Scientific Investigations Report 2006–5232

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5232

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Introduction

The Idaho National Laboratory (INL), which occupies about 890 mi2 of the eastern Snake River Plain (ESRP) in southeastern Idaho (fig. 1), is operated by the U.S. Department of Energy. INL facilities historically were used to develop peacetime atomic-energy applications, nuclear safety research, defense programs, and advanced energy concepts. Liquid-waste disposal resulted in detectable concentrations of several waste constituents in water from the eastern Snake River Plain aquifer (ESRPA) underlying the INL. The U.S. Geological Survey (USGS) INL Project Office provides an independent assessment of the migration and fate of waste constituents in water from the ESRPA. This report was prepared by the USGS in cooperation with the U.S. Department of Energy (DOE).

Ground-water-sampling protocols generally recommend that a well be purged prior to sampling (U.S. Environmental Protection Agency [USEPA], 1986). This recommendation is based on the assumption that the chemistry of stagnant water standing in a well casing or borehole is not the same as the chemistry of water in an aquifer. Three criteria commonly used to determine when a well has been purged sufficiently to yield representative water samples are: (1) removing an arbitrary number (usually three or more) wellbore volumes of water from the well; (2) monitoring field water-quality parameters—temperature, pH, specific conductance, turbidity, and dissolved oxygen—during purging until the values are stable; and (3) purging until hydraulic equilibrium takes place between well water and aquifer water (Gibs and Imbrigiotta, 1990; American Society for Testing and Materials, 1999). USGS policy specifies removing at least 3 wellbore volumes of standing water while measuring field parameters until they are within specified stability criteria for five successive measurements (temperature, ±0.2oC; pH, ±0.1 standard units; specific conductance, ±3 percent if greater than 100 µS/cm or ±5 percent if less than 100 µS/cm; dissolved oxygen (DO) concentration, ±0.3 mg/L; and turbidity, ±10 percent for NTU less than 100). Exceptions to the 3-wellbore-volume rule are allowed if site characteristics or study objectives require modifying the standard procedure by changing the number of borehole volumes removed from the well (Wilde and others, 1999). The American Society for Testing and Materials (1999, 2001, 2002) provide detailed information on additional well-purging criteria and combinations of the criteria described above, different types of equipment that can be used for purging and sampling ground water from monitoring wells, and alternative methods such as low-flow purging and sampling of ground water. Barcelona and others (2005) also described low-flow purging and sampling of ground water under high- and low-permeability conditions.

In 2000, an areal polygon was established downgradient of the Idaho Nuclear Technology and Engineering Center (INTEC) with ground water that contained Resource Conservation and Recovery Act (RCRA) listed waste (fig. 2). In August 2000, the State of Idaho Department of Environmental Quality (DEQ) granted a conditional No Longer Contained In Decision (NLCID) to DOE, which allowed for ground water removed from specific wells in the polygon to be discharged to the ground surface (Brian Monson, State of Idaho Department of Environmental Quality, written commun., August 21, 2000). The conditional NLCID was renewed by the DEQ in June 2002 (Brian Monson, State of Idaho Department of Environmental Quality, written commun., June 19, 2002), but was withdrawn in June 2003 (Brian Monson, State of Idaho Department of Environmental Quality, written commun., May 19, 2003). The USGS INL Project Office routinely collected ground-water samples from 30 wells inside the polygon that required purging 3 wellbore volumes and generated about 25,000 gal of purge water per sampling event. Beginning October 2003, the USGS was no longer allowed to discharge purge water to the ground surface. DEQ required that purge water be treated as a RCRA-listed waste, which necessitated pumping it into containers and transporting it to an approved disposal site.

In October 2003, the USGS began containerization of purge water removed from wells inside the polygon. Removing 3 wellbore volumes of purge water from many of these wells was not possible due to the large volumes of purge water and the size limitations of the containers. The USGS INL Project Office initiated a revised procedure to collect samples after purging 1 wellbore volume if field water-quality parameters stabilized. To evaluate the effects of decreased purging on the comparability of data, two techniques were used (1) a qualitative comparison of historical water-quality data from wells inside the polygon with water-quality data subsequently collected from the same wells after only 1 wellbore volume was purged, and (2) a quantitative comparison of data collected at selected wells (figs. 1, 2) after purging 1 wellbore volume with data collected at the same wells after purging 3 wellbore volumes.

