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Response to Public Comment
Listed below are summaries of comments by Southern Nevada Water Authority (SNWA), followed by a response from USGS. The summary of each comment is linked to the original document submitted by SNWA.
Comments from SNWA
Complete letter from SNWA (pdf)
RESPONSE
The USGS considers the draft summary report provided for public review on June 1, 2007 as meeting the requirements of Public Law 108-424. It is true that additional detail and analysis of BARCAS study data is provided in a series of USGS Scientific Information Reports (SIR) as well as several reports produced by the Desert Research Institute (DRI). The creation and production of these supporting reports were not suggested nor required by PL 108-424, but rather were produced on the internal initiative of the USGS and DRI as a method of releasing additional detail that would have been unwieldy within the summary report. It should be noted that the series of six USGS SIR reports over which the Secretary has authority were released simultaneously with the summary report. Nothing in the language of PL 108-424 granted the Secretary special authority over the Desert Research Institute or it's activities.
No mechanism exists within USGS policies or procedures to accept public comment on completed USGS Scientific Investigations Reports. Nothing in PL 108-424 suggests such a change was required in this circumstance.
All USGS SIR and DRI Publication Series reports related to studies undertaken as part of BARCAS study are presently available to the public and members of Congress.
RESPONSE
The BCM code has not been published at this time; however, many of the input data sets have been calibrated to varying degrees as application of the BCM code continues to be applied to study areas in southwestern United States.
For example, INFIL models developed for the Yucca Mountain Project were calibrated to stream flow measurements (BSC, 2004; Flint et al., 2000). Instead of calibration, the recharge model relies heavily on the correctness of inputs, sub-models, and underlying assumptions. Consequently, the validity of the BCM-derived recharge estimates depends solely on the validity of the recharge model inputs.
RESPONSE
Hydrologic data are sparse in the study area limiting the number of calibration targets available in the study area. However, the model has been calibrated to flows in other regions in southwestern United States where hydrologic data are more prevalent. Some text has been added to the summary report (SIR 2007-5261) describing the source of input data and comparisons to measured data.
SNWA4—Some of the input data are highly uncertain.
As stated by the authors of the draft Recharge Report, the greatest source of recharge model uncertainty is likely the saturated hydraulic conductivity of the bedrock. Few measurements of this parameter exist. In addition, the spatial distribution of this parameter is highly variable and is usually derived during model calibration. Since the recharge model was not calibrated, the hydraulic conductivity distribution used in the model may not be valid.
RESPONSE
As with most modeling efforts, seldom are data available at the spatial and temporal density of interest. As part of the modeling effort, parameters are varied to illustrate the impact on model output and provide a range of possible outcomes. SIR 2007-5099 provides a table of values for the saturated hydraulic conductivity—the range in values for the more permeable units (alluvium, eolian, limestone, and sedimentary rocks) typically spans a single order of magnitude. This implies that estimates of in-place recharge may be pretty good. The lower permeability units are more variable affecting the potential runoff estimates—a difficult parameter to accurately quantify.
SNWA5—The quantitative uncertainty analysis is limited.
The quantified uncertainty does not incorporate the uncertainties of all input parameters. BCM requires many inputs, and each of the inputs has some amount of uncertainty. However, in the uncertainty analysis, only the uncertainty from the percentage of recharge from runoff was quantified. This simplified uncertainty analysis resulted in large ranges in the estimated recharge.
RESPONSE
The numerous input parameters used by BCM preclude a rigorous uncertainty analysis. Because of the complexity of the BCM model, each BCM run requires several weeks. Thousands of realizations are typically evaluated for each parameter during uncertainty analysis. Most of the parameters were validated to some degree. The objective of the BARCAS study was to provide a reasonable assessment of the regional hydrologic system rather than to quantify uncertainty of each input parameter.
SNWA6—Uncertain recharge estimates result in uncertain estimates of interbasin flow.
