Frontiers

Wear and Tear

Senior Shirley Leung documents the consequences of soil erosion.
October 2011

We often attribute water pollution to trash or gasoline, waste that is irresponsibly discarded into natural habitats. Indeed this is a serious issue, but ironically enough, some of the decline in water quality can be pinned on nature itself—but that doesn’t mean humans get a pass. Soil erosion, primarily caused by urbanization, poor farming practices and a possible increase in the intensity of storm events and flooding due to climate change, is one of the main underlying offenders. Shirley Leung, a senior in environmental studies, is trying to get to the bottom of it.

“After listening to a lecture by Jane Willenbring, assistant professor in the Department of Earth and Environmental Science, I was determined to investigate erosion and the effect it was having on our ecosystem,” says Leung. “I was suspicious of the recognized methods of measuring for sedimentation and erosion rates, so I devised a new means of calculation.”

“When heavy rainfall occurs naturally in a forest or field, the water is dispersed evenly, and soaked up by plant life. But when even relatively small amounts of rainfall hit the city, the water begins a frantic rush to the nearest drain and eventually, the nearest stream.” – Shirley Leung

Leung shared Professor Willenbring’s dismay regarding the fact that over the past 30 years scientists have cited a continual, exponential climb in erosion rates, a conclusion that seemed too simplified to cover all bases. Instead of using carbon dating to chart soil dispersion, Leung proposed using the isotope Beryllium-10 to chart the movement of sediment. Because top soil is almost always rich in Beryllium, Leung theorized that it would be present in high doses throughout nearby bodies of water and wherever eroded sediment was ultimately being deposited. This would provide a framework through which erosion could be tracked, a process documented in their co-authored paper, “Using Meteoric Beryllium-10 to Understand Anthropogenic Influences on Soil Erosion and Deposition Rates.”

Unnaturally high rates of soil erosion can be catastrophic to the ecosystem. The Dust Bowl, which took place across American and Canadian prairie lands in the 1930s, is a prime example of this. When farmers scorched the ground in order to control weeds, it destroyed the greenery responsible for keeping the soil healthy and in place. This type of soil erosion is not only bad for farmers and our food supply, but can also severely harm fish and other aquatic life as the eroded soil deposits in downstream rivers and estuaries and chokes these ecosystems.

In order to conduct her studies, Leung needed control sites, regions where erosion rates were known to be low or non-existent. She researched uninhabited sites in New Zealand and South America, but eventually had to rule them out due to exorbitant travel expenses. She settled instead on Penn Oaks in Maryland, historical sites boasting William Penn’s namesake—it is rumored he used to visit these grounds and sit under the trees. Leung removed sample cores from these grounds, and from nearby streams and farms, so she could later compare the Beryllium-10 concentrations across these samples to work out where erosion seemed to be occurring most quickly.

Leung decided to focus much of her attention on the Chesapeake Bay—a dumping ground for runoff from multiple states. One of the main culprits when it comes to erosion in locations like the Bay is the design of the sewer systems. Though Philadelphia’s sewage does not directly intersect with the Bay, its infrastructure has similar issues. “Most of Philadelphia works on a combined sewer system. This means both sewage and rain water travel to the same wastewater treatment plants. But when water volume reaches its peak, the sewer begins diverting this combination of sewage and rainwater into local rivers and streams, posing a high risk for erosion.”

The underlying issue as it pertains to erosion is that the volume of water required for the system to start diverting is too high for its outlets to handle. Leung says urbanization is likely the main cause. When heavy rainfall occurs naturally in a forest or field, the water is dispersed evenly, and soaked up by plant life. But when even relatively small amounts of rainfall hit the city, the water begins a frantic rush to the nearest drain and eventually, the nearest stream. It is this type of “flashy stream” that overloads the system, eroding stream banks and nearby soil, a phenomenon Leung was able to document by tracing high amounts of Beryllium-10 in the water.

In order to prevent future erosion and sediment overload, Leung says new policies need to be enacted. Philadelphia is at the forefront of these initiatives. The Philadelphia Water Department is investing over a billion dollars into its Green City, Clean Waters campaign, which is focused on regulating new buildings and requiring they have a means to collect at least the first inch of rainfall during a storm, so that it can be distributed over time. This includes the use of cisterns—a method the new Penn Park utilizes—and green roofs, which capture and store rainfall in plants atop buildings.

In the future, Leung hopes to pursue a master’s in applied geosciences and eventually help implement clean water projects in developing countries. “My ultimate goal is to use my research to affect policy. It would be great to present the results and know that it helped to protect against the future consequences of widespread soil erosion.”