Stormwater on the Urban Landscape
Stormwater is excess rain or snowmelt that floods the land surface. On an open undisturbed landscape most rainwater soaks into the ground and the overflow eventually discharges into the nearest watercourse. However, in an urban environment, most land is impervious, i.e., covered with roads, buildings and parking lots, thus the volume of rainwater that can’t soak into the ground is overwhelming, creating extensive flooding, erosion, stream pollution, and physical damage to the stream bed. This is a problem that municipalities must manage.
When rainwater or snowmelt falls on permeable surfaces such as forests, fields and lawns, it percolates through the soil where it is filtered and cleaned. This clean groundwater collects underground. Eventually it enters the creek through springs that flow when the water table is higher than the level of the creek bottom. This is called the creek’s base flow and it supports the aquatic habitat. However, on an urban landscape, where much of the surface is impermeable, flooding rainwater picks up urban debris such as trash, road salt, motor oil, pesticides, and more, and flows over heaps of unconsolidated material deposited by activities such as construction and demolition. This dirty load floods down to the stream, eroding open ground as it flows by. Urban storm sewers capture some of this fouled water and discharge it, untreated, into the stream. Urban stormwater can damage the watercourse in many ways.
Impact on Streams
Chemical effects on the water are one type of harm that the stream ecosystem suffers. Some urban stormwater does enter the soil, but when the load is more than the soil can filter, pollutants arriving by that route reach the groundwater and enter the stream’s base flow. Pollutants can change the chemical balance of the stream water and the pH and can introduce poisons, like pesticides. Elevation in stream water temperature that results when large volumes of stormwater that has flowed over hot streets and sidewalks enter the stream also impacts the stream ecosystem. Some members of the aquatic community, such as coldwater fishes and invertebrates, will be unable to tolerate these effects and will die, thus diminishing biodiversity.
Dramatic changes to the stream hydrology, or flow, also disturb the ecosystem. High flows erode streambanks and scour streambeds, changing the stream’s subsurface profile. The force of the flow kills some of the aquatic life or sends it downstream. The increased sediment load is harmful to fish spawning beds and to other aquatic life. The moving sediment creates turbidity that reduces light penetration into the water, affecting plant life and periphyton (such as algae), which are at the bottom of the food chain for the fish. Streams with frequent high flows have lower biodiversity than those where high flows are less frequent. The level and diversity of biota can recover where disturbance is infrequent (Konrad and Booth 2005, p.161).
Research across the United States and abroad has been conducted to determine the impact of urbanization on watersheds. Few generalizations can be made about the results because there is so much variation in the layout of the developed areas and in the geography and geology of the watercourses.
Spring Creek Watershed
The Spring Creek watershed is located on slowly soluble carbonate bedrock that is characterized by sinkholes and underground conduits that allow surface water to penetrate more quickly than other types of bedrock. Stormwater flows into sinkholes, which reduces surface flow, but can also deliver the flood water’s pollutants directly to the groundwater, impairing the base flow into the creeks. However, this characteristic of voids in the bedrock allows the watershed to support a somewhat greater area of impervious surface than is noted elsewhere.
Within the Spring Creek watershed the total land surface is approximately 14% impervious, but the town center, which includes downtown State College and a large portion of Penn State University, is approximately 50% impervious. Spring Creek is by far the most urbanized of the eight watersheds in Centre County. Most of the county has a population density of less than 200 people per square mile. However, townships in and around State College reach densities of 25,001 to 50,000 per square mile (Figure 1). Future development will further stress the hydrology of the area (http://www.ms4partners.org/about-the-watershed.html) as the ratio of stormwater to base flow increases.
Figure 1. Population Density in Watersheds, Centre County, Pennsylvania
(Centre County Planning Office, 2017).
Segments of Spring Creek and its tributary Slab Cabin Creek have been assessed by the Pennsylvania Department of Environmental Protection (DEP) as being impaired by agricultural runoff, but other stretches of these streams are impaired by urban runoff from storm sewers. The pollutant is identified as siltation though other harmful chemicals come with it, but haven’t been specifically identified on these streams. The portions impaired by urbanization on Spring Creek and Slab Cabin Run flow through State College Borough, College Township and a small part of Benner Township (Figure 2).
Figure 2. Source Impairments in Spring Creek Watershed as identified by the PA Department of Environmental Protection.
