UOSA Contribution to Increasing Salinity in the Occoquan

The information is from the article cited below and a presentation to the ICPRB in 2021. 

Bhide, S.V., Grant, S.B., Parker, E.A. et al. Addressing the contribution of indirect potable reuse to inland freshwater salinization. Nat Sustain 4, 699–707 (2021). https://doi.org/10.1038/s41893-021-00713-7

A partnership between Virginia Tech, University of Maryland, Vanderbilt University, and North Carolina State University under a $3.6 million grant from the National Science Foundation has been studying freshwater salt pollution in the Occoquan Reservoir. This research is still in its first half examining the historical data from the Occoquan Reservoir.  So far the team has examined the 25 years of data collected by the middle of the last decade by the Occoquan Watershed Monitoring Lab’s mission of monitoring water quality associated with augmenting the water supply to the reservoir by adding the highly treated wastewater (reclaimed water) from the Upper Occoquan Service  Authority, UOSA, to surface waters from Bull Run and the Occoquan River.

The scientists quantified the contributions of the three salinity sources; UOSA, and two urbanizing watersheds; Bull Run and the Occoquan. This study is part of the National Science Foundation’s Growing Convergence Research (GCR) program, which “aims to catalyze solutions to societal grand challenges by the merging of ideas, approaches, and technologies from widely diverse fields of knowledge to stimulate innovation and discovery.” According to Dr. Stanley Grant Co-Director of the Occoquan Watershed Monitoring Lab and an affiliated faculty member at Virginia Tech, and his collaborators whose first half findings were published in Nature Sustainability last winter, the rising salt pollution in the reservoir is primarily from watershed runoff during wet weather and highly treated wastewater during dry weather. 

Across all timescales evaluated, sodium concentration in the treated wastewater is higher than in outflow from the two watersheds, but only during dry weather periods is the UOSA water the majority. Sodium in the treated wastewater originates from chemicals added during wastewater treatment, industrial and commercial discharges, human excretion and down-drain disposal of drinking water and sodium-rich household products. However, the source of more than half the sodium measured is unknown and could be attributed to the cyclical nature of the water supply.

The Occoquan Reservoir is an important part of our drinking water supply. The Occoquan supplies about 40% of the clean drinking water for around 2 million people and, in an emergency, can supply all for a short period of time. The reservoir’s current storage capacity is estimated by ICPRB to be 8.3 billion gallons. Prince William land accounts for 40% of the Occoquan watershed which contains 1,300 stream miles, Lake Jackson and Lake Manassas as well as the Occoquan Reservoir.  Water from the Occoquan Reservoir is distributed to customers in Fairfax and Prince William Counties. This water is essential, but the salinity in the reservoir has been rising over time and may be reaching a critical stage. 

Inland freshwater salinity, a phenomenon called the freshwater salinization syndrome (FSS), is rising nationwide. Though sodium mass loading to the reservoir is primarily from watershed runoff during wet weather and reclaimed water during dry weather, sodium concentration in the reclaimed water is higher than in outflow from the two watersheds in the current historical data, but development in the Bull Run and Occoquan watershed has been accelerating, increasing salinity. Currently, the Occoquan watershed is far less developed than the Bull Run Watershed, but Prince William is considering massive development in the watershed.  Rising salinity in the Occoquan Reservoir implies that its salt budge is out of balance and needs to change.

Salt sources during wet weather

salt contribution during dry periods

The first part of the NSF grant has been to examine the contribution of UOSA to the rising sodium level.  Hopefully by the completion of this phase of the research the scientists will have quantified all sources of the sodium in reclaimed water which originates from chemicals added during wastewater treatment, industrial and commercial discharges, human excretion and down-drain disposal of drinking water and sodium-rich household products like water softening systems. Thus, numerous opportunities exist to reduce the contribution of indirect potable reuse to sodium pollution at this site, and the FSS more generally. These efforts will require deliberative engagement with a diverse community of watershed stakeholders and careful consideration of the local political, social and environmental context.

Dr. Stanley hopes this research will foster collaborative learning and discovery leading to stakeholder driven bottom-up management of the salt budget for the Occoquan Reservoir. The researchers will be presenting their findings to date to the NSF later this year in hopes of obtaining phase II funding and the “moonshot” desire to host one of three NSF Engineering Research Center locations for the freshwater salinization research.