Submarine discharge of nutrient‐enriched fresh groundwater at Stinson Beach, California is enhanced during neap tides

Abstract

The influence of fortnightly spring-neap tidal variability on submarine discharge of fresh and saline groundwater was examined at Stinson Beach, California. Stinson Beach is a residential community that utilizes on-site systems for wastewater disposal. Fresh, shallow groundwater at the site contains high concentrations of nutrients (dissolved inorganic nitrogen [DIN], soluble reactive phosphate [SRP], and silicate) and human fecal bacteria. A groundwater-derived freshening and nutrification of the surf zone during neap tides was observed, followed by a 4-d increase in chlorophyll a concentrations. Analytical models and a freshwater budget in the surf zone were used to estimate the saline and fresh discharge of submarine groundwater. We estimate fresh groundwater discharge between 1.2 and 4.7 L min 21 m 21 shoreline during neap tides compared with 0.1 and 0.5 L min 21 m 21 during spring tides. This compares with 15.9 and 22.0 L min 21 m 21 saline groundwater discharge (forced by waves and tides) during neap and spring tides, respectively. Despite the smaller total (fresh + saline) flux of groundwater during neap compared with spring tides, the larger fresh discharge component during neap tides raises surf zone silicate, DIN, and SRP by 14%, 35%, and 27%, respectively, relative to spring tides. This observed fortnightly pulsing of fresh groundwater-derived nutrients was consistent with seaward hydraulic gradients across the fresh part of the beach aquifer, which varied due to aquifer overheight near 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.

Submarine Groundwater Discharge to a High-Energy Surf Zone at Stinson Beach, California, Estimated Using Radium Isotopes

“…Breakers with heights typically in the range of 0.5 to 2 m result primarily from northwest swells during winter and a combination of northwest and south swells in summer. A short description of local hydrogeology and human development in the area can be found elsewhere (de Sieyes et al 2008). Our research site is focused on the beach at the end of Calle de Resaca, a residential street in downtown Stinson Beach (37.899 N,122.646 W;Fig.…”

“…The work was conducted at Stinson Beach, a small beach community north of San Francisco, CA, USA. Groundwater at the site is contaminated with nitrogen (nitrate and ammonium, primarily), orthophosphate, and human fecal bacteria from septic systems, and previous research at the site has documented nutrification of the nearshore coastal ocean by fresh SGD in the summer (de Sieyes et al 2008). A previous experiment at this beach during dry summer weather estimated SGD using salinity as a groundwater tracer (de Sieyes et al 2008).…”

“…Groundwater at the site is contaminated with nitrogen (nitrate and ammonium, primarily), orthophosphate, and human fecal bacteria from septic systems, and previous research at the site has documented nutrification of the nearshore coastal ocean by fresh SGD in the summer (de Sieyes et al 2008). A previous experiment at this beach during dry summer weather estimated SGD using salinity as a groundwater tracer (de Sieyes et al 2008). The present study builds on that previous study but uses an entirely new radium tracer dataset consisting of over 500 measurements to investigate the effects of waves, tides, and season on SGD.…”

“…Tidal pumping has been explored on both the semidiurnal (AtaieAshtiani et al 1999;Robinson et al 2007) and fortnightly spring-neap timescales (Boehm et al 2004;de Sieyes et al 2008;Kim and Hwang 2002;Santos et al 2009;Taniguchi 2002). Michael et al (2005) documented a large, seasonally variable component of SGD controlled by precipitation/ evapotranspiration cycles.…”

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Fluctuations in SGD in response to the spring-neap tidal cycle has been widely reported [Kim and Hwang, 2002;Taniguchi, 2002;Robinson et al, 2007a;de Sieyes et al, 2008;Wilson et al, 2015]. While the salt distribution in a STE (including the configuration of an USP) typically does not oscillate considerably in response to semi-diurnal or diurnal tides (except for high permeability gravel beaches [Guo et al, 2010;Geng and Boufadel, 2017]), it does vary in response to the spring-neap signal.…”