Literature Review Methods For Artificial Recharge Environmental Sciences

It is the simplest, oldest and most widely applied method of artificial recharge. This method involves surface spreading of water in basins that are excavated in the existing terrain.

For effective artificial recharge, highly permeable soils are suitable and maintenance of a layer of water over the highly permeable soils is necessary.

When direct discharge is practiced, the amount of water entering the aquifer depends on three factors; the infiltration rate, the percolation rate, and the capacity for horizontal water movement.

Recharge by spreading basins is most effective where there are no impending layers between the land surface and the aquifer and where clear water is available for recharge.

The common problem in recharging by surface spreading is clogging of the surface material by suspended sediment in the recharge water or by microbial growth.

The estimated costs associated with the use of recharge basins are high since the basins depend on both infiltration rates and land values.

The estimated land required (ha) depends upon the volumetric rate of recharge and the infiltration rate i.e. Flow Rate (m3/d) / Infiltration Rate (m3/ha/d)

Vadose zone injection well

Recharge or injection wells are used to directly recharge water into deep water- bearing zones. Recharge wells could be cased through the material overlying the aquifer and if the earth materials are unconsolidated, a screen may be placed in the well in the zone of injection.

In some cases, several recharge wells may be installed in the same borehole.

Recharge wells are suitable only in areas where a thick impervious layer exists between the surface of the soil and the aquifer to be replenished.

They are also advantageous in areas where land is scarce.

A relatively high rate of recharge can be attained by this method.

The life cycle of vadose zone injection wells is very uncertain since they are an emerging technology.

However, they are more economical than recharge basins or direct injection wells as they provide some of the advantages of both recharge basins and direct injection wells.

Direct Injection Well

They can inject water directly into unconfined aquifers or confined aquifers.

Where unconfined aquifers are unavailable, direct injection wells are the only alternative for groundwater recharge and are capable of simultaneously injecting water into several aquifers.

However, direct injection wells are expensive, require advanced pre-treatment technology and advanced technology for maintenance.

Ground water recharge by direct injection is practiced

Where groundwater is deep or where the topography or existing land use makes surface spreading impractical or too expensive,

When direct injection is particularly effective in creating freshwater barriers in coastal aquifers against intrusion of saltwater,

When both in surface spreading and direct injection, locating the extraction wells as great a distance as possible from the spreading basins or the injection wells increases the flow path length and residence time of the recharged water. These separations in space and in time contribute to the mixing of the recharge water and the other aquifer contents, and the loss of identity of the recharged water originated from municipal wastewater.

Major Characteristics of Aquifer Recharge Methodologies

Recharge BasinsVadose injection wellsDirect injection wells

Aquifer Type

Unconfined

Unconfined

Unconfined or Confined

Pre-Treatment Requirements

Low Technology

Removal of solids

High Technology

Estimated Major Capital Costs US $

Land and Distribution System

$25000-75000 per well

$500000-1500000 per well

Capacity

1000-20000 m3/ha/d

1000-3000 m3/well/d

2000-6000 m3 /well/d

Maintenance Requirements

Drying and Scraping

Drying and Disinfection

Disinfection and Flow Reversal

Estimated Life Cycle

>100 years

5-20 years

25-50 years

Soil Aquifer Treatment

Vadose zones and Saturated zones

Vadose zones and Saturated zones

Saturated zones

(source: United Nations Environment Program)

History of direct injection wells in the United States

Widespread use of injection wells began in the 1930s to dispose of brine generated during oil production. Injection effectively disposed of unwanted brine, preserved surface waters, and in some formations, enhanced the recovery of oil,

In the 1950s, chemical companies began injecting industrial wastes into deep wells. As chemical manufacturing increased, so did the use of deep injection. Injection was a safe and inexpensive option for the disposal of unwanted and often hazardous industrial byproducts,

In 2010, the EPA finalized regulations for geologic sequestration of CO2. This final rule created a new class of wells, Class VI. Class VI wells are used solely for the purpose of long term storage of CO2.

(source: United States Environmental Protection Agency)

Types of Injection wellsClass 1

Class I wells are those that inject industrial, municipal and hazardous wastes below the deepest underground source of drinking water (USDW).

Class I wells can be subdivided by the types of waste injected: hazardous, non-hazardous, and municipal waste water.

Hazardous wastes are those industrial wastes that are specifically defined as hazardous in federal law. Many of these wells are located along the Texas-Louisiana Gulf Coast. This area has a large number of waste generators such as refineries and chemical plants as well as deep geologic formations that are ideal for the injection of wastes.

