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.
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.
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.