Print View From: Joe To: CC: Date: Wednesday - September 30, 2009 1:14 PM Subject: Snake Valley water situation and agreement w/ Nevada To those involved with the Snake Valley water agreement: I attended the Aug. 18th meeting at the DEQ building and have a few comments on the overall Snake Valley water problem. 1. I think the use of the terms "safe yield" and "sustainable yield" should be discouraged. These terms have been abandoned by hydrologists because they are misleading and very difficult to define. Basically, the terms are intended to mean ground-water withdrawals from a basin that are less than the average annual recharge to the basin, including both recharge from precipitation and other kinds of input such as subsurface inflow. The concept is that if withdrawals are limited to this amount, then there shouldn't be any permanent, continual decline in water levels, or ground-water mining. The problem with "sustainable yield" or "safe yield" is that they imply that there will be no problems until this amount is reached, and that is far from the case. To illustrate, when ground-water withdrawals, say from a well- field, begin in a basin, water levels around the wells will decline as water is pulled into the wells. At first, most of the water comes out of storage in the aquifer as the water levels drop, and a "cone of depression" forms around the wells. As withdrawals continue, the cone of depression widens, and as it does it intercepts water that was discharging naturally, such as from a spring, from a marshy or wet meadow area, or by subsurface outflow from the basin. As soon as the cone intercepts enough natural discharge to balance the amount being pumped, then water levels will stabilize and the system is back in balance--the amount pumped is balanced by an equal amount of intercepted natural discharge. If the amount being withdrawn from wells gets up to the amount of recharge ("safe yield"), then all natural discharge from the basin will end--recharge to the basin is balanced by well discharge, and water levels will stabilize. The problem with this is that all kinds of other problems likely have occurred--if pumping equals recharge, water levels in the vicinity of the well field can be lowered tens to hundreds of feet, and all the springs in the valley, the wetlands, and amounts going to other areas by subsurface outflow are gone. Other bad things also can occur, such as land-surface subsidence, and possibly inflow of poor-quality water from down-gradient areas. I realize that at this point, the Southern Nevada Water Authority is not proposing to pump anything close to the annual recharge, which could be as much as 160,000 acre-feet/year (including subsurface inflow from Spring Valley) according to the BARCAS study, and that full effects wouldn't be seen for more than 100 years, but even amounts much less than this can cause problems over shorter time periods. 2. I am a little confused about the figure of 132,000 acre-feet/year that is mentioned frequently--according to the BARCAS study, the recharge is 160,000 (111,000 from precipitation in the basin and 49,000 from subsurface inflow). The only 132,000 number is their estimate of water discharge by evapotranspiration. During the Aug. 18th meeting, the 132,000 was also mentioned in terms of water rights and reserves--66,000 for each state. I realize that the BARCAS numbers represent a fairly large increase in previous estimates, and Utah is reluctant to use them, but then why use their number of 132,000 as the recharge (at least it seems that is how it is being used)? The original estimate of recharge made in the '60s was 105,000 acre-feet/year, including 4,000 from subsurface inflow from Spring Valley; a more recent estimate which tried to balance the budgets for all basins in the Fish Springs ground-water flow subsystem included, for Snake Valley, total recharge of 121,600 acre- feet/year, which consisted of 104,000 from precipitation and 17,600 from subsurface inflow. So I'm not sure where the 132,000 acre-feet/ year came from. 3. I asked at the meeting how an "unacceptable" effect of pumping would be defined--one that would require some kind of change in how the project was operated. I realize that it is necessary to be flexible on this and deal with things as they develop, but I think it would be useful to at least make some effort to define unacceptable effects. One would be land subsidence, which I don't recall being mentioned--as a rule of thumb, this doesn't occur until more than 100 feet of water-level decline has occurred, and that decline isn't out of the question with the planned amount of pumping, although it would take a long time to occur. Another effect would be inflow of poor- quality water, but it might be good to have some kind of limit--such as an increase of 250 mg/l dissolved solids. A third would be loss of marsh/wetland/subirrigated pasture area--it might be useful to define how much is a problem, like 25% of the area. And then water- level decline--a limit such as 100 feet? Local water-rights holders can lower their pump if the water level declines and still get the water they need, but it costs more. I think some kind of attempt to define unacceptable effects of SNWA's project would be useful. 4. My last comment is on the overall uncertainty in the knowledge about the deep carbonate-rock aquifer. The shallow basin-fill aquifer is better known--not so much in Snake Valley with its relatively small number of wells and small discharge--but we do know much more about basin-fill aquifers in other parts of western Utah, and understand pretty well how they work. The carbonate-rock aquifer is a different matter. Except in a few areas, such as southern Nevada, we know little about it. The SNWA is doing, I presume, extensive drilling in Nevada and hopefully the data will be available to all. The Utah Geological Survey has completed a test-drilling program in Utah and has a lot of new, good information, but I think if you asked them, they would say they have just scratched the surface. We know little about the extent of the carbonate aquifer-- it may not be much of an aquifer in many places. We know little about the quality of water in the carbonates--in some places close to recharge areas, such as in Snake Valley, it is good, but in other places farther from recharge areas it is slightly saline or worse, and we don't know much about what's in between. We don't know much about the degree of interconnection between the basin-fill and carbonate aquifers, although so far available data indicate it's pretty good. We also know little about the transmissive and storage properties of the carbonates--the storage coefficient of consolidated rock is generally much lower than basin fill and this value is very important in how the system will respond to development. We also know little about how water moves through the carbonates--many workers think that it moves through the entire mass of the carbonate rock, while a few others think water moves primarily along fault/ fracture zones. Most of the existing models assume the first is the case, but if movement is primarily along fault/fracture zones and storage is much less, then the models won't predict effects well. If I were the SNWA, proposing to spend $3.5 billion on a project, I would want to know much more about the system--they are obtaining information in Nevada, but none in Utah, and the biggest part of the Fish Springs flow subsystem is in Utah. It would seem prudent to me to take 1 or 2% of that $3.5 billion and invest it in an extensive test-drilling and aquifer-testing program, with at least half in Utah. Finding out that the system doesn't work as was assumed, after having built the whole project, would be somewhat of a disaster. I would like to see such a program included in the Nevada/ Utah agreement. I appreciate having the opportunity to comment on the overall issue and the agreement--it's a good thing to get public input on an issue this important. Joseph S. Gates, Hydrologist 2560 Cavalier Dr., Cottonwood Heights 84121 801-943-0957