An investigation into the validity of using falling head percolation tests as part of a field assessment procedure for the design of on-site wastewater treatment systems

The infiltration capacity of soil under variably saturated conditions is critical knowledge that is essential in order to design effective engineered systems that rely on the percolation of water and/or wastewater as an integral part of the overall treatment process; for example, solutions for on-site wastewater treatment (septic tanks systems etc.) and storm water runoff systems associated with Sustainable Urban Drainage Systems. Such treatment systems are increasingly seen as appropriate sustainable solutions, which aim to attenuate both hydraulic and pollutant loads in order to protect surface and groundwater resources. Several different approaches can be taken to determine a soil’s hydraulic conductivity, either using percolation tests (carried out in the laboratory or in the field) or via soil size distribution. Each method yields different estimates of (saturated) hydraulic conductivity which can then be combined with knowledge of soil moisture retention curves, from which predictions can then be made of water flow under transient unsaturated conditions using, for example, the commonly adopted Richards equation.

In Ireland, falling head percolation tests are used to assess whether a site is suitable for an on-site wastewater treatment process for new developments in areas which lack access to centralised wastewater treatment systems. These tests are relatively easy to carry out, but suffer from lack of rigorous, standardised conditions during the test and so prove challenging when trying to convert the results into a rigorous metric that can be used for infiltration design. This research therefore carried out an international review of testing methods used in other countries of percolation characteristics of soils, including those based on constant head tests and/or soil texture. The advantages and disadvantages of each method are compared, as well as how the results are incorporated into soil treatment unit design.

In parallel to the international review, this research has evaluated results from over 800 falling head field tests carried out across a range of different subsoil types in Ireland. The data from each percolation test (water level drop and/or volume infiltrated) has then been modelled using both 2-D and 3-D numerical modelling code (Hydrus 2D) to derive saturated hydraulic conductivity values (K_s). The relationship between the field derived falling head saturated hydraulic conductivity results (K_sat)againstthe model derived K_s values has been plotted across the range of soil textural classes from fast percolating sandy soils to very slow clayey soils. Equally, the same comparison has been made between K_sat from a more limited number of field permeameter (constant head) tests against the model derived K_s values to allow direct comparison to be made between the two field methods via the same numerical modelling approach. Integrating the learnings gained from the assessment of different approaches and the modelling of field results in Ireland, the research concludes by recommending a staged move away from the falling head tests used in Ireland to a more hybrid approach based on soil texture and constant head (permeameter) tests.