Friday, September 28, 2007

Arguing with a Professional Geologist

Below is an article I wrote for the Cabarrus Neighbors. This was actually written earlier than the previous post. Below that is a reply from a professional geologist followed by my response. Both get fairly technical. So far I haven't heard back from him so I can't determine if he is right or if I am right. I know in general it isn't smart to disagree with people at thier own game but I couldn't resist. Would be glad to hear what you think.

Original article
Q. My neighbor's lawn is still green, so I know he is irrigating. We are both on wells. Can he dry up my well by irrigating, or could we be drawing from different aquifers, since everybody's well is a different depth? Also, my brother down the road went to the biweekly watering schedule, and his lawn turned brown. Since my neighbor's lawn is staying green, doesn't that mean he is watering more than twice a week?
First, you are on the same aquifer, so your neighbor can dry up your well. This has happened at some places in Cabarrus County, although it took more than one person to do it.
Basically, in your area, everybody pumping groundwater from between the major creeks is withdrawing the same water. The depth of the well doesn't change the source of the water.
My suggestion is to monitor your well by measuring the static water level. The static water level changes and has to be measured. Subtract the static water level from the depth of casing. The depth of casing doesn't change and should be on a plate attached to your well. The difference gives you an idea of how much groundwater is available.
Likely there is enough available water that you won't have to worry about talking to your neighbor. If you have only a few feet of water, you need to educate your neighbor about groundwater. Neither of your wives will be happy if you have to start hauling in water.
Your neighbor, however, could be following the biweekly watering schedule, or even a weekly watering schedule, and keeping his lawn green. Your brother's frequent and shallow irrigation regime developed a very shallow root system. This proved impossible to manage on a biweekly watering. A lawn with deeper roots can remain green on biweekly or even weekly watering.
One more thing: Be careful not to contaminate the well when measuring the static water level. I can measure the static water level in a nearby dug well. Beyond that I haven't found any written recommendations.
I think it could also be measured in my drilled well by dropping a nail and cork on a string down the vent pipe after modifying the vent cap so it is removable. I wouldn't use a lead weight.
If I had a laser tape measure on hand, I would try using that first. Without trying it, I just don't know how you can be sure you are hitting the static water level and not the walls. I would also make the measurement at a time when the well has had time to recharge.

Response from Geologist

I appreciate your willingness to take a stab at answering the questions, but I felt compelled to write to clarify a few things about wells and groundwater in the piedmont.
First, the well's static water level when it was drilled should also be listed on the well tag. During times of drought, well owners should expect that the water level will be inches to feet below that level under natural conditions. As you reference, pumping of that well and other nearby wells can also depress the water level. Examples of natural variation in groundwater levels due to current drought conditions can be found at: and you'll notice that crystalline bedrock wells in the western part of the state are typically a foot or two below their 5 year running average.
In this area of the state, most people rely on wells that are drilled into the bedrock. For bedrock wells, the well casing typically extends from just above the land surface to the top of the bedrock (no less than 20 feet by rule and often 40 to 80 feet or more in practice). Much of the groundwater supplied to a bedrock well results from the open borehole below the casing that intersects water bearing fractures in the bedrock. The volume of water available from a well is not dependent upon the casing depth, but really more on the placement of the submersible pump in the open hole interval below the casing. It is common to set submersible pumps 10 to 20 feet above the total depth or bottom of the borehole. Ideally, the well tag and pump tag will provide these numbers to a homeowner. Most of the supply wells that are being drilled these days are deep enough to almost be drought-proof, but common sense dictates that well owners should also be mindful of water conservation in times like this.
As you mention, it is possible to measure the depth to water using a string and a weight, but I'd really recommend that homeowners stick with the estimates provided by the well/pump tags or consult with a certified well driller or a hydrogeologist. If you have any questions, feel free to let me know.

My response
I agree that the tag contains a static water level number. That number should be good for a day. If it is good for more than a day there is no reason to monitor this data on a daily basis like they do on the website you referenced.
The static water level on my property changes a lot more than a foot or two. Since I have been on the property, I have measures a fluctuation of more than 22 feet.
To get a more scientific measurement I worked through the data on the website and this is the first example I come up with. Iredell County, North Carolina
Hydrologic Unit Code 03050101
Latitude 35°31'35", Longitude 80°52'42" NAD83
Land-surface elevation 803.08 feet above sea level NGVD29
The depth of the well is 28 feet below land surface.
The depth of the hole is 28 feet below land surface.
This well is completed in the Piedmont and Blue Ridge crystalline-rock aquifers (N400PDMBRX) national aquifer.
This well is completed in the SAPROLITE (110SPRL) local aquifer. In this example there was a fluctuation of 10 feet within one year. From September 2002 to September 2003.
In my article I said the volume available is the difference between the depth of casing and the static water level. If we have a disagreement this is the crux of our disagreement (or from your point of view, my misunderstanding). I think the amount of water in an underground aquifer is mostly above the bedrock. When I look at examples of bedrocks in mines and in mountain highway cuts, I don’t even see the fractures. My guess is that less than .1% of the total volume of bedrock is water. Once you get above bedrock you start getting 5 to 10% pore space. So the vast majority of underground water here in Cabarrus County is located in this 5 to 10% of the pore space from the bedrock to the static water level. Now the depth of casing in a properly constructed well is the same as bedrock. So it would be fair to substitute depth of casing for bedrock in the previous statement. This means the vast majority of the underground water here in Cabarrus County is contained between the depth of casing and the static water level. My educational goal is that people understand where this water is located and be able to measure and utilize it appropriately.

So far I haven't heard back from this individual. Let me hear what you think.

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