Fairfax County, Collecting Real Data to Model the Watershed

Potomac Watershed Round Table met on Friday, April 4th in the Fairfax County Herrity Building. The meetings are open so you are welcome to attend. As usual there were several stimulating presentations about programs operating in the Potomac Watershed and threats to our watershed. Shannon Curtis an Ecologist with the Fairfax Count Stormwater Planning Division spoke to the group about the evolution of the water monitoring program in Fairfax County Virginia and the long-term monitoring partnership between Fairfax County and the United States Geological Survey (USGS) that began in 2007.

Back in the 1980’s ecosystem monitoring by Fairfax County and others discovered that there is an ecosystem response time lag of 10-15 years (either positive of negative) to changes in the landscape. Traditional development practices cover large areas of the ground with impervious surfaces such as roads, driveways, sidewalks and buildings. Slowly, but surely this changes the ecosystem. The paved and impervious surfaces prevent rainwater from infiltrating into the ground, causing it to runoff site at velocities and volumes that are much higher than would naturally occur, carrying with it pollutants, oil and grease, and litter.

The collective force of high velocity rainwater scours streams and over time erodes stream banks carrying sediment and other pollutants into the streams, rivers, estuaries and bays. The US EPA believes that sediment and nutrient pollutions contained in runoff from urban areas is the largest source of water quality impairments to estuaries (areas near the coast where seawater mixes with freshwater) in the United States and has turned its water quality focus on these areas starting with the Chesapeake Bay Watershed and moving forward with the Gulf Coast estuaries.

Nationally, billions of dollars are being spent to implement stormwater best management practices and low impact development strategies based on computer simulations and models. In Virginia alone millions upon millions of dollars are expect to be spent on stormwater best management practices in the next 10 years. Fairfax county programs are helping to understand how well these programs work. The Fairfax County Stormwater Planning Division performed a baseline study of the condition of all the streams in Fairfax County in the late 1990;s and found at the time that three quarters of the streams were in fair, poor or very poor condition. The deterioration of the streams had resulted from the development of the county over the previous 40 years.

This finding was used to develop the stream protection and management plan. Then in 2007 Fairfax County Stormwater Planning Division and the USGS began a long-term monitoring effort to identify countywide conditions and trends in stream water quality and quantity. The first five years of data has been accumulated by the program. The information collected will be used to evaluate the benefits of past and future watershed improvement projects. There are currently twenty monitoring stations (recently expanded from 14) in the county collecting data. Fifteen of these sites are monitored manually on a monthly basis; the remaining five sites are equipped with automated stream gages which are monitored continuously.

Stream gage in Fairfax from USGS

Instruments at the five automated gages measure six indicators every 15 minutes and during storm events: water temperature, dissolved oxygen, pH, specific conductance (a measurement of the dissolved solids in the water), turbidity, and during the storm events sediment and nutrient (nitrogen and phosphorus) concentrations. The manual stations are sampled monthly. These gages cover and area of less than six square miles. It is hoped that this data will allow the USGS and Fairfax County to observe small and subtle changes over time.

The first five years of data (when there were only 14 gages in operation) has recently been accumulated and analyzed by the USGS and provides a baseline of the condition of the watershed based on real data and not US EPA’s Chesapeake Bay Model. The Urban loading modules of the Chesapeake Bay Model are believed to have the greatest uncertainties and this is a great opportunity to perform a “reality check” on the EPA’s oversight of the water quality in the Chesapeake Bay. EPA is using the Total Maximum Daily Load (TMDL) for nitrogen, phosphorus and sediment mandated to the six Chesapeake Bay Watershed states (Virginia, Maryland, Delaware, New York, Pennsylvania and West Virginia) and the District of the Columbia to manage contamination in the Chesapeake Bay Watershed.

The TMDL sets a total Chesapeake Bay watershed limit for the entire region of 185.9 million pounds of nitrogen, 12.5 million pounds of phosphorus and 6.45 billion pounds of sediment per year which is a 25% reduction in nitrogen, 24% reduction in phosphorus and 20 %t reduction in sediment from the current levels. The pollution limits are then partitioned to the various jurisdictions and river basins based on the Chesapeake Bay modeling tools. Now, the data that the USGS Gages Partnership with Fairfax County can provide a baseline of the condition of the watershed based on data not modeling by the EPA. So far they have discovered that high phosphorus in the western portion of the county is a naturally occurring deposit that was formed about 200-250 million years ago during the Triassic period and unlikely to be remediated by any stormwater or agricultural best management practices.

In addition, the monitoring has shown that stream conditions within the county have not changed much since 1998 when the Stream Protection Strategy base study was performed, though Mr. Curtis pointed out that there might be a subtle improvement in the data, but it could be a function of weather conditions, time will tell. However, during the past 15 years the county has grown in population and development increasing the pressure on the streams, so a steady stream condition might be a small victory.

Cleaning stormwater runoff is very expensive. Preventing stormwater runoff using green infrastructure and low impact development strategies appears to be effective, but is difficult to implement and maintain. Low Impact Development and green infrastructure are a series strategies for stormwater management emphasizing water capture and conservation using natural features to mimic as closely as possible natural hydraulic properties of a site. The idea is to reduce runoff with strategies like green roofs and rain gardens and move water slowly through open unpaved areas to allow infiltration of rain water into the earth. This reduces the quantity and velocity of stormwater as it leaves a site reducing the damage that uncontrolled stormwater runoff created by building roads, sidewalks, playgrounds, and structures and compacting soil can cause.

Nonetheless, the data gathering and work performed in Fairfax raises the question of whether it is possible for urban streams to ever fully recover. Those cleanup goals may not be realistic or attainable. Fairfax County is looking to discover what is the “best attainable conditions” for its streams. In addressing pollution from runoff each step requires consistent and sustained behavior modification of individual citizens working with government. Human behavior is very slow to change and maintaining stormwater best management practices is something each individual must do for the plan to succeed.