Groundwater Vulnerability

Groundwater Vulnerability in General

Groundwater Vulnerability Diagram, click the image to enlarge*

What is Groundwater Vulnerability?

Groundwater vulnerability is the susceptibility of underground water resources to pollution by various activities and contaminants. This vulnerability tends to be lower where soils and substrata are thicker and more organic and clay-rich, where there is a general absence of fissures, and where the water table is deeper (see illustration). These factors reduce and / or slow the amount of water moving downwards and are also important in stopping a wide range of contaminants reaching the water table.

There are a large number of activities with a potential to contaminate groundwater. Examples include use of pesticides and fertilisers in agriculture, septic tank discharge, sludge spreading and landfills. A technique that can appropriately subdivide land areas into areas of high and low groundwater vulnerability to a range of activities is therefore a useful tool in identifying locations where such activities have a higher risk of contaminating groundwater. Once identified, different areas of vulnerability can then be subjected to use restrictions, codes of practice or targeted for more detailed assessment.

* This image has been taken from the Groundwater Forum website (Image 19) at www.groundwateruk.org. Other useful images relating to groundwater issues can be found at that website.

Note: This definition was written by Nick whilst at Entec UK Ltd and comes from a project profile (See Reference #3 on Downloads Page) published on the Entec UK Ltd website and used with their permission.

Groundwater Vulnerability Assessment Framework Design

In 2002/3 Entec UK Ltd were commissioned by the Environment Agency national science team to develop a new framework for groundwater vulnerability assessment and mapping that would replace the maps published in the mid 1990s. Nick project managed this work and was the main technical developer of a new approach.

The project reviewed the different approaches to assessing groundwater vulnerability, which had been developed elsewhere in the world and concluded that none of the existing techniques fully met the Agency’s requirements. Nick developed a new framework which allowed vulnerability to be evaluated on the basis of predicted concentrations and travel times. The new framework allowed a range of tools or algorithms to enumerate data held as a series of GIS layers. It was intended that the tools / algorithms and data layers used for an activity would be selected on the basis of the properties of specific contaminants and the nature of the activity. Outputs would be produced as GIS layers that would be assessed against assessment criteria to determine groundwater vulnerability and would also be linked to appropriate planning responses, e.g. codes of practice. (For further information on this project undertaken by Nick at Entec UK Ltd see Reference #3 on Downloads Page). A Technical Report and Project Record were prepared for the Environment Agency, but these have not been published.

The Environment Agency took on these recommendations, but in subsequent work and GIS code development made some modifications to the framework design in terms of introducing scoring of different processes and datasets and addition and weighting of scores. This was done to help simplify communication of the principles of groundwater vulnerability to non-specialist users. Nick provided some technical review during this period (2004-7).

Nick project directed and managed a second Entec UK Ltd project for the Environment Agency on groundwater vulnerability in 2007-9. This was tasked with exploring groundwater vulnerability from non-mains sewerage (septic tanks and package treatment plants) and from agricultural use and disposal of pesticides to land. The generic GIS code and scoring approaches were developed further by others under Nick’s specifications.

Over the period 2009 to 2013, there were further developments on the Groundwater Vulnerability mapping approach. Data licensing issues prevented use of some data and it was viewed there was a need to simplify the final products to make them more accessible / understandable to the wider community. The Environment Agency released a report on their new mapping methodology in 2014 (see Reference #33 on Downloads Page) and, from late 2017, the new maps are to be made available on the Environment Agency What's in Your Backyard website.

Groundwater Vulnerability from Non Mains Sewerage

Non mains sewerage includes septic tanks from private dwellings and package treatment plants, which tend to deal with sewage effluent from a number of properties. Discharge of treated effluent from these systems is distributed out into an irrigation field which provides further filtration and attenuation. The residual treated effluent then infiltrates down to the water table. There are some useful cartoons on YouTube showing how septic tanks work and the Irish EPA have a lot of useful information on their website. As part of Entec UK Ltd’s project on Groundwater Vulnerability for the Environment Agency in 2007-9, Nick steered literature reviews and technical work by others and then translated the limited published science on these sewerage systems into scoring of different parameters for which the Agency had GIS data layers. Groundwater vulnerability from discharges from septic tank systems appears to be related to depth to water table and the nature of strata beneath the discharge with ammoniacal nitrogen being the key contaminant of concern. The soil zone is bypassed by the constructed infiltration systems (formerly loosely called soakaways). A technical report was co-authored by Nick and submitted to the Environment Agency as a record of the work.

Since 2009, Nick has been working on catchment scale investigations as part of the Defra / Environment Agency Evidence and Measures project. In some catchments, pollutant loading to the catchment from septic tanks / non-mains sewerage is high compared to water company treated sewage effluents and farming. Nick has also evaluated the impact from existing large soakaways and the potential impact from septic tank and other treated effluent discharges to ground.

Groundwater Vulnerability from Pesticides

Pesticides (herbicides, insecticides, fungicides etc.) form a large group of chemicals with widely ranging properties designed to tackle a number of farming challenges, help produce good farm yields and provide cost effective food to the wider public. They are widely used in agriculture, but also on railway lines, highway verges, moorland and forestry, and are unfortunately not uncommonly found in streams, rivers, lakes and groundwater. A groundwater vulnerability tool that would help the Environment Agency and DEFRA (Chemicals Regulation Directorate www.pesticides.gov.uk) identify which areas of England and Wales were at the highest risk of contamination from pesticide use and authorised disposal would be a powerful tool.

Nick led work to try and identify the key processes that would affect leaching of pesticides from soil that could then be used to inform groundwater vulnerability assessment. This is an extremely challenging area with no existing tool able to predict leaching of pesticides at a regional or national level and the best tools being exceptionally data hungry and time consuming to run. Monitoring data from the Environment Agency’s national monitoring network were examined. Nick’s work did not come to a full conclusion, despite significant efforts, but some important processes / challenges were identified. These include:

• factors (e.g. organic carbon content, moisture content, pH) affecting the degradation rate of different pesticides in different soils;
• the importance of clay content and most likely aeration capacity of soils in controlling bypass flow via macropores;
• the timing between application and the first significant rainfall event having a significant control on the concentration of pesticide resulting from bypass flow (See Reference #9 on Downloads page).

In addition to soil processes, protection of groundwater afforded by drift and large unsaturated aquifer thicknesses was considered, the evidence for pesticide degradation in the unsaturated zone is however not strong. A note summarising this work was submitted by Nick on behalf of Entec UK Ltd to the Environment Agency in Spring 2009.