September 28, 2016
I recently wrote about how saltwater intrusion was likely what killed the mammoth [Link to blog]. I mentioned that some coastal areas are using injection wells to pump fresh water into aquifers as a means of staving off saltwater intrusion.
Saltwater Intrusion 101
One of the easier ways to imagine saltwater intrusion is described here. But, I think also of the various science experiments I did in high school about osmosis. As the picture below shows, if you have equal levels of water and salt water, and a semi-permeable surface, osmosis will occur until the pressure equalizes.
This happens in nature, where groundwater levels (very possibly in underground aquifers) approach sea-level. The process of saltwater intrusion ceases naturally when there are higher levels of fresh water, because the fresh water exerts enough pressure to hold back the sea water. This relationship is described by the Ghyben-Herzberg principle.
Based on the Ghyben-Herzberg principle a separation in elevation between fresh and saltwater sources needs to occur to protect fresh water supplies from intrusion. For example, if the top of the groundwater level of Aquifer A is three feet above sea level, the depth to seawater needs to be approximately 120 feet below sea-level to protect that supply. If the groundwater rises to four feet above sea level (for instance, if a drought-stricken aquifer is replenished from rains), then the depth of seawater needs to be 160 feet below sea-level.
In some coastal areas, groundwater is pumped for use as potable water and for agricultural purposes. This can decrease the height of the fresh water table and cause an imbalance in the natural system creating the condition of saltwater intrusion into the fresh water supply. This condition results in rendering the supply useless as a source of fresh water supply to support human or plant life.
What does this look like in real life though? (Un)fortunately we have examples of that.
Case Study: Los Angeles County
During the first decades of the 20th century, in the Central and West Coast Basins (CWCB) of California, the rates of freshwater extraction occurred at twice the rate of natural replenishment. This has resulted in groundwater elevation 100 feet below sea level, and has increased the extent of saltwater intrusion inland. This condition is further exacerbated by the lack of stormwater infiltration to recharge these underground natural water supplies in the region’s urbanized areas, which is caused by excessive impermeable surfaces being used in development. To address the problem, the Los Angeles County Flood Control District performed an experiment to inject potable water into a previously abandoned fresh water well to test if pressure could be built up in a confined aquifer to block the saltwater intrusion. This test was successful, so a series of fresh water injection wells were created to form fresh water barriers that create pressure ridges greater than the pressure of the intruding seawater to help protect natural, regional fresh water supplies.
Currently, much of the water pumped into the CWCB projects is purchased (imported), and costs tens of millions of dollars per year (and that says nothing of the electrical costs and maintenance costs, which are sure to increase as this type of infrastructure ages).
How Do We Change It?
One way to decrease the costs and effects of depleting local water resources is to reduce how much water is pumped from these water supplies, allowing groundwater levels to rise. For instance, industrial sites might be able to utilize recycled water on their grounds, thus reducing how much pressure they are putting on the groundwater system. Sometimes other non-potable water sources like rain capture or gray water re-use can be utilized on suitable properties. Additionally, we can all do our part to conserve water use at the corporate and personal level by installing efficient water fixtures, using regionally appropriate plant species, and installing water conserving irrigation systems when supplemental water is needed in the landscape.
Additionally, ecological design can also provide solutions. Impermeable pavement can be replaced with permeable pavement systems. For example, turf block can be used for fire lanes; permeable pavers can be used in parking and plaza areas; and permeable concrete can be used to promote infiltration. In addition, infiltration swales (“rain gardens”) can be installed in planted areas. All of these measures are designed to temporarily retain water and allow it to slowly percolate into the soil and eventually seep into the aquifer, increasing groundwater levels. Another added benefit is that as more water is absorbed into the soil, the pressure on wastewater treatment systems decreases and makes spills of untreated water into natural water systems less likely during storm events.
These solutions are critical to helping decrease dependence on imported water supplies—and are solutions that can be applied to other places facing pressures from over-drafting of fresh water supplies. We should take these design and use considerations seriously, because when we rely on importing water we do so at the expense of communities hundreds of miles away.
Great Ecology Can Help You
Great Ecology has a history of developing innovative strategies for stormwater capture and reuse, as well as expertise in native plants across the U.S., which can be strategically planted to decrease irrigation needs. We can assess your site and provide recommendation to improve water conservation, capture, reuse, and infiltration.
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