New GIS mapping tool helps emphasize importance of groundwater recharge, helps landowners with decision making

The Scotia Barrens is an ideal location for groundwater recharge due to the heavy tree cover, sandy soil and limestone geology. Image provided by the Centre County Historical Society

Riverkeeper's note: This blog post was written by West Branch Regional Director Andrew Bechdel.

A hydrologist with the Susquehanna River Basin Commission, Graham Markowitz spends quite a bit of time discussing and measuring groundwater recharge - so much so, that helped develop a new tool to identify critical recharge areas.

Although a geologist by training, Markowitz said that hydrology and geology are interconnected.

“The amount of water that is stored underground or that makes its way into streams is governed by the geology of the landscape, the topography, the land uses," he said during an address at the Spring Creek Watershed's Second Annual State of the Watershed at Millbrook Marsh Nature Center in March, adding that “how much water you’re seeing in the streams is typically a result of the geology."

A new tool that identifies critical aquifer recharge areas
A native of Emporia, KS, Markowitz moved to central Pennsylvania to attend grad school at Shippensburg University. During his two-year grad program, Markowitz landed an internship with the Susquehanna River Basin Commission and has been there even since.

Markowicz’s work has culminated in the creation of a detailed GIS tool that “identifies optimal ground water recharge locations and critical aquifer recharge areas” within the Susquehanna River Basin. Markowicz hopes that this new tool can be used to help municipalities make well informed decisions that can increase the amount of water that infiltrates into the soil and is stored underground.

The emphasis on the mapping groundwater potential was initiated from an objective in the SRBC’s comprehensive plan update. At that time, there wasn’t a tool to illustrate areas that were critical for aquifer recharge. Since 2021, Markowicz and his colleague, Pierre MaCoy, began developing a GIS layer to identify the amount recharge potential in the Susquehanna River Basin.

With regards to identifying "critical aquifer recharge areas," Markowicz said that it’s about focusing on areas that have “increased development pressures or have water supply limitations."

According to the SRBC, these are the areas that should be prioritized and protected.

Check out the full interview with Susquehanna River Basin Commission hydrologist Graham Markowitz in the most recent episode of the Middle Susquehanna Riverkeeper Podcast:

What makes an area optimal for recharge?
Markowicz's model used multiple criteria including amount of impervious area (pavement vs. tree cover), soil type (percent clay vs. percent sand) and geology (limestone vs. shale and sandstone). They found that an ideal landscape for recharge includes natural tree and shrub cover, sandy soil and contains a karst geology.

The Karst landscape is abundant in limestone, providing many underground features where the water can be stored in aquifers. It is both abundant and a unique feature of the ridge and valley province in central Pennsylvania. 

According to Markowicz “at a Susquehanna River Basin scale, we typically look at the landscape infiltrating 20-30 percent of the precipitation that falls, but in the Spring Creek Watershed we’re seeing the portion of precipitation that infiltrates is about 50 percent."

Which makes Spring Creek one of the most productive aquifers in the state,

One of the most important recharge sites within this watershed is State Game Lands 176, otherwise known as the Scotia Barrens. Markowicz said that this location is ideal because it lacks impervious surfaces and is almost entirely forested. Additionally, the sandy soil permits rapid infiltration that gives way to the karst geology and aquifers that hold the groundwater. 

Why is groundwater infiltration important?
According to Markowicz, these underground resources are important for feeding cold water to streams during hot and dry periods.

“The more water you can get underground in the aquifers, the better you’ll be during a drought period," he said.

This is especially important during the summer when strong thunderstorms bring heavy rainfall that quickly infiltrates our streams. In urban environments, water falling onto hot pavement and other impervious surfaces can quickly discharge warm water and other pollutants into the stream system. The resulting warming of the water stresses fish and makes the habitat less suitable for coldwater species such as our native brook trout.      

By comparison, a stream with adequate groundwater inputs will remain cooler during the summer and filter out pollutants that would otherwise be discharged to the stream. Overall, it’s a boon to a vulnerable fish in mid-July!  

Speaking as a fly-fisherman, Markowicz emphasized the importance that groundwater plays in preserving habitat for the macroinvertebrate communities or “fish food” that live in the stream bed.

"By limiting stormwater runoff to streams, we can reduce the amount of sediment that clogs the spaces within the rocky substrate in which the aquatic insects (fish food) live and thrive," he said.

Creating a better future
When asked how municipalities and landowners can improve the landscape for groundwater recharge, he mentioned that landowners can take small scale actions such converting some of their lawn to a meadow.

On a larger scale, "it will require a collaborative effort amongst local engineers, planners and individuals in charge of the stormwater system," he said.

In addition, Markowicz suggested the SRBC’s new GIS tool can inform communities about recharge benefits and assist them in making decisions such as the purchasing of a conservation easement.

You can access the Susquehanna Atlas to view areas with high groundwater recharge potential in the Susquehanna River Basin. If you are  interested in learning more about the development of the GIS tool, you can read the 2023 report - Identifying Optimal Groundwater Recharge Locations and Critical Aquifer Recharge Areas Within the Susquehanna River Basin. If you would like access to more data, please access the Request for Information & Data page. 
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The Susquehanna River Basin Commission (SRBC) is an interstate agency that coordinates water resource management in the Susquehanna River Basin. To learn more, please visit www.srbc.gov.  ​