Groundwater is an essential water source, providing 35% of the fresh water used in California, and significantly more in drought years. However, when groundwater is used more rapidly than it is naturally replenished, actions must be taken to correct the imbalance, and one of the tools used by groundwater managers is managed aquifer recharge (or MAR).
Managed Aquifer Recharge (MAR) enhances the recharge rate by creating artificial streams and ponds where water trickles into the ground, or by using wells to directly inject water underground. MAR can also be used to improve groundwater quality and prevent some of the negative consequences of groundwater depletion, like ground sinking (subsidence) or the intrusion of salty groundwater from the oceans into coastal freshwater aquifers.
In an American Geosciences Institute webinar, Timothy Parker, principal hydrogeologist at Parker Groundwater, discusses managing groundwater storage and managed aquifer recharge in California. Next, Graham Fogg, from UC Davis discusses recharge and reservoir management and keys to water security.
TIMOTHY PARKER: Managing Groundwater Storage: Managed Aquifer Recharge in California
Timothy Parker, principal hydrogeologist at Parker Groundwater, began by discussing managed aquifer recharge in California. He noted that it was a perfect storm that led to the California legislature passed the Sustainable Groundwater Management Act (SGMA) in 2014. Five years of drought resulted in less surface water being delivered by the state and federal projects was compounded by environmental constraints that had evolved over the years; this led to more groundwater being pumped, and consequently more land subsidence impacts, particularly to water conveyance. There was also an increase in the visibility of nitrate contaminated groundwater that was especially affecting disadvantaged communities in the Central Valley.
SGMA requires all basins considered high or medium priority to establish a Groundwater Sustainability Agency (or GSA); the GSA must then create a Groundwater Sustainability Plan (or GSP) by either January 2020 if the basin is considered critically overdrafted, or by January 2022 for the other basins. The basin must achieve sustainability within 20 years. The state may intervene if the mandates are not met by the GSA. The map on the left shows the high and medium priority basins which are subject to SGMA; the map on the right shows the basins that are critically overdrafted.
“There are two things that make a difference between the critically overdrafted and the high and medium priority basins,” said Mr. Parker. “One, you have two years less to get the plan done. The other is that it looks like there’s going to be (and has been so far) more funding available to critically overdrafted basins.”
Currently, far more GSAs have formed than the number of basins subject to SGMA – which is 127. “What’s interesting is the law was designed to try to get people to form one agency ideally and one sustainability plan covering each one of these SGMA basins, but as we found out, some of them have up to 22 GSAs in a basin, which is going to make it more difficult to get their plans done and possibly cost more with coordination agreements and that kind of thing,” he said.
For a basin to become sustainable, they will have to increase supply, increase recharge, reduce demand, or a combination thereof. One of the challenges is that the state needs to do a lot more recharge projects to stabilize groundwater levels, especially in the San Joaquin Valley and incentives are needed to do so.
Mr. Parker said there are a lot of good projects and good management around the state; there are also areas where a lot more needs to be done. “SGMA raises the bar on groundwater management,” he said. “When the Governor signed this, it became very clear: put it on the locals and the locals want to do it. It’s not the state, but the state can intervene if they need to.”
California is facing a number of challenges to increasing recharge. A better understanding of the hydrogeology of the state is needed, including determining the most suitable soil, geologic setting, and aquifer space for recharge, as well as what rates can be achieved. Water managers need to know where, how much, and how frequently surface water will be available for recharge, and consider environmental flow needs and climate change. Changes in reservoir operations and additional conveyance are needed to maximize and optimize recharge opportunities, he said.
The potential benefits and possible adverse impacts of increasing recharge to water quality based on different land and water management strategies needs to be determined, as well as how compatible that would be with the existing regulatory system, he said. Regulatory permitting and legal water rights uncertainties need to be addressed with an emphasis on the public benefits or recharge and sustainability.
“Increasing groundwater recharge through changing management practices is recognized as a major part of the solution, but it’s going to take everything to get our basins into sustainability,” Mr. Parker said.
Managed Aquifer Recharge (or MAR) is the purposeful recharge of an aquifer under controlled conditions to store the water for later extraction, or to achieve environmental benefits. MAR requires five things: source water, conveyance, a suitable receiving aquifer, water rights, and to satisfy the regulations.
“Managed aquifer recharge is a tool to replenish aquifers and provide water supply security,” he said. “It’s cheaper than many of the other new forms of water supply available, and it can replenish depleted aquifers, enhance groundwater dependent ecosystems and address environmental issues, avoid saline intrusion, mitigate land subsidence, and help offset the costs of flood control and recycled water in urban areas as well.”
