What is an ‘impaired’ waterway and why are there so many of them in Pennsylvania?

More than a third of Pennsylvania’s lakes and a fifth of its streams are impaired, meaning that pollution has (or should have, anyway) hindered their use for drinking water supplies, fishing, recreation or the aquatic life that call the waterways home.

Perhaps the best illustration of the scale and scope of the problem is a map of so-called “Category 5” streams assembled as part of the state Department of Environmental Protection’s latest water quality monitoring report. A dense, interconnected network of squiggly red lines criss-cross the state, marking streams with the most severe impairment requiring limits on certain contaminants.

These “Category 5” streams include large swaths of the Juniata and Susquehanna rivers and many creeks, such as the Swatara in Dauphin County and the Yellow Breeches in Cumberland County. And those are just the examples in central Pennsylvania. You can learn about your local waterways here.

Pollution issues on these waterways have been known about for years, if not decades, but the DEP’s latest report determined that stretches of the Juniata from Huntingdon County to its mouth in Duncannon and the Susquehanna from Duncannon south to to the Route 462 Bridge near Columbia in Lancaster County are also impaired due to high pH levels.

In layman’s terms, high pH in a waterway is a sign of high alkalinity, which can adversely impact the health of aquatic life in the river and has to be accounted for when the water is used as a source for drinking water. Steelton, for example, sources its drinking water from the Susquehanna.

• Read the ‘TAPPED OUT’ special investigation of drinking water.

So far, the DEP hasn’t determined the source of this latest impairment along the Juniata and Susquehanna.

That’s not so uncommon. Of the 37,000 miles of streams that are subject to one impairment advisory or another (there’s some overlap here because one stream may be subject to multiple advisories), no source has been determined for 12,770 miles.

Among the waterways where sources of impairment have been determined, the most common sources are agriculture and abandoned mines.

Manure and other byproducts of agriculture add significant amounts of nutrients into the state’s waterways, leading to algae blooms and other issues. Mining, meanwhile, is responsible for discharges of heavy metals and other pollutants, including many substances that are toxic to both humans and aquatic life. Other key sources include urban and road runoff and habitat loss due to development.

According to the DEP, the leading cause of stream impairment is siltation, the process of fine particles accumulating in the water due to erosion or runoff. It’s followed by pathogens, metals, high or low pH levels and excess nutrients.

The sources of impairment in lakes is a bit different because they are largely closed systems: there’s nowhere for pollution in a lake to flow to except to evaporate (which tends to concentrate contaminants in the remaining water) or be absorbed underground.

According to the DEP, the most common known source is atmospheric deposition. That’s a process by which precipitation (such as rain and snow) or airflow will transfer substances from the atmosphere to surface water.

The other sources of lake contamination are mostly familiar culprits, such as agriculture and urban runoff.

Mercury, a toxic heavy metal that can cause serious health effects to humans, is the most common source of impairment at Pennsylvania’s lakes. It accounts for 31,793 of the roughly 85,000 total acres subject to various impairment advisories.

Following mercury are high or low pH levels, excessive nutrients and pathogens.

There is some good news contained in the latest DEP report, in the form of smallmouth bass. In 2014, the state issued an impairment advisory for the species to limit consumption after nearly a decade of diseases and fish kills that severely decreased fish populations in the Susquehanna. Ecologists believed that was due to excess phosphates, likely due to fertilizers running off into the river.

In 2005, 67 percent of all young-of-year smallmouth bass collected and surveyed from the Susquehanna between Sunbury and York Haven showed visible sign of disease. Last year, that figure had decreased to less than 10 percent.

This doesn’t necessarily mean the species is safe — monitors have observed spikes as recently as 2011 and 2014 — but the DEP and the Pennsylvania Fish and Boat Commission reported near-record smallmouth bass populations along the affected waterways.

The report does have its limitations.

First, the DEP focuses on impairment with these reports. Determining the actual source of impairment tends to be more difficult, as evidenced by the number of waterways where a source remains unknown.

Similarly, it’s simply not feasible for the DEP with its ongoing budget constraints to sample every single mile of waterway or test for every possible contaminant — including emerging ones like PFAS, which you may have read about recently — that could impact drinking water supplies and recreational waterways.

With those caveats aside, the DEP has made a wealth of data available about the state’s waterways.

For the first time, the report was presented in an interactive format that makes it more digestible for the average reader. You can also check out the condition of your local waterways via an interactive map.

Find something interesting? Let us know about it in the comments.

For more information about water contamination, read PennLive’s Tapped Out, an investigation of drinking water oversight in Pennsylvania.

Wallace McKelvey may be reached at wmckelvey@pennlive.com. Follow him on Twitter @wjmckelvey. Find PennLive on Facebook.