For each valley, the estimates of interbasin flow were calculated as residuals between recharge and evapotranspiration. Thus, recharge and evapotranspiration estimates drive the estimates of interbasin flow. For example, whereas the amount of recharge for Steptoe Valley was significantly larger than any previously reported estimate (Summary Report, Table 6, p. 48), the estimate of evapotranspiration was similar to previously reported values (Summary Report, Table 7, p. 49). This resulted in a large residual as an estimate of underflow. This underflow could easily be offset with the uncertainties associated with either the recharge estimates or discharge estimates.
RESPONSE
The BARCAS estimate of recharge for Steptoe Valley only is about 15 percent higher than previous estimates. As you state, the difference between the recharge and discharge (residual) incorporates uncertainties from both components; however, both geochemical modeling using NETPATH and water-budget accounting model (DSC) support the conceptualization of inter-basin flow across boundaries indicated in the report.
RESPONSE
An estimate of uncertainty for the interbasin flow component of the water budget is now provided in the Summary report. These estimates are the result of a Monte Carlo uncertainty analysis performed by Kevin Lundmark and presented in detail in DRI Pub. No. 41235.
SNWA8—There was no effort to quantify the uncertainty associated with water levels.
The water levels used to create Plates 1 and 2 were provided in SIR 2007-5089, but an uncertainty analysis of the water-level data was not presented to quantify the accuracy of the measurements. For example, USGS publication WRIR 02-4102 (D’Agnese et al., 2002) illustrated a method to quantify the observation error on hydraulic head measurements.
RESPONSE
An uncertainty analysis of the water-level data was outside the scope of work, however, attempts were made to limit the primary error (land surface datum) associated with hydraulic head measurements. For example, D’Agnese et al (2002) attributes land surface errors as often the largest error affecting the head observation accuracy. Data presented in SIR 2007-5089 minimized this source of error by utilizing a common datum derived from the USGS EROS Data Center, National Elevation Dataset of 1999 (see SIR 2007-5089, Appendix A).
SNWA9—The term "static" should be defined.
Page 3 of SIR 2007-5089 states that the study collected, compiled, and evaluated 418 water-level measurements to determine measurements that represent static water-level conditions in each aquifer. Page 5, however, states that historical water-level measurements that represent current ground-water conditions were used to develop the contour maps. It is uncertain what the water-level measurements actually represent. If the assumption is that the current measurements represent static conditions, this should be explicitly stated.
RESPONSE
Sections of the SIR are being revised per public comment. The term static was replaced by the term steady-state. While the data used to construct the maps were collected over multiple years, only data with minimal temporal change were used. Additional text was added to the summary report to define the criteria used to accept values for mapping purposes.
SIR 2007-5089 states that 76 basin-fill wells and 43 carbonate wells were used to create the potentiometric contour map. This means that over 60% of the control points do not penetrate the carbonate-rock aquifer system. As a result, contour lines near these control points are uncertain and should be depicted as such (i.e., dashed lines indicate "Uncertain" status). Also, page 5 of SIR 2007-5089 states that potentiometric surface and water-table maps published in previous reports were used as secondary guides in developing hydraulic head contours. These reports were not documented or discussed nor were the areas documented where these previous reports were used. In addition, contours in these areas should be dashed to indicate “Uncertain” status.
RESPONSE
Additional text has been added to the Summary report to describe why it is reasonable to construct a potentiometric surface for the carbonate-rock aquifer using water levels measured in basin-fill wells. Application of the criteria described by Bedinger and Harrill (2004) was used to constrain the heads used to map the potentiometric surface. Previously published potentiometric surface maps (Thomas and others, 1986; Bedinger and Harrill, 2004; Gates, 2004) and simulated potentiometric surfaces (Burbey and Prudic, 1991) guided map construction. These maps were relatively consistent; therefore, a solid contour line was used. So far, water-level data available from newly constructed wells do not violate current map construction. Refinements to the map will be made as additional data are available.
