Demonstration/Validation of the Snap Sampler Passive Groundwater Sampling Device
ER-200630
Background
Low-flow purging and sampling methods are commonly used to monitor groundwater but are expensive because of the time involved in waiting for purge parameters to stabilize, the capital cost of the dedicated equipment (pumps versus costly and questionably effective decontamination of the equipment between sampling events), and in many instances, the costs associated with disposing of the purge water. Also, low-flow sampling causes extensive mixing within the well and well bore, which prevents vertical profiling of the contaminant plume. Given the high costs associated with long-term monitoring for the Department of Defense (DoD) and the nation, a sampling method that is less labor-intensive and less costly but able to yield quality data is clearly needed.
Objectives of the Demonstration
The objectives of this project were to demonstrate that the Snap Sampler passive groundwater sampling device can provide technically defensible analytical data for the wide spectrum of analytes that are of concern to DoD at substantial cost savings.
Technology Description
The Snap Sampler passive groundwater sampling device can be used to obtain whole water samples in real time. This device is deployed in the well and left for an equilibration period that allows time for the well to recover from any disturbance caused by placing the device in the well, for the natural flow pattern in the well to be reestablished, and for the materials in the sampler to equilibrate with the analytes in the well water, thereby preventing losses of analytes due to sorption by the sampler materials. Also, by allowing time for the well to recover prior to collecting the sample, the well is less agitated during the sampling event and particles that are not normally mobile in the formation are less likely to be entrained in the sample when it is collected. Once the equilibration period is complete, the Sampler is triggered and the sample is sealed under in situ conditions. Thus, discrete depth within the well can be sampled.
Demonstration Results
Demonstrations were conducted at the former Pease Air Force Base (AFB) in Portsmouth, New Hampshire, and the former McClellan AFB in Sacramento, California. Ten sampling events were conducted at each site, and each monitoring well was sampled using Snap Samplers, Regenerated Cellulose (RGC) passive diffusion samplers, and the USEPA Region 1’s low-flow purging and sampling protocol. Analytes measured at the Pease site included total and dissolved concentrations of arsenic (As), calcium (Ca), iron (Fe), magnesium (Mg), manganese (Mn), potassium (K), and sodium (Na). At the McClellan site, samples were collected for a much broader range of analyte types. These included dissolved and total inorganics (non-metal anions, metalloids, and metals) and four volatile organic compounds (VOCs) (three chlorinated solvents and methyl tertbutyl ether [MTBE]). The performance criteria generally included the following: (1) the method could be used to collect samples for a range of contaminants, (2) the method provided reproducible results, and (3) there was agreement between the passive sampling methods and low-flow purging and sampling for the analytes of interest.
The Snap Sampler was able to collect adequate sample volume for all of the analyses, including requirements for additional quality assurance/quality control (QA/QC) samples. This was especially significant at the McClellan site, where samples were collected for several different analyte types, which required a relatively large volume of sample.
This sampling method provided reproducible data for the VOCs, dissolved inorganics, and total non-metal ions at both sites. However, at the McClellan site, this was not the case for several of the total metals, where both the Snap Sampler and low-flow samples had high variability between the field duplicate samples for chromium (Cr), Fe, and Mn. This was also true for both sampling methods for cobalt (Co), copper (Cu), and molybdenum (Mo), although concentrations of these analytes were near the reporting limit. The variability was also greater than the guideline for vanadium (V) with the Snap Sampler samples.
Generally, there was excellent agreement between analyte concentrations in the Snap Sampler and low-flow sampling, and these relationships were linear with the slopes equal to 1.0. There were no statistically significant differences between analyte concentrations in the Snap Sampler and the low-flow sampling for the VOCs, dissolved inorganics, total non-metal anions, and most of the total metals and metalloids. The exceptions to this were for total Fe (at both sites) and total Mn (at the McClellan site) where concentrations were significantly higher in the Snap Sampler samples.
Causes for the elevated concentrations of total Fe and Mn in the wells and in the Snap Sampler samples and the poor reproducibility of the two sampling methods for the total metals, which varied from well to well, may have included: (1) leaching of metal constituents of the stainless steel screens and low-carbon steel casing and screen; (2) corrosion of the well screens allowing fines to enter the well; and (3) agitation of the well caused when all the sampling equipment (i.e., bladder pump, baffles, Snap Samplers, and RGC samplers) was placed in the well. This agitation elevated the turbidity in the well and caused the formation of hydrous iron and possibly manganese oxides.
Because of the small pore size of the membrane in the RGC samplers, these samplers could not be used to collect samples for total inorganic analytes. For those analytes for which this sampler could be used, there was generally good agreement between the field replicate samples. The RGC sampler recovered equivalent concentrations of some but not all VOCs. MTBE and acetone were detected in the RGC samples but not in the low-flow or Snap Sampler samples, and trichloroethene concentrations were significantly lower in the RGC samplers than in the low-flow samples. For the dissolved metals and metalloids, there was good agreement between low-flow sampling and the RGC sampler for As, Ca, Cr, nickel (Ni), and V. Concentrations were significantly higher for barium (Ba), Mg, K, and Na in the RGC sampler although these differences were very small in magnitude.
Implementation Issues
This demonstration project showed that there does not appear to be any bias associated with using the Snap Sampler for sampling organic and most inorganic analytes. It is not clear, however, whether samples can be collected for some total metals, specifically total Fe and Mn. Inserting all the sampling equipment in the well elevated the turbidity in some of the wells, but it is not clear whether this would occur if only the Snap Sampler was placed in the well. Stainless steel and other steel casings and screens should not be used if analyzing for total metals that are constituents of the casing or screen material. This was true whether low-flow purging and sampling or the Snap Sampler were used.
The Snap Sampler was found to be relatively easy to use, and it provided lower sampling costs than low-flow sampling. Long-term monitoring costs were extrapolated for each site assuming that there were 50 wells and that quarterly sampling was conducted over 10 years. The cost savings associated with using the Snap Sampler was 46% at the McClellan site and 67% at the Pease site. The primary difference in the cost savings was attributed to the larger number of sample bottles that were needed at the McClellan site, where samples were collected for a broader spectrum of analyte types. Much of the cost savings were a result of the reduced sampling time needed to collect samples.
The RGC sampler does not have as broad an analyte capability as either the Snap Sampler or low-flow sampling. RGC samplers can only be used to sample for dissolved constituents so this prevents their use for total analytes such as total metals or highly hydrophobic organic analytes that can be particle borne. Because the RGC sampler can undergo biodegradation, using this sampler can necessitate two trips to the field: one to deploy the sampler and the other to retrieve the samples. However, the time needed for sampling is less than one-third of that needed for low-flow sampling. The cost savings for this sampler was 67% at the McClellan site and 71% at the Pease site.
Project Documents
Points of Contact
Principal Investigator
Ms. Louise Parker
U.S. Army Engineer Research and Development Center (ERDC)
Phone: 603-646-4393
Fax: 603-646-4640
Project Documents
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
- Guidance - Instructional information on technical topics such as protocols and user’s guides.
- Workshop Report - Summary of workshop discussion and findings.
- Multimedia - On demand videos, animations, and webcasts highlighting featured initiatives or technologies.
- Model/Software - Computer programs and applications available for download.
- Database - Digitally organized collection of data available to search and access.