USGS Publications Summarize Water-Quality Trends and Drivers in Urban Streams After 10 Years of Monitoring in Fairfax County, Virginia Active
Issue: Degraded water quality and ecology in urban streams has been widely documented, but explanations of changing conditions over time are often unavailable. A 15-year collaborative urban stream monitoring effort between the Fairfax County Stormwater Planning Division and the U.S. Geological Survey (USGS) is ongoing and has begun to shed light on this complex issue. In a new USGS report by Webber and others (2023), monitoring data collected from 2007 through 2018 were used to identify drivers of changing water quality and ecology in Fairfax County streams. This report characterized the use and evaluated the effects of stormwater management practices in Fairfax County. The report by Webber and others (2023) builds on a USGS report by Porter and others (2020) that described patterns and trends in Fairfax County stream conditions after ten years of monitoring. Findings from these reports provide insights for managing urban streams and will continue to be evaluated in future years through a continued Fairfax County-USGS partnership.
USGS Monitoring Program
In 2007, Fairfax County and the USGS established a long-term monitoring program to assess county stream conditions and the effectiveness of watershed management activities. This program is based on a stream-monitoring network that includes measures of water quality, water quantity, and ecology. The network monitors 20 watersheds (ranging from 0.6 to 5.5 square miles) that represent typical landscape conditions throughout the county (fig. 1). Impervious cover in these watersheds ranges from 7 to 50 percent, representing a mixture of residential and commercial land uses. Over the last decade, Fairfax County and the USGS have monitored these watersheds to evaluate how various water-quality and ecological metrics differ across the county and have changed over time. These metrics include streamflow, total nitrogen (TN), total phosphorus (TP), suspended sediment (SS), specific conductance (SC), and benthic-macroinvertebrate index of biotic integrity (IBI) scores. These metrics are measures of ecosystem health and are the focus of many ongoing management efforts in Fairfax County and throughout the Chesapeake Bay watershed.
A report by Porter and others (2020) provides a thorough characterization of the water quantity, water quality, and ecology of Fairfax County streams, including an assessment of trends in 14 watersheds that were monitored for 10 years. This report documents patterns and trends in nutrient and sediment loads, which are important measures of watershed condition and response. More recently, Webber and others (2023) evaluated drivers of water-quality and ecological responses in Fairfax County streams. Human activities, landscape conditions, and climatic factors are among the drivers that can affect the water quality and ecology of urban streams (fig. 2). These and other potential factors were evaluated, including the use of stormwater management practices designed to reduce nutrient and sediment loads. Statistical models were used to evaluate relations between these factors and monitored stream responses. These models, previous studies of urban streams, and local knowledge about Fairfax County water resources provided management-relevant insights about changing stream conditions.
Major Findings
The water quality and ecology of Fairfax County streams changed over time. Water-quality trends were assessed at 14 monitoring stations from March 2008 through April 2018 based on monthly samples. Over this time:
- TN trends were identified at 6 stations. TN concentrations decreased at 5 stations and increased at 1 station.
- TP trends were identified at 5 stations. TP concentrations decreased at 1 station and increased at 5 stations.
- SS trends were identified at 3 stations. SS concentrations decreased at 2 stations and increased at 1 station.
- SC trends were identified at 10 stations. SC increased at all of these stations.
Changes in ecology were assessed at 14 monitoring stations from 2008 through 2017 using benthic-macroinvertebrate samples collected every spring. Twenty ecological metrics were analyzed, including IBI, which is a composite measure of individual ecological metrics. On average, IBI scores increased throughout the 14-station network with increasing trends identified at 4 individual stations. These patterns represent an improvement in the ecological condition of Fairfax County streams.
The amount of management practices increased in Fairfax County over time. From 2009 through 2018, Fairfax County invested $87 million dollars in 245 practices that were expected to reduce nutrient and/or sediment loads. 59 of these practices were installed in the study watersheds. These practices include stormwater retrofits and stream restorations. Stormwater retrofits are designed to capture stormwater runoff in temporary storage areas and reduce loads through physical or chemical processes.
