Dregs means the water below the spigot was sampled on a separate filter. MP means a Millipore filter funnel was used to filter the water rather than in-line filtration because of the high concentrations. The Dregs Concentration is the mass filtered from below the bottle spigots divided by the volume of the water filtered from below the spigots, so it includes the particles in the water at the time the bottle was closed. It is assumed that the excess particles below the spigots were distributed throughout the bottle at the time of closure (probably in the form of aggregates). The measured volume of the water bottles used in NABE was 29.6 liters.