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Fairfax County Water Resources Monitoring Network


Study Overview







Spatial and Temporal Patterns in Streamflow, Water Chemistry, and Aquatic Macroinvertebrates of Selected Streams in Fairfax County, Virginia, 2007-18

By Aaron J. Porter, James S. Webber, Jonathan W. Witt, and John D. Jastram

Download the full PDF of this report from the USGS Publications Warehouse
View a presentation of findings included in this report

Urbanization substantially alters the landscape in ways that can impact stream hydrology, water chemistry, and the health of aquatic communities. Stormwater best management practices (BMPs) are the primary tools used to mitigate the effects of urban stressors such as increased runoff, decreased baseflow, and increased nutrient and sediment transport. To date, Fairfax County Virginia’s stormwater management program has made substantial investments into the implementation of both structural and nonstructural BMPs aimed at restoring and protecting watersheds. The U.S. Geological Survey (USGS), in cooperation with Fairfax County, Virginia, established a long-term water-resources monitoring program to evaluate the watershed-scale effects of these investments. Monitoring began at 14 stations in 2007 and was expanded to 20 stations in 2013. This report utilized the first 10 years of data collection to (1) assess water quantity and quality, as well as ecological condition; (2) compute annual nutrient and sediment loads; and (3) evaluate trends in streamflow, water quality, and ecological condition. Efforts are underway to link the biotic and abiotic patterns described herein to watershed management practices as well as factors such as land use change, public works infrastructure, and climate.

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Hydrologic, chemical, and benthic macroinvertebrate community conditions in the streams monitored were similar to those observed in other studies of urban streams. Multidecadal trends in baseflow indices and runoff ratios at long-term Chesapeake Bay Non-tidal Network streamgages (CB-NTN) indicate a decrease in groundwater recharge and increase in storm runoff as a result of urbanization. Streamflow yields varied spatially with land cover, geology, and soil characteristics, whereas flashiness was positively related to impervious area. Dissolved oxygen typically was lowest in the Coastal Plain and across all Triassic Lowlands streams, and highest in the Piedmont. Dissolved oxygen concentrations generally were above Virginia’s minimum criterion of 4.0 milligrams per liter (mg/L), most violations occurred at Paul Spring Branch in the Coastal Plain during the warmest months of the year owing to increased chemical and biological oxygen demand. Typical pH values of the monitored streams centered on neutrality (pH = 7); however, diurnal fluctuations were most prevalent in the continuous pH data at Flatlick Branch (FLAT; a Triassic Lowlands station), as a result of increased photosynthesis catalyzed by phosphorus-rich geology. Specific conductance (SC) varied spatially owing to geology (highest at Triassic Lowlands stations) and anthropogenic disturbance (watersheds with high impervious land cover). Specific conductance typically was inversely related to streamflow except in winter months following deicing road salt applications, when values increased by several orders of magnitude. A significant increase in SC of about 2 percent per year was observed from the combined trend result of all monitoring stations over the 10-year period. Significant SC increases occurred at nearly all monitoring stations. Increasing trends were observed during winter and nonwinter months, which suggests that salts applied to deice roadways and other impervious surfaces are stored in the environment and released year-round.

Suspended-sediment (SS) concentrations in monthly samples did not vary significantly between most stations, but typically were highest in the spring and lowest in the fall as a result of seasonal differences in streamflow and climate. Suspended-sediment yields ranged from 62 to 1,428 tons per square mile (ton/mi2), with a median of 302 ton/mi2. Annual loads were greatest during the wettest water years (October 1-September 30; 2008, 2011, and 2014), with the greatest interannual variability occurring at Difficult Run above Fox Lake (DIFF) and South Fork Little Difficult Run (SFLIL). Suspended sediment was primarily composed of silts and clays; however, the proportion of sand in suspended sediment was related positively to streamflow. Cross-correlation analyses suggested the dominant sources of SS were streambank erosion and resuspension of in-channel material at DIFF and FLAT; whereas, upland sources and erosion of upper streambanks were more common at Dead Run (DEAD), Long Branch (LONG), and SFLIL.

Median total phosphorus (TP) concentrations ranged from 0.016 to 0.077 mg/L, with a networkwide median of 0.022 mg/L, were highest in the warm season (April-September), and were composed primarily of dissolved phosphorous. Although TP concentrations were relatively low across the network, the highest concentrations were consistently at stations located in the Triassic Lowlands, owing to phosphorous-rich geology, and in the Coastal Plain, owing to the low-phosphorous sorptive capacity of those soils. A significant increase in TP concentration occurred in a few stations, but the combined trend results from all stations demonstrated a significant increase of about 4 percent per year. Networkwide increases were also observed in total dissolved phosphorus, orthophosphate, and total particulate phosphorus. The composition of TP shifted from dissolved to particulate as streamflow increased and for this reason loads primarily were composed of particulate phosphorous. Median annual TP loads were highest at FLAT and DEAD and ranged from 247 to 642 pounds per square mile (lbs/mi2) networkwide. Interannual variability in phosphorous yields was apparent at most stations; the highest loading years were also the wettest years during the study period and coincident with the highest peak annual flows.

