Artesian Aquifers – Water Quality
(Published July 2003)
Two artesian aquifers beneath Northumberland and Lancaster Counties provide potable water for about 75% of our citizens. Water from both artesian aquifers is being withdrawn at an unsustainable rate. But problems also exist with the water quality in the two aquifers, and ultimately, continued withdrawal of water from the deep aquifer will further degrade water quality.
The shallower artesian aquifer is encountered at depths between about 250 and 400 feet. Water quality is good to excellent in the northern part of Northumberland County, but toward the south the water is rarely used because it is often brown due to the presence of “tannic acids” and iron. The deep artesian aquifer is used by most citizens, and is encountered below about 600 feet. Both aquifers are found at greater depths toward the eastern part of the Northern Neck. As long as well casings are not cracked or broken, and the upper part of the casing is properly grouted, no problems with either bacteria or high nitrate should be encountered in artesian wells.
Several components are useful to describe the water quality, or chemical composition of the water, and two common components can cause health problems. Total Dissolved Solids (TDS) describes the total amount of dissolved substances in the water. TDS is normally expressed in parts per million (ppm), which is essentially the same as milligrams per liter (or kilogram) of water. If you multiply the TDS by 10,000, the units are weight percent. For reference, average seawater has a TDS (salinity) of about 35,000 ppm, or 35 parts per thousand, or 3.5%. Two components of concern with respect to our health are sodium (chemical symbol Na) and fluoride (F). EPA has never established a Maximum Contamination Level (MCL) for sodium, but some countries, including Canada, use 200 ppm. Health professionals recommend that individuals on a sodium-restricted diet do not drink water containing more than about 20 ppm sodium.
All our artesian water contains more than 20-ppm sodium. Fluoride is also of concern in causing mottling of teeth in children. The jury still seems to be out as to whether or not fluoride helps prevent tooth decay, or causes bones to be stronger, or more brittle, at least when the relatively low calcium concentration in our artesian water is taken into account. In any case, EPA’s Secondary Maximum Contamination Level (SMCL) of 2 ppm for fluoride is exceeded in the deep artesian aquifer in most of the southeastern part of the Northern Neck. The MCL of 4 ppm F is rarely reached.
(See maps with Sodium and Fluoride content at bottom of page.)
This table presents typical values for the two artesian aquifers in Northumberland and Lancaster Counties.
Clearly, the deep artesian aquifer presents more potential for health problems than does the shallow aquifer. But the deep artesian aquifer is also a “better” aquifer because it supplies water more readily than does the shallow aquifer. As a generalization, the TDS content of the water increases with increasing depth. There is no point in drilling much deeper because the water becomes too salty to drink. Sodium (fluoride, and just about all other dissolved species, especially chloride) increases in concentration toward the southeast as is shown on the accompanying map. The reason for this fact is just bad luck. Approximately 35 million years ago an object from outer space hit the ocean around Cape Charles. The impact pulverized the rocks, making it easier for water to flow through them, and seawater rushed in to fill the crater. Therefore salty water is not only closer to the Northern Neck than it would otherwise have been had the impact not occurred, but it is easier for salty water to move to the northwest as we continue to lower the pressure in our artesian aquifers by withdrawal of water. There is no evidence from Health Department records that the process of saline intrusion has begun anywhere in either Northumberland or Lancaster counties – yet.
The water in both our artesian aquifers is classified by geochemists as a “sodium-bicarbonate” type, which is common throughout the world. Bicarbonate (the HCO3- ion) is also commonly expressed as “alkalinity.” The water in our artesian aquifers is thousands or tens of thousands of years old. With time, as rain moved down the eastward-sloping aquifers from near the fall line, or vertically across the confining layers, chemical reactions progressed. Acid-producing reactions, including the decomposition of organic material, caused calcium carbonate (shell) to dissolve, producing a calcium-bicarbonate type of water. Neutralization of the acid by calcium carbonate resulted in a “hard” alkaline water with a pH of about 8.5. Uptake of calcium and release of sodium by clay minerals (ion exchange) accounts for the “soft” sodium-bicarbonate composition of the water today. In contrast to artesian water, rain is acid, with a pH typically around 4. Most of our plants are adapted to our acid soils, and the use of sodium-rich alkaline deep artesian water for irrigation can cause problems with plant growth.
Various kinds of water treatment processes are available to remove the sodium and other dissolved substances from the water, or to replace them with other substances. All kinds of water treatment are expensive and maintenance-intensive, and are unnecessary on a whole-house basis. Drinking bottled water may be the least expensive alternative for people worried about health effects of water from our deep aquifers, and rainwater harvesting is a simple and inexpensive solution to small-scale irrigation problems. Eventually, perhaps a century from now, water quality will have degraded and water levels in wells will have dropped to the point that alternative sources of water, like reservoirs, will be necessary.