SubsurfaceViewer: What the Subsurface Can Tell Us About Our Water

Water is something we mostly encounter at the surface: in rivers, lakes and springs, or after a heavy downpour. Yet a large part of the water cycle takes place where we cannot see it directly — underground. Rainwater infiltrates the soil, moves through sediments and rock, and accumulates in aquifers, geological formations that can store and transmit groundwater.

The paths water takes, how quickly it moves and where it is stored are all strongly influenced by the geological structure of the subsurface. So if we want to understand, protect or manage water resources sustainably, it is not enough to know where water is measured. We also need to understand which geological structures and properties control its movement. That is where a spatial understanding of the subsurface becomes essential.

Water follows geology

Below the surface, geological layers can have very different properties. Sand and gravel, for example, generally allow water to move through them relatively easily, while fine-grained, clay-rich layers can significantly slow down groundwater flow. Layer boundaries, faults and other geological structures also influence how groundwater is distributed and which flow paths can develop.

This is what makes hydrogeological questions so complex. Most information about the subsurface comes from individual observation points. A borehole shows which layers are present at one specific location. A groundwater monitoring well provides information on groundwater levels. Other investigations reveal more about the properties of the subsurface or the water itself. On their own, however, these data only provide snapshots. The real question is how the observations relate to one another between those known points.

Turning individual data points into a spatial picture

This is where interpretation begins. Does a water-bearing layer continue from one borehole to the next? Does it become thicker or thinner? Are there areas where rainwater can infiltrate more easily into deeper layers? Or is groundwater movement restricted by less permeable materials?

To explore these relationships, the SubsurfaceViewer brings different types of information together in a shared spatial environment. Borehole data, geological maps, digital terrain models, geophysical data and other measurements can be analysed side by side in maps, cross-sections and three-dimensional views.

From these individual data points, users can develop geological cross-sections and conceptual structural models. These models are not a direct picture of the subsurface. Instead, they represent a geologically informed interpretation of how formations may extend between known data points.

This turns a collection of separate observations into a spatial model — one that can be used to investigate the next key question: How easily can water actually move through these structures?

How permeable is the subsurface?

For groundwater flow, the location of a geological layer is only part of the picture. Its physical properties are just as important. One of the key parameters is hydraulic conductivity. Hydraulic conductivity, often expressed as the kf value, describes how easily water can move through soil or rock. In simple terms, coarse-grained sediments such as gravel and sand tend to be more permeable than fine-grained materials such as silt or clay. Direct measurements of hydraulic conductivity, however, are not available everywhere. In these cases, borehole descriptions can provide additional clues. The SubsurfaceViewer supports the structured analysis of lithological data — in other words, descriptions of the rock and sediment types present — and can help organise information such as grain size and other material properties.

This makes it possible to investigate potential spatial variations in permeability. It does not replace field measurements or expert interpretation, but it can help make better use of the data already available and identify where further investigation may be needed.

The geological structure can therefore be translated into a hydrogeological picture: Where can groundwater move relatively freely? Where is its movement restricted? And which flow paths are plausible under the given conditions?

When water carries more than water

These flow paths become particularly important when groundwater is not only transporting water, but also dissolved substances. If contaminants from legacy sites, industrial land or other sources enter the soil, they may infiltrate deeper into the subsurface and eventually be transported by groundwater.

An elevated measurement at a single monitoring point does not, by itself, explain where a contaminant came from or how it may spread. What matters is the geological setting in which that measurement occurs. Is the monitoring point located within a highly permeable layer? Is there a less permeable unit below it that could limit further migration? Do elevated concentrations appear at several locations within the same geological structure?

Looking at measurements together with boreholes, layer boundaries and geological models makes it much easier to investigate these relationships. The question is no longer simply, “Where was something detected?” but rather: How could that substance move through the subsurface?

From understanding to decision-making

At this point, geological and hydrogeological understanding becomes a practical basis for decision-making.

Water utilities, for example, need to understand the geological conditions surrounding a production well and how well the groundwater is protected from surface inputs. Municipalities may need to assess whether new developments could affect sensitive groundwater areas. Environmental investigations, meanwhile, often need to determine whether certain geological layers are likely to retain contaminants or allow them to migrate further.

These questions quickly become very concrete: Is a planned development located above an important aquifer? Are there low-permeability protective layers that help shield the groundwater from surface contamination? How do conditions change across the wider area? And where should further investigation be prioritised?

The SubsurfaceViewer can bring together different geofactors and assessment criteria in a spatial context. Multiple criteria can also be combined and weighted. This does not replace expert judgement, but it creates a transparent basis for comparing scenarios and setting priorities.

Making complex relationships easier to understand

When decisions are based on this kind of analysis, how the results are communicated matters as well. Geological and hydrogeological systems are complex, and not everyone involved works with borehole logs, monitoring data or subsurface models on a daily basis.

Cross-sections can show how a water-bearing layer may extend between several boreholes. Three-dimensional views can illustrate the spatial relationship between monitoring points, the ground surface and geological units.

This is useful not only for analysis, but also for communication between experts, public authorities, municipalities, planning teams and other stakeholders. Decisions are easier to understand when the spatial relationships behind them are visible. In that sense, the article comes full circle: much of what happens underground remains hidden from direct view, but by connecting and visualising the available data, those relationships can be made far more tangible.

To understand water, you need to look below the surface

Groundwater cannot be understood by looking at wells or monitoring points alone. What matters is the geological system through which it moves. Boreholes, measurements and geological observations all provide important pieces of the puzzle. Once they are brought together into a spatial picture, questions about groundwater flow, permeability and contaminant transport can be explored with far greater confidence.

The SubsurfaceViewer helps bring these different sources of information together and turn data into a clearer understanding of the subsurface — providing a stronger basis for water protection, planning and informed decision-making.

Because if we want to understand what happens to our water, we also need to understand what lies beneath it.