By Dr. Robert Puls
Oklahoma Water Survey
Norman, OK 73072
The water cycle or hydrologic cycle on earth involves the continuous transfer of water via evaporation, condensation, precipitation, movement over the land surface and infiltration (Figure 1). Water evaporates from the oceans and other open water bodies, moves across sky as water vapor in clouds, condenses and returns to earth as rain and snow, and then returns to the oceans and other open water bodies through rivers and underground pathways to start the cycle again. Some of the water that falls on the land evaporates from the soil or is transpired from plants back into the atmosphere. Water flows overland to stream channels, lakes, or the sea. Water infiltrates through soils and then resides in an unsaturated zone where it exists at or below atmospheric pressure, or in a saturated zone at above atmospheric pressure – this component is referred to as ground water. Once in the ground water system, the water moves slowly in response to hydraulic gradients until it reenters the surface part of the cycle. This can occur via springs, streams, wetlands, lakes, tidal waters and pumped wells.
Artificial recharge (AR) is a “process by which excess surface water is directed into the ground – either by spreading on the surface, by using recharge wells, or by altering natural conditions to increase infiltration – to replenish an aquifer” (NRC, 1994). Aquifer storage and recovery (ASR) is a subset of AR, where water is artificially recharged and stored in subsurface aquifers when water is readily available, and is then extracted at a later time when water demand is greater.
AR projects include controlling seawater intrusion in coastal aquifers, controlling land subsidence due to over pumping of aquifers, and maintaining environmental flows in streams. ASR projects cover a broad array of water resource management practices including:
- Increased supply of drinking water
- Environmental water supply to maintain in-stream uses
- Irrigation water for agricultural uses
- Capture and reuse of storm water runoff
- Reuse of treated wastewater
States, communities, industries and water suppliers are investigating ways to augment water supplies in the face of climate change, population increase and land development. An approach increasingly used with some success around the world is AR and in particular ASR. Spreading infiltration basins and injection wells are the most common means employed for AR. Many states are eager to use ASR technology to address water shortages during dry periods or peak water demand periods.
The New York City Department of Environmental Protection (DEP) has been involved in evaluating ASR to improve groundwater supplies by using deep aquifers to provide additional storage for surface water. Working with regional agencies, DEP is developing an Aquifer Storage and Recovery (ASR) project. Currently, the Lloyd Aquifer’s resources are depleting, mainly due to increased rate of consumption by Long Island communities. ASR would help to replenish the Lloyd Aquifer by injecting surplus water from New York City’s upstate surface water reservoirs into the aquifer. This water would be stored in the aquifer and, when necessary, the city could extract a portion of this potable water to supplement its drinking water supply. (http://home2.nyc.gov/html/dep/pdf/wsstate07.pdf).
The Lake Okeechobee Aquifer storage and recovery project, as part of the Comprehensive Everglades restoration program (CERP), is evaluating the use of ASR to address uncertainties associated with ASR and to determine the potential locations for future ASR wells in south Florida. The project will involve injecting 1.6 billion gallons of water per day into the upper Floridian aquifer during the summer, wet season. This multi-disciplinary, multi-purpose, multi-agency project will determine specific water quality characteristics of the receiving aquifer, water quality of recharged, stored and recovered water and the amount of recovered water. Available surface water is collected, treated to meet federal and state water quality standards, and pumped into the upper Floridian aquifer, which is separated from the overlying aquifer by layers of low permeability strata. The stored water is then recovered later by pumping for beneficial use. (http://www.saj.usace.army.mil/Documents/NewsReleases/archive/2006/NR0634.pdf.)
South Florida has a history of using ASR. Early 1980’s to mid 1990’s studies focused on engineering and water quantity considerations, but more recent studies have focused on water quality issues. Studies by the USGS and others have called attention to the fact that mixing of different oxidative waters can result in mobilization of metals and metalloids (e.g. arsenic) (http://water.usgs.gov/ogw/pubs/ofr0289/jda_mobilization.htm).
Water for the city of Wichita, Kansas comes from Cheney Reservoir and the Equus Beds Aquifer in south-central Kansas. Water levels in the aquifer have dropped more than 40 feet due to over pumping which has exceeded the recharge rate of the aquifer (about 6 inches per year). Because of over pumping, the aquifer is threatened from saltwater intrusion from the Arkansas River to the southwest and movement of oil field brine from the northwest. The city is evaluating the use of ASR to address these threats to the aquifer. An initial demonstration project showed that sufficient flow exists in the Little Arkansas River to allow for up to 150 million gallons per day during above base flow events and these diversions would only represent 15% of the river’s flow (http://www.wichita.gov/CityOffices/WaterAndSewer/ProductionAndPumping/ASR/FutureWaterSupply.htm). The project demonstrated that the water used for recharge could be economically treated to meet drinking water standards. The project also demonstrated that recharge wells, recharge pits and recharge trenches could be used effectively. Full-scale implementation is now underway to meet the area water needs by 2020.
