Response to Public Comment

Listed below are summaries of comments by Robin Davis, Licensed Professional Geologist, followed by a response from USGS. The summary of each comment is linked to the original document submitted by Robin Davis.

Comments from Robin Davis, Licensed Professional Geologist

Complete letter from Robin Davis (pdf)

RD1. The BARCAS study implies to support a massive groundwater extraction project proposed by SNWA.

RESPONSE

The BARCAS study was not designed to support or refute proposed ground-water development by SNWA.  The objective the of the study was to conduct a regional evaluation of the geology/structural characteristics, map the aquifer head distributions in the basin-fill and carbonate aquifers, and to re-evaluate estimates of the water budget components in the study area.

RD2. Precipitation data from very wet and non-representative period evaluated and provided too much ground water.

RESPONSE

Recharge was initially estimated for 1970-2004 because at the initiation of the study this was the available period of record for precipitation analysis.  Near the end of the study it was recognized that precipitation was about 5 percent greater during this period when compared to the long-term period of record (1895-2006).  Recharge estimates were adjusted (downward) to reflect the drier long-term period of record.

RD3. BCM assumes that all water from precipitation and snowmelt is available for infiltration and recharge for every time step, which were set at one-month intervals. 

RESPONSE

Net infiltration is the quantity of water that moves below the zone of surface evapotranspiration processes (Flint and others, 2004). Net infiltration occurs when enough water is made available to exceed the storage capacity of soil/rock. The authors’ state snow accumulated for several months provides enough moisture to exceed the soil storage capacity and exceed potential ET for the month or months during which snowmelt occurs.

A. Relying on minimum isotope tracer data to reveal an unprecedented discovery:  that the intermontane basin aquifers are significantly connected and therefore provide an abundance of groundwater.

RESPONSE

It is not an unprecedented discovery that aquifers are connected and interbasin flow occurs.  The earliest work, as early as 1909 by W.C. Mendenhall, and continuing through the evaluations by Eakin (1966), Harrill and others (1988), and Nichols (2000), realized that many of the basins (hydrographic areas) in the BARCAS study area generate more recharge than discharge. Additionally, down gradient HAs needed inflow to balance the ground-water discharge losses and support regional spring flow.

B. EPA has discovered that isotope tracer studies using only a single isotope can yield unreliable results on large scales.

RESPONSE

While this may be true—the DSC modeling provided a study-wide evaluation of the potential for ground-water to be exchanged among basins unlike other geochemical modeling such as NETPATH which evaluates exchange (flow paths) between point locations.  It should be noted that the DSC modeling results are supported by NETPATH results (multiple chemical species analysis). These results are presented in table 8 in the summary report (SIR 2007-5261).  Additionally, intrabasin flow within HAs, simulated by the DSC model, was supported by NETPATH models. None of the NETPATH models presented in table 8 refuted the DSC model results. 

RD4. Presenting results that show more harm than good for SNWA by overestimating available groundwater and contaminating their very supply by saltwater intrusion.

In other words, the regional adverse impact of groundwater extraction for the short-term benefit for a few in Nevada not worth the long-term risk.

RESPONSE

The USGS does not attempt to assess harmfulness or goodness for SNWA’s proposed action but rather to assess the response of hydrologic systems using new and existing data available at the time of this project.  Numerical modeling to evaluate the hydrologic system response to proposed ground-water pumping scenarios was not an objective of the study.  However, the authors state that “the remaining 80,000 acre-ft (net ground-water pumping in 2005) nearly equals the estimated quantity of ground-water outflow (90,000 acre-ft) from the study area.”  The similarity in these values does not imply an over-abundance of ground water.

RD5. Using the model PRISM when previously described in USGS reports as over-estimating precipitation particularly in eastern Nevada and mountainous areas.

RESPONSE

We agree that there are limitations when using the PRISM precipitation map for estimating recharge due to the over-estimation of precipitation particularly in higher elevation terrains.  However, it was beyond the scope of the study to develop a new precipitation distribution for the study area and so the readily available digital PRISM dataset was selected for use in the BCM. Additionally, PRISM estimated precipitation was compared to measured precipitation at 155 stations in Nevada and Utah. Annual measured precipitation for these stations averaged 12 inches and was about 1 inch less than the PRISM estimates. Differences between measured precipitation and PRISM estimates had a standard deviation of 4 inches. Therefore, errors resulting from using PRISM to distribute precipitation in the BCM were considered negligible.

RD6. BARCASS is inadequately comprehensive because it fails to consider all components of the hydrologic cycle.

The over abundance of groundwater reported in BARCASS that may cause deleterious effects on neighboring states is not responsible reporting or resource management.

RESPONSE

It is recognized that the BARCAS study does not include entire flow systems and that the estimates for water budget components tend to be slightly higher than the average of previous estimates; however, it was not an objective of the study to simulate (estimate) changes to the ground-water system (such as lowering of the water-level surface) due to future pumping nor to evaluate possible hydrologic responses to/changes to ecosystems from proposed pumping by SNWA.

RD7. Absent from the BARCAS analysis is the regional effect that aggressive ground water extraction would have including the expanding salt water wedge around the Great Salt Lake.

RESPONSE

The proposed analysis is beyond the scope of the study.  The Great Salt Lake is outside of the study area.  A calibrated ground-water flow and solute-transport model, and subsequent predictive pumping scenarios would be required to evaluate changes to the dynamics of a multi-density ground-water system. 

RD8. Geologic faults are mentioned in the BARCAS study and recognized for properties as a conduit to transmit water. 

Figure 16 accurately characterizes the basics such that normal faults result in surface springs that are vital to fending off a creation of a “wasteland”.

RESPONSE

Glad you liked figure 16.  Faults are discussed as potential conduits and barriers to flow in the BARCAS study.  The normal range-front faults that typify the study area may result in springs but the occurrence and magnitude of flow vary and typically are not the large regional springs.  Many of the range front springs form ephemeral streams that provide ground-water recharge downstream.