Salt in the Reservoir

This article is excerpted from the article cited below and the Virginia Tech news release.  

Bhide, Shantanu & Grant, Stanley & Parker, Emily & Rippy, Megan & Godrej, Adil & Kaushal, Sujay & Prelewicz, Greg & Saji, Niffy & Curtis, Shannon & Vikesland, Peter & Maile-Moskowitz, Ayella & Edwards, Marc & Lopez, Kathryn & Birkland, Thomas & Schenk, Todd. (2021). Addressing the contribution of indirect potable reuse to inland freshwater salinization. Nature Sustainability. 10.1038/s41893-021-00713-7.

Inland freshwater salinization historically was once thought to be a problem only in areas with arid and semi-arid climates, poor agricultural drainage practices, sodic soils and saline shallow groundwater. However, today we know that inland freshwater salinization is on the rise across many cold and temperate regions of the United States.  Inland freshwater salinization is particularly notable in the densely populated Northeast and Mid-Atlantic and agricultural Midwest regions of the country like here in Northern Virginia.

Freshwater salinization threatens freshwater ecosystem health and human water security. Chloride enrichment of streams is associated with declines in pollution sensitive benthic invertebrates and loss of critical freshwater habitat. Stream-borne salts can mobilize, nutrients and heavy metals that were previously sequestered into sensitive ecosystems and drinking-water supplies. Salinization of drinking-water supplies can mobilize lead, copper and other heavy metals from ageing drinking-water infrastructure through cation exchange and corrosion. It can also alter the perception of the quality of the water, a high enough concentrations, sodium and other salts degrade the taste of drinking water (coffee and tea).

Inland freshwater salinity is rising worldwide and is now called the freshwater salinization syndrome (FSS). Though increasing salinization is commonly attributed to winter deicing operations, winter application of brine and salt are only a part of the problem. Chronic salinization is primarily a result of increasing population and indirect potable reuse of wastewater-the practice of augmenting water supplies through the addition of highly treated wastewater and down river use of our freshwater resources. Releasing treated wastewater to surface waters and groundwaters has been growing and is encouraged by the EPA along with other forms of water reuse in their Water Reuse Action Plan.

In our own region, both indirect potable reuse of waste water from the Upper Occoquan Service Authority (UOSA) and human activities in the Bull Run and Occoquan River watersheds contribute to salinization of the Occoquan Reservoir in Northern Virginia. More than 95% of freshwater inflow to the reservoir is from the Occoquan River and Bull Run, which drain mixed undeveloped, agriculture, ex urban and urban and increasingly industrial landscapes.

Water from Bull Run includes baseflow (including from groundwater) and storm water runoff from the Bull Run watershed (34% of annual flow) together with highly treated wastewater discharged from UOSA (6% of annual flow) located just over a mile upstream of Bull Run’s confluence with the reservoir. Conceived and built in the 1970s, UOSA was the United States’ first planned application of indirect potable reuse and a model for the design and construction of similar reclamation facilities worldwide. Water discharged from the Occoquan River comes primarily from baseflow and stormwater runoff from the Occoquan River watershed (60% of annual flow).

The scientists found that possible sources of rising sodium concentration in the reservoir include deicer use in the rapidly urbanizing Occoquan River and Bull Run watersheds. Over the past 20 years salt has been added to UOSA’s sewer water from its >350,000 residential and commercial connections. Possible sources of sodium within UOSA’s sewershed include the down-drain disposal of sodium-containing drinking water and sodium-containing household products, use of water softeners in commercial and residential locations, and permitted and non-permitted sodium discharges from industrial and commercial customers.

from Bhide et al

The sodium concentration in UOSA’s effluent are consistently higher than sodium concentrations measured in Bull Run and the Occoquan Reservoir. Using probability analysis of the sodium mass load for the period 2010–2018 confirms that UOSA’s reclaimed water though small in volume dominates the sodium mass load entering the reservoir from the Occoquan River and Bull Run during dry and median weather conditions. UOSA’s contributes 60% to 80% of the sodium loading during dry periods, 30% to 50% during median and 5% to 25% during wet conditions. The Occoquan River and Bull Run watersheds exhibit the opposite pattern, contributing a greater percentage of the overall sodium load during wet weather periods. During wet weather, sodium mass loading from the Bull Run watershed is, on average, higher than sodium mass loading from the Occoquan River watershed, but both are dwarfed by UOSA. Across all timescales evaluated, sodium concentration in the treated wastewater is higher than in outflow from the two watersheds.

from Bhide et al

It begs the question, where does the sodium in UOSA’s reclaimed water come from? The scientists believe that the sodium in UOSA’s water comes from a variety of sources -watershed deicers, water treatment processes, household products, commercial and industrial discharges, drinking water treatment, and wastewater treatment. On the basis of data provided by UOSA they estimate that, on an annual average, 46.5% of the daily sodium mass load in UOSA’s reclaimed water is from chemicals used in water and wastewater treatment (for pH adjustment, chlorination, dechlorination and odor control), a single permitted discharge from the Micron Semiconductor facility and human excretion (our diets are salty). The source of the remaining 53.5% is unknown but the scientist believe it  includes contributions from the down-drain disposal of sodium-containing drinking water from Lake Manassas, the Potomac River and the Occoquan Reservoir, as well as sodium-containing house hold products that eventually end up in the sanitary sewer system.

Fairfax water has been exploring options to address the slowly rising sodium concentration in the reservoir, including the possible construction of a reverse osmosis treatment upgrade. Desalinating fresh water was estimated cost at least $1 billion, not including operating and maintenance costs and a vastly higher carbon footprint. This would include a tremendous loss of volume. Reverse osmosis looses about three quarters of the water. Which is the real problem.

The researchers envision at least four ways in which salt pollution can be reduced: limit watershed sources of sodium that enter the water supply (such as from deicer use), enforce more stringent pre-treatment requirements on industrial and commercial dischargers, switch to low-sodium water and wastewater treatment methods, and encourage households to adopt low-sodium products. 

"Addressing salinization of the Occoquan Reservoir requires working across many different water sectors, including the local drinking water utility (Fairfax Water), the wastewater reclamation facility (Upper Occoquan Service Authority), the state transportation agency (Virginia Department of Transportation), and city and county departments in six jurisdictions responsible for winter road maintenance, including the City of Manassas, City of Manassas Park, Prince William County, Fairfax County, Loudoun County, Fauquier County," said Dr. Stanley Grant  the director of the Occoquan Waster Quality Laboratory and one of the paper’s authors.