Water Well Journal
Issue link: http://read.dmtmag.com/i/543740
hypochlorite solution designed to deliver 300 ppm chlorine to a well if added to the top of the water level took as much as six hours to achieve satura- tion at the bottom of the well, and an additional six hours to distribute throughout the gravel pack. This indicated if the chlorine dose is added to the top of the water column and removed within the first six hours, less then half of the well surfaces are exposed to a biocidal concentration of chlorine. Thus for effective disinfection, you need to ensure you are delivering the prepared solution throughout the water column. Using a tremie pipe or similar means of application, you should spot the treatment solution evenly throughout the well, beginning at the bottom and working upwards. Let's walk though the disinfection process. When chlorinating a new or ex- isting well, several factors can improve the effectiveness. First, as with any procedure, you need to identify your goals and the well's condition. Evaluate the well from the surface to rule out potential contami- nation, vandalism, or any means of po- tential surface or near-surface influence on the well. For both new and old wells, it is advised to actively flush the well prior to treatment. For older wells, per- forming pre-treatment cleaning or evac- uation of the well prior to chlorination reduces the amount of biomass and de- bris that can readily degrade chlorine. Second, determine the pH and alka- linity of your mix water as well as iden- tify your desired chlorine concentration. By confirming the pH and alkalinity, you can identify the need for use of a chlorine enhancer and identify the correct dosage. This will allow you maximum production of hypochlorous acid—the biocidal form of chlorine— during treatment. Third, target a treatment volume suf- ficient to flood the entire well, borehole, and near-well aquifer with the chlorina- tion solution. We recommend using a value equivalent to three to four times the standing well volume. Fourth, do not shock the well! Target a chlorine concentration between 50 and 400 ppm for effective results. Using fresh sodium hypochlorite, in a 10% to 12% strength solution, is recommended over powdered calcium hypochlorite as it does not add any additional calcium to the well setting. Fifth, adding the chlorine into the well via a tremie pipe and making an effort to evenly distribute the solution throughout the entire well column will direct the solution to the areas it is needed. Applying agitation once the solution is in place will help disperse the solution throughout the targeted treatment area. Allow for sufficient contact time so the chlorine can go to work. As a rule of thumb, multiply the chlorine concentra- tion by the contact time, in hours, to a minimum of 1000 "contact units." Thus if you are using a solution of 250 ppm, you'll need a minimum of four hours contact time. When possible, allow the solution to stand overnight, downhole with periodic agitation. During chlorination efforts, we ad- vise regular monitoring of the downhole solution to ensure sufficient chlorine re- mains. There are several commercially available test strips that allow you to monitor the chlorine residual. It is rec- ommended you strive to maintain a min- imum active concentration of 50 ppm downhole during treatment. Once chlorination is complete, begin evacuation of the well from the bottom, working upwards, until all debris has been evacuated from the well. Monitor your chlorine concentration to ensure evacuation of the treatment solution. It is also advised you monitor con- ductivity and visual turbidity to make sure all disrupted material has been evacuated from the well. Once disinfec- tion efforts are completed, the well should be returned to an active operat- ing schedule as soon as possible. Proper Disposal Disposal of the chlorine solution is a growing concern, with regulations pres- ent in many states. The basic procedure is to capture the evacuated fluids and neutralize the chlorine content prior to disposal. There are a variety of chlorine- neutralizing products on the market including NW-500, sodium bisulfite, sodium thiosulfate, and ascorbic acid. Prior to evacuation, estimate the vol- ume of water you will discharge from the well to adequately flush the chlorine solution. Provide for a tank sufficient to hold the evacuated solution or a portion of it. Measure the chlorine level of the water and calculate the dose of the se- lected neutralizer required. Monitor the chlorine level within the tank to insure adequate neutralization. Once suffi- ciently de-chlorinated, the solution can generally be safely discharged. Check your local laws and regulation to make certain you are operating correctly. Well disinfection is a common but important form of maintenance. The hazards, costs, and potential impacts of chlorination dictate the procedure should be part of a scientific-based process. This is one where you take the time to evaluate the well—securing lab- oratory samples, pump data, or even a well video to correctly identify the con- dition of the well, the need for disinfec- tion, and the methods employed. And you are to do all this prior to beginning treatment. Misapplication of chlorine, a strong oxidizing chemistry, can damage new wells, cause failure of existing fouled wells, and present a potential hazardous condition for the crew and environment. Take the time to understand the prob- lem downhole and do it right. Michael Schnieders, PG, PH-GW, is a hydrogeologist and senior consultant for Water Systems Engineering in Ottawa, Kansas. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investi- gation and management. He can be reached at mschnieders@h2osystems.com. DACUM Codes To help meet your professional needs, this article covers skills and competen- cies found in DACUM charts for drillers and pump installers. DO refers to the drilling chart and PI represents the pumps chart. The letter and number immediately following is the skill on the chart covered by the article. This article covers: DOB-1, DOB-3, DOB-4, DOF-1, DOF-2, DOG-3, DOG-7, DOG-9, DOG-10, DOH-5, DOI-4, DOK-8 More information on DACUM and the charts are available at www.NGWA.org. WWJ WWJ August 2015 23 Twitter @WaterWellJournl