Artificial Recharge

Overpumped, overdrafted and in various states of depletion, Arizona's aquifers have suffered the consequences of past failure to manage this vital resource. Meeting a rapidly growing demand for water in the desert southwest has never been easy; but advances in groundwater pumping tech­nology in the first half of the 20th century made satisfying water demand easier than ever before. It is little wonder that the state of Arizona came to be addicted to groundwater. But with time, Arizona came to understand that unlimited groundwater use was indeed too good to be true. By the 1940s, statewide groundwater assessments were reporting gross overdrafts in many of the state's aquifers, resulting in rapidly falling water tables, reduced water quality, and subsidence of the land surface.

While the 1980 Groundwater Man­agement Act was a critical step in the right direction, Arizona's groundwater addic­tion could not be curbed overnight, nor could the damage wrought on its aquifers be quickly undone. (Even today, ground­water accounts for roughly 40 percent of Arizona's water use.) But what the Act did do was provide a framework for innova­tive ideas developed since 1980 to more effectively manage Arizona's water sup­plies.

Authors: Susanna Eden, Joe Gelt, Sharon Megdal, Taylor Shipman, Anne Smart, and Magdalena

Escobedo; Layout: Gabriel Leake and Melisa Kennedy

Arroyo Winter 2007

Artificial recharge is one such idea that has emerged over the past 20 years as a major water management tool for meeting water supply challenges. The concept of artificial recharge is simple: return water to aquifers and increase groundwater supplies. Yet the benefits are many. Besides storing water underground in wet years for use dur­ing dry years, it is also used to manage prob­lems of land subsidence, maintain base flow in streams, protect against salt water intru­sion, treat wastewater, and abate rising costs of groundwater pumping. For Arizona, it also provides a way for the state to achieve its goal of full utilization of its annual en­titlement to Colorado River Project water.

The State of Arizona has created a pro­gram to encourage and regulate the use of recharge as a water management tool. Ad­ministered by the Department of Water Re­sources in cooperation with the Department of Environmental Quality, this program already has had a significant impact on how water supplies are managed. The program provided the opportunity to experiment with new approaches and institutional struc­tures, and in the process to discover new uses and benefits from this versatile tool. To understand the significance of Arizona's wa­ter recharge program and related activities, it is necessary to understand artificial recharge, how it works, and what it can and cannot do.

What is Artificial Recharge?

R echarge is simply the process of adding water to an aquifer. Natural recharge results from natural processes such as precipitation and streamflow. It occurs along mountain fronts, in and along stream channels, and anywhere water is able to seep down to the water table. The water table defines the top of the saturated part of an aquifer. The area of the aquifer above the water table is referred to as the unsaturated or vadose zone. A good recharge site has permeable materials such as unconsolidated sand and gravel and adequate depth to water, to allow large quantities of water to move downward to the water table and ample storage capac­ity for recharged water. The geology of central and southern Arizona provides large aquifers particularly well-suited for recharge.

Incidental recharge is water entering the aquifer after various human uses; ex­amples are recharge of irrigation drainage, leakage from underground water lines, and treated wastewater discharges to channels.

In other words, the recharge is incidental to the use. Artificial recharge involves direct human intervention to enhance or create conditions for recharge.

Artificial recharge facilities or projects are constructed to control the movement and rate of infiltration, with the purpose of adding water to the aquifer. Artificial recharge projects generally are divided into two categories: surface methods and sub­surface methods. It is frequently described in the new hydrology literature as Man­aged Aquifer Recharge. Surface methods are further categorized into facilities that increase recharge in stream channels and projects built off-channel to which water is transported for recharge. Site-specific fac­tors such as land use, geology, hydrology, and water quality determine which of these methods is most appropriate.

In-channel artificial recharge facilities typically are built into a river or streambed that is usually or mostly dry to retain water so that more will infiltrate or percolate into the underlying aquifer. Such areas generally have high infiltration rates. Inflatable dams, gated structures, and levees, or other devices are installed or constructed to impede water flow, allowing time for infiltration.

Surface off-channel recharge facilities include spreading basins, trenches, ditch systems, or constructed water bodies such as wetlands, ponds, or lakes. For spreading basins, the top layers of soil are removed to reach more permeable layers sometimes as much as 20 feet below the surface. They are usually excavated, with the soils used to construct earthen berm walls. Surface spreading basins are by far the most com­mon recharge method in Arizona. They are relatively simple to construct and maintain at high infiltration rates and are less costly than subsurface methods if sufficient land is available.

Deep basins or pits can be converted from other uses (such as gravel pits) for recharge and also can serve as decorative lakes. In such cases, water levels of about 10 feet typically are maintained during opera­tion. Operating costs are usually low, since

Goats eat on the job to eliminate weeds in CAP groundwater recharge basins. Photo: Philip Fortnam, Central Arizona Project

Movement of recharged water underground

Winter 2007 Arroyo

basins are drained and maintained only once every year or two. Infiltration rates, however, are usually low due to build up of organic matter on the lake bottoms.

