Groundwater Risk Assessment

Dr.Harriet Nash

Honorary Research Fellow

UNESCO Chair on Aflaj Studies – Archaeohydrology

This article gives an outline of a framework for carrying out formal assessments of risks to groundwater. We carry out such assessments to:

• determine whether there are any impacts, real or potential on water quality and the scale of and risks to ecology and human health;

• to identify realistic options for dealing with any problems identified;

• if clean-up is needed, provide target concentrations to meet at the source to protect the receptor.

The source, pathway and receptor must all be present for a risk to exist.

This figure shows many potential sources on the surface and vertical migration pathways through the unsaturated zone, then lateral migration in groundwater.  

Only one receptor, an abstraction borehole is shown. However, groundwater itself and any springs and watercourses fed by baseflow from the aquifer are also receptors. In most countries, water quality standards are set for groundwater abstractions for drinking water and for protection of aquatic life and it is necessary to identify the most sensitive receptor to know which standards should be met. 

A typical risk assessment starts with a desk study: what activities or processes could have released contaminants and over what period? Is the surface permeable?  How much infiltration could take place (infiltration = rainfall – (evapotranspiration, surface flow and soil moisture deficit))?  What are the hydrogeological characteristics of the aquifer, including the unsaturated zone? Are there any low permeability strata limiting downward migration? What type of flow – intergranular or fissure flow? What and where are potential receptors? Is there any information on water quality (springs, boreholes, aflaj) and groundwater levels? The desk study provides the first stage for the site conceptual model, which is a diagram showing the source(s) pathways and receptors and text with their characteristics.

It is usually necessary to conduct a site investigation, first to identify the extent and severity of near-surface contamination and then to obtain aquifer characteristics. 

A tiered approach is the most cost-effective way to proceed. At Tier 1, if source concentrations, ie concentrations in soil or pore water, are lower than the relevant standard, no further action is needed. If they are higher, additional site investigation may be required to proceed. At Tier 2, if the concentrations after dilution in groundwater beneath the site are lower than the standard, no further action is needed. 

To achieve full protection and cost-benefit, it is necessary to address not only dilution but attenuation by degradation and/or adsorption. Most organic substances degrade in the environment, especially in aerobic conditions and attach to any organic carbon along the pathway, while inorganic substances adsorb to clay minerals. Degradation of organic substances is well documented, but adsorption of inorganic substances is specific to the lithology and at some point, there is no more capacity for adsorption. Tier 3 compares concentrations following such attenuation (biological or chemical), while Tier 4 includes more complex modelling of attenuation.  

If the concentrations, known or predicted, at the receptor exceed the standard, we then have to establish what concentrations at the source would give an acceptable concentration at the receptor. These are known as remedial targets. England’s Environment Agency provides a free spreadsheet and guidance for deriving remedial targets that can be downloaded at

https://www.gov.uk/government/publications/remedial-targets-worksheet-v22a-user-manual .

The spreadsheet is simpler than most commercial software: it treats the source as continuous and not declining, which is over-conservative for most organic contaminants. However, it is a good starting point with useful guidance on, for example, calculating air and waterfilled porosity in the unsaturated zone, the mixing depth in the aquifer within which dilution takes place, and dispersion along the groundwater flow path. It is also helpful in testing the water balance of the conceptual model: for example, the hydraulic gradient is adjusted in the spreadsheet to achieve a balance with infiltration. In this case, the values entered for both, and for the hydraulic conductivity should be reconsidered.

As with all models, the results should be confirmed by field data if possible. In northern Oman, springs and aflaj provide ready access to monitor groundwater quality and to detect pollution that may need to be remediated.