Posted: January 18, 2010     Author: John D. Bredehoeft

Snake Valley pumping very hard to monitor

Snake Valley pumping very hard to monitor

The Southern Nevada Water Authority proposes pumping groundwater from five desert basins in the Great Basin, including Snake Valley on the Nevada-Utah border, and piping the water south to Las Vegas. Under the proposed water-sharing agreement for Snake Valley, a monitoring plan is offered as a mechanism to control excessive adverse impacts.

The question is: Will monitoring work? The answer is that it is virtually impossible. Let me explain why.

Let's simulate monitoring the impacts of groundwater pumping in a hypothetical valley in the Great Basin. Let's say this hypothetical valley is 50 miles long and 25 miles wide and filled with typical permeable sand and gravel sediments. At the north end, mountain streams replenish the aquifer at an average rate of 100 cubic feet per second. At the south end is a spring that flows at the same rate. So the system is balanced. Inflow equals outflow.

Using a numerical groundwater model, let's simulate pumping in the central part of the valley, 25 miles from the spring, at the same rate that water comes in from the northern mountain streams. The flow of the spring will decline as it is impacted by the pumping. Fifty years in, the flow of the stream has dropped by 10 percent, so we stop the pumping.

But even though we've stopped pumping, the spring continues to decline at the same rate for another 15 to 20 years. Only 25 years after stopping does the spring start to recover. Ultimate recovery is so slow that even after 450 years, the spring has not regained its initial force, even though recharge to the aquifer is constant.

The fact that the spring flow continues to decline after pumping is halted surprises most people, even trained hydrogeologists. Suffice it here to say that pumping perturbs the system and it then takes some time for this disturbance to work its way through. Recovery takes a long time because the targeted aquifer is being replenished at only approximately 10 percent, or less, of the rate at which it was pumped.

Why? Think back to the experiment.

We depleted the aquifer at a rate of approximately 100 cfs for 50 years. While, after pumping, snowmelt and rain coming into the system might still occur at 100 cfs, and the depleted spring might flow at approximately 90 cfs, only the difference -- approximately 10 cfs -- will be recharging what was removed from the aquifer. In our model we know the exact cause of the decline -- pumping. The time delay we see in the response of the spring is because the wells are 25 miles away.

Now let's imagine that we are monitoring springs in Snake Valley. Our model suggests that the impact on a spring might be delayed by several years or longer, depending upon how far the pumping is from the spring.

As decline is spotted, enter the monitors, two from Nevada, two from Utah, to debate the cause. Undoubtedly, there will be wet and dry climate periods in the intervening years since pumping was initiated. There may be permanent climate change. Others will be pumping from the basin. There will be proponents for each of these factors as the principal cause in the decline. Defining causes could be further confused because the Southern Nevada Water Authority has reserved the right in its water-sharing agreement to move pumps around.

While modeling might suggest cause and effect, there will not be consensus about using model results.

Few will argue for totally stopping pumping, especially if large investments in infrastructure were made to support development.

The springs in Snake Valley are home to six unique species of fish that are of concern. Ultimately, a state or federal court, empowered by the Endangered Species Act, may restrict all the pumping thought to impact these creatures.

But once the genie of development is out of the bottle it will be virtually impossible to return it. Decline that is plainly predictable in careful modeling will persist in reality long after the monitors have stopped bickering or a judge has shut down the pumps. Adverse impacts -- ones that will be difficult, if not impossible, to pin on the pumping -- will undoubtedly follow.

One can envision a horror story.

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John D. Bredehoeft , who worked 32 years at the U.S. Geological Survey and currently consults on water resource and environmental projects, is an expert for the coalition opposing the Southern Nevada Water Authority groundwater development. A longer version of this article can be found at www.chanceofrain.com