Stream restorations are designed to reduce loads by stabilizing stream channels and reconnecting streams to their floodplains. Based on credits assigned by the Chesapeake Bay Program, these practices were expected to reduce nutrient and sediment loads by thousands of pounds. The increasing use of management practices occurred alongside dynamic landscape and climatic conditions in Fairfax County. The county’s human population increased during the study, requiring the construction of new homes, roads, and stormwater and wastewater infrastructure. These changes, along with year-to-year differences in rainfall and temperature can affect the water quality and ecology of Fairfax County streams.
Expected management-practice effects were not consistently observed in monitored stream responses. Several lines of evidence led to this conclusion:
- Changes in monitored nutrient and sediment loads did not clearly align with expected management-practice load reductions. Monitored loads were adjusted to account for differences in annual streamflow, but most loads did not decrease over time.
- Changes in storm-event hydrology were not consistent with expected management-practice effects. Many practices were designed to reduce peak streamflow and slow the delivery of runoff to streams but changes in these hydrologic measures were not consistently observed.
- Expected management-practice effects were not related to monitored changes in water-quality or ecology. Management-practice effects were evaluated after accounting for effects of many landscape and climatic conditions.
Landscape and climatic conditions explained some of the water-quality and ecological responses in Fairfax County streams (fig. 3). In addition to potential effects of management practices on the water quality and ecology of Fairfax County streams, this study considered potential effects of nearly 100 landscape and climatic factors. Statistical models were used to identify factors that had the greatest effect on monitored responses. These influential landscape and climatic factors are described below.
- TN concentrations were higher and increases over time were larger in watersheds with elevated septic-system density.
- TP concentrations were higher in watersheds with more turfgrass; concentrations were lower, but had larger increases over time, in watersheds with deeper soils.
- SS concentrations were higher in watersheds with greater stream channel densities.
- SC was higher in watersheds with more developed land and shallower soils.
- Benthic-macroinvertebrate IBI scores were lower in watersheds with high road density and had larger increases over time in bigger, more developed watersheds.
- Climatic conditions affected the water quality and ecology of Fairfax County streams. Annual variability in TN and TP concentrations and benthic-macroinvertebrate IBI scores was affected by precipitation. Annual variability in SS concentrations and specific conductance was affected by air temperature.
Management Implications
Management-practice effects were not directly identified in monitored stream responses, but this study provided insights about the future evaluation and use of management-practices. Many factors may have affected an ability to identify management-practice effects. These factors were carefully considered in this study and may become less influential in future years. Potential factors include:
- The amount, timing, and location of management practices. Practices were completed in only half of the study watersheds, typically towards the end of the monitoring period. Management-practice effects may be more readily identified in watersheds with greater practice amounts, larger practice treatment areas, and longer amounts of post-restoration monitoring.
- Unmeasured landscape and climatic factors. This study considered the effects of many landscape and climatic factors, including pressures from ongoing urban growth. However, an incomplete or inaccurate representation of how factors like precipitation and land use have changed over time may obscure potential management-practice effects.
- Uncertain management-practice effects. This study considered credited management-practice load reductions, but there is a large amount of uncertainty in expected management-practice effects. Management-practice load reductions likely follow non-linear trajectories over time and may not occur immediately upon practice completion. These complexities were underrepresented in this study.
Overall, this study highlighted that controlling for variability caused by geology, human activities, precipitation, and/or air temperature is an important consideration for future evaluations of management-practice effects in urban streams.
For More Information
Porter, A.J., Webber, J.S., Witt, J.W., and Jastram, J.D., 2020, Spatial and temporal patterns in streamflow, water chemistry, and aquatic macroinvertebrates of selected streams in Fairfax County, Virginia, 2007–18: U.S. Geological Survey Scientific Investigations Report 2020-5061, 106 p. pubs.er.usgs.gov/publication/sir20205061.
Webber, J.S., Chanat, J.G., Porter, A.J., and Jastram, J.D., 2023, Evaluating drivers of hydrology, water quality, and benthic macroinvertebrates in streams of Fairfax County, Virginia, 2007–18: Scientific Investigations Report 2023–5027, 198 p. pubs.er.usgs.gov/publication/sir20235027.
