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GROUNDWATER INJECTION AND PROBLEM PREVENTION
An On-Line Version of a Column First Published in:
The National Environmental Journal Sept./Oct. 1993 Vol. 3 No. 5 Pages 24-25
by: David B. Vance
With few exceptions groundwater remediation requires groundwater recovery, treatment and disposal. Storm drains, sanitary sewers, and surface streams have been, and will continue to be, receptors for treated water. However, permit requirements and volume based user fees often contribute significantly to the cost of groundwater remediation systems.
In rural settings storm or sanitary drains may not be available; and increasingly in urban settings Publicly Owned Treatment Works (POTWs) are reluctant (or unable) to accept recovered groundwater. In urban settings water quality is not the issue, rather it is stress on the hydraulic capacity of the POTW system. In these instances the most viable option for water discharge is reinjection.
Reinjection can be less expensive (at least to set up) than permit fees and flow based discharge fees. As such, it is sometimes implemented even when other options are available. Lastly, an injection regime may be part of a program to obtain hydraulic control of a site.
The most common source of problems for groundwater recharge systems is plugging. Any recharge system will eventually foul and the design of the system should provide for that eventuality. The goal is to minimize the rate at which the fouling occurs. The purpose of this column is to examine some of the pitfalls that can occur when the recharge option is selected. Without exception it is much easier to prevent damage from occurring than it is to remedy a damaged condition.
Suspended solids are a major cause of plugging. In most instances solids will collect within the first ½ inch of a well pack. At a flow rate of 10 GPM and total suspended solids (TSS) at 5 mg/L, approximately 2/3 of pound of solids will be introduced per day. In a year this would total 240 pounds, or about 2 cubic feet of solids. This would be capable of fouling about 100 square feet of injection face (30 linear feet of a 1 foot diameter well). Generally infiltrated water must have a TSS level < 2 mg/L in order to maintain adequate infiltration rates over a long period.
Entrained air can impact recharge capacity through simple physical blockage or through what is termed the "Jamin Effect". The Jamin Effect results when capillary forces act on small advective channels that contain alternating air bubbles and water. These channels are then capable of responding to finite pressure gradients without allowing fluid flow, meaning that surging an impacted well will not displace the entrained air. Once in a formation, air can be extremely difficult to remove.
The problem is particularly exacerbated by allowing injected water to fall into a well (rather than piping below the groundwater surface), thus mixing with air that can be transported into the adjacent formation. Saturated zone in-situ bioremediation systems using injected air or hydrogen peroxide can also be susceptible to air entrainment problems.
Injection of water that is cooler than the receiving aquifer will cause degassing as the two waters mix. The solubility limit of oxygen in pure water at 40o F is 13.1 mg/L, at 50o F 11.3 mg/L. Over a 24 hour period, at a flow rate of 10 GPM, approximately 0.20 pounds of oxygen would degas upon contact with the formation water. This represents about 2.5 cubic feet of gas entrained in the pore spaces directly adjacent to the injection zone (as depth increases the gas volume would decrease). Assuming a one foot diameter well, and that entrainment of air through a 1 foot section of the formation would decrease injection efficiency through the Jamin Effect, a well with a 30 foot screened interval would have flow inhibited within 10 days.
To prevent microbial growth care must be taken to insure there is no source of carbon or nutrients (nitrogen and phosphorous) in the injected water. The carbon content of water should be evaluated through a Total Organic Carbon (TOC) analysis, not just contaminants of concern (such as BTEX). Bacteria will exploit any carbon resource, not just those under regulatory discharge limits. TOC must be < 10 mg/L to insure that reasonable infiltration rates can be maintained over cost effective durations.
In instances where the groundwater is iron or sulfate rich, iron bacteria or sulfate reducing bacteria may also create problems.
Chemical reactions between the injected and formation water are common. Typical reactions result in the formation of precipitants in the mixing zone. These reactions are complex and temperature sensitive. Differences in redox potential (usually due to dissolved oxygen levels) and carbonate chemistry are most often responsible for problems.
When injecting into clay rich soils, ionic reactions can also come into effect. Clay particles can be dispersed (to cause plugging deeper in the formation) or swell when exposed to recharge water with ionic character different than the formation water. These reactions can be induced through exposure to water of different ionic concentration or through exposure to water with different cations present (for example water discharged from a caustic or lime precipitation system).
Iron precipitation can be a serious problem in groundwater systems that are high in soluble ferrous iron. Iron present in recovered water may precipitate forming TSS that must be removed before reinjection. However, iron precipitation is also possible in the mix zone if the redox conditions of the recharge water are significantly different than the formation water.
Lastly, injection efficiency has been observed to be reduced by as much as 50% from compaction of the gravel/sand pack around a well through over zealous surging during well development, pumping tests or re-development performed to alleviate some of the problems described above. This is a common problem. In this instance more is not better, well development should be judiciously applied based on the composition of the pack around the casing.
Impact from all of the above can be minimized in an injection system during the design stage of a project. It requires a thorough evaluation of the chemistry of the discharge that includes determination of ionic character, redox condition, chemical composition, and temperature. The same must be done for the formation water. Potential reactions and problems should be evaluated by a competent geochemist. It may be determined that a different method of water treatment would be more cost effective (i.e. eliminating air stripping to prevent precipitation reactions driven by elevated dissolved oxygen).
Remember, the cliché "an ounce of prevention is worth a pound of cure" is most applicable to groundwater injection.
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