Subsurface recharge includes drywells, frequently referred to as vadose zone wells, of various designs and injection wells that inject water, often several hundred feet, into an aquifer. These methods are typically used if land needed for surface methods is un­available and/or if an impermeable layer lies between the land surface and the aquifer.

The well shaft penetrates the impermeable layer enabling the recharge water to reach the aquifer. Costs of construction, op­eration and water pre-treatment can make this an expensive method of recharge. In addition, recharge volumes are low com­pared with basins, making the unit cost of recharge more expensive. In Arizona, injec­tion wells represent a very small percentage of the permitted recharge capacity.

Constructed recharge projects require maintenance to maintain infiltration rates and ensure smooth operations. For example, buildup of fine sediments and organic mate­rial can form a clogging layer, reducing re­charge rates in surface spreading basins and trenches. Drying out a basin often solves the problem by allowing the clogging layer to dry and crack. A more intensive solution involves using tillers or scrapers to break up the clogging layer.

Because clogging of recharge wells is a more difficult problem to solve, subsurface projects usually are designed to prevent clogging. Injection wells installed into the aquifer can be rehabilitated by pumping out water and with it the clogging material in the borehole. In vadose zone wells or drywells, however, if clogging occurs, it is usually per­manent.

Growth of weeds at recharge sites also is a problem. The flooding of spreading basins creates fertile seedbeds for weeds to take root and grow. The weeds increase evapotranspiration of water that otherwise would be available to replenish the aquifer. The weeds also attract pests such as midge flies. The Central Arizona Project has used goats at recharge facilities as a weed control strategy instead of chemicals; the goats are more ecologically friendly and their use is less expensive than applying chemicals. CAP has used goats at about half the cost of her­bicides.

Tracking the Hidden Waters

T he defining characteristic of groundwater is that it is underground and out of sight. To design, operate or regulate a recharge facility, a sufficient understanding of unseen subsurface conditions and water movement patterns is imperative. The effective man­agement of such facilities requires answers to some important questions: Where does the recharged water go once underground? What water quality changes might occur to water that is recharged? What effect might recharged water have on its subsurface envi­ronment? Arizona recharge permit regula­tions require detailed investigation of these factors before a project is developed, as well as on-going monitoring throughout the life of the project.

Water managers often rely on sophisti­cated computer models to predict the move­ment of recharged water and to minimize off-site impacts as much as possible. For example, models have been used to discern if a proposed recharge project would release pollution from known sources such as land­fills and dumps. The models, however, are only as good as the available data and the current scientific understanding of physical, biological, and chemical processes. Some degree of uncertainty must be accepted with any model of subsurface conditions.

This uncertainty is one reason ground­water monitoring is an important part of recharge projects. The Lower Santa Cruz Recharge Project provides a good example of using on-going monitoring to ensure the safety and effectiveness of a project (see LSCRP sidebar on following page). Water level rises under the Tangerine Road Land­fill are carefully monitored. Recharge activi­ties would be adjusted to prevent the water level from rising into the landfill liner and mobilizing potential contaminants.

Problem of Overdraft

T he story of artificial recharge in Arizona begins with groundwater overdraft and the resulting depletion of many of the state's aquifers. Hand dug wells and windmills, with their shallow subterranean range, provided early settlers with rather limited groundwater supplies. Soon, however, steam powered pumps, in use by the end of the 19th century, allowed greater access to groundwater. In 1899, the Tucson Wa­ter Company's first steam driven pumping plant could pump 1,250 gallons per minute from a 40 foot well. Groundwater — the buried treasure once out of reach — now appeared to be an accessible and plentiful resource. The 1920s were boon times for Arizona farmers. Not only were pumps becoming more efficient, but the power to work them was inexpensive. Low produc­tion costs and high market prices induced farmers to plant more cotton and, as a result, to pump more groundwater. De­spite concerns raised during a drought in the 1930s, groundwater use increased. By the early 1940s, various proposals were made in the Arizona Legislature, for study­ing, writing and passing a groundwater code. Realizing it was to its benefit, the agricultural sector took a special interest in the passage of a groundwater code, with the Arizona Farm Bureau Federation calling for a code as early as 1942. Yet the pumping continued. After World War II, advances in pumping technology made it economically feasible to pump water from depths of as much as 500 feet to irrigate cotton, vegetables and citrus. The result was an increase in irrigated acreage, from 768,000 acres in 1945 to 1,279,000 acres in 1953, occurring mostly in areas of the state dependent on groundwater.

Despite continued overdraft, legislative efforts to manage groundwater pumping made little headway, and for all practi­cal purposes, no effective regulation of groundwater pumping was in place until the passage of the Groundwater Manage­ment Act in 1980.