Evaluación de Primer Año de Ejecución del Proyecto Competitividad Empresarial

Driscoll, F.G. (1987). Groundwater and Wells, 2nd Edn. Johnson

Division, St. Paul, MN.

Ground Water Manual, A Water Resources Technical Publication. (1985). U.S. Department of the Interior, Bureau of Reclamation. Parmley, R.O. (1992). Hydraulics Field Manual. McGraw-Hill,

New York.

Powers, J.P. (1992). Construction Dewatering, 2nd Edn. John Wiley & Sons, New York.

WATER WELL DRILLING TECHNIQUES

JAMESA. JACOBS

Environmental Bio-Systems, Inc. Mill Valley, California

Water well installation, for groundwater monitoring projects or for water supply wells for irrigation or human consumption involves a variety of drilling techniques. Rig selection is related to accessibility, time and cost of project, sediment type (consolidated rock or unconsolidated soils), sample type (undisturbed vs. disturbed), and sample integrity. For environmental projects, designing wells and evaluating groundwater also involves assessing the soil, sediments, or rocks above and below the water table.

The vadose zone includes the zone immediately above the water table to the surface. The lower vadose zone includes the capillary fringe, the zone where water is drawn upward by capillary force. The water table is the top of the saturated or phreatic zone within the aquifer.

More detail regarding water wells, aquifers, and drilling techniques is provided in Driscoll (1), Testa (2), Sisk (3), and DWR (4,5). There are numerous differences in drilling techniques, methods and equipment, many regional variations, and specific local requirements. A generalized summary of selected water well drilling techniques follows.

ROTARY DRILLING

Rotary drilling techniques include direct mud rotary, air rotary, air rotary with a casing driver, and dual- wall reverse circulation. Rotary drilling techniques are commonly used for deep water supply wells. Direct mud rotary drilling uses fluid, which is pumped down through the bit at the end of the drill rods and is circulated up the annular space back to the surface. The fluid at the surface is routed via a pipe or ditch to a sedimentation tank or pit, then to a suction pit where the fluid is recirculated back through the drill rods. Air rotary drilling uses air as a circulation medium instead of water. In unconsolidated deposits, direct mud or air rotary can be used, providing that a casing is driven as the drill bit is advanced. In dual- wall reverse circulation, the circulating medium (mud or air) is pumped downward between the outer casing and inner drill pipe, out through the drill bit, then up the inside of the drill pipe.

Rotary drilling techniques are commonly limited to consolidated deposits of rocks (Fig. 1). In mud rotary, a mud filter cake develops along the borehole wall, poten- tially reducing aquifer permeability. Where a resistant

Figure 1. Rotary drilling rig used to assess shallow spillage from train derailment area at mile 262 (Photo P045; EPA).

layer such as overlying basalt flows or conglomerate strata exists at shallow levels within the vadose zone and above the target depth, an air rotary rig can be used to drill to a predetermined depth followed by another more suitable drilling method (2).

CABLE-TOOL DRILLING

Cable-tool drilling is the oldest drilling technique available and is for installing water supply wells in selected locations. It is not used often in the environmental field, as the technique is slow, noisy, and dusty. The exception is the use of cable-tool drilling in glacial environments containing large cobbles in the Pacific Northwest portion of the United States or in young volcanics such as in Hawaii. Cable-tool rigs, called percussion or spudder rigs, operate by repeatedly lifting and dropping the heavy string of drilling tools in the borehole, crushing larger cobbles and rocks into smaller fragments. During cable-tool drilling, the hole is continuously cased by an unperforated steel casing with a drive shoe. The casing is attached on top by a rope socket to a cable that is suspended through a pulley from the mast of the drill rig. The process of driving the casing downward a few feet is followed by periodically bailing the borehole of the broken rocks and accumulated soils from the bottom of the borehole. Formation water or added water is used to create a slurry at the bottom of the borehole.

WIRE LINE CORING

Coring is the drilling method that produces cylindrically shaped cores. A rotary rig is used in conjunction with water, drilling mud, or air. Cutting is accomplished by drill bits located at the end of the rotating barrel or tube. The barrel gradually slides down into the annular opening. The core is then separated from the rest of the formation mass, and the barrel containing the core is retrieved.

