Geology Can Protect Groundwater Supply Wells from Nearby Contamination

Protecting groundwater resources and users from contamination is often a complex problem that I'd like to highlight during National Groundwater Awareness Week - #GWAW. It is relatively easy to choose a location for a groundwater supply well. Previously installed groundwater supply wells and the operation of those wells generates knowledge and data that subsequent groundwater supply developers can use with a high degree of confidence to predict good locations for supply wells. This is because even though the subsurface has been created by complex geological processes acting from small scale to a large scale, the ability of the aquifer to supply water to a well is related to the large-scale properties of the aquifer. For example, a few layers of clay in an otherwise sandy aquifer won’t change the productivity of the well.

On the other hand, when contamination exists or is released in the vicinity of a supply well, smaller scale geological features can accelerate or inhibit contaminant migration to the supply well. Common examples are supply wells in the vicinity of septic systems and retail gas stations with historical releases. We know that the groundwater quality in the vicinity of these types of locations is impaired, yet it is not always the case that nearby supply wells are affected. The small-scale clay layer(s) that don’t materially affect the productivity of the well may be enough to cut off migration of contaminants to the well. Understanding these smaller-scale geological features can help groundwater supply developers take appropriate action to mitigate the risk of installing and operating well(s) in the vicinity of contamination.

#TeamGEI helped a client who was both responsible for contamination in the aquifer and a user of the aquifer for water supply. Their water supply wells were in the vicinity of the path of contaminant migration. Investigation of the aquifer revealed that there are ancient river beds buried under ground (the thin blue lines highlighted in orange on the image), and while these buried river beds only slightly affected the productivity of the water supply wells, they were the pathways that contamination was flowing along. GEI created a relatively simple groundwater model, shown below, that demonstrated that there was only one well that was potentially threatened by contamination (the blue flooded contours). The contamination that migrated furthest was restricted to the ancient riverbeds, so it was only one well (W6) that was near one of these buried riverbeds that was threatened. The buried riverbeds protect the other supply wells by containing the contamination within them. GEI concluded that the rest of the supply wells could continue to be operated. Mike Hawthorne, PG , Jake Riedel, and I will be at the AEHS Foundation 34th Annual International Conference on Soil, Water, Energy, and Air in San Diego next week talking this project and other ways we help protect groundwater resources.