Studies in the Central Ground Water Basin in Los Angeles County have demonstrated that treated municipal wastewater effluent placed in large holding ponds or spreading grounds can be used to successfully recharge downgradient aquifers with no impact on water quality (http://water.usgs.gov/ogw/pubs/ofr0289/ras_transport.htm) . These studies evaluated the transport and fate of wastewater constituents over long travel distances and long time frames (decades) from the point of recharge to points of withdrawal. California law is more stringent than the EPA common drinking water standards for applications of AR with wastewaters as source waters. The state additionally requires that no more than 1 mg/L organic carbon and nitrogen in the ground water be of wastewater origin. Because of this requirement, the proportion of recycled wastewater is limited to 50% annually and to 35% within three contiguous years.
In F2004 the San Antonio Water System Board approved an ASR system for the Carrizo-Wilcox Aquifer. It was developed to capture surplus water during wet months and store it underground for drought management. Water from the Edwards Aquifer is disinfected to meet drinking water standards and is then pumped into the Carrizo-Wilcox Aquifer for storage. The volume in storage at the end of 2011 was over 87,000 acre-feet.
In South Australia, an ASR trial demonstrated that urban stormwater could be injected into a brackish limestone aquifer to create a useful water resource with only passive pretreatment and no disinfection (Pavelic et al. 2006). Stormwater was captured and passively treated using constructed wetlands and then injected into confined aquifers for later beneficial use. Recovery efficiency was on the order of 60%. The injected water was highly turbid (suspended solids from 29-169 mg/L) and some clogging around the well bore occurred but was mitigated by redevelopment of the well. The degree of clogging was small considering the quality of the injected water and moderate aquifer transmissivity (180 m2/day). The recovered water was used successfully for irrigation.
TECHNICAL SUMMARY, STATE OF PRACTICE
AR systems employ infiltration basins, constructed wetlands and bank filtration systems. Infiltration basins are used where there is sufficient distance between the base of the system and the water table and there are no impermeable layers in between. These are often used for capturing stormwater runoff. Constructed wetlands are often used for wastewater treatment and beneficial reuse. Bank filtration is the acceleration, by pumping, of the influx of surface water to ground water storage near the banks of the surface water body. This is often used to improve overall water quality by removing particles, bacteria, viruses, and organic and inorganic contaminants.
ASR systems specifically relate to the use of a well. ASR systems can use single wells for both injection and recovery and multiple wells where injection and recovery wells are separated by some distance. These systems have gained increasing acceptance around the world. Separate injection and withdrawal wells are used where hydrogeologic conditions indicate treatment of injected waters can be enhanced using an appropriate flow path distance for attenuation of some contaminants. Single well systems simply inject water which then displaces existing water in the aquifer and some mixing occurs prior to withdrawal.
ASR systems are used for seasonal storage and recovery, long-term storage, emergency storage and for reclaimed water reuse (stormwater, wastewater). The benefits of these systems are many and include, restoration of ground water levels, prevention of saltwater intrusion, reduction of land subsidence, enhancement of base flow to streams, and pathogen or contaminant reduction (i.e. passive treatment systems).
TECHNICAL ISSUES, RESEARCH NEEDS
A number of technical issues have been raised regarding the successful implementation of AR and ASR strategies. These include aquifer vulnerability to the introduction of non-native waters (e.g. mobilization of metals), potential impairment of existing water rights, well clogging due to chemical precipitation reactions, well clogging due to increased turbidity or microbial growth, poor recovery efficiency, pump failures, maintenance of infiltration capacity in basins, and sustainability of constructed wetlands and other engineered systems. Surface infiltration systems can function well over a large range of water quality conditions. Injection wells, on the other hand, are much more sensitive to water quality variations, except in the case of karstic limestone or fractured rock. Ecological effects relate mainly to downstream flows; will diversions or abstractions have impacts on ecosystem services downstream (water availability, recreation etc.)?
Research is needed to better predict geochemical reactions between waters of different chemistries to prevent well clogging and metal mobilization and thus the overall water quality of the recovered water. Differences in the oxidation-reduction potential of injected and receiving waters have resulted in the leaching and mobilization of arsenic and other elements (e.g. Fe, Mn)(Arthur et al. 2002) and can lead to precipitation reactions which can clog the system. Fate and transport studies are needed for virus survival and transport and organic and inorganic colloidal transport in aquifers. Risk assessment models and decision support systems would be extremely beneficial for environmental decision makers. These should include models that more accurately predict well clogging, alterations to aquifer permeability, recovery efficiency and water storage capacity. The use of stable isotopes has been shown to be useful to assess changes in hydrologic properties of aquifers before, during and after ASR implementation (Izbicki 2002).
