Submarine Discharge of Nutrient-Enriched Fresh Groundwater at Stinson Beach, California Is Enhanced during Neap Tides

• Published in: Limnology and oceanography 2008-07, Vol.53 (4), p.1434-1445
• Main Authors: de Sieyes, Nicholas R., Yamahara, Kevan M., Layton, Blythe A., Joyce, Elizabeth H., Boehm, Alexandria B.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Aquifers; Beaches; Biological and medical sciences; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fresh water; Fundamental and applied biological sciences. Psychology; Geochemistry; Groundwater; Hydraulics; Marine; Mineralogy; Ocean tides; Oceans; Pollution, environment geology; Salinity; Sea water; Sea water ecosystems; Silicates; Synecology; Water geochemistry
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2008.53.4.1434

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.