Over the last several decades, scientists have conducted a number of studies to address the nature and magnitude of groundwater flow across the INL. As a result of these studies, a substantial volume of geochemical data can be utilized to interrogate flow velocity at the INL. This section addresses the question of how fast groundwater flows beneath the INL, and utilizes the data collected from previous studies to extract this information. The velocities calculated from this exercise are at best an
approximation, due to uncertainties in identifying exact flow paths between given well sets. The velocities are based on an assumption of one-dimensional flow paths between wells used in the analyses and as such represent minimum values. However, it is important to note that for each area of the aquifer for which data is available and calculations were made, the variation in velocity is small. Therefore, the results can be assumed to be internally consistent and can be utilized in model calibration activities.
Flow velocity estimations were made based on the aquifer domains defined by Roback et al. (2001) (Figure 2-34) as these domains were based on regions of the aquifer for which uniform flow
characteristics exist. Additionally, the OU-10-08 modeling effort has established its flow domains based on this same set of geologic and geochemical information. In addition to the natural isotope and
geochemistry data that are available to conduct this exercise, anthropogenic contaminant data also are available to support identification of groundwater flow paths/zones. The most compelling reason to utilize groundwater chemistry to define the velocity domains is that the data are consistent with the geologic conditions that prevail in the eastern Snake River Plain.
The following subsections are a compilation of many data sets that can be used to extract information regarding flow velocity for the areas of the aquifer defined by Roback et al. (2001). The primary source of chemistry data for these regions are reports published by Roback et al. (2001) and Lou et al. (2000). As a means of calibrating the velocity calculations obtained from these studies, data from reports by Robertson (1974) and Cecil et al. (2000) were used to make supporting calculations.
Range
Figure 2-34. Interpreted preferential flow paths from stable isotope and thermal signatures (after Roback et al. [2001]).
2.2.4.1 Chlorine-36. Chlorine-36 released from INTEC provides valuable insight into groundwater velocity in the lower one-half of the Birch Creek flow path (Figure 2-35 shows both the Birch Creek and Little Lost River flow paths). Because chloride behaves conservatively in the aquifer and moves
downgradient at near the same velocity as does the groundwater, first arrival of this constituent at a downgradient well provide an estimate of minimum velocity. Cecil et al. (2000) calculated the velocity of water traveling from INTEC to Wells USGS -011 and -014 located south of the INL boundary. By modeling the first arrival and concentration of Cl-36 arrivals at these two wells he calculated a travel time of approximately 28 years. Yielding a flow velocity of approximately 3 m/day, however, analysis of radioactivity in earlier water samples collected from these wells indicate that Cl-36-bearing water may have arrived at these wells much earlier. Cecil et al. (2000) determined that if peak Cl-36 production occurred in 1958, as well as first arrival of C1-36 at USGS -011 and USGS -014, then the velocity in this region of the aquifer could be up to 2 times faster or 6 m/day and could indicate a maximum flow velocity of up to 6 m/day (Cecil et al. 2000).
2.2.4.2 Natural and Anthropogenic Tracers. Robertson (1974) looked at natural and anthropogenic chemical data collected from 1959–1972 in an attempt to characterize the Snake River Plain Aquifer beneath the Idaho National Laboratory, then known as the National Reactor Testing Station.
Although Roberson did extensive analysis of the natural chemical data available at that time, none of it yielded any insight into the aquifer flow velocity. They did however make some calculations of flow velocity based on contaminant transport. The velocity estimates from this work range between 1.5 and 6 m/day in the southern portion of the INL. Generally speaking, the 6 m/day estimate was for the southern portion of the Little Lost River flow path and the lower values were for the area located proximal or within the western slow flow region near the toe of the Lost River Range.
2.2.4.3 Isotope Velocity Estimates. The most comprehensive INL-wide geochemical data set available is from a series of reports conducted by Lou et al. (2000), Roback et al. (2001), and
Johnson et al. (2001) as part of a DOE Environmental Management Science Program. Although these reports did not investigate flow velocity per se, they do contain data that can be used to calculate velocity.
The chemistry-derived velocity estimates are based on a few simplifying assumptions. First, the chemistry data do not provide any information regarding the path that water takes between two wells; therefore it is assumed that the water flows in a direct path between the upgradient and downgradient wells. Second, because the data represent the amount of time that water has been in contact with the host rock, one can simply divide the time of reaction by the distance traveled to obtain the travel time. These assumptions are necessary due to the absence of a sufficient well spacing and should be considered a minimum velocity. Perhaps the most important assumption, however, is that the water along the flow path is chemically isolated from other water. That is, water traveling between two wells moves through a homogeneous media and does not contact water either from depth or from other flow paths. This assumption is problematic in that most of the sampling points are located in the uppermost part of the aquifer and do not have correlated open intervals. It is therefore appropriate to attribute these velocity estimates to the upper portion of the aquifer and any extrapolation to the vertical dimension remains problematic.