Although the authors state that the approach used to generate the contour map is similar to that published in the Death Valley Regional Flow System (DVRFS) for southern Nevada, the control points appear to be quite different. For example, only two regional springs were used in the entire BARCAS map, whereas the DVRFS map contains many springs for the same area. For example, on Plate 1 of Belcher (2004) there are at least 6 spring locations (Bastian, South Millick, Shoshone, Minerva, Swallow Canyon, and Unnamed spring), in Spring Valley alone, that were used for the construction of water-level contours.
RESPONSE
While the springs are not shown on the maps, the land surface (assumed to be the spring head) was used to guide map construction using the criteria that regional spring head/potentiometric surface is higher than the water table and local spring head/water table is higher than the potentiometric surface (Wilson, 2007).
SNWA12—The geologic evidence does not support large groundwater flow volumes along this path.
The recent work performed by SNWA analyzed previous investigations conducted by the Nevada Bureau of Mines and Geology, University of Utah, Stanford University, and USGS (Hose and Blake, 1976; Tschanz and Pampeyan, 1970; Loucks et al., 1989; Best et al., 1989; Best et al., 1993; Gans et al., 1989; Lumsden et al., 2002; Mankinen et al., 2007; McPhee, et al., 2005; McPhee, et al., 2006; Scheirer, 2005; Dixon et al, in prep.; Poole and Sandberg,1977; Willis et al., 1987; Drewes, 1967) and concluded that the northern portion of the Fortification Range is complexly faulted and contains repeated sections of the Chainman Shale, an aquitard, beneath the surface and water table. The southern portion of the Fortification Range contains volcanic rocks that SNWA interprets to be intracaldera rocks of the Indian Peak caldera complex. SNWA believes that the combination of Chainman Shale and intracaldera rocks most likely restricts groundwater flow through the range.
RESPONSE
Neither the characterized hydrographic area boundaries (fig. 15) or the potentiometric-surface map (Plate 3) prohibit flow along the flow path. Previous water budget estimates resulted in minimal excess recharge and therefore, examining subsurface outflow routes would not have been considered.
SNWA13—Flow arrows are not consistent with water-level contours.
The potentiometric surface map (Plate 3) includes flow arrows that depict flow going from southern Steptoe Valley to Lake Valley. However, the potentiometric contours indicate a groundwater gradient that is steeper in the direction of southern Steptoe Valley to Cave Valley. The permissibility of flow characterization (Plate 3) also indicates a preferential flow path in the direction of Cave Valley. This information is contradictory to the postulated flow from Steptoe Valley and to the commonly accepted delineations of groundwater flow systems in the Great Basin (e.g., Harrill et al., 1983; Harrill et al., 1988; Harrill and Prudic, 1998; Nichols, 2000). Furthermore, in USGS publication HA 694-B (Thomas et al., 1986) the potentiometric contours for the rocks of the carbonate-rock province clearly indicate groundwater flow from southern Steptoe Valley to northern Steptoe Valley not Lake Valley.
RESPONSE
Agreed, flow arrows have been removed from Plate 3. The large recharge mound over Steptoe and other valleys, and smaller higher mounds over the Egan and Schell Creek Ranges result in this region being the headwaters of numerous flow systems. Intra-basin boundaries, defined as shallow depths to basement rock, split Steptoe into multiple sub-basins. These intra-basin boundaries may act as divides, shunting water in differing directions within Steptoe Valley. Lundmark and others (2007) note that “the southern portion of Steptoe Valley is an important area because it has the highest average potentiometric surface and does not appear to travel northward in Steptoe Valley because the ground water is isotopically heavier in southern Steptoe Valley”.
SNWA14—Geochemical modeling does not support the flow path directions and/or volumes.
