Q&A: Aren’t sediments just wet soils? Is sand capping appropriate to isolate contaminated sediments?

Sediments are NOT just wet soils.

Sediments deposited underwater in modern geological times are categorically different than wet soils.  Ignoring those differences could lead to major errors in remedial investigations, evaluation of regulatory requirements, and assessment of environmental liabilities.  These differences may include:

  • High susceptibility to water transport (e.g. tides, storm induced flows)
  • Temporal variations existing over hours to decades
  • Large spatial heterogeneities, and
    • Low percent solids
    • High organic carbon
    • Wide range in particle sizes
    • Strong biological influences
    • Debris
  • Contaminant reservoirs in many forms
  • Complex geochemical changes over short (centimeter scale) vertical intervals
  • Ground water and surface water pathways
16 minutes later with 2, 4, and 6-inch sand caps

16 minutes later with 2, 4, and 6-inch sand caps; and sharp interfaces (<2 millimeters)

Soft, fluffy sediment

Soft, fluffy sediment

Sand caps can be utilized to isolate contamination even in very soft “fluffy” sediment.

The key to designing and constructing sand caps on top of very soft sediment is to use fine sands to minimize initial density differences and to add the sand very gently.  Soft sediment is easily disturbed and can be displaced in mud “waves” when sand is introduced quickly in large volumes or clumps.  These clumps impact the soft sediment as large single “stones” instead of a gentle rain of small particles.  The following figures illustrate the very sharp interface (<2 millimeters) that can occur between a sand cap and very soft sediment using a well-designed and engineered approach to sand placement.

About Rick Beach
Rick Beach brings nearly 45 years of experience with sediment projects and programs where he has managed, directed, and implemented site investigations, data gap evaluations, feasibility studies, remedial designs, remediation services, project optimizations, third-party reviews, potentially responsible party (PRP) identification, litigation support, and expert testimony.