What are the advantages of MAR over surface reservoirs? Mr. Parker said MAR projects have a smaller impact on land use; there aren’t evaporative losses or algal blooms to deal with. There is high natural attenuation; it dampens the quality and temperature, and lower costs. MAR can also be scaled up over time, so you can start small and build it up so you don’t have to pay it all at one time.
There are challenges, too. The aquifer must be suitable for the project. Aquifers are not water tight; there can be losses in aquifers as well as many wells in it, and it can be hard to sort out. Surrounding saline water may mix; there also can be reactions with source water with the groundwater and aquifer matrix. Infiltration recovery rates can be limited by plugging, and there can be potentially higher operations and maintenance costs, he said.
Managed Aquifer Recharge (or MAR) is a proven and well demonstrated technology. MAR projects are usually economical; they typically cost less than alternative water sources, and can be implemented in stages. MAR projects can be implemented as mutually beneficial projects for environmental and water quality improvements, along with supply resiliency and security. MAR projects are adaptable to different settings, opportunities, and constraints, including freshwater, brackish, or saline receiving aquifers; MAR projects can use drinking water, recycled water, stormwater, or groundwater as the source of water for recharge. He noted that in some places, they are pumping it out of one aquifer and putting it into another.
The design of MAR projects is governed by the hydrologic setting and hydrology. There can be infiltration using spreading basins; aquifer storage and recovery wells where one well puts water in and another well takes it out; or aquifer, storage, treatment, and recovery where if there is a long travel distance, you can get some treatment out of the aquifer. There are dry wells, riverbank filtration, standard check dams, and distributed projects, including Flood MAR. There is low impact development which uses a variety of distributed options to put water into the ground in urban settings.
With respect to design considerations, detailed hydrogeologic characterization is key. Source and receiving water characterization is important; tests must be done because the project is going to put different water into the subsurface and it’s going to react – it can be a benefit or it can be negative.
Another challenge is clogging; whether it’s physical, chemical, mechanical, or biological, clogging can be managed that through design but you have to know what you’re designing for by doing detailed studies up front, he said. There are treatments, such as strainers, filtration, microfiltration, disinfection, or chemicals.
EXAMPLES OF MAR PROJECTS IN CALIFORNIA
Santa Clara Valley Water District
The Santa Clara Valley Water District is located in the southern part of the San Francisco Bay Area. The district was formed to deal with land subsidence and flooding in the San Jose area due to overextraction or groundwater mining. The District has nearly 400 acres of recharge ponds, 91 miles of controlled instream recharge, and they recharge approximately 100,000 acre-feet per year.
Sacramento Regional Water Authority
The Regional Water Authority is comprised of 17 districts. The region is surrounded by the American River and the Sacramento River so there is a lot of surface water. The Authority cross-connected all the districts, and now has a very extensive conjunctive use program where they can exchange surface water for groundwater. As a result, they have actually successfully stored nearly 150,000 acre-feet of water over the last ten years; they’ve put a water accounting framework in place, and they’ve set the instream flows to avoid fisheries impacts. Mr. Parker noted that they’ve done a good job of reducing the amount of water they are using.
Kern Water Bank
The Kern Water Bank, located in the southern part of the San Joaquin Valley, is a true groundwater banking project that started operating in 1994. The project provides multiple ecosystem benefits with 70 shallow recharge ponds over 70,000 acres. The average recharge rate is about 1/3 foot per day. They have 84 recovery wells and an annual recovery capacity of about 240,000 acre-feet.
Los Angeles Basin
In the Los Angeles Basin, they have three seawater intrusion barrier projects and spreading basins. With seawater intrusion as the catalyst, the Water Replenishment District in 1959. The District recharges about 70,000 acre-feet by spreading and another 30,000 acre-feet by injection.
Orange County Water District
The Orange County Water District was established in 1933; they have over 100,000 acres of spreading facilities. They established a replenishment assessment and then built the most advanced groundwater replenishment system in the world with an operational capacity of about 112,000 acre-feet per year. 30 MGD goes to the seawater intrusion barrier, 70 MGD to recharge and approximately 250,000 acre-feet is recharged per year.
