Sediment (silt) and Hydro-Soil Surveys

Sediment (silt) and Hydro-Soil Surveys

When considering restorations and mitigations to aquatic environments it is important to understand the impacts that the silt and hydro-soil have on the ecology of both running but in particular still and standing water such as ponds and lakes. Carrying out a survey is or should be one of the first and initial steps in the process.

What is sediment (silt) ?

Sediment which also commonly termed  as ‘silt’ is often particulate in nature especially when it enters a waterbody with an inflowing stream or indeed as run-off from the surrounding catchment.  It can be both organic and inorganic.  The sediment will also be made up of organic matter such as leaf litter, dead algae and plant matter in various stages of decomposition.  It is often nutrient rich and also has a tendency to be oxygen poot (anaerobic)

What is hydro-soil ?

Hydro-soil is simply the compacted soil base of a waterbody that consists of the original base that was created by nature or excavated by man.  It is a combination of the original base material with varying depths of accumulated sediment layered above.

Hydro-soil survey

Surveying the hydro-soil and the sediment proportion  allows an understanding of  the processes and conditions prevalent in the water environment.  A survey should include the depth of the Sediment contrasted against the depth of the water. This can help to understand any sedimentary processes and the possible need for dredging, draining and sediment removal.  It can also be very informative to understand the different percentage composition of the sediment between organic and inorganic.

Organic versus inorganic sediment

A survey should also include providing an appreciation and understanding  of both the organic and inorganic component of the hydro-soil. This is particularly important to understand as these natural processes are often critical in achieving the much sought after ‘natural balance’ of the entire waterbody.  It is possible to bioremediate the organic proportion of the sediment with significant benefit to the hydro-soil and the entire waterbody.

Carrying out a survey to obtain a physical sample is key to be able to perform a Waste Acceptance Criteria “WAC Test” to check if sediment is suitable to be removed from a waterbody and be taken to landfill or for re-use.

Waste Acceptance Criteria (WAC)

WAC sets the parameters for different types of waste and determines criteria for waste to be accepted to a particular landfill type.  A WAC test will essentially analyse the leaching ability of contaminants in the soil to identify how contaminants will act when in landfill. So, after soil waste has undergone basic characterisation to classify it as non-hazardous or hazardous it can then undergo WAC testing.

When is WAC testing required

It's important to remember that a WAC test can only be carried out after waste has been classified as non-hazardous or hazardous. The landfill will require 'Basic Characterisation' of the soil and you have a 'Duty of Care', as the waste producer, to carry out this characterisation.

To achieve this basic characterisation you must carry out a waste classification test, once this has been completed, soil waste can undergo a WAC test. A waste classification test is often carried out in conjunction with a WAC test to give you all the information you would need to provide the landfill operator. 

WAC analysis (specifically leaching test results) must not be used for waste classification and hazardous waste assessment purposes. This analysis is only applicable for landfill acceptance and does not give any indication as to whether a waste may be hazardous or non-hazardous. - Waste Management 3 (WM3)

If waste is classified, through a waste classification test, as non-hazardous it may be desirable to carry out WAC testing in order to confirm the waste meets inert landfill criteria, as inert landfill disposal is generally cheaper than non-hazardous. To be accepted to inert landfill it must undergo inert WAC testing and meet certain criteria on contaminant leaching and other characteristics.

If the silt/soil waste fails an inert WAC test it cannot go to inert landfill and must be sent to non-hazardous landfill. Alternatively, if waste is classified as hazardous it must then undergo hazardous WAC testing to identify it can accepted to hazardous landfill or non-reactive hazardous landfill. If hazardous soil waste fails hazardous WAC testing then it must be treated prior to disposal, to ensure it meets the necessary criteria for hazardous landfill.

Don’t worry let us do the dirty work

With over 30 years of experience, A.G.A. Group Consultancy is in an ideal position to offer all of the services and advice as detailed above …. Give a consultant a call today; 01953 886824