Total nitrogen (TN) concentrations typically were low throughout the network with exceptions occurring at stations located in watersheds with a high density of septic infrastructure. Elevated TN concentrations also were observed in some watersheds without a high density of septic systems and may be attributable to geologic and soil properties that limit denitrification as well as other unknown anthropogenic inputs. Total nitrogen typically was dominated by nitrate during baseflows; however, the proportion of particulate nitrogen increased during stormflows. Total nitrogen yields were similar across stations, with medians ranging from about 3,600 to 6,300 lbs/mi2 and were related to annual streamflow volume. Total nitrogen concentrations and flow-normalized concentrations decreased over the 10-year period at 7 stations, with median reductions of about 2.5 percent. Increasing trends were observed at the two stations with the highest median TN concentration (Captain Hickory Run and SFLIL, 3–5 mg/L), both watersheds contain a high density of septic infrastructure. The combined trend results from all stations revealed no trend in TN and a declining trend in nitrate of about 2 percent per year.

Overall, benthic community metrics indicated that streams throughout Fairfax County were initially of poor health; however, many metrics show an improving trend (from poor to fair based on the Fairfax County Index of Biological Integrity [IBI]). Significant increasing trends in IBI occurred at the network-scale and at 4 individual stations; additionally, scores improved by at least 1 qualitative category (for example, poor to fair, fair to good) at 11 of the 14 stations between 2009 (the first year all 14 stations were sampled) and 2017. Changes in all metrics suggest that the biodiversity, function, and condition of streams in Fairfax County are improving, but some of these improvements are driven by increased diversity and percent composition of organisms that are tolerant of the urban environment.

Streamflow, Water Quality, and Aquatic Macroinvertebrates of Selected Streams in Fairfax County, Virginia, 2007 to 2012

By John D. Jastram

Download the full PDF of this report from the USGS Publications Warehouse

Efforts to mitigate the effects of urbanization on streams rely on best management practices (BMPs) that are implemented with the intent of reducing and retaining stormwater runoff. A cooperative monitoring effort between the U.S. Survey and Fairfax County, Virginia, was initiated in 2007 to assess the condition of county streams and document watershed-scale responses to the implementation of BMPs. Assessment of the data collected during the first 5 years of this monitoring program focused on characterizing the hydrologic and ecological condition of 14 monitored streams.

Hydrologic, chemical, and macroinvertebrate community conditions in the streams monitored were found to be consistent, overall, with conditions commonly observed in urban streams. Hydrologically, the monitored streams were found to be flashy, with flashiness positively related to road cover in the watershed. Typical pH values of streams throughout the network centered around neutrality (pH = 7) with strong daily fluctuations apparent in the continuous data. Patterns in specific conductance were largely representative of anthropogenic disturbances - watersheds having the greatest percentage of open space and estate residential land-use had the lowest typical specific conductance values, and specific conductance variability was less than what is observed in watersheds that are more intensively developed.

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In watersheds having greater road coverage, and more development in general, increases in specific conductance over several orders of magnitude were observed during winter months as a result of the application of de-icing salts on impervious surfaces. Dissolved oxygen conditions were typically within the range required to support healthy biological communities, although occasional departures during summer months at some sites fell below the impairment threshold for streams in Virginia.

Nitrogen (N) and phosphorus (P), concentration patterns were largely consistent across the network, with few exceptions. Nitrogen concentrations in monthly samples were generally low and dominated by nitrate. Exceptions to the generally low N concentrations occurred at Captain Hickory Run, which had a median total N concentration of approximately 4.9 milligrams per liter (mg/L), compared to the network-wide median of approximately 1.7 mg/L, and at Popes Head Creek Tributary, where total N concentrations spiked to 6 - 8 mg/L during low-flow periods in August or September of each year. Phosphorus concentrations in monthly samples were generally low and dominated by the dissolved fraction. Two monitoring stations in the network, Flatlick Branch and Frog Branch, are notable for having median total P concentrations that were, on average, approximately three times greater than the median total P concentration of 0.02 mg/L observed at the other 12 stations in the network.