GRAHAM FOGG: Recharge and Reservoir Management: Keys to Water Security
Graham Fogg, Professor of Hydrogeology and Hydrogeologist at the Department of Land, Air, and Water Resources at UC Davis, began by giving his key points. “Water storage is water security; the question is where to store the water,” he said. “Climate change is already making surface storage less reliable in California and other places and this puts even more emphasis on the need for subsurface storage in California. By far the largest space to store water is underground. Winter recharge on farms and floodplains offers massive though largely yet unrealized opportunities, and alternative management of reservoirs and groundwater is key. We haven’t really operated these two systems together very much yet.”
In California, there are four major stores of water: snow, mountain groundwater, surface reservoirs, and alluvial groundwater basins. The state generally depends on the snow to get through the annual drought period, which is spring, summer, and fall, but climate change means much less snow in the mountains.
California has a Mediterranean climate which is characterized by rain and snow in the winter, with hardly a drop of rain or snow between May and October. California has dealt with this by having a reservoir of snow in the mountains that slowly melts through the spring and the summer. However, climate change will bring more prolonged droughts, more intense winter storms, earlier snow melt, and loss of snow storage. Climate change models project that the snowpack will be diminished appreciably and in the future century or half century, we can expect to have much less snow storage, he said.
While the total precipitation for the state has not changed much over the last century, the state is already seeing the effects of the loss of snowpack. He presented a slide showing snowmelt runoff for the Sacramento River and the San Joaquin River between April and September as a fraction of total runoff in the state. He noted that total runoff has not diminished, but the all important snowmelt runoff from April to September when reservoirs historically have been filled to get through the dry summer and fall is diminishing.
Mr. Fogg explained that the snow is melting sooner and there are more rain-on-snow events, so it isn’t that the total water coming into the state is changing, but that snowmelt runoff is diminishing appreciably. This makes it harder to store water in the reservoirs, which has been the main mode of water storage in the past; plus in a drought, the reservoirs only last two or three years, he said.
“We’re losing surface storage effectiveness, so what do you do? By far the largest place to store water is underground,” he said, presenting a graph of groundwater overdraft trends and pointing out that this creates room for groundwater storage in the Central Valley.
“California’s average annual groundwater overdraft is about 2 MAF. … This has resulted in available space for storage if water. If we look at available Central Valley storage volume in the state as a whole, there’s 140 reservoirs that can store about 42 MAF of water; that’s a lot of water. In the Central Valley subsurface, conservatively, there’s room for another 140 MAF to store water, so if you’re looking for space to store water, and you want to advocate for surface storage, some of that might be useful, but the big opportunity is in the subsurface. The question is how to make that happen.”
Utilizing the empty space in the Central Valley aquifers would require massive increases in recharge, and winter recharge on farms and floodplains offers massive opportunities. He presented a slide with a depiction of the San Joaquin Valley both pre and post development. Predevelopment, there was groundwater discharge into rivers, there were higher water tables and natural vegetation in the basin. Post-development, pumping from wells primarily for irrigation and water supply drew down the water table, and now most of the streams are losing.
“A consequence of this was a massive increase in recharge and much of the water for the irrigation also came from surface water reservoirs from the Sierra Nevada,” Mr. Fogg said. “The irrigation increased the recharge by a factor of 2 to 3. Predevelopment, Central Valley recharge was about 2.6 MAF. Post development, Central Valley recharge was more than double that, 5.6 MAF. The differential is even greater in the southern part of the Central Valley than the northern part. That’s due to irrigation; that’s a two fold increase in recharge.”
There’s also recharge in the rivers, he continued. “Before development, the Central Valley floor rivers were receiving base flows so there was a negative recharge of negative 1.2 MAF. Post development, the Central Valley treams became losing, so we have net recharge from the streams to the detriment of some of those streams ecosystems, but we have about 1 MAF of gain, so that’s a 2.2 MAF swing in the river recharge.”
Irrigation can be very efficient for recharge, although there are downsides to it, but if we’re trying to store water, using irrigated lands is one option, he said. The idea is take the excess winter flows from the rivers that comes from the earlier snowmelt in the winter and spring, as well as those from more intense storms due to climate change, and distribute it on irrigated lands for recharge during the winter. Using a technique like this could potentially significantly augment the recharge on a regional scale, he said. Rivers and floodplains also can recharge aquifers by using reservoir releases and floodplain management.
“In all of this, alternative management of reservoirs and groundwater is key,” he said. To illustrate the point, Mr. Fogg discussed the a project recently completed as part of the University of California Water Security and Sustainability Research Initiative which studied the reoperation of Folsom Reservoir and downstream aquifer management to maximize total water storage. The study area was on the American and Cosumnes rivers; the groundwater basins were located in Sacramento which is in the lower watershed. The study considered reoperating the reservoir for multiple objectives that maximizes total water storage and hydropower in the reservoir for a time period of 1994 – 2003. “The upshot of the study is that they were able to reoperate the reservoir to achieve similar or a little bit greater reservoir storage, just optimizing based on the weather and climate,” he said.