106 WATER WELL DRILLING TECHNIQUES HOLLOW-STEM AUGER DRILLING

Hollow-stem continuous flight auger drilling techniques are commonly used for subsurface environmental projects (Fig. 2). Hollow stems consist of a series of continuous, interconnected hollow auger flights. The hollow-stem flight augers are hydraulically pressed downward and rotated to start drilling. Soil cuttings are rotated up the outside of the continuous flighting in the borehole annulus. A center rod with plug and pilot bit are mounted at the bottom. The plug is designed to keep soil from entering the mouth of the lead auger while drilling. Upon reaching the sampling depth, the center rod string with plug and pilot bit attached is removed from the mouth of the auger and replaced by a soil sampler.

The soil sampler is lowered into the borehole through the hollow stem of the auger (the center tube), and sampling is started. Samples can be continuously retrieved but are typically collected at 5-foot intervals, at hydrologic or lithologic changes, or at intervals of obvious contamination.

HORIZONTAL DRILLING

Horizontal or lateral radial wells are used more in subsurface environmental remediation although they have been used by the oil industry for decades. Based on the configuration in map view, horizontal water wells emanate from a center hub well. The most obvious application is where the area of concern, such as a contaminant plume,

Figure 2. Close-up of a hollow-stem auger rig (Courtesy of Joe Ryan, University of Colorado).

Figure 3. Direct push technology rig (Courtesy FAST-TEK).

is inaccessible due to aboveground structures, tankage, roads, or subsurface structures such as landfills, lagoons, pits, pipelines, or wells.

DIRECT PUSH DRILLING

Direct push technology (DPT) rigs are used almost exclusively in the environmental field (Fig. 3). DPT rigs rely on the static weight of the vehicle, typically a pick-up truck or van, combined with the percussion of the onboard industrial jackhammer as the energy for advancing the soil and groundwater samplers. DPT rigs are designed for easier accessibility than larger more conventional drilling rigs. DPT rigs can install small-diameter driven well points or piezometers. Unlike rotary auger rigs, DPT rigs do not generate large amounts of drilling derived wastes. DPT rigs, including cone penetrometer technology (CPT) rigs, can also drive various sensors tools to obtain data about subsurface conditions (6).

WELL INSTALLATION

Well materials for water supply wells and monitoring wells must be chemically compatible with potential contaminants as well as water geochemistry, such as pH, iron content, turbidity, alkalinity, and other parameters (Fig. 4). Casing, both blank and screen sections, can be constructed of fiberglass-reinforced plastic, stainless steel, concrete, or thermoplastic which include polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), and styrene rubber (SR). Based on cost, availability, and chemical compatibility, the most common casings and screens used for shallow drilling projects in the environmental field are made of PVC. Deeper wells are typically constructed of either thicker gauge PVC or steel. Well screen openings are commonly slotted, lowered, or created through a wire wrapping process. Slot sizes can be determined by previous well installations in the area or by a grain size sieve analysis. Centering devices may be fastened to the casing to assure even distribution of filter material and grout within the borehole annulus.

GROUNDWATER DYE TRACING IN KARST 107

Christy box (concerete) or traffic rated flush well cover Confining layer or unsaturated zone Water-bearing zone Confining layer (imperocable) Bentonite plug PVC end cap Static water level 0.010" slot 2" diameter schedule 40 PVC 2" Diameter schedule 40 PVC solid Locking top (plug)

Concrete

Grout (cement type I, II, portland 5-5% bentonite)

Minimum 1ft bentonite seal

Sand Maximum 20 ft of slots into water Bearing zone If aquitard (confining layer) is not encountered

Minimum 2" Annulus

1– 5 ft

Figure 4. Groundwater monitoring well construction diagram.

BIBLIOGRAPHY

1. Driscoll, F.G. (1986). Groundwater and Wells, 2nd Edn. Johnson Filtration Systems, St. Paul, MN, pp. 268–339. 2. Testa, S.M. (1994). Geologic Aspects of Hazardous Waste

Management. CRC, Boca Raton, FL, pp. 145–187.

3. Sisk, S.J. (1981). NEIC Manual for Groundwater/Subsurface Investigations at Hazardous Waste Sites. EPA-330/9-81-002, U.S. Environmental Protection Agency, Denver, CO.

4. California Department of Water Resources. (1981). California Well Standards, Bulletin 74–81, Sacramento, CA.

5. California Department of Water Resources. (1990). California Well Standards, Bulletin 74–90, Sacramento, CA.

6. Jacobs, J. (2000). Monitoring well construction and sampling techniques. In: Standard Handbook of Environmental Science, Health, and Technology. J. Lehr (Ed.). McGraw-Hill, New York, pp. 11.46–11.68.

READING LIST

American Society for Testing Materials. (1984). Standard Practice for Description and Identification of Soils (Visual-Manual Procedure), Method D 2488-84. Philadelphia, PA.

