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Shallow Well Water

(Published October 2001)

The water we all use in Northumberland County was once rain. About 2/3 of our rain is utilized (transpired) by plants. Because of the porous nature of our soils, very little water runs off as overland flow (streams), but rather, most rain water percolates down through the soil to the water table. Now called groundwater, the water flows laterally through the ground toward lower elevations at rates of about 100 meters each year. Almost all our shallow groundwater discharges into the nearest waterway and carries with it substances which have dissolved along the flow path. This water-table aquifer is penetrated by numerous shallow ("dug") wells for domestic use.

Approximately a quarter of Northumberland County's homes derive their water from shallow wells. The County receives, on average, about 44 inches of rain each year. This means that each acre annually receives over one million gallons of rain! An extravagant person uses about 100 gallons of water each day, so each acre can potentially provide the water needs for about 30 extravagant people. Despite a renewable and seemingly inexhaustible supply, many problems exist with this source of water. Shallow wells can be pumped dry during times of drought. Additionally, limited volumes of water from shallow wells are inadequate where large numbers of people are living in close proximity, or for industrial uses.

The quality of the water in our shallow aquifers is more of a problem than its quantity. The most common complaint is that the water "smells of sulfur". Sulfur smell is the result of hydrogen sulfide (H2S, or "rotten egg gas"), which usually forms because the well has been contaminated with organic material such as leaves, insects, or scum on the walls of the well. To prevent contamination, the cap on the well must be tight enough to prevent particles from falling into the water. The cap must also be light tight so as to prevent the growth of algae in the well. If there is organic material in the well, bacteria begin to decompose it (it “rots”). This reaction (CH2O + O2 ----> CO2 + H2O, where the formula CH2O is chemical "shorthand" for organic material) is called respiration, and it consumes oxygen gas (O2), which is dissolved in the water. If the dissolved oxygen is exhausted, other bacteria begin to utilize oxygen in the dissolved sulfate ion (SO4=), which is abundant in our water, producing “reduced” sulfur in the form of (odoriferous) H2S. Once the oxygen is removed from the water, dissolved, or “reduced” ferrous iron (Fe++) can also form. After the water is pumped from the well, the reduced iron reacts with oxygen gas in the atmosphere to form iron oxide “rust”, which precipitates and stains the water and anything it contacts.

Stagnant wells are much more susceptible to these processes caused by oxygen depletion than are wells which are in constant use. The harmless kinds of bacteria that cause the reduction of iron and sulfur are ubiquitous, and should not be confused with harmful kinds of bacteria. The most common, harmful bacteria in our water are the "fecal coliforms” found in the intestinal tracts of warm-blooded animals (including people). They are present in manure and septic systems and, if they enter a well, health problems typically result. Because water always flows "down hill" a shallow-water well should never be located between a septic system (or privy, or animal pen) and the nearest waterway. Bottom line – be sure your shallow well is properly capped, and use it enough to keep the water from becoming stagnant and oxygen depleted.

Another water quality problem is dissolved nitrate (NO3--). Nitrate is a nutrient, and an important component of most kinds of fertilizer applied by agricultural practices and lawn and plant fertilization. Septic systems also contribute considerable amounts of nitrate to our shallow groundwater. The average nitrate concentration of shallow groundwater in Northumberland County is approximately 5 ppm (milligrams of nitrogen as nitrate per liter). Nitrate levels in excess of 1 ppm (about half of our shallow wells) are potentially harmful to infants, and unless shallow well water has been tested, infants should be given only bottled water. Recent research suggests that nitrate concentrations in excess of about 4 ppm (about one third of our shallow wells) may contribute to an increased risk of Non-Hodgkin's lymphoma and perhaps bladder cancer in women. About 8% of Northumberland County wells exceed EPA’s Maximum Contamination Level of 10 ppm nitrate. If you use a shallow well for domestic purposes, you should have it analyzed for both fecal coliform bacteria and nitrate, and discuss the results with a health professional.

Equally important is the fact that our shallow groundwater and its contained nitrate discharges into local waterways (330,000 gallons per acre per year). Once the nitrate enters the waterways it fertilizes the growth of suspended algae just as surely as does the “point source” discharge from wastewater treatment plants. As we seek to reduce the input of nitrate to Chesapeake Bay, so as to de-list the Bay from EPA's "impaired waters" list by 2010, the high nitrate concentration in our shallow groundwater poses a difficult challenge.


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