Suspended-sediment and nutrient loads and yields were similar to those of urbanized watersheds in other studies, although the yields from these urbanized basins were greater than, or within, the upper quartile of yields observed throughout the Chesapeake Bay watershed. Annual suspended-sediment loads ranged from 289–10,275 tons, with a median of 1,311 tons, and corresponding yields ranged from 107 - 2,827 tons per square mile (ton/mi2), with a median of 277 ton/mi2. Annual total N loads ranged from 8,014 - 36,413 pounds, with a median of 21,314 pounds, and corresponding yields ranged from 3,361 - 8,360 pounds per square mile (lb/mi2), with a median of 6,200 lb/mi2. Annual total P loads ranged from 380 - 6,558 pounds, with a median of 1,874 pounds, and corresponding yields ranged from 140 - 1,562 lb/mi2, with a median of 543 lb/mi2.

Benthic macroinvertebrate community metrics indicated that streams throughout Fairfax County are generally of poor health. One station, Castle Creek, was an exception with results indicating relatively high quality aquatic health.

Six additional monitoring stations were added to the network in 2012 to improve spatial coverage throughout Fairfax County. Monitoring activities are expected to continue at all 20 stations for the foreseeable future as BMP implementation is conducted.


Publications about this study
  • Streamflow, water quality, and aquatic macroinvertebrates of selected streams in Fairfax County, Virginia, 2007 - 12: U.S. Geological Survey Scientific Investigations Report 2014 - 5073; Jastram, J.D., 2014, 68 p.,
  • 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; Porter, A.J., Webber, J.S., Witt, J.W., and Jastram, J.D., 2020, 106 p.,
  • Water-Quality Monitoring in an Urban Watershed:The Influence of Septic Systems on Nitrate Concentrations and Spatial Patterns; Presentation at the 2016 National Water Quality Monitoring Conference, Tampa, FL, by J.S.Webber. Download
  • 2020 Monitoring Program Update, presented to Fairfax County Staff on August 19,2020. Presentation Files; Video.
Publications supported by data from this study
  • Bank-derived material dominates fluvial sediment in a suburban Chesapeake Bay watershed. River Research and Applications 2018;1-13; Cashman, M.J., Gellis, A., Sanisaca, L.G., Noe, G.B., Cogliandro, V., Baker, A.. 2018.
  • Spatial and temporal variation of stream chemistry associated with contrasting geology and land-use patterns in the Chesapeake Bay watershed - Summary of results from Smith Creek, Virginia; Upper Chester River, Maryland;Conewago Creek, Pennsylvania; and Difficult Run, Virginia, 2010 - 2013. U.S. Geological Survey Scientific Investigations Report 2016 - 5093, 211 p.; Hyer, K.E., Denver, J.M., Langland, M.J., Webber, J.S., Bohlke, J.K., Hively, W.D., and Clune, J.W.. 2016.
  • Storms, channel changes, and a sediment budget for an urban-suburban stream, Difficult Run, Virginia, USA. Geomorphology 278: 128 - 148; A.C. Gellis, M.K. Myers, G.B. Noe, C.R. Hupp, E.R. Schenk, L. Myers. 2017.
  • Recent and historic sediment dynamics along Difficult Run, a suburban Virginia Piedmont stream. Geomorphology 180 - 181: 156 - 169; Hupp, C.R., G.B. Noe, E.R. Schenk, and A.J. Benthem. 2013.
  • Effects of distributed and centralized stormwater best management practices and land cover on urban stream hydrology at the catchment scale; Loperfido, J.V., et al. 2014;
  • Quantitative characterization of stream turbidity-discharge behavior using event loop shape modeling and power law parameter decorrelation; Water Resources Res., 50, 7766 - 7779, doi:10.1002/2014WR015417; Mather, A. L., and R. L. Johnson 2014, .
  • Developing a new stream metric for comparing stream function using a bank-floodplain sediment budget: a case study of three Piedmont streams; Earth Surface Processes and Landforms 38: 771 - 784, DOI: 10.1002/esp.3314; Schenk, E.R., C.R. Hupp, A. Gellis, and G.B. Noe. 2013.
  • Fluorescence-based source tracking of organic sediment in restored and unrestored urban streams; Limnology and Oceanography 60: 1439 - 1461, DOI: 10.1002/lno.10108; Larsen, L., J. Harvey, K. Skalak, and M. Goodman. 2015.
