2025 Water Quality Monitoring in Prince William Forest Park

Looking for macroinvertebrates near Quantico Creek

NPS Photo

Water Quality Monitoring Program

Water can provide habitat, quench thirst, create power, offer a medium for recreation, and is essential for life to flourish. Prince William Forest Park (PRWI), a unit of the National Park System, is home to the headwaters of Quantico Creek, which is a large tributary of the Potomac River.

The Quantico Creek Watershed is approximately 30 square miles and is located in the southeastern part of the County. Much of the watershed includes Prince William Forest Park and Marine Corps Base Quantico. PRWI plays a crucial role in the monitoring and protection of Quantico Creek’s water quality. Monitoring the quality of water in the headwaters of the park helps us to have a better understanding of what changes and processes affect its quality downstream. In other words, water quality is reflective of activities taking place upstream of surface waters.

The park’s Natural Resource Management branch is responsible for analyzing water samples to measure key Water Quality Indicators (WQI) – ensuring public safety by both testing lakes and streams for Escherichia coli (E. coli) and sampling for aquatic macroinvertebrates weekly. The water quality monitoring program has three main components, 1) assessment of benthic macroinvertebrate populations, 2) testing for presence of fecal coliforms, and 3) analysis of water chemistry data. Together, these components make it possible to quantify the park’s overall stream health.

Stream Health & Aquatic Macroinvertebrate Communities

Aquatic macroinvertebrates provide insight into stream health as some species are more sensitive to poor environmental conditions than others. The presence of more sensitive species, such as mayflies, stoneflies, and caddisflies, indicate good stream quality as they are more sensitive to higher temperatures and pollution levels. Alternatively, an overabundance of organisms that are tolerant of poor conditions, such as certain types of dragonflies, worms, and non-biting midges, may be a sign of habitat degradation.

Benthic macro invertebrate (stream-bottom insect) populations are highly vulnerable to deteriorating stream conditions, so their populations make good indicators of water quality.

Prince William Forest Park uses a protocol called the Izaak Walton League’s Save Our Streams Protocol (SOS) in which macro invertebrates are collected, identified, counted, and then returned to their collection site. A rating system, based on the number of different insect species and their tolerance to environmental conditions, determines water quality.

Trends in the overall health of the park’s streams are positive as shown in the chart below; compared with previous years, average stream health index values have remained stable or improved at most of the 32 macroinvertebrate sampling sites throughout the park. Two sites located along the park boundary adjacent to a housing development regularly score lower quality than the rest of the park.

The bar graph shows sites adjacent to the development highlighted. Tests above the Purple line show acceptable ecological conditions. Tests below the orange line show unacceptable ecological conditions. Tests between lines are a gray zone, and conditions cannot be determined at this time.

NPS Graph

Stream Health & Water Quality Testing

The park’s Natural Resource Management branch conducts regular water quality monitoring by analyzing water samples collected from various locations within the park. This includes testing for indicators such as coliform bacteria, which are essential for assessing public safety and ecological health.

Fecal coliforms, such as E. coli, are the bacteria found in the intestines of warm-blooded animals. To protect public safety, the Virginia Water Control Board has specified its minimum standard for surface water as a maximum of 235 colonies of coliforms per 100 milliliters of water. Prince William Forest Park tests its six lakes for coliforms once each week. It also collects data at an additional six stream sites biweekly. If a sample taken from a lake has an excessive number of coliform colonies, the lake is closed to visitor use until subsequent test results meet the standard. Fortunately, the occurrence of excess coliform levels is rare, and it usually follows heavy storm events with high volumes of surface runoff.

Below, you will find Figures A and B, which provide some insight into season 2025 water quality trends and E. coli results.

Figure A. Prince William Forest Park E. coli samples and results for lakes from June 2025-November 2025

NPS Graph

Our lakes and streams in and around the park are used recreationally throughout the year. To ensure the safety of our visitors, we make sure to test our E. coli (Escherichia coli) levels weekly throughout the summer and into early fall. If our test results come back at an unsafe level, which is above 235mpn (most probable number), then the location is retested. If the retest shows levels are still too high, the location will be closed to visitors until levels return to normal. In this case, we need to further investigate what is causing these spikes in our E. coli levels, to see if there is a possibility that the camps in use are having some sort of effect. Once the cause is determined and addressed, then the area can be reopened for recreation.

For the summer of 2025, most test levels were well under the unsafe threshold. We did have to retest some locations when levels spiked, but they always came back within normal limits. Sometimes we notice these spikes happening in the lakes and streams after heavy rains, which is why we go back out and retest, to be sure the levels shown are not the norm for that body of water. Our lakes have the highest tendency to spike, as you can see from the graphs above, but they drop quickly back to ‘normal’ levels. From late June to early July, Lakes 1, 2 and 4 had a short spike in their E. coli levels which quickly returned to normal. Another small jump happens for Lake 1 in late July but not to an unsafe level. Our final high spike for the lakes happens in September with Carters Pond and Lake 1, which returned to a safe level by the delayed next sampling in November.

Figure B. Prince William Forest Park E. coli samples and results for streams from June 2025-November 2025

NPS Graph

Our streams tend to stay much more consistent in their E. coli levels. We can see from the graph above that they all start rather high in early July and by mid-month are back below the 235mpn mark. We do see a small spike happen in mid-September, but again our levels slowly return to normal.

Data collected by Michelle Gutierrez, Alyssa Cavallaro and Jeremy Strube, 2025 PRWI Natural Resource Management Interns

Tags: prince william forest park, water, water quality, water quality data, water quality monitoring, quantico creek, stream health, macroinvertebrates, fecal coliform, watershed, stream monitoring