Effect of ATES systems on the spreading of contaminants

What is the problem?

Historical soil and groundwater contamination frequently occurs on a large scale in central city areas. Indeed, in some Dutch cities the aquifer thermal energy storage (ATES) systems (open geothermal energy storage systems) can only make use of aquifers which contain contaminants. In urban areas the demand for subsurface space for the storage of thermal energy often exceeds supply, which explains why the geothermal energy wells are frequently installed close to each other. It has to date been unclear how the interaction of several ATES systems within such areas influence the spreading of contaminants, and what factors determine the risks for surrounding areas, such as water abstraction sites. To gain such insights, KWR has performed modelling studies that closely consider all the relevant spreading processes on both a local and regional scale.

The key spreading mechanisms

One important spreading mechanism is the extraction of contaminants from one well and their direct infiltration into another well (recirculation effect). Since ATES systems in practice often work with several warm and cold wells, the contaminants extracted for example from a single warm well, will be infiltrated in diluted form, in all cold wells simultaneously. This spreading to several infiltration wells accelerates the pace of the contaminant spreading and extends its range.

Another important mechanism is the ‘jumping’ of the contaminants from one ATES system to another. In areas with a number of subsurface activities, ATES wells are positioned on the basis of their (expected) thermal radius of influence (solid lines). Because the hydraulic radius of influence (dotted lines) is approximately 1.5 times larger than the thermal radius, and contaminants in groundwater usually travel within the radius of the hydraulic radius of influence, they can jump from one ATES system to another. Combined with the recirculation effect, contaminants can, within 1.5 years of arriving in one ATES well, jump to a well of another system hundreds of meters away.

Moreover, in areas with numerous ATES wells the hydraulic and thermal influence areas are not perfectly circular. As a consequence of the interaction between head changes  in neighbouring ATES wells, short-circuit flows can occur: this reinforces the effect of the overlapping hydraulic radii.

Case study: Utrecht city centre

Utrecht city centre was selected as a case study for modelling on a large spatial scale. Grateful use was made of the support provided by Vitens and the municipality of Utrecht in the form of data, models and practical insights. The case study revealed that a point source of contamination had spread to most of the area in a period of ten years. The speed with which the contaminants spread through the city centre area was about ten times faster with ATES systems than without them.

Implications for practice

In busy areas the spreading of contaminants because of the ATES systems present is boosted to the point that, within 5-10 years, all contaminants are, in significantly diluted form, spread throughout the entire area. The greater flow caused by the ATES systems also mean that source areas with layers of sunken liquid contaminant present (liquids that are heavier than water, also known as DNAPLs) generate more contamination. In this manner the contaminant load in the area is increased.

Better modelling

With the standard modelling software it is not really possible to properly simulate the recirculation effect of ATES systems with several wells. Previous studies have therefore systematically underestimated the spreading of contaminants by ATES systems of this kind. KWR has nonetheless developed a method which is able, despite the limitations of the modelling software, to calculate the spreading truthfully. Moreover, KWR is currently exploring the possibility of structurally improving the modelling tools in this regard.

In the current situation, the faster spreading of contaminants due to ATES systems does not lead to a noteworthy shortening of the contaminant transport times to the Vitens drinking water abstraction areas, which are situated away from Utrecht. The accelerated spreading and dilution only occur in the area within which the ATES systems have hydraulic influence on each other. The abstraction areas are sufficiently far away from the area within which ATES systems cause the accelerated spreading of contaminants, so that the impact on the overall transport time is limited. It is therefore important to maintain a sufficiently large non-ATES buffer area around aquifer abstraction sites where ATES systems and contaminants are also present. This is necessary as a means of preventing a short-circuit flow from being established to the abstraction area from a particular ATES source area. At the same time, it is possible to continue installing ATES wells in busy subsurface areas on the basis of their thermal influence areas: it helps keep the area in which the contaminants spread compact.