Understanding surface water and your well

By Medge L. Carter
Special to the Rappahannock News
With recent spring rains, some area homeowners may notice an increase in sediment in their private wells.

Besides being an annoyance, this increase in sediment may signal that the well is influenced by surface water that is somehow finding its way into the well and compromising the water supply. Surface water can contain pathogens and contaminants commonly found on the ground’s surface. The purpose of this article is to explain some of the reasons a well may be surface-influenced, and to suggest a few possible approaches to alleviating the problem.

In the Virginia Piedmont, groundwater moves through fractures in the rock. Most newer drilled wells here are non-flowing Artesian wells, which means that the pressure on the aquifer from above causes the water to rise above the depth at which it enters the well. We draw on this stored water when we turn on the tap.

We are lucky in that usually, when one drills a few hundred feet down in any given location, one encounters one or more water-bearing fractures that are adequate to support a household. But because we don’t know the precise hydrogeology of any given site, we don’t know exactly where that water is coming from. It can be surface-influenced somewhere along its path. In general, the shallower the source, the more likely it is surface-influenced.

Possible problems can also occur during well construction. To explain those, an understanding of basic well construction is required. When a well is drilled, a large bore-hole is drilled until the driller hits hard rock. Sections of casing (nowadays usually six-inch steel or PVC pipe) are then inserted into the hole, the steel welded and the PVC glued together.

The purpose of the casing is simply to hold the hole open so the well does not collapse in on itself, cutting off access to groundwater.
Once the casing is set, the driller drills through the hard rock with a smaller bit inserted through the casing. Typically, the driller drills until they hit at least three to five gallons per minute of water (or, if no water is found — which is rare — until the property owner cries “uncle”).

The yield is frequently composed of water from more than one “water zone.” With low yielding wells, the driller may go further to provide some storage in the well. Once sufficient water has been found, or a decision has been made to stop drilling, the well is grouted. This is a process by which the “annular space” (the space between the casing and the soil) is sealed shut using a slurry of cement or bentonite clay. The purpose of grouting is to protect the well from surface influence or contamination.

Identifying problems

So, problems with well construction can include:

• The well is not cased into solid rock (the borehole is wearing away or, in a worst-case scenario, it collapses, but you’d know it as you’d be pumping mud)
• The well is not grouted properly
• The grout fails over time

There are a few things a homeowner can check. By removing the well cap and shining a flashlight down into the well, one can check to be sure the casing is still intact and not corroded.

Occasionally, older steel casings will be so pitted and corroded at the ground’s surface that soil is oozing through the holes and into the well. Shine the light around the pitless adaptor (the fitting where the water line comes through the casing below the frostline and goes down to the pump, so-named because it eliminates the need for a pit to house the well-head).

If the seal on the pitless adaptor has failed, or if the hole drilled through the casing is too large for the fitting or if the pitless adaptor was set off-center in the hole, soil may be oozing through at that spot.

Finally, make sure there is positive drainage away from the wellhead (the part you can see above ground). On flat or slightly concave landscapes, ponding water sitting around the top of the well can ooze down through a poor grout and get into the well. For obvious reasons, this is more likely to occur during a heavy rain. In situations like this, it may help to crown the soil around the wellhead and create or enhance drainage away from the wellhead.

The appropriate fix depends on the exact nature of the problem, which can often be difficult to determine. In my opinion, one of the best chances for fixing a surface-influenced well is to install a liner in the well, effectively casing and grouting it deeper.

This may alleviate sediment from either a shallow water zone or a problem with the grout. Most wells in Rappahannock are cased approximately 20 to 30 feet, because 20 feet is the minimum allowed by code, hard rock is generally pretty shallow here – and it is the most affordable option.

In many cases (no pun intended), it is possible to insert a narrower casing (four-inch) inside of the six-inch casing, with a “boot” on the bottom to hold it in place, and then grout between the two casings. That will serve the dual purpose of sealing off some shallower sources, as well as providing a secondary grout on the well.

The risk to this procedure is: If the only water zone feeding the well is shallow, casing it off with a liner may cut off the water supply to the well, rendering it useless. The situation should be carefully evaluated by a reputable well driller before proceeding.

Another option is to drill a new well. In choosing this option, consider casing and grouting to a depth of at least 50 feet in an effort to prevent the same thing from happening with the second well.

A final option is to decide to live with it. Various water treatment devices can help alleviate the problem. As I said earlier, the concern with surface influence is that bacteria and other pathogens can get into the well. To determine if this is happening, consider testing the water for the presence of bacteria during one of these heavy rain events.

Each situation is different. Some may require the services of a licensed well driller; some may require the services of a qualified water treatment contractor. Feel free to call with questions and I will assist as best I can: 540-675-3516.

Medge L. Carter is the environmental health officer at the Rappahannock County Health Department. This article was adapted from a recent post on Rappnet, the county’s email list-serve (www.rappnet.org).

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