The deuterium-calibrated discrete state compartment (DSC) model, developed by the Desert Research Institute (Lundmark, 2007, Master's Thesis; Lundmark et al., 2007), is based on the same series of water samples used for the geochemical modeling described in Hershey et al. (2007). The DSC model is based on a single geochemical parameter (deuterium), whereas the geochemical modeling is a more rigorous process using multiple chemical parameters including deuterium. Flow paths developed using the DSC model should therefore be supported by the geochemical modeling in order to be considered viable. This is not the case for the Steptoe Valley to Snake Valley flow path. The relatively high flow rates from Steptoe Valley to Lake Valley and from Lake Valley to Spring Valley (Summary Report, Plate 3) are inconsistent with the geochemical modeling results. Although geochemical modeling does not support flow from Steptoe Valley into Lake Valley (Hershey et al., 2007, p. 74), flow of 20,000 acre-feet per year is shown in Plate 3. Although only 0 to 5 percent of the groundwater in southern Spring Valley was determined to be from Lake Valley (Hershey et al., 2007, p. 69), flow of 29,000 acre-feet per year is shown in Plate 3.
RESPONSE
Agreed, flow arrows have been removed from Plate 3. The flow path from southern Steptoe to southern Snake is possible according to both the DSC (fig. 41) and NETPATH (fig. 45) isotopic/chemical model results. While the NETPATH model from Steptoe to Lake Valleys was inconclusive—flow from southern Steptoe to Spring, from Lake to Spring, and from Spring to Snake Valleys were all feasible.
Without knowing the locations of the initial (or mixing) waters, their chemical compositions, and their relative contribution as a mixing end-member, the validity of the model in supporting interbasin flow cannot be verified. In addition, in Table 8 of the Summary Report the ratio of initial and recharge components for the Lake Valley to Spring Valley flow path is misleading because the initial water (95 to 100 percent) is from Spring Valley and not Lake Valley as suggested in the table.
RESPONSE
The authors’ state in Hershey and others (2007) that the water chemistry and isotopic database used for water-rock modeling consisted of 397 complete water analyses. Sampling sites are located on figure 17. Chemical data are listed in the appendices of Hershey and others (2007).
SNWA16—The magnitude of interbasin flow is not always supported by geochemical modeling.
The Summary Report states that the magnitude of interbasin flow for selected HA boundaries was supported by geochemical modeling (p. 73, last sentence). The selected HA boundaries should be specified and the reason for this selection presented. Only a limited number of the flow paths were tested using geochemical modeling (interbasin flow between Spring and Snake, Steptoe, and Lake basins and between Steptoe and Spring, Lake, and Cave basins) (Hershey et al., 2007, pp. 39 and 40). Out of these modeled flow paths, the magnitude of interbasin flow is not always supported by the geochemical modeling results. For instance, geochemical modeling does not support 20,000 acre-feet per year flow from Steptoe Valley into Lake Valley; (1) no valid models were found for this flow path (Hershey et al., 2007, p. 74); (2) nor is 29,000 acre-feet per year flow from Lake Valley into Spring Valley supported; and (3) the contribution of Lake Valley groundwater to Spring Valley was determined to be 0 to 5 percent (Hershey et al., 2007, p. 69). The geochemical modeling results indicated that the contribution of southern Spring Valley groundwater to southern Snake Valley was indeterminate (Hershey et al., 2007, p 67).
RESPONSE
See response to question 14 above. Geochemical modeling includes both the isotopic modeling (DSC) and NETPATH. The primary reason for only a select number of flow paths being evaluated is lack of data. In the Hershey and others (2007) abstract it is stated “water-rock reaction models could not be developed for other basins in the study area because of the limited number of locations with water chemistry and isotopic data”. Text has been added to the interbasin flow discussion to clarify the selection of interbasin boundaries of interest and the limitation for evaluating every boundary. Both of the geochemical models are non-unique and rely on numerous assumptions about initial water chemistry, degree of mixing, and “openness” of the system. These models can not definitively identify flow paths; rather model results are used to eliminate unlikely (unrealistic) flowpaths.
SNWA17—DSC model results are not consistent with the previous work of Thomas et al. (2001).