  • Fine particle retention within stream storage areas at base flow and in response to a storm event; Water Resources Research 53: 5690 - 5705, DOI: 10.1002/2016WR020202; Drummond, J. D., L. G. Larsen, R. González-Pinzón, A. I. Packman, and J. W. Harvey. 2017.
  • Disrupted carbon cycling in restored and unrestored urban streams: Critical timescales and controls; Limnology and Oceanography 62: S160 - S182, DOI: 10.1002/lno.10613; Larsen, L. G. and Harvey, J. W.. 2017.
Publications from efforts associated with this study
  • The influence of microtopography on soil nutrients in created mitigation wetlands; Restoration Ecology 17: 641 - 651; Moser, K., C. Ahn, and G. Noe. 2009.
  • Measurement of net nitrogen and phosphorus mineralization in wetland soils using a modification of the resin-core technique; Soil Science Society of America Journal 75: 760 - 770; Noe, G.B. 2011.
  • Interactions among hydrogeomorphology, vegetation, and nutrient biogeochemistry in floodplain ecosystems; In Shroder, J.F. (Editor in Chief), Butler, D.R., Hupp, C.R. (Volume Eds.), Treatise on Geomorphology, Vol. 12, Ecogeomorphology. Academic Press, San Diego, CA. pp. 307 - 321; Noe, G.B. 2013.
  • Hydrogeomorphology influences soil nitrogen and phosphorus mineralization in floodplain wetlands; Ecosystems 16: 75 - 94; Noe, G.B., C.R. Hupp, and N.R. Rybicki. 2013.
  • Science Summary—Sediment and Nutrient Trapping in the Flood Plain of Difficult Run, Virginia, and Implications for the Restoration of Chesapeake Bay; U.S. Geological Survey; Noe, G.B., C.R. Hupp, E.R. Schenk, and N.R. Rybicki. 2013.
  • Simulating stream transport of nutrients in the Eastern United States, 2002, using a spatially-referenced regression model and 1:100,000 - scale hydrography, USGS Scientific Investigations Report 2013 - 5102; Hoos, A.B., R.B. Moore, A.M. Garcia, G.B. Noe, S.E. Terziotti, C.M. Johnston, and R.L. Dennis. 2013.
  • Characteristic length scales and time-averaged transport velocities of suspended sediment in the Mid-Atlantic Region; U.S.A. Water Resources Research 50: 1 - 12, doi:10.1002/2013WR014485; Pizzuto, J., E.R.Schenk, C.R. Hupp, A. Gellis, G. Noe, E. Williamson, D. Karwan, M. O’Neal, J. Marquardt, R. Aalto, D. Newbold. 2014.
  • Soil greenhouse gas emissions and carbon budgeting in a short-hydroperiod floodplain wetland;  JGR Biogeosciences. DOI: 10.1002/2014JG002817, Batson, J., G.B. Noe, C.R. Hupp, K.W. Krauss, N.B. Rybicki, and E.R. Schenk.  2015.
  • Metformin and Other Pharmaceuticals Widespread in Wadeable Streams of the Southeastern United States;  Environmental Science & Technology Letters. DOI: 10.1021/acs.estlett.6b00170, Bradley,P.M., C.A. Journey, D.T. Button, D.M. Carlisle, J.M. Clark, B.J. Mahler, N.Nakagaki, S.L. Qi, I.R. Waite, and P.C. VanMetre.  2015.
Publications of interest for this study
  • 1983, Chemical quality of ground water in the Culpeper Basin, Virginia and Maryland; U.S. Geological Survey Miscellaneous Investigations Series Map I - 1313-D; Posner, A., and Zenone, C.
  • 1983, Map showing geologic provinces, landforms, drainage basin characteristics, and flooding in Fairfax County and vicinity, Virginia; U.S. Geological Survey Miscellaneous Investigations Series Map I - 1421; Froelich, A.J., and Langer, W.H.
  • 1985a, Maps showing geologic terrane, drainage basins, overburden, and low flow of streams in Fairfax County and vicinity, Virginia; U.S. Geological Survey Miscellaneous Investigations Series Map I - 1561; Froelich, A.J., and Zenone, C.
  • 1985b, The relation of water quality to geology and land use changes in Fairfax County and vicinity, Virginia; U.S. Geological Survey Miscellaneous Investigations Series Map I - 1534; Froelich, A.J., and Zenone, C.
  • 1987, Low-flow characteristics and chemical quality of streams in the Culpeper geologic basin, Virginia and Maryland; U.S. Geological Survey Miscellaneous Investigations Series Map I - 1313-H; Lynch, D.D., E.H. Nuckels, and C. Zenone
  • 9, Maps showing geologic and hydrologic factors affecting land-use planning in the Culpeper Basin, Virginia and Maryland; U.S. Geological Survey Miscellaneous Investigations Series Map I - 1313-J; Froelich, A.J.