Non-hazardous wastes are any other industrial wastes that do not meet the legal definition of hazardous wastes and can include a wide variety of fluids.

Municipal wastes, which are not specifically defined in federal regulations, are wastes associated with sewage effluent that has received treatment.

Site Selection and Distribution

Site selection for a Class I disposal well is dependent upon geologic and hydrogeological conditions, and only certain areas are suitable. Most of the favorable locations are generally in the mid-continent, Gulf Coast, and Great Lakes regions of the country, though some other areas are also safe for Class I well sites.

The process of selecting a site for a Class I disposal well involves evaluating many factors. To take in consideration first is the determination that the underground formations possess the natural ability to contain and isolate the injected waste. One important part of this determination is the evaluation of the history of earthquake activity. If a location shows this type of instability in the subsurface, it may mean that fluids will not stay contained in the injection zone, indicating the well should not be located in that particular location.

A second important factor is determining if any improperly abandoned wells, mineral resources that provide economic reserves or underground sources of drinking water are identified in the area. These resources are evaluated to ensure that the injection well will not cause negative impacts.

A detailed study is conducted to determine the suitability of the underground formations for disposal and confinement.

The injection zone in the receiving formation must be of sufficient size (both over a large area and thickness) and have sufficient porosity and permeability to accept and contain the injected wastes. The region around the well should be geologically stable, and the injection zone should not contain recoverable mineral resources such as ores, oil, coal, or gas.

Operating and Monitoring Requirements

The operating conditions for the well are closely studied and are limited in the permit to make sure that the pressure at which the fluids will be pumped into the subsurface is safe, that the rock units can safely receive the volume of fluids to be disposed of, and that the waste stream is compatible with all the well construction components and the natural characteristics of the rocks into which the fluids will be injected.

Class I injection wells are continuously monitored and controlled, usually with sophisticated computers and digital equipment. Thousands of data points about the pumping pressure for fluid disposal, the pressure in the annulus between the injection tubing and the well casing (that shows there are no leaks in the well), and data on the fluid being disposed of, such as its temperature and flow rate, are monitored and recorded each day. Alarms are connected to sound if anything out of the ordinary happens, and if unusual pressures are sensed by the monitoring equipment, the well automatically shuts off.

Class 2

Class II injection wells have been used in oil field related activities since the 1930's. Today there are approximately 170,000. Class II injection wells located in 31 states.

Class II wells are subject to a regulatory process which requires a technical review to assure adequate protection of drinking water and an administrative review defining operational guidelines.

Class II wells are categorized into three subclasses: salt water disposal wells, enhanced oil recovery (EOR) wells, and hydrocarbon storage wells.

Salt Water Disposal Wells: As oil and natural gas are brought to the surface, they generally are mixed with salt water. Geologic formations are selected to receive the produced waters, which are reinjected through disposal wells and enhanced recovery wells. These wells have been used as a standard practice in the oil and gas industry for many decades and are subject to authorization by regulatory agencies.

Enhanced Oil Recovery Wells(EOR): are used to increase production and prolong the life of oil-producing fields. Secondary recovery is an EOR process commonly referred to as water-flooding. In this process, salt water that was co-produced with oil and gas is reinjected into the oil-producing formation to drive oil into pumping wells, resulting in the recovery of additional oil. Tertiary recovery is an EOR process that is used after secondary recovery methods become inefficient or uneconomical. Tertiary recovery methods include the injection of gas, water with special additives, and steam to maintain and extend oil production. These methods allow the maximum amount of the oil to be retrieved out of the subsurface.

Hydrocarbon storage wells: are generally used for the underground storage of crude oil and liquid hydrocarbons in naturally occurring salt or rock formations. The wells are designed for both injection and removal of the stored hydrocarbons. The hydrocarbons are injected into the formation for storage and later pumped back out for processing and use.

Operations

Typically, oil, gas, and salt water are separated at the oil and gas production facilities. The salt water is then either piped or trucked to the injection site for disposal or EOR operations. There, the salt water is transferred to holding tanks and pumped down the injection well. For EOR, the salt water may be treated or augmented with other fluids prior to injection. In some EOR cases, fresh water, or fresh water converted to steam, is injected to maximize oil recovery.

Injection well operations are regulated in ways to prevent the contamination of USDWs and to ensure fluid placement and confinement within the authorized injection zone. This includes limitations on factors such as the pressure that can be used to pump the water or steam into the well, or the volume of the injectate.