The results of the DSC model are also inconsistent with those obtained using a similar approach by Thomas et al. (2001) and appear to be quite dependent on the boundaries of the model. For instance, the flow directions in the model by Thomas et al. (2001) are south from Lake Valley to Patterson Valley and southwest from Cave Valley to Pahroc Valley. In the DSC model, the flow is east from Lake Valley to Spring Valley (with no southward flow into Patterson Valley) and southwest from Cave Valley to White River Valley (with no southward flow to Pahroc Valley). Lake Valley is on the northeast boundary of the Thomas et al. (2001) model and is on the southern boundary in the current DSC model. Similarly, Cave Valley is on the northern boundary of the Thomas et al. (2001) model and is on the southern boundary of the current model. Therefore, it seems worthwhile to extend the DSC model boundaries further south to determine the impact on the modeled flow direction and magnitude.
RESPONSE
The current DSC model uses a different data set than Thomas and others (2001) used. As stated by Lundmark and others (2007), the optimal or best model was defined by achieving a minimum difference (residual) between the simulated and observed deuterium concentrations and ET rates. But it is important to note that other models may yield similar residuals yet have different flow patterns. Reducing uncertainty will require isotope data from areas that currently have sparse or non-existent sampling sites and expanding the modeled area to include explicit sinks for water of known magnitude.
SNWA18—The geologic evidence does not support large groundwater flow volumes along this path.
The recent work performed by SNWA analyzed previous investigations conducted by the Nevada Bureau of Mines and Geology and USGS (Hose and Blake, 1976; Brokaw and Shawe, 1965; Brokaw and Heidrich, 1966; Brokaw, 1967; Brokaw and Barosh, 1968; Lumsden et al., 2002; Ponce, 1992; Scheirer, 2005; Mankinen et al., 2006; Dixon et al., in prep.; Kleinhampl and Ziony, 1985) and determined that the presence of volcanic, plutonic, and clastic rocks would most likely prohibit flow from Steptoe Valley to White River Valley.
RESPONSE
Neither the characterized hydrographic area boundaries (fig. 15) nor the potentiometric surface map (Plate 3) prohibit flow along the flow path. Previous water budget estimates resulted in minimal excess recharge and therefore examining subsurface outflow routes would not have been considered.
Given the major departure from conventional thought, this flow path should have been evaluated to help support or refute this new idea or the idea should have been abandoned since there appears to be no new data or evidence to support this interpretation.
RESPONSE
In the Hershey and others (2007) abstract, it is stated “water-rock reaction models could not be developed for other basins in the study area because of the limited number of locations with water chemistry and isotopic data”. In some cased NETPATH models could not be developed with the available data but this condition does not necessarily mean that the flowpath is invalid.
SNWA20—The sources of information used in the interpretation of the hydrogeology are limited.
For example, Plate 1 (hydrogeologic map) of the Summary Report was compiled only from digital versions of the 1:500,000-scale state geologic maps for Nevada and Utah (Summary Report, p. 13 and Plate 1), with additional sources used for determination of caldera boundaries and boundaries of highly extended terrains. Other more detailed maps were not considered, such as those prepared by the Nevada Bureau of Mines and Geology in cooperation with the USGS (Hose and Blake, 1976; and Tschanz and Pampeyan, 1970; Coats, 1987; Kleinhampl and Ziony, 1985) or those prepared by the Utah Geological Survey in cooperation with the USGS (Hintze and Davis, 2002a and b).
RESPONSE
The geologic maps cited as sources for Plate 1 are those sources for which digital spatial data were available in order to combine the NV and UT geology into a consistent set of hydrogeologic units. Such a combined map based upon digital data was needed not only for geologic interpretation but for the needs of infiltration and recharge modeling and other aspects of the project. Project resources did not allow for the creating of a new geologic compilation that incorporated more recent geologic mapping (such as the 1:24,000-scale mapping of the northern Snake Range by Elizabeth Miller and students) into the hydrogeologic plate. More detailed geologic maps were certainly consulted in the conceptualization of the system and the development of subsurface interpretations. Most of the publications cited by the reviewers can be found in the reference list.
SNWA21—The hydrogeologic framework section lacks interpreted cross-sections.
