HYDROGEOLOGY

It is important when carrying out a site investigation to determine the nature of any pollutants present in the groundwater. Regulatory authorities are likely to insist on this, particularly if the site is in close proximity to an aquifer, a source protection zone or licensed abstraction.

(An aquifer is defined as a geological formation or group of formations containing sufficient saturated permeable material to yield significant quantities of water to wells and springs). Environment Agency - Source Protection Zones

Regulatory authorities may also require leaching tests to be carried out on samples of soil.

The purpose of these determinations is to quantify the contamination status of the groundwater, and to identify the potential for further pollution to occur by soil and water interaction, and hence assess the risks to groundwater supplies.

Identification of contaminants in the groundwater is not the end of the story, however, since the site being investigated may not be responsible, although it is likely to be blamed. A neighbouring site may be the culprit. Contamination may have resulted from a non-replenishable one-off occurrence, eg a fuel spill, or the result of on-going pollution, eg leachate from a landfill site.

Where contamination has occurred, a number of factors affect the concentration of the contaminants in the groundwater. These include distance from the contaminant source, the amount of contaminant reaching the aquifer, its solubility and density, and the groundwater flow. These characteristics can be more fully understood and explored through the application of groundwater modelling techniques.

It is important therefore to determine the hydraulic properties of the aquifer and the corresponding controls on the groundwater. Using industry standard computer packages such as MODFlow, hydrogeologists can construct a model of groundwater heads (the heights to which water can raise itself above a reference level or datum) at various points throughout the site to predict the movement of plumes of pollutants through the aquifer. Modelling and calculation of flow rates using Darcy's Law (Q=KAI) are helpful in identifying the likely source of contamination and the risk to locally protected sources.

Once the properties of the aquifer are understood, it is possible to plan comprehensive and effective remediation measures, required to manage the situation. Examples include removal of groundwater for treatment (pump and treat) and monitored natural attenuation of pollutants.

By careful planning, the investigation can minimise the costs involved, so that boreholes required for these measurements can also be used for geotechnical, soil sampling and gas monitoring purposes.

The availability of hydrogeologists within the group adds a further dimension to Eclipse Scientific's site investigation capabilities.