Friday, March 18, 2011

NW Kansas (Ogallala) Hydrology 101

Basically an aquifer is any underground formation that stores and can transmit pore water to a well in volumes that can be used. In nature, aquifers come in all shapes and sizes and usefulness. They can consist of fractured shales that yield just a few gallons per minute or day, to productive gravels that can yield multi-thousands of gallons per minute. Here in NW Kansas, our Ogallala Aquifer is middle of the road when compared to the possibility of aquifer extremes.

It’s also important to understand that even within a single aquifer, there easily can be considerable variability due to the aquifer make-up itself – how it was deposited, what materials it is made up of, how much cementing has occurred, how thick it is, how well or poorly sorted it is – you get the idea. Remember, groundwater exists in the relatively minute pores in between the aquifer material. Usually only 10% - 25% of a non-artesian aquifer’s saturated volume is water, and rarely is an entire non-artesian aquifer fully saturated. All this to make the point that a volume of water taken out of an aquifer will cause a water table decline that appears much larger.

It’s also possible that the aquifer characteristics change with depth. If they do, taking out a specific volume of water year after year (in a declining situation) will likely result in different rates of decline as different sections of the aquifer are dewatered. Add to this dynamic the variations from year to year in how much natural recharge is taking place from precipitation, underflows and/or leakage from other aquifers both above and below and you can see that it's no simple task to correlate withdrawals with changes in aquifer levels.

And what makes the system work? Gravity. The land surface, aquifer and water table in our NW Kansas portion of the Ogallala are all tilted slightly to the East. This means the groundwater elevation West of Colby is higher than it is to the East. This gradient – usually about 10 - 15 feet per mile – means the water is naturally moving through the aquifer pore spaces to the East (down gradient). The rate of this flow is dependent on the gradient and the interconnectedness of the aquifer pores – the steeper the slope and the more interconnected the pores are, the faster the flow rate. At our natural slope, our groundwater is moving about 75 feet per year. It’s our situation - small and fairly restricted connectivity between pore spaces and a relatively flat gradient - that dictate well yields, the size and shape of pumping cones, how fast the system flushes and how long contamination persists.

I hope this fairly general description has been helpful. If there are other aspects of groundwater hydrology you’d like to know about, don’t hesitate to ask.

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