Exploring Coastal Groundwater with Dr. James Heiss

Recently, I had the opportunity to interview Dr. James Heiss, a coastal groundwater researcher at UMass Lowell. Dr. Heiss has spent much of his career studying how groundwater beneath coastal regions interacts with seawater. His work is especially important because it helps scientists and communities better understand how to protect coastal drinking water supplies.

What Is Coastal Groundwater?

When people think about water, they usually picture lakes, rivers, or oceans. However, there is another important and often overlooked water source beneath coastal areas: coastal groundwater. This groundwater is stored in the spaces between sand and silt particles underground. For many coastal communities, this groundwater is a major source of drinking water.

Despite its importance, coastal groundwater is highly vulnerable to contamination. One of the biggest threats is saltwater intrusion. As Dr. Heiss explained, saltwater intrusion happens when saltwater from the ocean moves into freshwater aquifers underground. When this occurs, the groundwater becomes salty and can no longer be used safely for drinking or other purposes.

The Causes of Saltwater Intrusion

Saltwater intrusion can be caused by both natural processes and human activities. Dr. Heiss described three main factors that contribute to this problem. First, during droughts, there is less freshwater flowing from land toward the ocean. This allows saltwater to push farther inland underground. Second, sea level rise increases the pressure of ocean water, forcing saltwater deeper and farther into coastal aquifers. Finally, excessive pumping of freshwater from coastal wells can pull saltwater upward into those wells, contaminating the freshwater supply. As these pressures increase, the amount of usable freshwater decreases.

Tracking the Mixing Zone: Where Fresh and Salt Water Meet

A major focus of coastal groundwater research is the mixing zone, which is the area where freshwater from land meets salty seawater from the ocean. This boundary is not fixed — it can move inland or toward the sea depending on factors such as rainfall, groundwater pumping, and sea level changes.

Dr. Heiss explained that many coastal regions use monitoring well networks to track the location of this mixing zone. By measuring groundwater salinity over time, scientists can tell whether saltwater is moving farther inland. This information helps communities take action before drinking water supplies become contaminated.

Field Research: Studying the Impact of Storms

To better understand how natural events affect coastal groundwater, Dr. Heiss and his team conducted field research in Duck, North Carolina, in the Outer Banks. Their study focused on how coastal storms impact the mixing zone between fresh and salt water. With climate change increasing the frequency and intensity of storms, understanding these effects is becoming more important.

The team installed groundwater wells along the beach and across the barrier island to measure changes in groundwater salinity during storm events. These wells were made from PVC pipes with small slots that allow groundwater to enter. Even though installing and maintaining these wells in a hurricane-prone area is challenging, the data they collected is helping scientists better understand how storms influence coastal groundwater systems.

Storms vs. Tides: Two Different Influences

Dr. Heiss also explained that storms and tides affect groundwater in different ways. Tides are predictable and mainly affect the area close to the shoreline. In contrast, storm surges can push saltwater far inland, flooding roads, marshes, and other low-lying areas. This saltwater can seep straight down through the soil and reach the groundwater, causing direct contamination from above. Unlike tides, storm surge creates sudden and more intense changes to groundwater salinity.

Biogeochemical Processes in Coastal Groundwater

The mixing of freshwater and seawater also triggers important chemical and biological processes underground. One key process Dr. Heiss discussed is denitrification. In this process, bacteria convert nitrate in groundwater into nitrogen gas. This is beneficial because nitrate is a common pollutant that can contribute to harmful algal blooms when it reaches coastal waters. Denitrification helps remove this pollutant before it enters the ocean.

Preventing Saltwater Intrusion: Possible Solutions

Although saltwater intrusion is a serious problem, Dr. Heiss emphasized that there are ways to reduce its impact. One important strategy is managing groundwater pumping. By limiting pumping near the coast during high-demand periods, such as during the summer tourist season, communities can reduce the risk of pulling saltwater into freshwater wells.

Another possible solution is building subsurface barriers, sometimes described as underground dams, to block saltwater from moving inland. In addition, using permeable pavement in urban areas can allow more rainwater to soak into the ground. This helps recharge groundwater, keep the water table higher, and reduce the risk of saltwater intrusion.

Final Thoughts

Interviewing Dr. James Heiss was a valuable opportunity to learn about an important but often invisible part of coastal systems. Coastal groundwater plays a critical role in providing drinking water, yet it is highly vulnerable to saltwater contamination. As climate change continues to affect sea levels, storms, and drought patterns, protecting this resource will become even more important. Dr. Heiss's research highlights how much of what sustains coastal communities lies beneath the surface — and why careful monitoring and responsible management are essential for protecting our water supply.