Monday, December 29, 2014

The Causes of Reduced Well Flow

If your household water is supplied by a well, responsibility for maintaining your water supply falls to you, and there are many potential causes of what seems to be a loss of water pressure or water volume. In a well, a diminished water supply or well yield can be caused by drop in water level due to drought or over pumping of the aquifer, the well could be failing or fouling or there might be an underlying well construction or design problem. There are also equipment problems that seem to mimic a failing well- a leak in the pitless adaptor or pipe to the house or a worn or damaged pump impeller could reduce well flow or water pressure. Remember that equipment problems are the most common cause of well problems. So let’s start there.

The essential components of a modern drilled well system are: a submersible pump, a check valve (with an additional valve every 100 feet), a pitless adaptor to bring the water to the house below the frost line, a sanitary sealed well cap to keep out vermin and bugs, electrical wiring including a control box, pressure switch, a pressure tank to literally push the water throughout the house and an interior water delivery system known as your plumbing. There are additional fittings and cut-off switches for system protection, but the above are the basics. To keep the home supplied with water each mechanical component in the system and well must remain operational.

A leak in the piping from the well to the house could reduce the well flow, a damaged pump or the components in the basement that provide consistent water pressure and the electrical switch that turns on the pump. Look for indication of moisture, and subsidence to find a leaking pipe between the well and the house. In the house water goes into the pressure tank. Inside the pressure tank is an air bladder that becomes compressed as water is pumped into the tank. The pressure in the tank moves the water through the house pipes so that the pump does not have to run every time you open a faucet. The pressure tank typically maintains the water pressure between 40-60 psi or 30-50 psi for smaller tanks. After the pressure drops below the cut in pressure (typically 40 psi), the electrical switch turns on the pump and the pressure in the tank increases as the tank fills. If however, the pump is not delivering water fast enough the pressure tank could fail to regain its head while the water is in use. Also, jiggle the tank to make sure that there is not a hole in the bladder and the area above the bladder is not filling with water and becoming water logged. Sometime just draining the pressure tank, bleeding the air out and recharging it will improve a situation, but like any mechanical piece of equipment pressure tanks do wear out.
from USGS

The well itself can also be the cause of reduced well flow. The well has a casing (a metal or plastic liner) that may extend the length of the well, or at least to the bedrock and then have some sort of slotted casing, screen or “sock” around the pump impeller to keep debris, sand and sediment out of the system. In Virginia, there have been well regulations in place since 1992 to prevent poorly designed and developed wells but, it still happens and there are still a huge number of wells that predate regulations.

If you are having a well drilled check to make sure that the well driller is licensed and that the well is built according to regulations (if your location does not have well construction regulations check the regulations in other states or provinces to make sure you get a quality well). Always use a local well driller with experience in your immediate vicinity, the type of well construction must be matched with the geology and the characteristics of the aquifer. Experience is often helpful (but not everyone is capable of understanding and learning from experience). An understanding of geology and hydrology, very local and detailed regulation, or enough experience of knowing what has worked before is essential when choosing between a perforated well casing or well screen, identifying the right size slotting or screening to use, the placement within the borehole of the screening or perforated liner, whether a sand pack is necessary and where to locate the pump in the well. Poor choices in any of these items could cause problems with excess sediment in your water or reduced well yield.

When you drill a well, mud and bore hole cuttings can partially plug the well. This material must be removed to allow water to freely enter the well during well development. A good well driller will do a better job of this, a less than good well driller will tell you that excess sediment in your new well needs a sediment filter and will happily sell you a new pump when the first one fails prematurely. Sediment does tend to reduce in the first year because not all of the cuttings are removed during well development. If the well has not been fully and properly developed, the well will often produce excess amounts of sediment or have a low water production yield. Though not every well drilled has the potential to provide enough water for a household (even in my water rich part of Virginia), poor choices in well completion design can render even a good well a poor producing well.