Purpose and Scope

The purpose of this report is to present results of the qualitative and quantitative comparisons of water-quality data to determine if a change from purging 3 wellbore volumes to 1 wellbore volume has a discernible effect on data comparability. Specific objectives of the study were to: (1) plot and visually compare historical water-quality data for 30 wells in the RCRA-listed waste polygon with data collected after the change to purging only 1 wellbore volume; (2) use a simple statistical equation to determine if water-quality data collected from 17 long-pumping wells (after pumping 1 and 3 wellbore volumes) were statistically the same at the 95-percent confidence level; and (3) use the same simple statistical equation to re-evaluate if water-quality data reported by Bartholomay (1993) from 11 long-pumping wells at the INL (after 1, 2, and 3 wellbore volumes were pumped) were statistically the same at the 95-percent confidence level.

Geohydrologic Setting

The ESRP is a northeast-trending structural basin about 200-mi long and 50- to 70-mi wide. The basin is bounded by faults on the northwest and downwarping and faulting on the southeast. The basin is filled with basaltic lava flows interbedded with terrestrial sediments (Whitehead, 1986). Individual basalt flows range from 10- to 50-ft thick, although the average thickness probably ranges from 20 to 25 ft (Mundorff and others, 1964, p. 143). Sedimentary deposits consist mainly of lenticular beds of sand, silt, and clay with lesser amounts of gravel. Locally, rhyolitic lava flows and tuffs are exposed at the land surface and may exist at depth under most of the ESRP. A 10,365-ft-deep test hole at the INL penetrated 2,160 ft of basalt and sediment and 8,205 ft of rhyolitic volcanic rocks (Mann, 1986).

Basaltic lava flows and interbedded sedimentary deposits combine to form the ESRPA, the main source of ground water on the ESRP. Depth to water in the ESRPA at the INL ranges from about 200 ft in the northern part to more than 900 ft in the southern part. Direction of regional ground-water flow in the ESRPA generally is from the northeast to the southwest. Water moves horizontally through basalt interflow zones and vertically through joints and interfingering edges of the interflow zones. Infiltration of surface water, heavy pumpage, geologic conditions, and seasonal fluxes of recharge and discharge locally affect the movement of ground water (Garabedian, 1986).

Previous Investigations

Many previous studies evaluated well-purging criteria to determine when chemistry of water withdrawn from wells was representative of the chemistry of water in the aquifer. Most studies evaluated how changing well-purging criteria affected organic compound concentrations, especially in shallow sand and gravel aquifers. Dumouchelle and others (1990), Gibs and others (1990), and Herzog and others (1991) provide a comprehensive list of reports presenting results of these studies. Robin and Gilham (1987) used conservative inorganic tracers (sodium bromide and sodium chloride) to evaluate the number of purge volumes required to achieve chemical equilibrium between well water and aquifer water and concluded that less than three wellbore volumes was sufficient under the conditions of their tests. Robin and Gilham (1987) also pointed out that the findings of studies evaluating well-purging criteria are site specific and depend on local hydraulic, hydrologic, and geochemical conditions.

Barcelona and others (2005) presented results of a case study to illustrate low-flow sampling performance under minimal (low permeability) and significant, but stable (high permeability) drawdown conditions. They concluded (for the conditions of their study) that constituent concentrations stabilized after 2 wellbore volumes for a low-permeability well and after one-half wellbore volumes for a high-permeability well. They also concluded that stabilization of field water-quality parameters more effectively indicated when to collect samples than the number of wellbore volumes purged from a well.

The effect on concentrations of inorganic, organic, and radioactive constituents in water from deep aquifers like the ESRPA that result from changing well-purging criteria has not been studied extensively. However, Bartholomay (1993) evaluated changes in concentrations of tritium and strontium-90 after purging 1, 2, and 3 borehole volumes. Samples were collected from 11 ESRPA wells and analyzed by DOE’s Radiological and Environmental Sciences Laboratory (RESL) for tritium and strontium-90. Bartholomay (1993) presents statistical comparisons of analytical data from samples collected at each well. A pair of equations derived from a test method recommended by the American Society for Testing and Materials (1988) was used for the comparisons. Wells included in his study were selected because they required 1 or more hours to purge 1 borehole volume of water. Bartholomay (1993) concluded that tritium and strontium-90 concentrations in water samples from wells with purge times greater than 3 hrs at the INL are not measurably affected by purging either 1, 2, or 3 borehole volumes.

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