The inclusion of cross-sections, similar to those provided in Sweetkind et al. (2001) for the Death Valley region, could have supported other elements of the study and provided insights into the conceptualization of the hydrogeologic framework. Cross-sections provided on the bottom of Plate 1 of the Summary Report are only diagrammatic and are not referenced to the map. In addition, inconsistencies exist between the map and the cross-sections. For example, fault depiction is not consistent.
RESPONSE
A suite of published cross sections such as those published by Sweetkind and others (2001) was beyond the scope of the enabling legislation and the time and budget constraints of the project. We agree that a cross section-based approach or the construction of a detailed 3D framework model is the logical next steps for future study. Absent those types of subsurface interpretations, the BARCAS study relied on published reports, geologic map data, and geophysical data to develop subsurface interpretations.
SNWA22—The BARCAS hydrogeologic framework describes many of the basins as half grabens.
SNWA considers most of the basins in the area to be asymmetrical horsts, bounded on each side by asymmetrical grabens. This is supported by the many geophysical studies completed by the USGS Geophysical Unit of Menlo Park (Scheirer, 2005; Mankinen et al., 2006; Mankinen et al., 2007; McPhee et al., 2006; McPhee et al., 2007). These studies could have been used to explain the structural framework as well as depict the faults on Plate 1.
RESPONSE
The authors of the BARCAS report were aware of these studies that were in progress at the time; in fact, some of the authors of these reports were also authors of parts of the BARCAS report. We assume the reviewers mean that the ranges are asymmetrical horsts, rather than as the review comment is written. We feel that this comment is to some extent a matter of semantics and the level of detail at which individual basins are viewed. The half-graben concept is modeled after the seismic work in extensional terranes elsewhere (such as Lake Tanganyika) where a broad graben-like valley is seen to contain opposed or en-echelon half-grabens that tend to link in complex ways. This is a more detailed interpretation of the gravity data than simply defining the sides of a generalized graben.
SNWA23—Depiction of the transverse zones shown on Plate 1 of the Summary Report are too general.
Additional work performed by Rowley (1998) and Rowley and Dixon (2001) could be used to improve the transverse zones shown on the hydrogeologic map.
RESPONSE
Rowley (1998) is in the reference list and was consulted during the development of the transverse zone discussion. The depiction of the transverse zones was left generalized in part due to the suggestions of the project hydrologists that the hydrologic data do not demand that these features play a significant role in the overall flow system.
SNWA24—Care should be taken in using Hess et al. (2004) to describe basin fill.
In Table 3 of the Summary Report, there are several references to Hess et al. (2004), describing a lack of volcanics in a particular valley. Hess et al. (2004) document oil and gas well data. As the basin fill is generally not a target in drilling oil and gas wells, distinguishing between true valley-fill and volcanics is often not a priority, and therefore the different rock types penetrated while drilling through the basin-fill are typically aggregated into a general basin-fill category. Little Smoky Valley is an example of this type of interpretation error. The valley is surrounded by volcanics, which are described in Table 3 of the Summary Report, and has a caldera underlying the southern subbasin; therefore, it almost certainly contains volcanics that were not described in the borehole data.
RESPONSE
The reference to Hess and others (2004) is primarily to give the reader of the report a published reference to turn to when trying to access drill hole data in the study area. For Table 2, our intention was to summarize for the reader available published data. The reviewer’s assertion that “[the basin]… almost certainly contains volcanics that were not described in the borehole data”, while perhaps accurate, is an interpretive assertion that extends beyond the purpose of Table 2. Such an assertion would then necessitate an explanation of why the driller’s logs do not identify volcanic rocks. In the interest of brevity and allowing readers to evaluate published data sources, we attempted to base the entries in Table 2 on published sources.
SNWA25—Geochemical information used in any analysis should be presented in the report.
Statements made in the Summary Report (p. 78), specifically “some geochemical data were available for the study area” and “additional geochemical information was inferred” leave the reader to question how much data was real and how much was inferred. This is also not clear from Hershey et al. (2007). Much of the data used for the geochemical models is not presented in any of the BARCAS study reports (only data acquired for the BARCAS study are presented in Appendix A of Hershey et al., 2007). Although the data may be published in databases, the “best” individual analyses used for water-rock reaction modeling (Hershey et al., 2007, p. 33) are not known for several of the flow paths evaluated, and thus the validity of these models cannot be verified.