Issue: Degraded water quality and ecology in urban streams has been widely documented, but explanations of changing conditions over time are often unavailable. A 15-year collaborative urban stream monitoring effort between the Fairfax County Stormwater Planning Division and the U.S. Geological Survey (USGS) is ongoing and has begun to shed light on this complex issue. In a new USGS report by Webber and others (2023), monitoring data collected from 2007 through 2018 were used to identify drivers of changing water quality and ecology in Fairfax County streams. This report characterized the use and evaluated the effects of stormwater management practices in Fairfax County. The report by Webber and others (2023) builds on a USGS report by Porter and others (2020) that described patterns and trends in Fairfax County stream conditions after ten years of monitoring. Findings from these reports provide insights for managing urban streams and will continue to be evaluated in future years through a continued Fairfax County-USGS partnership.
USGS Monitoring Program
In 2007, Fairfax County and the USGS established a long-term monitoring program to assess county stream conditions and the effectiveness of watershed management activities. This program is based on a stream-monitoring network that includes measures of water quality, water quantity, and ecology. The network monitors 20 watersheds (ranging from 0.6 to 5.5 square miles) that represent typical landscape conditions throughout the county (fig. 1). Impervious cover in these watersheds ranges from 7 to 50 percent, representing a mixture of residential and commercial land uses. Over the last decade, Fairfax County and the USGS have monitored these watersheds to evaluate how various water-quality and ecological metrics differ across the county and have changed over time. These metrics include streamflow, total nitrogen (TN), total phosphorus (TP), suspended sediment (SS), specific conductance (SC), and benthic-macroinvertebrate index of biotic integrity (IBI) scores. These metrics are measures of ecosystem health and are the focus of many ongoing management efforts in Fairfax County and throughout the Chesapeake Bay watershed.
A report by Porter and others (2020) provides a thorough characterization of the water quantity, water quality, and ecology of Fairfax County streams, including an assessment of trends in 14 watersheds that were monitored for 10 years. This report documents patterns and trends in nutrient and sediment loads, which are important measures of watershed condition and response. More recently, Webber and others (2023) evaluated drivers of water-quality and ecological responses in Fairfax County streams. Human activities, landscape conditions, and climatic factors are among the drivers that can affect the water quality and ecology of urban streams (fig. 2). These and other potential factors were evaluated, including the use of stormwater management practices designed to reduce nutrient and sediment loads. Statistical models were used to evaluate relations between these factors and monitored stream responses. These models, previous studies of urban streams, and local knowledge about Fairfax County water resources provided management-relevant insights about changing stream conditions.
Major Findings
The water quality and ecology of Fairfax County streams changed over time. Water-quality trends were assessed at 14 monitoring stations from March 2008 through April 2018 based on monthly samples. Over this time:
- TN trends were identified at 6 stations. TN concentrations decreased at 5 stations and increased at 1 station.
- TP trends were identified at 5 stations. TP concentrations decreased at 1 station and increased at 5 stations.
- SS trends were identified at 3 stations. SS concentrations decreased at 2 stations and increased at 1 station.
- SC trends were identified at 10 stations. SC increased at all of these stations.
Changes in ecology were assessed at 14 monitoring stations from 2008 through 2017 using benthic-macroinvertebrate samples collected every spring. Twenty ecological metrics were analyzed, including IBI, which is a composite measure of individual ecological metrics. On average, IBI scores increased throughout the 14-station network with increasing trends identified at 4 individual stations. These patterns represent an improvement in the ecological condition of Fairfax County streams.
The amount of management practices increased in Fairfax County over time. From 2009 through 2018, Fairfax County invested $87 million dollars in 245 practices that were expected to reduce nutrient and/or sediment loads. 59 of these practices were installed in the study watersheds. These practices include stormwater retrofits and stream restorations. Stormwater retrofits are designed to capture stormwater runoff in temporary storage areas and reduce loads through physical or chemical processes.
