Surely this is a simple question, but I've yet to find a good answer...

How do I determine the depth of the water table? I imagine it's different from the depth required to dig a well. Also, how do I determine if digging into the water table will be sufficient water to fill a pond?

I think it's called "head" to well drillers. They drill the test hole and then see how high the water rises above the original depth at which it was encountered. This represents the (I think) transmissivity and pressure of water within the aquifer. The depth to the water minus the head equals the elevation at which your pond would fill.

Wow, quick reply! I'll look back at the well-drilling logs, but I think this would work out to about 178 feet if I remember correctly...a little too deep for a pond

If you have a nearby creek or stream the water table will likely be closer to the surface, however a pond built close to a waterway is at a higher risk of flooding and the subsequent introduction of unwanted fish species.

IMHO, it IS different. The water table we worry about wrt ponds is "surface water," which can vary a fairly quickly due to rain, dryness, etc. The water table we worry about wrt wells is "ground water," which does not vary much due to short term rain or dryness, but can of course decrease noticeably from long-term drought or excessive pumping out of the aquifer.

At my farm, surface water will cause most postholes to fill up about 10 months a year. But to get a good ground water flow for our wells, we're down 100-150 feet.

This has always been my position on the water tables, which I have been unable to get developers and zoning commisions to understand. I would appreciate being straightened out by anyone who knows I'm wrong. (Really.)

This is not one question but I can help out with some general definitions.

"Water table" is an ambiguous term. It can be a perched water table or the "piezometric" surface which can be thought of as a regional water table. Hydrogeologists can use the terms interchangeably. The general concept for a water table is that it is "unconfined" meaning that it does not have an impermable unit (clay, shale, etc.) above it.

If an aquifer is confined, the measured water level is called the potentiometric surface, which is generally above the actual pemeable layer due to the pressure of the overlying materials. The pressure can be significant causing the water level to rise a great distance above the aquifer, even above the ground surface(artesian well).

Head is simply the measured water level or elevation. If a well is placed into saturated material, the measured water level is the head. The differences in head create the potential for flow to occur from areas of higher head to those with lower head.

Except for being visible on the surface, prone to evaporation, and vulnerable to contamination, surface water only has one basic difference with groundwater to a hydrogeologist making flow calculations. Instead of being contained in a medium such as sand that has 35% pore space and limited permeability, water can be thought of as having 100% pore space and unlimited permeability. Otherwise surface water heads are handled similar to groundwater heads in flow calculations.

Surface water can be perched such as in a lined pond or connected to the groundwater table such as an unlined gravel pit pond. Head can be measured the same way in a pond based on the elevation of the water surface.

If the water level in the pond is higher than the water level in an aquifer, there will be potential for flow from the pond to the aquifer (leak). The opposite is also true (spring).

I have several water table ponds. We finally had some rain recently and you can see the initial increase in water levels directly from rain to the pond surface, followed by several days of groundwater recharge to the pond as a result of increased head from the rising water table due to infiltration of rain in the drainage basin. The opposite occurs during evaporation, which lowers the water table in the vicinity of the pond. Liners just slow down the process.

Theo, in your case where groundwater is so deep, the surface water you refer to is most likely all perched water so your position is correct. It wouldn't necessarily stand up at my place, where those same post holes are right into the water table. The water table at my property fell almost 4-feet this summer as did my ponds but they are always connected with the aquifer unless it falls 12-feet where it would be below the pond bottom. Keep in mind that even clays have limited permeability so differences in head will cause flow through the clay, however, it is very slow (by the way, hope the Bucks pound the skunks this Saturday).

There are also capillary effects due to surface tension that cause soil moisture in a thin zone above the water table but I don't think that was the question.

Crisw, you will probably need professional help to calculate your water budget. To really do it right, you balance all of the water input/output of the drainage basin, not just your pond (precipitation, groundwater recharge, runoff, etc.) minus the outflow (evapotranspiration, streamflow, seepage,well withdrawals,stream discharge, etc).

A simplified approach can be made if you know the annual precipitation, pan evaporation rate, aquifer thickness, permeability, and water level in the aquifer. Digging a water table pond can have negative effects that can get you trouble with upgradient users.

