Wednesday, August 17, 2011

Groundwater Movement in NW Kansas

As we have been discussing what to do in our high priority areas (HPAs) the question of basic hydrology comes up again and again - How fast is the groundwater moving through the aquifer?  There are beliefs that it is a fast moving river all the way to the fact that it doesn't move at all because it's in a very large bathtub.  The issue is important to the regulated community because they want to understand where the benefits to any groundwater conservation they accomplish will accrue.  Will the conserved water be available to them and the next generation? Or flow on to benefit folks elsewhere who are not conserving.

Based on these concerns the Kansas Geological Survey (KGS) did a water budget study on a legal township scale for a typical GMD 4 HPA in Thomas County.  (This is a larger report, but the TH-5 water budget portion starts on page 104)  There is about 300 feet of elevation drop from West to East across this 4-Township area whose cross section is 30 miles in length.  Since there are natural surface elevation and bedrock gradients from West to East, there is likewise a natural groundwater gradient as well.  So the bathtub theory is dispelled pretty quickly.

As it turns out, the hydology calculations show that the average pore velocity of groundwater coming into the West boundary of the study area is 1.05 feet per day.  They also found that the average pore velocity of groundwater leaving the East boundary is .56 feet per day - almost exactly half the incoming rate.  This has to do with all the pumping taking place inside the heart of the HPA area and its affect on natural groundwater flow gradients.  Essentially, this heavier pumping over time has depressed the gradient in the middle which has tended to speed up the upgradient pore velocities and slow down the downgradient velocities.  Anyway, there go the raging river groundwater flow myths as well.

They conclude several things: 

"Based on the values tabulated above, the long term flow in the area takes approximately 15-20 years per mile.  While the effects of local pumping might speed this up slightly, we consider it very unlikely that volume of groundwater underneath a township could be replaced in less than 50-60 years.  This means that the first and greatest effects of either conservation or depletion will be experienced in the immediate area."

So, while we can't guarantee every drop of conserved groundwater will be available forever to those making the sacrifices, it does appear that the the vast majority of the benefits will accrue to them for at least a generation or so.  And keep in mind, that every aquifer is different, so natural groundwater flow rates can be very different as well - ranging from very, very slow to veritable underground rivers.  That's just not the case here. 

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