Subterranean Solutions: Tracking Groundwater Recharge
By Gordy Slack
The demand for fresh water in California outstrips supply by millions of acre-feet per year, and that discrepancy will continue to grow along with population and development. In decades past, the state’s Department of Water Resources (DWR) might have constructed new reservoirs and dams on the state’s rivers, but for practical and political reasons, that is no longer an option.
Unable to tap new sources, California’s cities, farms, and industries turn to the only other available option: ground water. In fact, much of the five million acre-feet of annual water supply deficit is made up by overdrafting water from the state’s underwater aquifers, leading to serious consequences. In parts of the San Joaquin Valley, for example, the ground subsided dozens of feet, damaging the storage capacity of the aquifer itself. In other places, salt water is sucked from the ocean into aquifers that have been pumped dry, contaminating remaining freshwater supplies. Rivers, creeks, and wetlands can run dry, too, as the water that would fill them seeps instead into the underlying aquifers. All of these effects have dire consequences for the state’s human population and well as its wildlife and ecosystems.
“It is not sustainable,” says Fisher. Hence the Recharge Initiative, which focuses efforts to protect, enhance, and improve the availability and reliability of ground water resources.
In its 2003 California Water Plan Update, the DWR focuses on several ways to take control of the state’s water deficit. The approach with the most immediate potential is conservation: use less. “That will be essential,” says Fisher, “but alone, it won’t be enough.”
The second DWR recommendation is less intuitive: “enhanced use and storage of groundwater.”
Posing ground water as a solution to the state’s water shortage seems odd because overdrafting and its effects are already signature symptoms of the crisis. But the DWR is not proposing to simply draw more water out of the state’s underground supplies. Rather, it is hoping to use these spaces more creatively.
In particular, it may be possible to use the large and growing empty spaces left in overdrawn aquifers as storage areas, and possibly water treatment “plants,” for water that can be diverted to them during times of abundance.
The overdrafting of the past, says Fisher, may have a “small silver lining” in the storage areas it makes available for the future.
The basic idea: take excess winter flows from rivers, streams, and wetlands, storm water runoff from impervious urban surfaces, recycled water, and even desalinated water, and divert it into natural underground aquifers where it would be available for use during the dry season.
For example, in Santa Cruz, where Fisher conducts his experiments, there is abundant precipitation in the wet winter season, and very little in the dry season. If the rainwater and snowmelt could be captured and put back into the ground, a process known as recharging, the natural aquifers could store it handily until the dry season.
Every water basin is different, cautions Fisher, and each must be understood in its own context. Some will be more amenable to recharge than others. Some will recharge quickly and improve the quality of the water put into them, others may recharge slowly and may even degrade the water. But overall, the potential for storage in many parts of the state is tremendous. It will be a key part of any long term California water solution, he says.
But before groundwater can be efficiently managed, we have a lot to learn about the dynamics and biochemistry of groundwater storage, says Fisher. Some systems clog up, for example, and there are a half dozen different reasons for that. Those conditions can be found in all different combinations and configurations at different sites. Understanding what’s going on where, in different kinds of groundwater systems, will be key to making optimal use of their capacity.
And taking water from one supply, where one interest group may have specific rights to it, and putting it into the ground, where its legal status may be altered or lost, will only work if the water can be tracked and monitored. Even so, heavily manipulated and managed groundwater systems will require a great deal of cooperation and buy-in from agriculture, municipalities, industries, and the state’s other big water users. So in addition to its research component, the Recharge Initiative focuses, too, on forging cooperative relationships between local water users.
Since 2007, Fisher has studied a model recharge project in Watsonville, an agricultural community halfway down Monterey Bay between Santa Cruz and the City of Monterey. This project was created and is managed by a regional water agency, which is permitted to divert water during the winter from the Harkins Slough and place it into an infiltration pond, a sandy-bottomed area where standing water percolates down through the soil and into the aquifer below.
In 2008, Fisher, UCSC Engineering Professor John Vesecky, and colleagues received a seed grant from CITRIS to help devise and deploy sensors that would help to monitor recharging water as it descends from the pond into the aquifer below. Those sensors, which were built for the project from scratch, are embedded into columns deployed below the pond. They have to integrate with digital acquisition and telemetry systems so they can measure not only the rate of movement of the water downward (employing an ingenious method that measures the temperature of the water at different levels and tracks the penetration of diurnal thermal waves) but also the water quality at various stages.
Before the CITRIS-supported system, thermal probes were completely autonomous and would be deployed only when the pond was dry and then they would collect and store data. When the pond dried up again, eight months after their installation, the sensors would be removed and their data dumped.
Today’s sensors will transmit—via RF transmission—data as it is collected and Fisher, Vesecky, and colleagues are devising a way to analyze that data in real time. That capacity opens the door not only for the study of the recharge process, but also for its real-time management. “Theoretically, you could log into a website and get seepage rates in numbers per day,” says Fisher.
As important as figuring out how to map and track the recharge, and thus manage and better predict its potential and actual capacity at any given time, is understanding the dynamics of water quality changes as water percolates down. Before Fisher’s studies, it was assumed that for water quality to improve substantially as it filtered through the pond, it would have to move slowly. But Fisher’s results belie that prediction. Denitrification, a process by which microorganisms permanently break down nitrate, a nutrient added to the water in this watershed as fertilizer, and turn it into di-nitrogen, which escapes to the atmosphere, is occurring at “extremely high rates.” That’s good news for water treatment.
Denitrification rates and seepage rates will be different from basin to basin, but the results from Fisher’s experiments are very encouraging.
The tools Fisher is developing will be as important as the data he gleans from his own research, he says. “A big part of our project is figuring out how to export this kind of study so that water districts can optimize their own recharge systems across the state.”