Additional source water characteristics that can affect AR and ASR implementations include suspended solids, dissolved gases, nutrients, biochemical oxygen demand, microorganisms, sodium, organic compounds, and pathogens. While some negative impacts to water quality of receiving waters have been observed, there are technical and engineering fixes to avoid such problems. For example, pre-injection treatments could include the following:
- Removal of oxygen to reduce redox reactions and match injectate and receiving waters more closely
- Treat injectate to National Primary Drinking Water Standards
- Filter injectate to remove colloidal materials to reduce clogging issues
At the federal level, the U.S. Environmental Protection Agency regulates ASR through the Underground Injection Control (UIC) Program. ASR wells are regulated as class V injection wells under this program. EPA may directly implement an ASR program or the state may have ‘primacy’. An ASR well owner must submit basic inventory information to the state or EPA and demonstrate that the well can be operated to protect a USDW. If the regulatory authority is satisfied it may authorize the well by rule or require a Class V well permit. As of February 2009, there were approximately 630 operating ASR wells in the U.S. (http://water.epa.gov/type/groundwater/uic/aquiferrecharge.cfm). Most of these were located in the southwest or southeast regions of the U.S.
There is some variation in state requirements. Nine states require that water used for ASR injection be treated to national or state drinking water standards or state groundwater standards. Some states allow the injection of treated effluent, untreated surface or groundwater, and reclaimed water subject to state recycled water criteria. However, EPA regulations state in 40 CFR 144.12 that “no owner or operator shall construct, operate, maintain, convert, plug, abandon or conduct any other injection activity in a manner that allows the movement of fluid containing any contaminant into underground sources of drinking water.”
In Australia, ASR guidelines were established in 1996 (Dillon and Pavelic). While the guidelines adhere to internationally accepted principles, they are different for two reasons. One, they do not presume injected water to be of drinking water quality as an essential and sole objective; and two, they allow for demonstrated sustainable attenuation of contaminants by natural processes in aquifers. The overarching principle is to remove or reduce contaminants at the surface especially those resistant to degradation in the aquifer. Pretreatment methods may include both passive and engineered systems. In other words, if a scientifically credible case can be made for the aquifer to accomplish some level of additional cleanup to help meet drinking water quality standards, then it will be considered.
The Oklahoma Comprehensive Water Plan (OCWP) released in 2011 evaluated the use of artificial aquifer recharge and ‘marginal quality water use’, sometimes referred to as ‘reuse’ to supplement water demands in different parts of the state. The plan identified and recommended five sites across the state where recharge demonstration projects could be most feasible using 12 specific scoring criteria. These included: demand and source water proximity, source and native ground water quality, geochemical interactions, transmissivity of the aquifer, residence time and cost. The top rated site from this analysis was located near the town of Ada in south central Oklahoma. The Blue River would be the source of recharge water for this site. It has had minimal MCL (maximum contaminant level) exceedences over time and low TDS (total dissolved solids). This indicates that perhaps no pretreatment of the source water would be required for a project in this location. The OCWP concluded that artificial recharge was a viable option for augmenting Oklahoma water supplies in the future. More research is needed in this area.
The Oklahoma Water Survey will work with other state and federal agencies, academia, municipalities and tribes to promote and assist with the development of AR and ASR demonstration projects in the state. Oklahoma spans seven different precipitation regimes from east to west (Figure 3). Given the unique location of Oklahoma and its recent history of extreme weather events (tropical storm Erin, 2007; record heat and drought 2011; Christmas blizzard, 2009; blizzard 2011; ice storm 2007), climate impacts on water supply and water quality in the state have garnered renewed attention on the part of researchers, state and federal agencies responsible for the water resource management, businesses, municipalities, tribes and citizens. It is imperative that that the state provide research, data and information to protect ecosystems and the services they provide and better manage and predict water availability for future generations given the alterations to the water cycle caused by climate change and human activities. The use of AR and ASR are options to better mange both water quality and water availability and their use is more likely as water demands increase to meet future needs.
Arthur, J.D., A.A. Dabous, and J.B. Cowart. 2002. Mobilization of Arsenic and Other Trace Elements During Aquifer Storage and Recovery, Southwest Florida. USGS Open File Report 02-89.
Dillon, P. and P. Pavelic, 1996. Guidelines on the Quality of Stormwater for Injection into Aquifers for Storage and Reuse. Urban Water Research Assoc. of Aust. Research Report No. 109.
Izbicki, J. 2002. Using Chemical and Isotopic Tracers to Assess Hydrogeologic Processes and Properties in Aquifers Intended for Injection and Recovery of Imported Water. USGS Open File Report 02-89.
NRC, Ground Water Recharge Using Waters of Impaired Quality, 1994, Commission on Geosciences, Environment and Resources.
Oklahoma Comprehensive Water Plan. 2012. Oklahoma Water Resources Board. http://www.owrb.ok.gov/supply/ocwp/ocwp.php
Pavelic, P., P.J. Dillon, K.E. Barry, N.Z. Gerges. 2006. Hydraulic Evaluation of Aquifer Storage and Recovery with Urban Stormwater in a Brackish Limestone Aquifer. Hydrogeology Journal 14:1544-1555.