EPA Municipal Separate Storm Sewer System (MS4) program
The federal Environmental Protection Agency (EPA) has developed regulations for managing stormwater under the National Pollutant Discharge Elimination System (NPDES) permit program which addresses water pollution by regulating the many different sources (industrial, agricultural, etc.) that discharge pollutants. The program is administered by state governments, in Pennsylvania by the Department of Environmental Protection. The particular program that we are interested in here is the Municipal Separate Storm Sewer System (MS4) because it regulates stormwater from the municipalities that discharge into the Spring Creek watershed. To prevent harmful pollutants from being washed or dumped into this storm sewer system, operators must obtain an NPDES permit and develop a stormwater management program.
Several municipalities have partnered in the MS4 program that includes the Spring Creek watershed. They are State College Borough, College, Ferguson, Harris, and Patton Townships, as well as Penn State University. The program involves six minimum control measures. Two measures involve public education and public involvement, in effect instructing the public about what not to deposit in storm sewers. Two measures involve construction and post-construction runoff management, and two involve illicit discharge detection and elimination, which are methods of enforcing the first four. Plans to implement these measures must be documented and submitted to the EPA. Permits to release stormwater into streams must be renewed every five years.
As the program develops, it is becoming more complex, including setting standards for Total Maximum Daily Loads (TMDLs). A TMDL is the calculated maximum amount of a specific pollutant that a waterbody can receive and still meet its water quality standards. Where portions of the Spring Creek watershed are impaired by urbanization, the current designation of impairment is siltation. When the TMDLs are determined, levels of particular pollutants will be identified and where above the standard, they can be reduced.
The water in creeks in our watershed eventually enters the Chesapeake Bay. Another focus of the MS4 program in the upcoming cycle will be a Chesapeake Bay Pollution Reduction Plan (CBPRP) which each of the partners will submit. The goal of the CBPRP is to reduce the amount of suspended solids by 10%, total nitrogen by 3%, and total phosphorous by 5% over a five year period. To simplify calculations, DEP is allowing the assumption that the reduction goals for nitrogen and phosphorous are met if the sediment reduction is achieved.
In addition to the MS4 project, other groups are working on locating and mitigating impairment and protecting water quality in our watershed, particularly the County Conservation District, Clearwater Conservancy, Trout Unlimited, and Pennsylvania’s Centre County Senior Environmental Corps.
Spring Creek runs through the Borough of Bellefonte, population about 7,000. It is grappling with problems caused by stormwater. It is a hilly town, the county seat, and was thriving in the 1800’s when much of its sewer system was installed. Recent unusually heavy rainstorms have resulted in flooding that has required extensive reconstruction. For example, one spot where the sewers failed was on Lamb Street, which slopes down to Spring Creek and becomes a bridge over the water. The antique Lamb Street sewer was built of stone. It receives the water from the system of sewers from streets upslope. Ultimately all this stormflow is discharged into Spring Creek at the base of the Lamb Street bridge. Over time, however, the stone structure collapsed, blocking the flow of stormwater and causing the asphalt street to buckle. The aged infrastructure and the intense storms have necessitated reconstruction of the system, now completed, on Lamb Street and two other streets.
Nearby, next to a derelict plant that once crushed limestone, stand large piles of gravel and lime dust. In heavy rains, this material flows into Buffalo Run, a tributary of Spring Creek, introducing siltation and raising the pH. And uptown (in the summer of 2018), the high school is constructing a large athletic field. The field has been stripped of all vegetation. Around the field runs a silt sock, a long, soft tube about eight inches in diameter filled with mulch. The EPA, under the NPDES permit program, requires the use of a silt sock to filter the soil from the runoff during a rain. However, the silt sock doesn’t do the job. When it rains, a “brown river” pours down the adjacent road. The storm drains catch some of it. The rest creates flooding that interferes with traffic and damages buildings. It is the burden of the borough to figure out how to mitigate these situations and how to pay for them.
These examples highlight the challenge for municipalities to protect the streams and rivers that initially attracted people to come and build towns along them. These waters are now threatened by our continual transformation of the landscape through which they flow.
Konrad, C. P. and D. B. Booth. 2005. Hydrologic changes in urban streams and their ecological significance. in Brown, L.R., R.H. Gray, R.M. Hughes, M.R. Meador, editors. Effects of Urbanization on Stream Ecosystems. American Fisheries Symposium 47: 157-177.
Acknowledgements to Justin Kozak, Centre County Conservation District Water Specialist, and Don Holderman, Bellefonte Borough Assistant Manager, for their generous contributions of time and information to this paper.
Betsie Blumberg, before retirement, was a science reporter for the National Park Service.