What Is “Urban Karst” and How Might It Affect Stormwater in Your Community?
“Karst” is defined as “landscape underlain by limestone that has been eroded, producing ridges, towers, fissures, sinkholes, and other characteristic landforms.” Lancaster County and other communities in south-central Pennsylvania experience karst geology and the issues that can come with it. But what is meant by the term “urban karst” and why should you care about it? That is what we will explore in this article.
Stormwater in the City
Our urban areas are covered by a lot of impervious surfaces (e.g. roofs, roads, and pavement). This ground cover prevents rain from infiltrating into the ground, resulting in increased stormwater runoff. To manage that runoff, communities use a variety of stormwater best management practices (e.g. rain gardens, swales, retention ponds, etc.) that allow rain to slowly infiltrate into the ground. However, very little is known about what happens to that water under the ground, especially in urban areas where it may come in contact with sanitary sewer pipes, underground utilities lines, and other man-made structures. The interaction between groundwater flow and those structures is what is known as “urban karst”.
Urban Karst and its Impact on Groundwater
Researchers Jeremie Bonneau, Tim D. Fletcher, Justin F. Costelloe, and Matthew J. Burns, from the School of Ecosystem and Forest Sciences and the Department of Infrastructure Engineering, at the University of Melbourne, Australia, published a study of urban karst and its effect on stormwater infiltration in The Journal of Hydrology (Vol. 55 (2017) 141-150). They found that some of the infiltrated stormwater could travel along the artificial pathways created from the urban karst (through the man-made trenches and rock-filled spaces with many pores) and potentially infiltrate into some of the pipes, thus affecting groundwater.
In a more rural setting, rain typically takes a long time to seep through the soil before reaching groundwater. This time allows contaminants to better settle out before it can pollute the groundwater. However, in urban areas with the larger pore spaces and unnatural conditions created by the urban karst, as well as the faster stormwater runoff flows created by all the impervious surfaces, allows the rain water to infiltrate faster, increasing the chances of pollutants reaching the groundwater. In fact, their research showed that in urban areas infiltration can take place in under two weeks, as opposed to the multiple years it may take in undeveloped areas.
What Does This Mean for Urban Communities?
The researchers concluded that more information and case studies are needed to be able to confidently decide whether infiltrated stormwater contributes to groundwater flow and pollution. They do note that the benefits of using stormwater BMPs are well documented – so this is not to say that communities should discontinue their use. Landstudies has built over 30 floodplain restoration projects in karst geology over the years. Our track record for water quality and quantity has been monitored and documented with excellent results. If an urban community’s main desire is to replenish their groundwater via stormwater runoff infiltration, they should still remain aware of the potential shortcomings of the technology, but don’t let it discourage you from moving forward with your project.