Exploring Coastal Groundwater with Dr. James Heiss

Recently, I had the opportunity to interview Dr. James Heiss, an expert in coastal groundwater research at UMass Lowell. He has dedicated his career to studying how groundwater beneath coastal regions interacts with seawater, and this provides insights critical to protecting our drinking water supplies.

What is Coastal Groundwater?

When most people think of water bodies, they imagine lakes, rivers, and oceans. But beneath the surface of coastlines around the world, there is another hidden water body: coastal groundwater. This groundwater exists in the spaces between sand and silt particles beneath the coastline. Coastal groundwater is an essential resource for many communities, serving as a critical source of drinking water. However, this groundwater is vulnerable to contamination, particularly from a process known as saltwater intrusion. As Dr. Heiss explained, saltwater intrusion occurs when saltwater from the ocean moves into freshwater aquifers beneath the land, contaminating them with salt. This can make the groundwater unsuitable for drinking and other uses.

The Causes of Saltwater Intrusion

Both natural and human factors can cause saltwater intrusion. In our conversation, Dr. Heiss highlighted three primary causes. First, during droughts, less fresh groundwater flows from land to sea, allowing saltwater to move inland beneath the coastline. Second, as the sea level rises, the greater pressure of the ocean water can push saltwater further inland into coastal aquifers. And finally, excessive freshwater pumping from coastal wells can draw saltwater upwards into the wells, contaminating them with salt. As these processes intensify, the availability of fresh drinking water decreases.

Tracking the Mixing Zone: Where Fresh and Salt Water Meet

One of the most important aspects of coastal groundwater research is understanding the mixing zone — the area where fresh groundwater from the land meets salty seawater from the ocean. This boundary can move inland or seaward over time, influenced by factors like groundwater pumping, precipitation, and sea level changes. Dr. Heiss explained that some coastal regions use monitoring well networks to track the location of this mixing zone. By measuring groundwater salinity, they can detect whether the saltwater interface is moving inland, which can help them take preventive measures.

Field Research in Coastal Groundwater: Studying the Impact of Storms

To understand how coastal groundwater is affected by natural events, Dr. Heiss and his team conducted a field study in Duck, North Carolina, located in the Outer Banks. Their focus was on how coastal storms impact the mixing zone between fresh and salt water. As extreme storms become more frequent with climate change, understanding their effects on groundwater becomes even more critical.

Dr. Heiss’s team installed groundwater wells along the beach and across the barrier island to monitor how groundwater salinity changed during storm events. These wells, made of PVC pipes with slots to allow water in, provided data on how the mixing zone responded to the storms. Despite the challenges of installing and maintaining these wells in a hurricane-prone area, the team has begun to analyze valuable data that could improve our understanding of storm impacts on coastal groundwater.

Storms vs. Tides: Two Different Coastal Influences

One of the key distinctions Dr. Heiss emphasized was how storm surge impacts groundwater differently from tides. While tides are predictable and mainly influence the intertidal zone, storm surges bring saltwater far inland, covering roads and marshes. This saltwater can seep down through soils and reach the groundwater, creating a direct, vertical form of contamination. In contrast, tides create a more stable, predictable pattern of mixing.

Biogeochemical Processes in Coastal Groundwater

Dr. Heiss also explained that the mixing of freshwater and seawater in coastal aquifers triggers biogeochemical reactions. One of the most important of these is denitrification, where bacteria convert nitrate in groundwater to nitrogen gas. This process is beneficial because it removes nitrate, which is a common contaminant that can cause harmful algal blooms when it reaches the ocean.

Preventing Saltwater Intrusion: Possible Solutions

While saltwater intrusion may seem like an overwhelming threat to our coastal groundwater, there are effective actions we can take to protect this vital resource. Dr. Heiss highlighted several practical solutions, such as managing groundwater pumping. By reducing pumping near the coast during high-demand periods, like the summer tourist season, we can prevent saltwater from being drawn into freshwater wells. Another possible solution is installing physical barriers, like underground dams, called subsurface barriers, to block saltwater from moving inland. Moreover, using permeable pavement in urban areas can help rainwater infiltrate the groundwater more easily. This would keep the water table higher and protect against saltwater intrusion.

Final Thoughts

Interviewing Dr. James Heiss was a fascinating opportunity to learn about a hidden but crucial aspect of our environment. Coastal groundwater is an essential resource, but it is also a vulnerable one. As climate change continues to impact coastal regions, understanding and protecting this resource will only become more important. Dr. Heiss’s work is a reminder that some of the most critical aspects of our environment lie beneath the surface — and that protecting them requires careful study and responsible management.