Testing and Monitoring

After placing Class II injection wells in service, ground water protection is assured by testing and monitoring the wells. Injection pressures and volumes are monitored as a valuable indicator of well performance. Effective monitoring is important since it can identify problems below ground in the well so that corrective action can be taken quickly to prevent endangerment of USDWs.

Class 3

They are related to mineral extraction.

The techniques these wells use for mineral extraction may be divided into two basic categories: solution mining of salts and sulfur, and in situ leaching (in place leaching) for various minerals such as copper, gold, or uranium.

Solution mining techniques are used primarily for the extraction of salts and sulfur. For common salt, the solution mining process involves injection of relatively fresh water, which then dissolves the underground salt formation. The resulting brine solution is pumped to the surface, either through the space between the tubing and the casing in the injection well, or through separate production wells. The technique for solution mining of sulfur is known as the Frasch process. This process consists of injecting superheated water down the space between the tubing and the casings of the injection well and into the sulfur-bearing formations to melt the sulfur. The molten sulfur is extracted from the subsurface through the tubing in the injection well, with the aid of compressed air, which mixes with the liquid sulfur and airlifts it to the surface.

In situ leaching is commonly used to extract copper, gold and uranium. Uranium is the predominant mineral mined by this technique. The uranium in situ leaching process involves injection of a neutral water solution containing nontoxic chemicals (e.g., oxygen and carbon dioxide) down the well. This fortified water is circulated through an underground ore body or mineral zone to dissolve the uranium particles that coat the sand grains of the ore body. The resulting uranium-rich solution is then pumped to the surface, where the uranium is extracted from the solution and the leaching solution is recycled back into the ore body through the injection well.

Class 4

Class IV wells have been identified by the Regulatory Bodies as a significant threat to human health and the environment since these wells introduce very dangerous wastes into or above a potential drinking water source. The Regulatory Bodies has banned the use of these wells for many years. However, due to both accidents and illegal intentional acts, Class IV wells are still periodically found at various locations.

Regulators evaluate site conditions, determine what actions need to be taken to clean up the well and surrounding area, and permanently close the well so additional hazardous wastes cannot enter the subsurface through the well. This well class may include storm drains where spills of hazardous wastes enter the ground or septic systems where hazardous waste streams are combined with sanitary waste.

Although otherwise banned, there is one instance where Class IV wells are allowed. In these cases the wells are used to help clean up existing contamination. Sites exist where hazardous wastes have entered aquifers due to spills, leaks or similar releases into the subsurface.

Some remediation technologies require the contaminated ground water to be pumped out of the subsurface, treated at the surface to remove certain contaminants, and then pumped back into the contaminated formation. The process essentially creates a big treatment loop for the ground water.

(source: ground water protection council)

Advantages of Artificial Recharge

The use of aquifers for storage and distribution of water and removal of contaminants by natural cleaning processes which occur as polluted rain surface water infiltrate the soil and percolate down through the various geological formations.

Groundwater recharge is preferred because there are negligible evaporation losses, the water is not vulnerable to secondary contamination by animals or humans, and there are no algae blooms resulting in decreasing surface water quality.

In rock formations with high, structural integrity, few additional materials may be required (concrete, metal rods) to construct the well.

Groundwater recharge stores water during the wet season for use in the dry season when demand is highest.

Aquifer water can be improved by recharging with high quality injected water.

Aquifers provide large amounts of storage capacity that can be made available through auifer recharge hence increasing the sustainable yield of the aquifer.

Most aquifer recharge systems are easy to operate.

Disadvantages of Artificial Recharge

In the absence of financial incentives, laws, or other regulations to encourage landowners to maintain drainage wells adequately, the wells may fall into disrepair and ultimately becomes sources of groundwater contamination.

There is a potential for contamination of the groundwater from injected surface water run-off, especially from agricultural fields and road surfaces. In most cases, the surface water runoff is not pre-treated before injection.

Recharge can degrade the aquifer unless quality control of the injected water is adequate.

Unless significant volumes can be injected into an aquifer, groundwater recharge may not be economically feasible.

(source: Spandre R- EOLSS)

Artificial Recharge in Mauritius

The aquifers in Mauritius are mainly of the leaky type (geology of Mauritius). A leaky aquifer can be confined or unconfined and it can lose or gain water through aquitards bounding them from either above and/or below.

There are five main aquifers and the increase in demand for groundwater has caused extraction of freshwaters from aquifers.

The freshwater has been lowered to such an extent that seawater has invaded permeable bottom layers bearing freshwater. This phenomenon is known as seawater intrusion.

The aquifer becomes contaminated with salt which may become very difficult and costly to treat the water."