There should be someone at the loccal NRCS office who can help. The easiest way to figure it out is to investigate other pond projects near you. Practical experience in your area may prevent you from wasting money on a no brainer.

Thanks, Steve! Great post. Five stars.

Thanks to all for the replies!

The area where we are thinking of building the pond is in La Center, WA. Plenty of people in the area have ponds, so I should be able to get some info. The site we are looking at is on gently sloping ground at the bottom of the property, so the drainage field is pretty good, and we are looking at a pretty small project- 1/4 to 1/2 acre. We live in San Diego, CA right now- ponds are a real rarity here! We won't be beginning contruction for 4-5 years, so we have plenty of time to look into things.

Good post Steve and nice follow up link Robinson. Thanks to both of you.

Best way I know is by offsets. Check with all your neighbors that may have (or did have) wells, and ask how deep they were.

If you have a lot of high ground around you, you have a fair chance of hitting shallow ground water, a seep, or a spring. If you do have hills above you, you can usually find spots that stay damp well into the summer. If you find one, maybe dig a small test hole and see what it does over time.

That said, relying on ground water is always an iffy situation. It's very lucky indeed when you have a spring or ground water that adds more water year around than it takes away.

This is interesting. So I have a question. If I dig a hole and after some short time it fills partially up with water (not caused by rain), does this mean I've dug down into the water table? That's how I've always thought about it. Or is this still considered surface water? In the case of my pond I dug it out of a low lying area that was always wet and the pond filled up regardless of rain does that mean I've dug it below the water table and thus the water table is sustaining my water level? I think that's what I've done and I guess I should consider myself blessed in that regardless of how little or much rain we get my pond level only fluctuates about 1 foot. Underground water has always intrigued me.

LBuck I think you're confusing "static water level" in a well with the "water table." The static water level is more related to the actual pressure in the aquifer and how much it causes the water to come up in the well pipe. I.e. I can hit water at 3 feet on my property. However the static water level in one of my wells is 16 feet. The other one I use for aquaculture on higher ground is about 30 feet.

This all depends on several things. Springs and seeps come from higher ground, and can actually fill a pond year around. (or at least it would not drain the pond in dry weather).

If you have a lot of surrounding high ground, you can actually hit water bearing sand very close to the surface. In this case, your pond would stay pretty full year around. When the water table drops in dry weather, the pond level probably would not drop a lot. Sounds like that's your situation, you lucky dog.

Think of the three scenarios described above, groundwater hydraulics is pretty simple.

perched water - limited supply, dry beneath, usually occurs above a shallow clay or silt layer but it could be unfractured rock, shale etc., can be very localized.

confined aquifer - static water level will rise above the aquifer due to pressure. Impermeable layer above the aquifer (silt or clay).

unconfined - water table aquifer, static water level in well is same elevation as water table.

There are only a few exceptions like Karst and then there are also leaky confined aquifers but we don't need to go there.

In Cecil's example he probably has some perched water at 3-feet, then a clay or silt that holds the perched water. The wells are screened in a permeable zone that is deeper. He didn't say what the static water level in the wells is but I'd guess that it is above the well screen based on the presence of the perched zone at 3-feet which indicates a confining unit.

bz, sounds like you are right on. bobad's right the high ground creates the head that provides flow into your pond. Same situation that we have with our ponds. The key with water table ponds is having adequate supply from the drainage basin to offset evaporation. At least indirectly, all fresh water comes from precipitation. Water table ponds just "borrow" groundwater as it migrates by.

Steve,

Actually we have water under a confined layer here which is probably the clay that causes the water to perch on -- not too far below the surface. Apparently even with the thick clay there are areas that water can perculate down to recharge the aquifer as we have abundant supplies of water in sand and gravel aqifiers in the area. My sturated gravel layer is at least 60 feet thick and some areas around here can support the pumping of 2000 gpms.

Recharge probably takes place through sand and gravel veins and other permiable areas higher up which I have encountered on the property.

Most of the wells in my area are classified as artesian wells but don't have enough pressure to bring the water above ground to become flowing artesian wells. However the water will come up in the well pipe far above where contact is made with the aquifer and above where the well screen is postioned.

Sure wish mine was flowing! I do have flowing wells some within a mile of me. Darn!