Land Capability Assessment

What is the main objective of a Land Capability Assessment?

The main goal is to assess if the wastewater arising in the proposed or existing household/ hotel/ restaurant/school etc. can be managed with any of the available onsite wastewater treatment systems within the site boundaries without causing any damage to nature and humans.

What are the most important information that a Land Capability Assessment report should tell you:

• the recommended treatment system
• the Land Application Area (LAA) required for sustainable on-site wastewater treatment

In the report while defining the size of the required Land Application Area and treatment system the Assessor considers several Site Features, Key Chemical and Physical Soil Features, water and nutrient (if required) balance calculations and gives recommendations for design, construction, operation and maintenance to achieve a sustainable wastewater treatment system.

A critical element of the land capability process is to adequately characterize  the soil profile. If this characterization is not done satisfactorily, the LCA is worthless and your onsite sytem will be designed by guesswork.

There are 3 critical elements:

1. Profile thickness (including topsoil interval)
2. Pofile hydraulic properties (including colloid stability)
3. Nutrient uptake and pathogen attenuation ability.

Profile thickness and soil description

To adequately perform the soil description it requires significant experience and training to be accurate and consistent.

During a site assessment the assessor should dig a hole until 2 metres and lay the soil profile out to be able to get familiar with the physical properties of different soil layers and thus be able to tell where the limiting soil horizon lies (the soil horizon with the lowest permeability).

Assessors should do onsite dispersion soil test to find out if any of the cleyey soil horizons show signs of dispersion or not. If you have a dispersive clay soil layer after a short period of time it will completely clog and become totally impermeable which also a limiting factor in the downward movement of the wastewater.

During the field investigation we also can encounter shrink-swell clayey soils which under saturated conditions can swell to a degree that it will not let the water flow through it, they create an impermeable soil layer.

Profile hydraulic conductivity properties

The Darcy equation states that velocity of a liquid through a porous medium is the product of the hydraulic conductivity and the hydraulic gradient. Hence, knowing the hydraulic conductivity allows the estimation of deep seepage and flow times which allows confident disposal system design and demonstrates the adequacy of buffer distances to sensitive areas and entities.

Hydraulic conductivity of many soils can easily be measured onsite!!

… and coupled with some (simple) laboratory testing to determine colloid stability  (dispersion and swell potential) provides a high degree of certainty in hydraulic design.

Onsite constant head permeability testing is considered best practice in the Code of Practise.

In that it is clearly stated that „However, should there be a dispute or any doubt or uncertainty regarding the soil category derived by visual/tactile methods, in situ permeability testing MUST be undertaken”.

The Code of Practise also states that the permeability testing is to be undertaken in the limiting layer. In Victoria, this limiting layer will mostly be a clay subsoil.

In situ permeability testing is not rational for the extreme Type 6 soils (medium to heavy clays)

Nutrient uptake and pathogen attenuation 

Several processes affect nitrogen levels within soil after application of effluent . Alternate periods of wetting and drying with the presence of organic matter promotes reduction to nitrogen gas. Plant roots absorb nitrates at varying rates depending on the plant species, however nitrate is highly mobile readily leached and can enter groundwater via deep seepage.

To ensure adequate attenuation of nitrogen, a nitrogen balance is used with conservative estimates of the nitrogen uptake by different plants. Sufficient Land Application Area should be used to encourage wetting/drying cycles within the effluent field to stimulate microbial attenuation of nitrogen.

A small amount of nitrogen, as nitrate will inevitably reach the groundwater. However, this nitrogen from effluent would be insignificant in the context of the nitrogen routinely applied in common farming practises in the vicinity.

Removal rate of virus during percolation depends on loading rate. Irrigate  a little a time and none make it through to ground water.

Soil amelioration – Type 6 soils

For swelling, sodic and dispersive soils, gypsum is commonly used to create and/or maintain water-stable peds thus creating/maintaining an appropriate design hydraulic conductivity.