“The key part of this is the diversion of excess flood flows from those reservoir releases to accomplish an increase in total system storage with both the reservoir reoperation and recharge of divertable high magnitude flows,” he continued. “These are flood flows or high magnitude flows coming down the river and through the reservoir that are mostly above the 90 percentile.”
He presented a graph showing that over time, if year by year, you accumulate the increases in total storage benefit, it’s an increase of about 4 MAF over the ten-year study period of storage in the reservoir. “These are annual improvements in reservoir storage; you don’t actually get the cumulative storage in the end,” he said. “But in the groundwater, we’re assuming all the divertable water is recharged. You get almost another 4 MAF in the groundwater system if you try to recharge that water. So keep in mind, the state water overdraft deficit is about 2 MAF. Even if these are highly optimistic, this indicates the number potential for recharge is quite large.”
Mr. Fogg acknowledged that the numbers don’t include the consequences of trying to recharge the groundwater system or the hydrogeologic limits on recharge.
He presented a slide, noting that the graph on the upper left of the slide shows that during the simulation period, there is a baseline without effects on groundwater recharge or efforts to recharge the groundwater and there was essentially decreasing groundwater storage. “With the recharge, just over the 20 year period, we see significant increases, not as high as the total water diverted from Folsom Reservoir, but still quite significant,” he said. “There are increases in stream base flow in the upper right, and some excess flows into adjacent basins.”
Mr. Fogg concluded by noting that soils and geology are key to successful recharge of aquifer systems, but there wasn’t time left in his presentation to address that.
QUESTIONS AND ANSWERS
Question: Recharge opportunities in the San Joaquin Valley are huge, but how does agricultural drainage or infiltration from agriculture and salt management factor in to that?
“The groundwater quality in the Central Valley is deteriorating because of recharge from irrigation, so the only way to stabilize that water quality is to try and curtail that as best as possible, but also to recharge with cleaner water,” Mr. Fogg said. “In terms of salt management, the biggest problem is that the basins are closed basins because of the way they’ve been managed – that is, any salt coming into the basin from within the system through rock-water interaction for example, doesn’t get out because the main exit for the water is now irrigation evaporation. So filling these basins up to the point that the water and the salt has natural exits again is in my view, really the only solution for keeping the salts in balance. These basins are currently accumulating salt. And filling them up and developing more natural discharge points is really the only way to stabilize that. That’s happening on a century timescale.”
“There is a very large effort that’s been ongoing for a number of years called the Central Valley Salts program (or CV-Salts), which is a salt and nutrient management plan that’s being developed on a very large scale,” said Mr. Parker. “Part of the reason for that is to help the salt management in the basin. It’s well recognized and it’s being dealt with through this kind of a process, but increasing recharge is needed and so we’ve got a little bit more work to do on the science side to get there, and a lot more work to do in terms of designing projects, really massive projects that can help address that.”
Question: Could you talk briefly about how recharge and storage occur where the land has subsided? Does the aquifer expand again?
“The short answer is no,” said Mr. Fogg. “But the part the aquifer system that is compacted is not the aquifer sands and gravels themselves, it’s the silts and clays. There can be a little bit of expansion, but not much.”
“We have to think about these aquifer systems a little more differently than we have in the past,” added Mr. Parker. “The fine grained units, the clays store a lot more water than the coarse grained material so there’s a lot of storage in the Central Valley as a whole in these clays, but it’s slowly squeezed out over time when you get compaction. We’re losing that storage, so you can start to think about that as some of the main storage in these large aquifer system and the more coarse grained units as the conveyance or conduits for moving water.”
Mr. Fogg agreed, and added, “Our research shows that contrary to what people assume, when you recharge that system, since more than half of it is not aquifer – that is it’s silts and clays, most of the increase in the groundwater storage happens in those fines – even if they’ve previous compacted. It doesn’t mean they expand, but it means that the fine grained part of the system is a heretofore unappreciated mechanism in the storage of groundwater.”
“The other thing is they are more reactive, so you can have more reactions going on and you can actually have things moving out of that that you don’t necessarily want in your water supply,” said Mr. Parker.
“Some of the arsenic contamination has been attributed to subsidence-driven compaction in squeezing out waters in the clays that might be high in arsenic,” said Mr. Fogg.