RESPONSE
Portions of the geochemistry section have been revised. The DRI report, Hershey and others (2007) is now properly cited because it was published after the release date for the draft Summary Report. The reviewers are referred to the numerous tables and appendices in Hershey and others (2007) that provide the data used in the geochemical modeling.
The tables summarizing exceedances of drinking water standards (Summary Report, Table 5; Hershey et al., 2007, Table 1) are not consistent, and thus it is unclear whether the same data set was used. This information should be consistent between the two reports, especially because the Summary Report is presented as a summary of the work presented in the other BARCAS reports (i.e., Hershey et al., 2007).
RESPONSE
There are a couple of differences between the tables found in the summary report and Hershey and others (2007) report. The differences are due to the data sets used to construct the tables. The table in Hershey and others (2007) used only data sets with a complete suite of major ion analyses; whereas, the summary report includes all data for a specific constituent of interest.
SNWA27—Flow from north-central Spring Valley to northern Spring Valley is not likely.
A geochemical model was developed and concluded that a flow path from central Spring Valley to northern Spring Valley was possible (Hershey et al., 2007, p. 49 to 53). The Yelland Playa acts as a discharge area for groundwater and streams in north-central Spring Valley. This interpretation is shown in SNWA's (SNWA, in prep.) own work as well as that of the USGS in Thomas et al. (1986).
RESPONSE
Both of the geochemical models are non-unique and rely on numerous assumptions about initial water chemistry, degree of mixing, and “openness” of the system. These models can not definitively identify flow paths; however, the NETPATH modeling did not refute the potential flow path.
SNWA28—Summary Report Table 8 heading incorrect.
The heading in Table 8 of the Summary Report (p. 79) should read "Inorganic Carbon Groundwater Flow Velocity" rather than "Inorganic Groundwater Flow Velocity".
RESPONSE
Column headings in Table 8 have been revised.
SNWA29—Summary Report Table 5 heading incorrect.
The heading in Table 5 (Summary Report, pg. 46) should be changed from “With Constituent” to “Constituent Detected.” The constituents are naturally occurring and probably present in the groundwater, albeit, at low levels. The “presence” of the constituent is thus dependent on the analytical detection limit.
RESPONSE
Column heading in Table 4 (previously Table 5) has been revised.
SNWA30—Incorrect Reference given.
Thomas et al. (2001) is listed in the reference section of the Summary Report (p. 94) as “Age Dating Groundwater …”, but is probably actually referring to the report “A Deuterium Mass-balance Interpretation of Groundwater Sources and Flows in Southeastern Nevada,” (Publication No. 41169).
RESPONSE
The reference has been revised.
SNWA31—The process by which water-level contours were created requires additional discussion.
Page 3 states that potentiometric and water-level surfaces are represented by spatially interpolated contours of hydraulic head. A discussion documenting how the contours were created is needed.
RESPONSE
Additional text has been added to the Summary report to describe why it is reasonable to construct a potentiometric surface for the carbonate-rock aquifer using water levels measured in basin-fill wells. Application of the criteria described by Bedinger and Harrill (2004) was used to constrain the heads used to map the potentiometric surface. Previously published potentiometric surface maps (Thomas and others, 1986; Bedinger and Harrill, 2004; Gates, 2004) and simulated potentiometric surfaces (Burbey and Prudic, 1991) were used to guide construction of the maps. These maps were relatively consistent; therefore a solid contour line was used. So far, water-level data available from newly constructed wells do not violate current map construction. Refinements to the map will be made as additional data are available.
SNWA32—What is the significance of Figure 4 in SIR 2007-5089?
Why is it important to highlight that most water-level measurements were collected in 1990 and 2005? The significance of Figure 4 and the 1990 and 2005 years of data collection should be discussed.