California Regional Water Quality Control Board. (1989). Leaking Underground Fuel Tank (LUFT) Field Manual: Guidelines for Site Assessment, Cleanup, and Underground Storage Tank Closure. Sacramento, CA.

California Regional Water Quality Control Board. (1990). Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites. Sacramento, CA.

Munsell Color. (1988). Munsell Soil Color Charts. Munsell Color, Baltimore, MD.

GROUNDWATER DYE TRACING IN KARST

DAVIDM. BEDNAR, JR. Michael Baker Jr., Inc. Shreveport, Louisiana

Groundwater dye tracing has proven to be an effec- tive tool for aquifer characterization, protection, and to provide remediation strategies in karst areas. Tracing

108 GROUNDWATER DYE TRACING IN KARST

groundwater flow routes with fluorescent dyes is highly successful because they are water soluble and inexpen- sive. Fluorescent dyes are usually the best tracers to use in karst because they are easily detected in concentra- tions that are one to three orders of magnitude less than those at which nonfluorescent dyes can be measured spec- trometrically (1). Additionally, they are safe to use, work effectively in different hydrogeologic settings, and can be used effectively to trace water from subsurface to surface water bodies. Dye tracing in karst has been used success- fully to (1) delineate spring recharge areas and subsurface basins, (2) determine site-specific hydrology, (3) estimate groundwater flow velocities, (4) trace groundwater flow from areas of recharge to discharge, (5) delineate wellhead protection areas, (6) map and characterize conduit flow routes, (7) detect leakage from residential sewage disposal systems, (8) identify sources of potential pollution from hazardous waste sites, (9) detect leakage from dam sites, and (10) characterize groundwater flow routes to impor- tant springs and cave streams along highway corridors. WHAT IS KARST?

To understand why groundwater dye tracing is an effective tool for characterizing karst groundwater resources, one must have a basic understanding of karst and the unique hydrologic characteristics of its landscape. Karst refers to lands primarily underlain by limestone and dolomite where surface water is integrally connected to the ground- water system through preferential flow routes, which results in the formation of distinctive surface landforms and hydrologic features as well as subsurface features. Lack of these landforms and features does not mean that karst is not present.

A characteristic feature of karst areas is the some- times rapid interconnection of surface water with the groundwater flow system. Even in the absence of sur- face streams, a karst region is a zone of drainage into the aquifer; the entire area can be a recharge zone (21). In most karst areas, two general types of recharge have been recognized: discrete and diffuse recharge. Discrete recharge, also known as concentrated recharge, is char- acterized by relatively rapid movement of water through localized areas (such as through sinkholes, losing streams, or other areas) toward the groundwater flow system. Areas of discrete recharge transport much of the water through preferential flow routes that commonly trans- port water at rates several orders of magnitude greater than those encountered on nonkarst groundwater sys- tems, support turbulent flow, are too large to provide effective filtration for most pathogens, and provide min- imal adsorption or other natural cleansing processes (4). Substantially greater quantities of water per unit area enter the groundwater system through discrete recharge areas than through diffuse recharge (5). For this reason, the groundwater system is highly vulnerable to contami- nation from accidental spills and poor land use practices. Groundwater flow velocities in many karst areas vary as much as 10 to 1500 ft per hour between the same two points—the latter, in response to storms—and are tens of thousands to several million times faster than those characteristic of many granular aquifers (1).

Diffuse recharge refers to the general and relatively slow seepage and percolation of recharge toward the groundwater system.

THE EPIKARST

The epikarst is the uppermost portion of the bedrock that consists of fissures and cavities formed by dissolution. The dissolution features in the epikarstic zone are organized to move infiltrating water laterally to downgradient seeps and springs or to collector structures such as shafts that conduct the water farther into the subsurface (6). The epikarst can vary from essentially zero to 30 meters or more (4) and is controlled by such factors as climate, depth of groundwater circulation, and bedrock structure.

Aley (7) summarized general dye recovery results for karst aquifers where dye was introduced directly into locations of discrete recharge that ultimately discharged to springs that passed through the epikarst. Aley recognized three hydrologically distinctive epikarstic zones in karst areas; rapid draining epikarsts, seasonally saturated epikarsts, and perennially saturated epikarsts. Rapid draining epikarsts commonly occur in areas of high topographic relief in bedrock of high solubility and negligible sediment infiltration. Rapidly draining epikarsts are saturated with water for short periods of time, especially after storms, and have little water storage or detainment. Seasonally saturated epikarsts occur in areas of moderate relief where the solubility of the bedrock has resulted in the development of soil and residuum thickness and at elevations greater than local perennial streams. Water is typically stored seasonally and after major storms lasting periods of weeks to months. Perennially saturated epikarsts occur in areas of low to moderate relief along perennial streams and are mostly saturated with water.

