| Performance Comparison: Direct-Push Wells Versus Drilled Wells |
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| Government Study Finds Direct Push Wells Comparable to Conventional Drilled Wells |
EnviroEquip
News, May 2002
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Port Hueneme, CA
EXECUTIVE SUMMARY
Introduction
A comparison between ground water monitoring alternatives (direct-push installed monitoring wells and hollow stem auger drilled monitoring wells) was conducted on the leading edge of a methyl-tertiary butyl ether (MTBE) plume located at Naval Base Ventura County (NBVC) Port Hueneme, California. The purpose of this effort was to determine whether representative chemical and water table data could be generated using properly designed direct-push monitoring wells.
An advisory committee comprised of experts from industry, government regulatory entities, and academia assisted with the project design and review of the work plan and all reporting efforts. |
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Methodology
Field efforts included piezocone measurements, collection of core samples, pre-installation collection of water samples from selected depths, installation of customized monitoring well test cells, and sampling of the wells in triplicate. Laboratory efforts included chemical analysis of water samples (for MTBE and various inorganic materials and parameters), determination of permeability for selected core samples, and determination of grain size distribution for selected samples.
From February 8 to February 14, 2000, a total of 32 wells were installed in two cells. Twelve wells were installed in Cell A, while a total of twenty wells were installed in Cell B. Specific well screen design (sand filter pack and slot size) was determined using several criteria. To evaluate performance of wells adhering to the ASTM specifications (ASTM D5092), grain size distribution curves were used to determine filter pack grain size and corresponding slot size recommendations. For Cell A, each of the wells was designed using ASTM specifications. For Cell B, two additional well designs were also employed to account for the most common well installation designs used by drillers and direct-push device operators.
Sampling
The effluent from the pump was run through the flow-through cell at the established flow rate. The well was purged for a minimum of 5 minutes while simultaneously monitoring for stabilization of dissolved oxygen, specific conductance, temperature, and pH in accordance with ASTM D5463. If stabilization was not observed within 5 minutes, pumping and monitoring continued until stabilization was achieved. The field team noted that 10-minute purge times are about the average duration.
Once stabilization occurred and readings were noted in the field logbook, the pump was turned off. The depth to ground water was checked for water table drawdown before sampling. If the drawdown was greater than 0.5 feet, pumping would continue at a reduced rate to allow the well to recover. For low-flow sampling, drawdown must be minimal (Puls and Barcelona, 1996). To date, drawdown has been negligible in all the wells during purging.
Initially the geo-chemical sample bottle was filled directly from the pump tube (at the established flow rate), then the pump speed was turned down to less than 100 mil per minute and the turbidity sample bottle filled. Turbidity was measured using a Portable Turbidity Meter. Finally, a set of three 40-ml VOA sample bottles was filled for the MTBE analysis.
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Analysis
An extensive statistical effort was conducted to compare the performance of the different well designs for the Port Hueneme hydrogeologic regime. Analysis of variance (ANOVA) was selected as the best technique for analyzing data consisting of categorical factor predictors and a continuously varying response variable.
