field ranged from 1.6 to 70.6 moles day-1, depending on season and transect location. Except for