Groundwater dye tracing in the epikarst is more com- plex than that reported in most karst groundwater tracing reports. Aley (7) stressed that dye tracing in the epikarst requires extensive sampling and quantitative analysis, more detailed and quantitative analysis of background fluorescence characteristics, and simultaneous use of mul- tiple dyes with dye quantities and analytical approaches selected to minimize the chance that small dye recoveries are obscured by another dye.

Fluorescent Dyes

Many fluorescent dyes have been used in groundwater dye tracing to characterize flow routes in karst areas. For general problem solving in karst areas, eosine, fluorescein, pyranine, rhodamine WT, and sulforhodamine B are the most useful fluorescent dyes (8). The characteristics of these dyes are well documented in Aley (9), Smart and Laidlaw (10), and Kass (11). The selection of dyes to be used, the location of dye introduction points, the manner in which the dyes are introduced, the sampling strategy employed, and the analytical approach used must be tailored to the hydrogeologic setting, the issues of concern, and the quality and credibility of the data needed for the study (12).

GROUNDWATER DYE TRACING IN KARST 109 Sampling Locations

In karst landscapes, water that goes down into the subsurface eventually comes up at a spring or a series of springs. Springs represent the final terminus or discharge points of groundwater flow in karst areas and serve as excellent sampling locations to monitor for dye introduced into the groundwater system (surface streams, pumping wells, and monitoring wells can also be used).

It is equally important to know where the dye does not go as where the dye does go. All possible discharge points should be sampled. All springs within a radius of perhaps 5 to 15 miles from a facility, especially those within±90◦ of the vector of the hydraulic gradient from it, should be monitored during dye traces (13). For maximum results, dye tracing should be conducted during low and high flow conditions. Groundwater movement during periods of high flow may be diverted to higher preferential flow routes that may discharge at springs in adjacent groundwater basins. For this reason, it may be necessary to monitor 10 to 40 sites during a single trace to ensure that dye is detected. However, due to time and budget constraints, dye trace tests are often designed to maximize current hydrologic conditions.

GROUNDWATER TRACING METHODS

Activated carbon samplers are often used to recover dyes. Activated carbon samplers (also called charcoal samplers or passive detectors) consist of a few grains of activated coconut charcoal placed in heat-sealed packets of fiberglass screening. They are placed at sampling locations to adsorb continuously and thus accumulate specific fluorescent dyes (Figs. 1 and 2). As an illustration, a charcoal sampler in place in flowing water containing fluorescein or rhodamine WT for a week will typically contain about 400 times more dye upon analysis than the mean dye concentration in the water being sampled (14).

The sampling interval is based on site-specific condi- tions and the questions that need to be answered by the study. Weekly sampling intervals are appropriate for most

Figure 1. Activated charcoal packet placed at Big Spring, Hardy County, West Virginia. Spring discharges near red house located in the upper left corner of photo.

Figure 2. Activated charcoal packet attached to rock prior to placement in Waites Run, Hardy County, West Virginia.

studies. Once dye introduction points and sampling loca- tions have been identified, Aley (7) recommends one or more rounds of sampling at most (and preferably all) of the sampling locations to characterize background fluores- cence before the final selection of dye types and quantities are determined. The amount of dye used for tracing studies is typically based on professional experience, as there is no credible standard equation for estimating dye quantities needed for groundwater tracing work (8).

SELECTION OF DYE INTRODUCTION POINTS

In karst areas, sinking streams (Fig. 3) and sinkholes (Fig. 4) are commonly used as dye introduction points. However, the selection of appropriate dye introduction points depends on the type of study being performed. On some occasions, it is often difficult to encounter overland flow at ideal introduction points. This problem can be resolved by using ‘‘dry sets.’’ A dry set involves the placement of a dye so that it will be flushed into a surface drainageway, sinkhole, culvert, or roadside ditch by the first storm flow (8).

Additional successful techniques used for introducing dye, especially at hazardous waste sites, include epikarstic dye introduction points (EDIPs) and dye introduction trenches (DITs). EDIPs are vertical boreholes that enter the top of the bedrock. DITs are constructed with a backhoe, typically 17 to 33 feet long, and extend into the epikarst. The construction and use of EDIPs and DITs, in addition to determining water quantities used to flush dye into the subsurface, are described in Aley (7,8).

Groundwater monitoring wells are typically poorly suited for use as dye introduction points primarily because