The most striking result of the statistical analyses of the seven inorganic geochemical elements evaluated was that there were no strong systematic variations observed which were based on well type. There were significant temporal variations in many cases that were shared between Cell A and Cell B. However, there were no clear, consistent patterns among the temporal trends for the different elements. In a number of instances depth zone was also a significant factor with common trends for both cells. Once again, depth-related trends varied for different elements. Indications are that spatial and temporal variations in chemical concentrations are much larger than variations related to well type. |
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Table
5. List of Laboratory Methods
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| Analyte/Parameter |
EPA
Method |
| MTBE |
8260 |
| Color |
110.1 |
| Specific
Conductance |
120.1 |
| Hardness
(CaCO3) |
130.2 |
| Odor |
140.1 |
| PH |
150.1 |
| Total
Dissolved Solids |
160.1 |
| Turbidity |
180.1 |
Metals
(via Inductively
Coupled Plasma) |
200.7 |
| Ions |
300 |
| Alkalinity |
310.1 |
| Fluoride |
340.1 |
Surfactants
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425.1 |
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| Table 6. Cost and IDW
comparisons for direct-push and rotary installed wells |
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Direct-Push Wells |
Rotary Installed Wells |
| Well Diameter |
2" and 3/4" |
2" |
| Maximum Well Depth |
20' (6.1m) |
20' (6.1m) |
| Average No. Installations/Day |
6 |
4 |
| Average Cost (Equipment and
Labor) |
$20/ft |
$23/ft |
| Average Well Material Costs |
$3/ft* |
$6/ft |
| Solid Waste Generated |
0 drums |
6 drums (for 8-2" wells) 0.75
drums/well |
| Decon Rinseate Generated |
0.5 drums/day (5 total
for 24 wells) ~0.2 drum per 3/4" well (16)
~0.3 drum per 2" well (8) |
2 drums/day
(8 total for 8 wells)
~1 drum/well |
| Average Development Water Volume |
21 gal/well
(~10 gal/well per 3/4" well) |
45 gal/well
(~15 gal/well per 2" well) |
| *Stainless steel prepack
screens (2") cost $28/ft; Prepack schedule 40 PVC screens
(3/4") cost $10/ft. |
| Prices are in USD |
Summary and Conclusions
Installation of drilled wells requires more
time, and therefore more expense, than for direct-push wells. However, the
largest cost differences in the well installation efforts are associated
with the generation of solid and liquid waste. Solid soil cutting waste is
not generated for direct-push wells, except when required to set wellhead
traffic protection boxes. However, this relatively small amount of
material is generally not hazardous unless situated at the location of a
surface contaminant release. For this project, liquid waste generation
during well installation and development was 3 to 4 times higher for
drilled wells.
When planning for use of direct-push wells,
a significant cost advantage can be realized when coupling monitoring well
installation activities with site characterization activities associated
with solute plume delineation. Since many direct-push monitoring well
installation devices can be used to deploy direct-sensing probes,
expedited site characterization activities can be augmented with
direct-push wells without an additional mobilization requirement. This
approach significantly reduces the time and labor associated with report
review, contracting, and permitting activities often required when plume
delineation field efforts are limited to field screening and reporting
activities. In addition, the plume delineation field screening data can be
best utilized to determine appropriate ground water monitoring locations
while the investigators remain in the field.
Although a comprehensive hydraulic
evaluation was not conducted, water level values also appeared to yield
comparable results for the different well designs. Since the study
duration was limited to approximately 6 months, a longer observation
period may be required to evaluate the long-term and seasonal (greater
than 1 year) performance of direct-push wells.
In summary, no significant performance
differences were observed between the direct-push wells and hollow stem
auger drilled wells. Within experimental error, the performance was
comparable for the hydrogeologic setting of Port Hueneme, California. More
specifically, the chemical variability among the different well types was
less than that displayed by spatial heterogeneities associated with well
screen depth differences and temporal variability.
Downloads
Complete
Report (3.72MB - suggest
"right-click, save target as" to your desktop)
Part 1
- Executive Summary, Table of Contents, Introduction (1.18MB)
Part 2
- Description of Field and Lab Efforts, Analytical Results, Water Levels
(940KB)
Part 3
- Installation Costs, Statistical Analyses, Summary and Conclusions
(279KB)
Part 4
- Acknowledgements, References, Appendices (1.07MB)
Part 5
- Appendices cont., Images (645KB)
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By Mark Kram (NFESC), Dale Lorenzana (Intergraph), Dr. Joel Michaelsen (UCSB) and Ernest Lory (NFESC).
Naval Facilities Engineering Command
Washington DC 20374-5065
January 2001
NFESC Technical Report TR-2120-ENV |
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