Groundwater supply can change because groundwater systems are dynamic. In the Valley and Ridge of Virginia (west of 95 and before the Appalachian Plateau) the geology is characterized by unconsolidated overlay underlain by fractured rock. In the Piedmont region the fractured rock tends to be sedimentary rock and is carbonate rocks within the areas of karst terrain. Fractured rock systems tend to be water rich areas of Virginia, but not uniformly so. In the fractured rock systems of the Valley and Ridge wells draw groundwater from fractures in the bedding plane which run parallel to the vertical fractures. Fractures can run dry or become encrusted. In unconsolidated sediments of the coastal plain ground water is pulled from the saturated zone, but the wells needs to be screened. In the Appalachian Plateau which is a flat layered rock system with horizontal fractures, the coal seams are typically the aquifer and groundwater is typically shallow. Coal country is the location of many shallower dug wells which easily go dry during times of drought.

While many well problems are caused by poor construction, development or operation of the well, the geology can also be a source of problems. Reduced well yield can be caused by lack of recharge. The water withdrawn from an aquifer can be increased by building homes and increased use for irrigation, domestic watering of gardens and/or reduced recharge. The more land area that becomes covered with pavement, and buildings the less water percolates into the ground and recharges the aquifer. If water is withdrawn from a well faster than the aquifer is able to produce, the well is over-pumped and that is reported to be the most common cause of premature well failure. Over-pumping not only depletes the groundwater, but it rapidly increases the rate of sediment drawn into the well by the pumps suction, causing plugging of the perforated area where water flows into the well. It can also cause corrosion, incrustation and biofouling or the aquifer to compact which further restricts water flow to the well.

Sometimes a decline in water level is seasonal or due to a drought. Typically water levels are higher in spring and lower in the fall. Extended dry periods can also impact water levels, especially in shallow aquifers supplying dug wells. Checking the water level in your well or a nearby proxy monitoring well is a way to identify water level trends and aquifer depletion before the problem becomes serious. If you have the opportunity to install a level monitor for your own well, it is a way to identify a failing well or diagnose a problem, but in most instances it is not practical. For years I have coveted a water level monitor (also a Viking stove- but I don’t have either).

Mineral incrustation is a common problem in some aquifers where there is an abundance of dissolved minerals including calcium, magnesium and iron, as well as iron bacteria. If you have hard water, you well can become encrusted when minerals precipitate or settle out during the pressure changes in the pumping process. This causes scale deposits on the casing, liner and screens. Over time incrustation can reduce the flow of a well. If you have scale formation within the well a well can be treated with chemicals or acid or in some geology gently hydraulically fractured. To do this “right” takes equipment and knowledge. There are well treatment specialists and lots of people who have no clue. Be award that an old metal casing may not survive chemical or mechanical treatment and the well may collapse.

Installing and pumping a well often introduces bacteria into the subsurface and increases the level of oxygen and nutrients in the well and surrounding aquifer. Naturally occurring bacteria, such as iron bacteria or sulfur reducing bacteria, may thrive under these conditions. Iron bacteria, sulfur reducing bacteria and related bacteria can form a gel-like slime that captures chemicals, minerals and other particles such as sand, clays and silts. "Biofouling" occurs when the accumulations of gunk are sufficient to reduce water flow through screens and slats or impair the pump. This can mean reduced well yield and water quality. Iron bacteria buildup is a problem that I have dealt with. A couple of years ago I chlorinated the heck out of the well (800 ppm chlorine- I kid you not). That single treatment has kept my house and hopefully my well iron bacteria free since. I keep an eye out for slime build-up on the toilet flappers and will treat the system again when necessary- before I have reduced well yield or pump damage.

It is important to understand what is going on with your well before you begin replacing pumps or drilling new wells. Sometimes it’s just a loose wire or a waterlogged pressure tank, other times you have much bigger problems.

No comments:

Post a Comment