Coastal septic systems and submarine groundwater discharge [electronic resource] : a case study

The focus of this dissertation is submarine groundwater discharge (SGD), the direct flow of groundwater from the seabed to the sea, and onsite wastewater treatment systems in coastal California. The research focuses primarily on a single coastal community in central California, Stinson Beach, where conventional onsite treatment systems, or septic systems, are used exclusively for wastewater disposal. The overarching goal of the work has been to quantify the magnitude and timing of SGD at the site and to provide insight into how onsite wastewater treatment at Stinson Beach affects local groundwater quality and, via SGD, surface water quality, all with the broader goal of informing and guiding future development along the California coast. The dissertation includes four research chapters, each focused on one or more important aspects of the issue of SGD and coastal septic systems at Stinson Beach. The first research chapter entitled "Submarine discharge of nutrient-enriched fresh groundwater at Stinson Beach, California is enhanced during neap tides" describes a 14-day study investigating the influence of fortnightly spring-neap tidal variability on submarine discharge of fresh and saline groundwaters at the site. Fresh, shallow groundwater at the site was observed to contain high concentrations of nutrients including dissolved inorganic nitrogen, soluble reactive phosphate, and silicate, as well as human fecal bacteria. A groundwater-derived freshening and nutrification of the surf zone was observed at neap tide, and was followed by a 4-day increase in chlorophyll a concentrations in the surf zone. Analytical models and a fresh water budget in the surf zone were used to estimate the saline and fresh SGD at both neap and spring tides. Fresh SGD at the site was estimated at between 1.2 and 4.7 L min-1 m-1 shoreline during neap tides compared to 0.1 and 0.5 L min-1 m-1 during spring tides. This compares to saline SGD estimates of 15.9 and 22.0 L min-1 m-1 during neap and spring tides, respectively. Despite the smaller total discharge of groundwater during neap compared to spring tides, the larger fresh discharge component during neap tides raised surf zone silicate, DIN, and SRP by 14%, 35%, and 27%, respectively, relative to spring tides. The observed fortnightly 'pulse' of fresh groundwater-derived nutrients into the surf zone was consistent with seaward hydraulic gradients across the fresh part of the beach aquifer, which varied due to aquifer overheight, or the mounding of groundwater due to variable infiltration of salt water during tides, adjacent the beach face. Darcy-Dupuit estimates of seaward fresh groundwater flow in this area agreed well with the fresh discharge results of the mass balance.vvThe second research chapter of the dissertation entitled "Submarine groundwater discharge to a high-energy surf zone at Stinson Beach, California, estimated using radium isotopes" describes and compares results from a pair of two-week long experiments conducted in the dry season (July 2006) and wet season (March 2007) to examine tidal, wave-driven and seasonal variability of SGD at the site using natural radium tracers. Tide stage, tide range, breaker height and season each explained a significant degree of radium variability in the surf zone. A mass balance of excess radium in the surf zone was used to estimate SGD and associated nutrient fluxes during each season, confirming larger discharge rates during the wet season. Our results indicate median groundwater discharge rates of 6 to 8 L min-1 m-1 in July 2006 and 38 to 43 L min-1 m-1 in March 2007. SGD from 200 m of Stinson Beach in March 2007 was shown to contribute a flux of phosphate and dissolved inorganic nitrogen approximately equal to that associated with all local creeks and streams within 6 km of the study site at that time. The third research chapter "Fresh submarine groundwater discharge from a coastal aquifer forced by the Mediterranean climate of central California" 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 enterococcus, the behavior of nitrogen and microbial pollutants in the field was consistent with results from laboratory experiments, which demonstrated low denitrification potential in slurry tests, but fast fecal indicator bacteria and virus attenuation rates in saturated column experiments. Comparisons of total nitrogen flux to the subterranean estuary in this study agree well with SGD-associated nutrient flux estimates from prior studies at the site, suggesting that septic systems at the site are a persistent source of nitrogen to the subterranean estuary and may at times also be a source of enterococci. Denitrification potentials measured at the site suggest a possible role for in-situ remediation strategies to optimize nutrient removal in the beach aquifer.

Advisor: Boehm Alexandria primary advisor. Thesis advisor
Fendorf Scott advisor. Thesis advisor
Freyberg David L. advisor. Thesis advisor