Saltwater Intrusion

Heavier saltwater from the Gulf of Mexico is moving up the Mississippi River as a growing wedge beneath the freshwater moving downstream. This is primarily because of the lack of rainfall in the Midwest (in the tributary valleys and along the Mississippi itself) during August and September. This lack of rainfall results in a reduced river flow that is not powerful enough and deep enough to prevent the denser and heavier saltwater from moving inland and upriver.

When the Mississippi River flow falls below 300,000 cubic feet per second, it cannot prevent salt from coming up from the Gulf of Mexico. Flows on the Mississippi River fell to 145,000 cubic feet per second at the end of September and is expected to fall even lower in coming weeks. It would require approximately 10 inches of rain across the Mississippi and Ohio River valleys, to return the river to a high flow rate capable of driving the saltwater wedge back to the Gulf.

The saltwater wedge is expected to reach the New Orleans water intake area in the next couple of weeks. When it does the drinking water will exceed the U.S. EPA Safe Drinking Water Act standard of 250ppm sodium chloride (salt)  and  people will not want to drink it due to taste, and those on low-salts diets should not exceed 20 ppm sodium chloride. Desalination is not part of this or any river water treatment systems and the New Orleans water treatment system cannot remove the salt. New Orleans is looking to piping or barging fresh water from up river to mix with the New Orleans water and dilute the salt.

Saltwater intrusion is the leading edge of climate change ahead of sea level rise. Saltwater intrusion precedes tidal inundation of low-lying lands, and dramatically changes the chemistry of tidal freshwater wetlands. Although there always has been a swath of coastal land adapted to salt, interactions among sea-level rise, climate change and coastal water infrastructure (overuse of groundwater and surface water) is causing saltwater to reach further and further inland.

As our climate is changing we are seeing dramatic pictures of increases in storm driven flooding, and higher-amplitude tidal inundation associated with sea-level rise. The effects of saltwater intrusion on ecosystem services, has received less attention, probably because changes in water chemistry are invisible to the public. However, the addition of marine salt to previously freshwater systems have profound impacts not only on our drinking water systems but on ecosystem balance, leading to coastal forest loss, species replacements, reductions in agricultural productivity, declines in coastal water quality, and marsh migration.

Saltwater intrusion and the degree of upland salinization are driven by five main factors: the position of sea-level relative to the land and water table, the frequency and magnitude of storms and tides, the frequency and duration of drought, surface and groundwater water withdrawals for drinking water and irrigation, and hydrologic connectivity the presence of tide gates, levees, agricultural diversions, and reservoirs.

In the U.S., the Chesapeake Bay region is the third most vulnerable area to sea level rise, behind Louisiana (New Orleans) and southern Florida.  Our region’s coastal plain is subsiding along with sea level rising. Over the past 100 years, due to the combination of global sea level rise and regional land subsidence sea level has risen by approximately one foot within the Chesapeake Bay.  This combined with the low lying flat geography with increase saltwater intrusion and inundation.  

The Potomac Watershed of the Chesapeake Bay watershed has the Interstate Commission on the Potomac River Basin (ICPRB) to help manage the Washington metropolitan area water supply system by coordinating withdrawals from the Potomac River and off-river reservoirs and recommending releases from upstream reservoirs when forecasted flow in the river is not sufficient to meet expected needs and prevent saltwater intrusion. The river flow for this is measured at Little Falls dam near Washington, D.C. and must meet the water utilities demands and an environmental flow-by of 100 million gallons per day (MGD). Hopefully, this system will prevent the Washington Metropolitan Area from experiencing the same problems as New Orleans is facing right now.

Sea level rise is occurring and will continue to do so into the future. The salinity effects from sea level rise could potentially be mitigated to some extent but cannot be reversed. Coastal storms and associated flooding occur several times each year, but are increasing in intensity and frequency with each passing decade.  Depending on the intensity of coastal storms, salinity effects due to over wash of tidal waters onto land can last for several months. The frequency of droughts are expected to remain the same occurring every few years, but are expected to increase in duration causing worsened salinity impacts due to reduced freshwater flows during those times. However, those effects will reverse once precipitation returns to normal levels. Conversely, periods of excessive precipitation that we are also forecast to have will mitigate salinity impacts.

Tidal  saltwater intrusion is not are only salt problem. We are experiencing inland salinization and have had saltwater intrusion into the Potomac aquifer from overuse. The Potomac River and Occoquan Reservoir are experiencing salinization.  Analyses from three different studies at multiple locations have found increasing freshwater salinization in Northern Virginia and the Occoquan Reservoir. Regionally, as salt levels have risen, WSSC is seeing discolored water problems related to winter deicing when chloride levels were observed to spike from 40mg/L to 100 mg/L. Increasing chloride levels is from sodium chloride (salt) due to rising sea levels, increased direct and indirect potable reuse of wastewater, the increased amount of pavement and the salting of roads in the winter. Nearly all road salt is eventually washed into adjacent rivers, streams, and groundwater aquifers - road salt is considered the largest contributor to rising salt levels.  

The ICPRB, the Virginia Department of Environmental Quality (VDEQ) and the Northern Virginia Regional Commission have joined together to develop a voluntary Salt Management Strategy published in 2020 to reduce the largest source of salt/ chloride to the Potomac, its tributaries and the Occoquan Watershed, but this alone may not slow the increasing salinization of our source water for drinking as road construction continues at an alarming pace. While trying to encourage the adoption of the voluntary salt management strategy, we keep building roads and paving over the county.

Road salt impacts not only potability of water, but also impact drinking water infrastructure in terms of lifetime and leaks. Water contamination is an emerging and increasing problem for both private well owners and municipal water suppliers. Salt, sodium chloride, spikes have caused changes in water chemistry triggering the lead in solder to be released into the water. Chloride is an aggressive ion that exacerbates corrosion, especially galvanic corrosion in hot water heaters and at solder points where pipes are joined.