Stream restorations are designed to reduce loads by stabilizing stream channels and reconnecting streams to their floodplains. Based on credits assigned by the Chesapeake Bay Program, these practices were expected to reduce nutrient and sediment loads by thousands of pounds. The increasing use of management practices occurred alongside dynamic landscape and climatic conditions in Fairfax County. The county’s human population increased during the study, requiring the construction of new homes, roads, and stormwater and wastewater infrastructure. These changes, along with year-to-year differences in rainfall and temperature can affect the water quality and ecology of Fairfax County streams.
Expected management-practice effects were not consistently observed in monitored stream responses. Several lines of evidence led to this conclusion:
- Changes in monitored nutrient and sediment loads did not clearly align with expected management-practice load reductions. Monitored loads were adjusted to account for differences in annual streamflow, but most loads did not decrease over time.
- Changes in storm-event hydrology were not consistent with expected management-practice effects. Many practices were designed to reduce peak streamflow and slow the delivery of runoff to streams but changes in these hydrologic measures were not consistently observed.
- Expected management-practice effects were not related to monitored changes in water-quality or ecology. Management-practice effects were evaluated after accounting for effects of many landscape and climatic conditions.
Landscape and climatic conditions explained some of the water-quality and ecological responses in Fairfax County streams (fig. 3). In addition to potential effects of management practices on the water quality and ecology of Fairfax County streams, this study considered potential effects of nearly 100 landscape and climatic factors. Statistical models were used to identify factors that had the greatest effect on monitored responses. These influential landscape and climatic factors are described below.
- TN concentrations were higher and increases over time were larger in watersheds with elevated septic-system density.
- TP concentrations were higher in watersheds with more turfgrass; concentrations were lower, but had larger increases over time, in watersheds with deeper soils.
- SS concentrations were higher in watersheds with greater stream channel densities.
- SC was higher in watersheds with more developed land and shallower soils.
- Benthic-macroinvertebrate IBI scores were lower in watersheds with high road density and had larger increases over time in bigger, more developed watersheds.
- Climatic conditions affected the water quality and ecology of Fairfax County streams. Annual variability in TN and TP concentrations and benthic-macroinvertebrate IBI scores was affected by precipitation. Annual variability in SS concentrations and specific conductance was affected by air temperature.
Management Implications
Management-practice effects were not directly identified in monitored stream responses, but this study provided insights about the future evaluation and use of management-practices. Many factors may have affected an ability to identify management-practice effects. These factors were carefully considered in this study and may become less influential in future years. Potential factors include:
- The amount, timing, and location of management practices. Practices were completed in only half of the study watersheds, typically towards the end of the monitoring period. Management-practice effects may be more readily identified in watersheds with greater practice amounts, larger practice treatment areas, and longer amounts of post-restoration monitoring.
- Unmeasured landscape and climatic factors. This study considered the effects of many landscape and climatic factors, including pressures from ongoing urban growth. However, an incomplete or inaccurate representation of how factors like precipitation and land use have changed over time may obscure potential management-practice effects.
- Uncertain management-practice effects. This study considered credited management-practice load reductions, but there is a large amount of uncertainty in expected management-practice effects. Management-practice load reductions likely follow non-linear trajectories over time and may not occur immediately upon practice completion. These complexities were underrepresented in this study.
Overall, this study highlighted that controlling for variability caused by geology, human activities, precipitation, and/or air temperature is an important consideration for future evaluations of management-practice effects in urban streams.
For More Information
Porter, A.J., Webber, J.S., Witt, J.W., and Jastram, J.D., 2020, Spatial and temporal patterns in streamflow, water chemistry, and aquatic macroinvertebrates of selected streams in Fairfax County, Virginia, 2007–18: U.S. Geological Survey Scientific Investigations Report 2020-5061, 106 p. pubs.er.usgs.gov/publication/sir20205061.
Webber, J.S., Chanat, J.G., Porter, A.J., and Jastram, J.D., 2023, Evaluating drivers of hydrology, water quality, and benthic macroinvertebrates in streams of Fairfax County, Virginia, 2007–18: Scientific Investigations Report 2023–5027, 198 p. pubs.er.usgs.gov/publication/sir20235027.