Chapter 3 - The Interstitial Environment

Abstract

The interstitial system is defined by sand granulometry, which determines pore space or porosity, and drainage rates or permeability. Interstitial water flow is driven by groundwater discharge and by water filtration through the beach face from the input by waves and tides. Reflective beaches, with coarser sands and high permeability, rapidly filter large volumes of water and are consequently well flushed and oxygenated. Dissipative beaches have finer sands with lower permeabilities and filter smaller volumes of water at low rates. Waves are more important in reflective systems and tides in dissipative cases. Groundwater advection of solutes is also an important process in this system. Interstitial chemistry is primarily determined by the balance between inputs of oxygen and organic materials. Anaerobic conditions and strong chemical gradients are more likely to develop in dissipative situations, especially where organic inputs are large, whereas conditions in reflective beaches are more likely to be physically dynamic with strong interstitial flows, high oxygenation, and thorough drainage during low tide.