RESPONSE
Additional text will be added to the report to clarify the purpose of figure 4.
SNWA33—Incorrect definition of Measurement Altitude in Appendix A.
The definition for Measurement Altitude (feet) states that altitudes are rounded to nearest tenth of a foot. The data in Appendix A are clearly rounded to the nearest 10 feet.
RESPONSE
This definition has been corrected and file will be updated.
SNWA34—Please describe your use of the greater-than or less-than signs in Appendix A.
The circumstances that lead to an altitude being qualified with a '<' or '>' symbol should be described. Do these qualifiers represent dry holes or flowing wells? For example, site 380120114120701 was checked with the National Water Information System Database (http://waterdata.usgs.gov/nwis/) such that the land surface is at 5,770, so why is the value reported as greater than 5,730 ft?
RESPONSE
The greater-than and less-than signs indicate whether or not the mapped water level altitude for the carbonate-rock aquifer would likely be higher or lower than a water level measured in a well penetrating the basin fill. It is based on criteria stated in the report.
SNWA35—The study would have benefited from field verification.
The spring database is a nice compilation of existing USGS site location databases (such as the National Hydrography Dataset and National Water Information System, as well as published USGS topographic maps). However, the effort would have benefited from actual field investigations to verify these databases.
RESPONSE
It was beyond the time and budget constraints of the study to do field verification of all springs in the BARCAS study area. The database, as presented, gives some idea of the number of springs, and when available, the associated spring flows and chemical characteristics are provided. The BARCAS database will compliment the more detailed but relatively fewer sites included in the spring evaluation presented in the baseline report for the EIS.
SNWA36—State that comparisons with NDWR data were only performed for three valleys.
Page 67 of the Summary Report states that delineated acreages were compared to available NDWR crop inventories. The text implies that this was done for the entire study area, when in reality it was done for only Steptoe Valley, Newark Valley, and the northern part of Little Smoky Valley.
RESPONSE
The crop inventory evaluation that supports the conclusions in the summary report is given in detail in DS 273 by Welborn and Moreo (2007). Reviewers can find the information in question in DS 273. The summary report has been reviewed by the authors and the implication of concern to the reviewer is not evident in the current version.
For example, the statement “...if 375 ac-ft is required by the crop, then 425 ac-ft needs to be withdrawn from the well …” is vague and leaves the reader unclear as to the method used to determine the crop requirements. In addition, the text does not mention the supplemental nature of groundwater rights in these basins. Namely, a cursory examination of water rights from the Nevada Division of Water Resources reveals that several of these basins (including Spring, Snake, and Steptoe) contain supplemental groundwater rights. The text also combines spring discharge and groundwater pumped from wells into one category (i.e., groundwater), while the NDWR consider springs to be surface water.
RESPONSE
The cited example is intended only to illustrate how irrigation return flow is calculated, not crop consumptive use. The paragraph following equation 2 in the summary report explains the derivation of crop consumptive use estimates. These sections were revised and hopefully clarified.
The fact that some wells are used on a supplementary basis is readily acknowledged in DS 273 (Welborn and Moreo, 2007). However, the authors consider regional springs to be part of the regional ground-water flow system, and local springs and snowmelt runoff to be part of the local flow system. Regional springs are supported by regional ground water which is also the source to most of the ground water pumped from wells. NDWR administers Nevada water law. The purpose of this study is not to administer water rights but to develop an accurate conceptualization of the ground-water flow system of White Pine County.
SNWA38—Geodatabase analysis and field verification need to be described.
No discussion was provided in the report to document how the geodatabase that was constructed was analyzed to determine the irrigation water source, irrigation system, and crop type. Was this all determined by field verification? And, if so, additional information should be provided to describe the field verification efforts.
RESPONSE
The following statements can be found in the “Field Verification” section of DS 273 (Welborn and Moreo, 2007):"Fields were visited to confirm that irrigation had occurred in remotely delineated polygons. The irrigation method, crop type, and water source for each field also was inventoried." DS 273 is a final report.