Flow velocity for the entire Little Lost River flow path (Figure 2-35) is 4.0 m/day. This means that over a length of 58 km originating at the mouth of the Little Lost River (transition to the eastern Snake River Plain) to Well USGS-124 south of the southern INL boundary, the water moves at a rate of 4.0 m/day. The upper one-third of the flow path starting from the transition to the eastern Snake River Plain down to Well USGS-17 is approximately 2.6 m/day. This assumes that water collected from Well USGS-17 is contained within the fast flow path originating at the mouth of the Little Lost River. Based on current understanding of the data, Well USGS-17 is on the edge of this flow path and may be affected by the slow flow area located to its east. This being the case and due to a lack of sampled wells between
Figure 2-35. Interpreted groundwater velocities along preferential flow paths.
the mouth of the Little Lost River and the central INL, one must look to Wells USGS-83 and USGS-107 for the next velocity estimate along this flow path. Calculated ages of these waters are 33 and 21 years respectively. Water from Well USGS-83 has a calculated age of 33 years indicating that water originating at the mouth of the Little Lost River traveled an average of 3.1 m/day to arrive at the well. Well
USGS-107 located more closely to the center of the fast path has an age of 21 years. If all of the water in this well has its origin in the Little Lost River Valley, then the water flow velocity between the mouth of the Little Lost River and USGS-107 is 4.8 m/day. The distal one-third of this flow path or the southern end from Well USGS-107 to Well USGS-124 is more constrained than the other sections of this zone due to the locations of many of the INL’s contaminated facilities. An estimate of flow velocity along this section of the flow path can be made between Wells USGS-107 and USGS-124 located approximately 13 km south of the INL boundary. These two wells are located approximately 18 km from each other and are located within the recognized Little Lost River flow path. The travel time for water between
USGS-107 and USGS-124 is 19 years and covers 18 km, yielding an average flow velocity of 2.6 m/day.
This is approximately 3 m/day slower than the velocity estimates derived from Cecil et al. (2000) and Robertson (1974). The differences between these estimates could be due to (a) scale effects, (b) not considering dispersivity, or (c) the travel distance between the wells was simply assumed to be the linear distance between the two locations.
Slow flow zones located on the east and the west of the Little Lost River flow path have calculated water ages approximately 6–9 years older than the waters of similar latitude in the Little Lost River flow path. Because so few wells exist in these regions, it is necessary to extrapolate between available wells, which yields average flow velocities of 0.3 to 0.4 m/day. This estimate assumes that all of the
equilibration of the water chemistry is a result of in situ digenetic activities, and that no significant
upwelling of deep equilibrated waters impacts water chemistry (a very questionable assumption according to McLing, Smith, and Johnson [2002]). Calculating flow velocities in these regions is severely hampered by a lack of well control and head data.
Calculations using the entire length of the Birch Creek flow path from the mouth of Birch Creek (transition to the eastern Snake River Plain) to the southern boundary of the INL yields an average flow velocity of 2.6 m/day. The upper one-third of this flow path passing through TAN yields a flow velocity of approximately 8.5 m/day. Velocity estimates based on chemistry for the area upgradient of TAN should be treated skeptically, as the location of the boundary between the Birch Creek Valley and the eastern Snake River Plain is not well understood. Additionally, there are numerous sources of young recharge water in this area, including the entire return flow from the Birch Creek which flows onto the Site just upgradient from TAN during the winter months. This is a problem in that the aquifer in this area is confined between the PQ and the QR interbeds into a 60-m thick aquifer, effectively increasing the impact of current year water. From TAN downgradient the velocity estimates for this flow path are better constrained due to the potential influence of the regional aquifer. However, the distal reaches of this region contains few control wells. Assuming that water flowing beneath TAN eventually contacts Well USGS-2, then it takes water approximately 42 years to travel within the flow path from TAN to Well USGS-2 a distance of 35 km. These data give an average flow velocity of 2.6 m/day, indicating that the velocity estimates from the upper section of the eastern flow path are affected by local conditions, including the arbitrary location of the Birch Creek Valley-eastern Snake River Plain contact.
Although this exercise yields some velocity estimates that are not internally consistent (such as the upper reaches of the eastern flow path), when all sources of data are placed into context, the variation in flow velocities between multiple data sets is generally very small. And in all cases, even the most extreme ranges are far less than an order of magnitude. In addition, the data collected from anthropogenic sources and the data collected from natural groundwater chemistry yield data that are remarkably similar. These velocity estimates provide valid field-based constraints on the simulated velocities that result from the flow model.
2.2.5 Estimates of Specific Discharge Based on Deep Well Temperature Profiles