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

de Sieyes, Nicholas R., Kevan M. Yamahara, Blythe A. Layton, Elizabeth H. Joyce, Alexandria B. Boehm

Limnol. Oceanogr., 53(4), 2008, 1434-1445 | DOI: 10.4319/lo.2008.53.4.1434

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^-1 m^-1 shoreline during neap tides compared with 0.1 and 0.5 L min^-1 m^-1 during spring tides. This compares with 15.9 and 22.0 L min^-1 m^-1 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.