Water Well Journal
Issue link: https://read.dmtmag.com/i/557876
strength low alloy steel, and stainless steel. Low carbon steel is 98%+ iron providing low tensile strength and low corrosion resistance with low cost. High strength low alloy steel pro- vides good mechanical properties (weld- ability), better corrosion resistance than low carbon, excellent strength to weight ratio, and a mid-range comparative cost. Stainless steel is available in more than 150 different grades based on chromium content. The chromium forms an inhibiting passivating film, chromium oxide, on the metal surface. The two most prominent grades of stainless steel used in well construction are 304 and 316 based on the ratio of chromium to nickel. Stainless steel pro- vides the highest corrosion resistance of the three common steel types used and correspondingly is the highest cost. An alternate to steel casing and screen components are PVC (polyvinyl chloride), HDPE (high density polyeth- ylene), and fiberglass. These materials all exhibit excellent corrosion resistance and ease of handling and installation due to their light weight. PVC, being the most used of these alternates, offers multiple coupling op- tions for easier installation. However, there are some collapse strength limita- tions to be considered in evaluating the use of these materials. Cost benefits of thermoplastics can be positive over steel, but screen design issues can impact this benefit. Combinations of stainless screen and PVC casing are cur- rently being used to address this issue. Filter pack component Many wells completed in unconsoli- dated or poorly consolidated aquifers re- quire a gravel pack to improve the well performance, eliminate sand production, and aid in well construction. There are two ways to develop a well gravel pack: (1) through development and removal of fine-grained aquifer material surround- ing the well (natural pack), or (2) to in- stall an artificial pack. The U.S. Army Corps of Engineers researched the use and design of gravel packs and concluded the particle size distribution curve for the pack should closely parallel the curve for the aquifer material in order to minimize movement of fine-grained aquifer material into the gravel pack and well. It also came up with ratios between the filter pack and aquifer material to use as design criteria. Recent commercially available gravel packs or pre-packs that are composed of more uniform sands or glass beads are easier to install and more permeable, but may not provide adequate filtering properties. The composition of the gravel packs can have a significant impact in a well's lifespan, particularly how the well holds up to rehabilitation. Typical packs com- posed of natural materials may contain minor secondary minerals such as mica, pyrite, feldspar, or carbonates which may impact stability, water chemistry, and operational life. Gravel pack standards specify they are composed of hard, rounded grains, no less than 95% silica. Recent studies show gravel packs that are not all silica (>90%) or have pitted, irregular-shaped grains degrade under acid treatments and foul more rapidly. The more rounded, less pitted nature offers less potential for the accumulation of biofilm which often acts as the catalyst for mineral scale development and more advanced fouling. Since its debut in Europe in 2007, a replacement to natural geologic materi- als comprised of medical-grade glass beads (Silibeads by Sigmund Lindner) is being promoted for use in wells. This manufactured material eliminates the in- clusion of natural undesirable minerals with higher percentages of silica (>96%), greater uniformity coefficient (1.09), and a more rounded and less pit- ted surface (reported sphericity of 85%- 95%). As with traditional gravel pack selection, care should be taken to design the pack the same as its natural counter- part. The complete life cycle The process of evaluating well con- struction material selection based on site-specific conditions is an exercise to evaluate and positively impact the complete life cycle cost of the well. Although there is an extremely large database of information on water wells from a variety of sources, there is little comparative information on complete life cycle cost. The 2012 National Ground Water Research and Educational Foundation William A. McEllhiney Distinguished Lecture Series presentation by Marvin Glotfelty, RG, "Life-Cycle Economic Analysis of Water Wells—Considera- tions for Design and Construction," pre- sents the most comprehensive analysis. This study does not consider thermo- plastics as those materials were not ap- plicable to the wells evaluated. Using cost information for low carbon steel, high strength low alloy (HSLA) steel, and stainless steel along with filter pack, drilling equipment, labor, engineering, and energy cost—a well "unit cost" was established. Developing a standard for com- parison, from the wells observed, the parameters of an 18-inch diameter, 1200-foot depth were selected. Based on these specifications the initial well costs were: Low carbon steel = $512,314 HSLA steel = $566,974 Stainless steel = $776,362 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: DOA-4, 5, 6, 7; DOB-1, 2, 3, 4; DOC-3, 4; DOE-4, 5, 6, 9, 10; DOF-1, 2; DOG- 1, 2, 5, 8, 9; DOH-1; DOJ-1, 3, 6, 7 More information on DACUM and the charts are available at www.NGWA.org. Oxidation Subject at Expo Session Coauthor Roger Miller will be leading a session at the Groundwater Expo titled The "Oxidation Reaction": Friend or Foe to the Groundwater Industry? It takes place Tuesday, December 15 from 9:15- 10:15 a.m. It will cover the general description of the oxidation reaction, outline where and how the reaction occurs, what creates the reaction, and what positive or negative effects it has on groundwater systems. WWJ September 2015 23 Twitter @WaterWellJournl CRITERIA continues on page 24